Y Uprety

M Chiao

M R Collier

T Cravens

M Galeazzi

D Koutroumpa

K D Kuntz

R Lallement

S T Lepri

W Liu

D Mccammon

K Morgan

F S Porter

K Prasai

S L Snowden

N E Thomas

E Ursino

B M Walsh

SOLAR WIND CHARGE EXCHANGE CONTRIBUTION TO THE ROSAT ALL SKY SURVEY MAPS

DXL (Diffuse X-ray emission from the Local Galaxy) is a sounding rocket mission designed to estimate the contribution of Solar Wind Charge eXchange (SWCX) to the Diffuse X-ray Background (DXB) and to help determine the properties of the Local Hot Bubble (LHB). The detectors are large-area thin-window proportional counters with a spectral response similar to that of the PSPC used in the ROSAT All Sky Survey (RASS). A direct comparison of DXL and RASS data for the same part of the sky viewed from quite different vantage points in the Solar system and the assumption of approximate isotropy for the Solar wind allowed us to quantify the SWCX contribution to all 6 RASS bands (R1-R7, excepting R3). We find that the SWCX contribution at l = 140 • , b = 0 • , where the DXL path crosses the Galactic plane is 33% ± 6%(statistical) ± 12%(systematic) for R1, 44% ± 6% ± 5% for R2, 18% ± 12% ± 11% for R4, 14% ± 11% ± 9% for R5, and negligible for R6 and R7 bands. Reliable models for the distribution of neutral H and He in the Solar system permit estimation of the contribution of interplanetary SWCX emission over the the whole sky and correction of the RASS maps. We find that the average SWCX contribution in the whole sky is 26% ± 6% ± 13% for R1, 30% ± 4% ± 4% for R2, 8% ± 5% ± 5% for R4, 6% ± 4% ± 4% for R5, and negligible for R6 and R7.

INTRODUCTION

The ROSAT All-Sky Survey (RASS) provides the best maps of the soft diffuse X-ray background (Snowden et al. 1997). These maps are consistent with previous all-sky measurements of the diffuse background but have far superior statistics and angular resolution, and after more than 20 years are still the benchmark for all diffuse emission studies at energies less than 2 keV.

It was widely accepted that the diffuse X-ray flux observed in the ∼1/4 keV (R12) band originated largely in a ∼100 parsec (pc) scale million-degree interstellar plasma in the Local Hot Bubble (LHB) with a significant contribution at high latitudes from patches of million-degree emission in the Galactic Halo (McCammon & Sanders 1990;Kuntz & Snowden 2000;Bellm & Vaillancourt 2005). The 3/4 keV (R45) band arises in 1.5 -3 × 10 6 K plasma located more extensively throughout the Galactic disk and halo, with 30-50% of the flux at intermediate and high latitudes due to AGN and a modest but poorly-determined contribution due to stars close to the Galactic plane and toward the center (Snowden et al. 1997;Mushotzky et al. 2000). Shadowing studies using molecular clouds located near the outer boundary of the LHB showed that most of the observed R45 flux originated beyond these clouds, and that the several percent that was "local" was concentrated in the OVII lines near the low-energy limit of the band (Snowden 1993;Smith et al. 2007;Henley & Shelton 2008;Gupta et al. 2009). The X-ray flux in the R67 bands above 1 keV is thought to be almost entirely from AGN. During the course of the six-month ROSAT survey it became apparent there was a significant source of background that was strongly variable on a time scale of several hours to a couple of days in the R12 and R45 bands (Snowden et al. 1994). The source was unknown, but since the time variation was short compared to the observing time for each part of the sky, these "long-term enhancements" (LTEs) could be empirically removed. The good agreement with the zero points of earlier surveys gave some confidence that there were no large residuals (Snowden et al. 1995). Much later, after the discovery of X-ray emission from comets (Lisse et al. 1996), it was recognized that the LTEs were largely due to Solar Wind Charge eXchange (SWCX) in the outermost parts of the Earth's geocorona (Geocoronal SWCX) [START_REF] Cox | The Local Bubble and Beyond[END_REF]Cravens et al. 2001). SWCX refers to the process by which the heavy ions of the solar-wind pick up electrons from neutrals (mostly H or He) to emit EUV or low energy X-rays (Cravens 1997). It was soon realized that there would also be X-rays from SWCX on interstellar neutral H and He passing through interplanetary space (Heliospheric SWCX) [START_REF] Cox | The Local Bubble and Beyond[END_REF]. However, the much slower time variations in this component due to the extended spatial distribution of the target neutrals largely precluded its being removed in the survey analysis.

Despite numerous efforts, an accurate estimation of the heliospheric SWCX contribution has been hindered by the poorly known cross sections for producing the many X-ray lines from SWCX, limited data on heavy ion fluxes in the Solar wind, and the general spectral similarity of SWCX and thermal emission. The best current calcula-tions show that SWCX could generate anywhere from 25% to 100% of the R12 band emission in directions of the minimum observed flux, e.g., the Galactic plane (Cravens 2000;Koutroumpa et al. 2009b). As a result, studies of soft X-ray emission from the ISM and beyond were plagued by the uncertainty of an unknown SWCX contribution, and even the existence of the LHB was held by some to be in doubt (e.g., Welsh & Shelton (2009)).

The DXL mission was designed to reduce these uncertainties by direct measurement of the interplanetary SWCX contribution (Galeazzi et al. 2011(Galeazzi et al. , 2012;;[START_REF] Thomas | Gamma-Ray Space Instrumentation for Astronomy XVIII[END_REF]Galeazzi et al. 2014;[START_REF] Uprety | [END_REF]. This was done by comparing the X-ray flux measured during a scan across a high density feature in the interstellar neutral distribution called the "helium focusing cone" with observations of the same part of the sky made 22 years and 3 months earlier during the ROSAT survey, when the lines of sight passed through more typical parts of the solar system. Given well-established models for the 3-dimensional distribution of neutral H and He in interplanetary space (Koutroumpa et al. 2006;Lallement et al. 1985aLallement et al. ,b, 2004)), all of the highly uncertain atomic parameters for production of many X-ray lines by charge exchange and the poorly known fluxes of various heavy ions in the solar wind could be rolled into a single parameter to be fit to the data (see § 3). An additional parameter was introduced to allow for differences in the Solar wind flux between the ROSAT and DXL observations. The model has to assume isotropy of the Solar wind, but both ROSAT and DXL observations were made close to maximum of the Solar cycle where this is not unreasonable, particularly for the small region near the ecliptic plane where the observations were made.

Our initial analysis of these measurements indicates that SWCX contributes about 40% of the R12 band on the Galactic plane near l = 140 • , where the R12 flux is close to its minimum value (Galeazzi et al. 2014). In this paper we extend these results to the other RASS bands, and use the best fit coefficients for the atomic and Solar wind characteristics to predict interplanetary SWCX contributions to the RASS maps over the entire sky. In § 2 we briefly describe the characteristics of the DXL mission, in § 3 we focus on the data analysis, § 4 contains the result of the investigation, and conclusions are discussed in § 5.

DXL

The technical details of the DXL mission were extensively described in Galeazzi et al. (2011). To summarize, DXL is a sounding rocket mission designed to measure the contributions of the LHB and SWCX through the spatial signature of their emission. The main instrument on the payload is DXL, which consists of two large area proportional counters mounted on an aluminum frame and supported by the rocket skin. The instrument was inherited and refurbished from the Wisconsin All Sky Survey program (McCammon et al. 1983). The payload also carried the first prototype of a wide field-of-view soft X-ray camera using micropore reflector technology (Collier et al. 2015). DXL was successfully launched from White Sands Missile Range in New Mexico on December 12, 2012.

The DXL instrument is composed of two co-aligned, large area (1, 000 cm 2 physical area per counter) pro- -We matched DXL bands (dotted) as closely as possible to the ROSAT bands (solid) for direct comparison. The DXL bands are labeled as D1 (black), D2 (red), D4 (green), D5 (blue), D6 (cyan) and D7 (magenta) in correspondence to ROSAT's R1 (black), R2 (red), R4 (green), R5 (blue), R6 (cyan) and R7 (magenta) bands. The bands are defined by the grasp (a product of effective area to solid angle in cm 2 sr as a function of energy.

portional counters (CI and CII) that provide excellent counting statistics when scanning the sky in the limited observing time of a sounding rocket flight. The grasps of the two DXL counters are shown in Figure 1. When coupled with the optics (6.5 • FWHM mechanical collimators, electron blocking magnets, and carbon-based thin windows with supporting mesh for the proportional counter gas), the grasp (effective area-solid angle product) of each counter in the 1/4 keV range is ∼6 cm 2 sr.

The counters use a wire-wall design and are filled with P10 gas (90% Argon, 10% methane) at about 800 Torr. The X-rays are mechanically collimated using 2.5 cm thick, 3 mm cell honeycomb. Ceramic magnets embedded in the collimator provide a magnetic field of 150 Gauss within and above the collimators which intercept electrons that could either generate X-rays when impacting the collimator structure, or mimic X-ray events in the detectors. The detector employs a three-sided veto for cosmic-ray rejection using anodes at the sides and back of each main counter.

One side of each counter, facing the mechanical collimators, is covered with a thin X-ray window (25 cm × 50 cm). For the first DXL flight, we used thin (80-90 mg cm -2 ) plastic windows supported by a 25 µm thick, 100 lines-per-inch nickel mesh to retain the counter gas while allowing soft X-ray photons to enter the counter. The windows are composed of Formvar (the registered trade name of the polyvinyl formal resin produced by Monsanto Chemical Company) with an additive, Cyasorb UV-24 made by American Cyanamide, to absorb stellar ultraviolet photons that could otherwise generate a large non-X-ray background. The windows were originally manufactured for the DXS shuttle mission (Sanders et al. 2001) and DXL used the spares that remained preserved. The windows were completely recalibrated for the DXL mission. While the energy resolution of the proportional counters is relatively poor, the carbon edge at 0.284 keV in the filter response allows a clear separation of the events into different bands, roughly corresponding to those used for the RASS. Figure 2 shows the normalized DXL (D1-D7) and RASS (R1-R7) bands for comparison.

To separate the SWCX contribution from the other components of the DXB, the DXL mission uses the spatial signature of SWCX emission due to the "He focusing cone", a higher neutral He density region downwind of the Sun (Michels et al. 2002). Neutral interstellar gas flows at ∼25 km s -1 through the solar system due to the relative motion of the Sun and the local interstellar cloud. This material, mostly hydrogen atoms with about 15% helium, flows from the Galactic direction (l,b)∼(3 • ,16 • ), placing the Earth downstream of the Sun in early December. The trajectories of the neutral interstellar helium atoms are governed primarily by gravity, executing hyperbolic Keplerian orbits and forming a relatively high-density focusing cone downstream of the Sun about 6 • below the ecliptic plane. Interstellar hydrogen, on the other hand, also experiences significant impact from radiation pressure and ionization, creating a neutral hydrogen cavity around the Sun, with negligible focusing.

To maximize the signal from SWCX, DXL started scanning from l = 140 • , b = 0 • , to l = 185 • , b = -17.5 • and returned, to scan through the predicted location of the He focusing cone twice (Figure 3). It then performed a fast 360 degree scan of the sky to normalize the data outside the cone and to estimate the instrument background (while looking at the Earth). After the fast scan, the mission again performed two slow scans between l = 185 • , b = -17.5 • , and l = 140 • , b = 0 • (Galeazzi et al. 2014). Figure 4 shows the center of the DXL scan path as a function of time during launch.

3. DATA ANALYSIS Both DXL and ROSAT measured a combination of SWCX and non-SWCX emission. For the December 2012 DXL observation, the Earth was in the He focusing cone and the line of sight was chosen to pass close the greatest SWCX contribution from the focusing cone, while avoiding bright point sources. The ROSAT observation in the same direction, taken from the RASS occurred in September of 1990, when the Earth-Sun line was perpendicular to the He focusing cone. The RASS line of sight at that time did not pass through the focusing cone (although it did, like all lines of sight include heliospheric SWCX emission). Thus the DXL and ROSAT data contain the same cosmic emission but very different levels of SWCX emission. Given a model of the neutral density distribution in the heliosphere and measurements of the solar wind flux during both observations, one can solve for the true cosmic emission.

The DXL scan path was designed to run close to the Galactic plane to minimize any non-local contribution to the diffuse X-ray emission. For both DXL and ROSAT, the observed flux (F ) contains contributions from the heliospheric SWCX (S(t)) and the non-SWCX (mostly LHB -L) emission. The ROSAT data may also include residual Geocoronal SWCX (G), although Kuntz et al. (2015) has shown that this residual is small. Any Geocoronal contribution to DXL is negligible due to the look direction, which is directly away from the Sun. We can therefore write the flux for each mission as a function of the pointing direction ( , b) as:

F RASS ( , b, t) =S RASS ( , b, t) + L RASS ( , b) + G F DXL ( , b, t) =S DXL ( , b, t) + L DXL ( , b) (1)
where the SWCX count rate S( , b, t) is the integral along the line of sight of the product of the ion flux, neutral densities, cross sections for producing charge exchange, and the efficiency summed over all of the lines that fall within the bandpass:

S( , b, t) = i j n i (t)n He v rel (t)σ i b ij g j ds + i j n i (t)n H v rel (t)σ i b ij g j ds, (2)
where i represents the solar wind species and j the emission lines for the species, σ i is the neutral-dependent in-teraction cross sections for individual species, b ij is the neutral-dependent line branching ratio, g j is the instruments response to line j, and v rel (t) is the relative speed between solar wind and neutral flow (the quadrature sum of bulk and thermal velocities). With ion density n i in terms of the proton density n p at R 0 = 1AU , and assuming that it scales as one over the distance R from the Sun squared and that solar wind ion neutralizations are minimal, we can define the compound cross-section as (Galeazzi et al. 2014) :

α = i j n i (R 0 ) n p (R 0 ) σ i b ij g j (3)
In the case of constant solar-wind conditions, the solarwind flux can be removed from the integrals, and the total charge exchange rate with H and He can be written as:

S( , b, t) = n p (R 0 , t)v rel (t)α He n He R 2 ds+ α H α He n H R 2 ds (4
) where n R 2 ds is the integrated neutral column density along the line of sight, weighted by one over the distance from the Sun squared ( 1R 2 ) to reflect the dilution of the solar wind as it flows outward. Note that in the equation above, the assumed stationarity of the ion flux is an approximation because the flux is known to vary strongly on timescales of about a day. These fluctuations, however, are smoothed when averaging over the few-week transit time through the relevant interplanetary region, as is evident from the very good agreement between sky surveys performed by different missions in different years (Snowden et al. 1995). We can therefore approximate the time dependent parameters n p (R 0 , t) and v rel (t) as isotropic and "constant" in time for the duration of each observation, but assume that they can change by a collective adjustable factor over the 20-year interval between the DXL and ROSAT observations.

For simplicity, we can then rewrite Eq. 4 in terms of two parameters:

S( , b, t) = β(t) × N ( , b) (5)
where

β(t) = n p (R 0 , t)v rel (t)α He (6)
depends on the solar wind properties and the cross section with neutrals, and

N ( , b) = n He R 2 ds + α H α He n H R 2 ds (7)
depends on the pointing direction and the ratio between cross sections with H and He. Direct comparison of DXL and RASS count rates would require, in general, detailed instrument responses as well as the spectral shape of the emission. However, because the DXL and RASS responses are very similar, the ratio can be parametrized by a single parameter. For example, the ratio between the DXL D1+D2 and the RASS R1+R2 count rates is adequately parameterized as a function of R2/R1, almost independently of the underlying spectral model or abundance ratios (see Fig. 5). The DXL to RASS count rate ratio shown in The red dots are for counter 1, blue for counter 2. The DXL rates were calculated by folding various thermal and abundances models for a wide range of temperatures of the unabsorbed component (to cover the widest range of ROSAT R2/R1 ratio) through the DXL response. The temperature and normalization of the absorbed component, and the photon index and normalization of the power law were fixed to the best fit values obtained by fitting ROSAT data at the center of the DXL slow scan region.

Fig. 5 as a function of R2/R1 was calculated from spectral model fits to the RASS data along the DXL scan path. The model included an unabsorbed thermal component (SWCX+LHB), an absorbed thermal component (Galactic halo), and an absorbed power law (unresolved point sources). We evaluated the neutral hydrogen column density value, required to model the absorption coefficient, using the Infrared Astronomy Satellite (IRAS) 100 µm data. For the contribution of unresolved point sources, we used a power law with photon index fixed at 1.4 (Masui et al. 2009;Yoshino et al. 2009). For the unabsorbed component, we tested the APEC (Smith et al. 2001), MEKAL (Mewe et al. 1985(Mewe et al. , 1986;;Liedahl et al. 1995) and Raymond-Smith (Raymond & Smith 1977) thermal models11 , and Savage & Sembach (1996) and Anders & Grevesse (1989) abundance models. Once we had determined the best fit, we varied the foreground LHB+SWCX component over a wide range of temperatures and abundances to determine the dependence of the conversion function from ROSAT to DXL upon R2/R1 (D(R2/R1), shown in Fig. 5). Similar procedures have been used for the individual R1 and R2 band, R4 and R5 (using the R5/R4 ratio), and R6 and R7 (using the R7/R6 ratio).

We can then rewrite Eq. 1 as:

F RASS ( , b) = β RASS (t) × N RASS ( , b) + L RASS ( , b) + G (8) F DXL ( , b) =C × D(R2/R1)× (β DXL (t) × N DXL ( , b) + L RASS ( , b)) (9)
where C is a constant parameter to account for differences between the laboratory measured response function and the flight response (should be consistent with 1). Notice that in Eq. 9 the parameter D is a function of R2/R1, R5/R4, or R7/R6, depending on the band that is being fit.

For the analysis of the data, we reduced the DXL data for each band as a function of Galactic longitude (a good proxy for scan path) using 1 degree bins along the slow scan path, and 5 degree bins along the fast scan path. We also extracted ROSAT data in the six bands for direct comparison with DXL. The ROSAT rates were weighted over the DXL collimators using the same set of longitude bins used for DXL. Since the RASS data as published no longer contain the point sources, point source count rates were added to the ROSAT rates prior to data fitting to account for the contribution of the point sources to the DXL rates. The point source rates were obtained from the ROSAT bright and faint source catalog in the DXL look direction. 12 . The source rates were weighted by the DXL collimator response, and corrected for vignetting before adding to the RASS band rates. We found that the point sources contribute between 1% and 5% to the R12 band, and between 2% and 15% to the R45 band along the DXL scan path.

Following the fitting procedure of Galeazzi et al. ( 2014) we performed a simultaneous fit to both DXL counters using Eqs. 8 and 9. We used, as free parameters, the constants C i and C ii for each counter, the RASS SWCX parameter β RASS , and the ratio r = β DXL β RASS . Note that we decided to use r instead of β DXL as it simplifies the fitting procedure and can be more easily interpreted (it is just a constant accounting for the difference in solar wind flux between the time of the DXL and ROSAT measurements). According to solar wind data from NASA's multi-source solar wind database OMNI 13 , r should be close or slightly smaller than 1, as solar wind flux was somewhat weaker during the DXL campaign).

The ROSAT (solid blue line) count rates for R1, R2, R4 and R5 bands, and DXL (black dots) count rates for D1, D2, D4 and D5 bands are plotted against Galactic longitude in figures 6 through figure 9. The respective figures also show the best fit lines (solid red) to DXL data, along with SWCX contributions to DXL (dashed red) and ROSAT (dashed blue) bands.

RESULTS

DXL Results

The result for the combined RASS R12 band has already been reported and discussed in Galeazzi et al. (2014). Here we report our results on all the RASS individual bands and their implications. In Galeazzi et al. (2014), two possible contributions from geocoronal SWCX and two different ratios of H to He compound cross-sections were used. For this work, we also tested different values of the contribution G from geocoronal SWCX to the individual R1 and R2 bands, and we tested different values of the H to He compound cross-section ratio α H α He from 1 to 8, as there is theoretical indication that the ratio may be higher than the maximum 12 http://www.xray.mpe.mpg.de/rosat/survey/ 13 http://omniweb.gsfc.nasa.gov c Using data from the WIND satellite we calculated the DXL solar wind proton flux np(Ro)v rel = 2.7 × 10 8 cm -2 s -1 .

β RASS = np(R 0 )v rel α He (RU cm 3 AU) c 1040±352 1810±341 2712±465 123±79 144±113 218±131 r = β DXL
TABLE 3 SWCX Contributions to ROSAT bands with statistical and systematic errors respectively. c The R12 and R45 results come from independent fits of the data in the two combined bands and are consistent with the sum of the SWCX contribution from the individual bands. c The all-sky average were derived from the values of β RASS and R from Table 2 and the model of the neutral distribution from Koutroumpa et al. (2006) value of 2 considered in the previous analysis. (Vasili Kharchenko, private communication). The comparison between Galeazzi et al. (2014) and Kuntz et al. (2015) also indicates that the ratio may be higher. The fit result depends weakly on the ratio α H α He , preventing us from successfully fitting independently for α H and α He . For example, in the R12 band, the SWCX contribution on the Galactic plane slowly changes by less than a factor of 2 while the ratio is increased by a factor of 8. Table 1 shows the individual parameters for some of the combinations in R12 band. Similar or weaker trends have been found in all other bands. As a consequence, from here on, we show one nominal set of values and include the variation due to different values of G and α H α He in the systematic error (up to a ratio of 4). The systematic error also includes uncertainties in the laboratory calibration (represented by C i and C ii ), differences in the response function between ROSAT and DXL (represented by D(R2/R1)), and the effect of the data range used for the fits (for example, using only the slow scan region vs. the full scan, half the data, etc.).

SWCX a b R1 R2 R12 c R4 R5 R45 c l ∼ 140.5 • , b ∼ 0 • ) (RU) 56±11±20 79±10±7 131±14±30 4±2±2 4±4±3 7±4±4 l ∼ 140.5 • , b ∼ 0 • ) (%) 33±6±12 44±6±5 38±4±8 18±12±11 14±11±9
As expected, the SWCX contribution to the RASS R6 and R7 bands was found to be compatible with zero, therefore, for the rest of this paper, we focus only on the R1-R5 bands.

The fitting parameters obtained in all the bands are presented in Table 2. The table shows the representative data for the condition when the ratio of H to He cross-sections was 2 and the geocoronal contribution to the RASS was assumed to be 50 RU (ROSAT Units or 10 -6 counts s -1 arcmin -2 ) for the R12 band -evenly split between the two bands -and 0 RU for the R45 band. As we previously discussed, changes in the results due to different assumptions are included in the systematic error (up to a ratio α H α He =4). Following the same convention used in Galeazzi et al. (2014), the parameters C i and C ii are the ratios of the fitted DXL response to the nominal value from laboratory conditions for the two DXL counters. The errors in the table are 1 σ. We note that the values of χ 2 are larger than optimal. The DXL mission was designed to focus on the slow scan region, as it is the region that constrains the SWCX to LHB ratio near the Galactic plane. In that region the reduced χ 2 is well within statistically acceptable values for all fits. However, outside the slow scan region, particularly at very low longitude, there are additional effects not included in the model that affect the value of χ 2 , but do not affect the result of the investigation (this was verified by using different fitting ranges).

To better understand the contribution of SWCX to the RASS bands and, more in general, to the diffuse X-ray emission, we use the parameters from Table 2 to calculate both absolute (in RU) and fractional SWCX rates. In particular, in Table 3 we report the SWCX contribution at l ∼ 140.5 o , b ∼ 0, which is were the DXL scan path intersects the Galactic plane. We point out that, according the the ROSAT maps, this is a point where the total diffuse X-ray emission is close to the minimum across the whole sky, and where we therefore expect the highest relative SWCX contribution. Although we used an independent (and slightly different) procedure to reduce the data, the results in the combined R12 band are consistent with those already reported in Galeazzi et al. (2014). In the R12 band, we find that the total SWCX contribution is 131±14(statistical)±30(systematic) RU at the Galactic plane. The combined SWCX contribution is 38% ± 4% ± 8% of the ROSAT count rate at the location.

Extrapolating the DXL Results

We also used the expected distribution of neutral H and He from Koutroumpa et al. (2006) to estimate the SWCX contribution over the whole sky (Table 4). The composite emission coefficients fit in the area of the DXL scan were used for all directions and for the six month duration of the survey, placing considerably more stress on the assumptions of isotropy and constancy in time of the Solar wind. The systematic uncertainties given for the all-sky averages do not include these additional factors, but since the Solar wind is expected to be most isotropic at Solar maximum when the survey was performed, we estimate that these are small compared to those from the poorly constrained ratio of α H /α He . The model for the 3-D distribution of neutral H and He over the Solar system has been tested by Solar backscatter and direct in-situ measurements and should not be a major contributor to the uncertainties. Using this method, the Aitoff-Hammer projection of SWCX contribution in the RASS R12 band is presented in Figure 10. The estimated all-sky average emission from SWCX in the R12 band is 168 ± 20 ± 53 RU, which is 27% ± 3% ± 9% of the total R12 emission. We have also used the estimated SWCX emission of Figure 10 to remove its contribution from RASS. The "cleaned" ROSAT R12 map is shown in Figure 11.

Similarly, in the R45 band, the SWCX contributions at l ∼ 140.5 o , b ∼ 0 is 7 ± 4 ± 4 RU, or 13% ± 8% ± 8% of the observed ROSAT counts. As most charge exchange models indicate that the SWCX contribution to the R45 band is dominated by oxygen emission (mostly OVII), we also used this result to get upper limits on the SWCX OVII+OVIII emissions. Specifically, if we assume that all the R45 SWCX emission is due to OVII+OVIII, we can set an upper limit on the emission of 3.2 ± 1.7 LU. This is consistent with predictions from Koutroumpa et al. (2009a) and with observations in the shadows of nearby molecular clouds (Gupta et al. 2009;Henley & Shelton 2013) showing that the small residual contribution to the R45 band from less than the ∼150 pc distances to these clouds is dominated by OVII line emission. Both the apparent variability of these lines in multiple observations of the same cloud and the very small OVII fluxes predicted by the best emission model fits to the LHB suggest that this is largely SWCX emission. Using the average temperature of 0.0903 keV for the LHB, and an emission measure of 2.55 × 10 -3 cm -6 pc from Liu et. al. (in preparation -see also Snowden et al. (2014)), we calculated that the LHB contributes about 0.37 LU to OVII+OVIII, or ∼3 RU to the R45 on the Galactic plane, which is ∼4% of the average ROSAT rate on the plane. The bulk of the R45 flux must come from non-local emission, identified with Galactic Halo or non-Galactic emission at high latitude. The origin of the bulk of R45 emission on the Galactic plane (where Galactic Halo and non-Galactic emission are absorbed) remains an open question. It seems that an additional, not yet identified source of X-rays must be responsible for about 90% of the total emission on the Galactic plane in the R45 band. This is a long-standing issue in X-ray astronomy, pre-dating the discovery of SWCX (McCammon & Sanders 1990).

The Aitoff-Hammer projection of the SWCX contribution to the RASS R45 band is presented in Figure 12. We updated the ROSAT R45 map by removing the SWCX emission as shown in Fig. 13. Notice that the contribution from SWCX is very small, and its effect on the RASS map is barely noticeable. When extrapolated over the whole sky, the average SWCX contribution is 9±6±7 RU, or 6% ± 4% ± 4% of the total observed rate.

We also used the "AtomDB Charge Exchange (ACX)" model (Smith et al. 2014) to compare our result with the analysis performed by Smith et al. (2014) using data from the DXS mission (Sanders et al. 2001). Specif-ically, we performed a global fit to our estimated allsky SWCX contribution to the R1-R5 bands with "solar wind temperature" (i.e., the temperature of the solar corona responsible for the solar wind) and normalization as free parameters. We find a solar wind temperature of 1.22 ± 0.08 × 10 6 K, within the range of temperatures obtained by Smith et al. (2014). However, we note that Smith et al. (2014) find that SWCX is the main contributor to the total 0.1-0.4 keV flux, with the LHB contributing only 26%±4%, which is inconsistent with the results reported here and in Galeazzi et al. (2014). Smith et al. (2014) relied on an extremely approximate model of the charge exchange spectrum (due to the lack of good atomic data), and errors in the underlying model could have caused the discrepancy. Using data from the DXL sounding rocket mission, we have made quantitative estimates of the SWCX contribution to the RASS maps of the diffuse X-ray background. This contribution was removed from the RASS maps to produce "clean" maps of the interstellar and extragalactic diffuse emission. Averaged over the sky, we found about 27% of the observed flux in R12 and 6% in R45 is due to SWCX. Even at a point in the Galactic plane where the observed fluxes are near their absolute minimum values, SWCX accounts for ∼38% of the R12 band and ∼13% of R45.

CONCLUSIONS

  Fig. 1.-DXL CI (red) and CII (blue) grasp (effective area times solid angle).

  Fig.2.-We matched DXL bands (dotted) as closely as possible to the ROSAT bands (solid) for direct comparison. The DXL bands are labeled as D1 (black), D2 (red), D4 (green), D5 (blue), D6 (cyan) and D7 (magenta) in correspondence to ROSAT's R1 (black), R2 (red), R4 (green), R5 (blue), R6 (cyan) and R7 (magenta) bands. The bands are defined by the grasp (a product of effective area to solid angle in cm 2 sr as a function of energy.

Fig. 3 .

 3 Fig. 3.-DXL scan path into the He Focusing Cone. The color map shows the central region of the cone, with the colorscale corresponding to the He density distribution weighted by one over the distance from the Sun squared ( 1 R 2 ) to reflect the dilution of the solar wind as it flows outwards.

Fig

  Fig.5.-DXL/ROSAT rate vs R2/R1 ratio for the R12 band. The red dots are for counter 1, blue for counter 2. The DXL rates were calculated by folding various thermal and abundances models for a wide range of temperatures of the unabsorbed component (to cover the widest range of ROSAT R2/R1 ratio) through the DXL response. The temperature and normalization of the absorbed component, and the photon index and normalization of the power law were fixed to the best fit values obtained by fitting ROSAT data at the center of the DXL slow scan region.

  Fig.6.-DXL (black dots) count rates in D1 band and ROSAT (solid blue line) count rates in R1 band. The solid red line shows the best fit to the DXL count rate. The solar wind charge exchange contributions to DXL (dashed red line) and ROSAT (dashed blue line) are also shown.

  Fig. 7.-Same as Fig. 6 for the D2 band.

  Fig. 8.-Same as Fig. 6 for the D4 band.

  Fig. 9.-Same as Fig. 6 for the D5 band.

a

  Errors are 1 sigma b The results shown are calculated using α H α He =2, G = 50 RU for the R12 band, G = 25 RU for the R1 band, G = 25 RU for the R2 band, and G = 0 RU for all other bands.

13±8±8a

  Statistical errors are 1 sigma, systematic errors are absolute. b The results shown are calculated using α H α He =2, G = 50 RU for the R12 band, G = 25 RU for the R1 band, G = 25 RU for the R2 band, and G = 0 RU for all other bands.

a

  Statistical errors are 1 sigma, systematic errors are absolute. b The results shown are calculated using α H α He =2, G = 50 RU for the R12 band, G = 25 RU for the R1 band, G = 25 RU for the R2 band, and G = 0 RU for all other bands.

Fig. 10

 10 Fig.10.-The Aitoff-Hammer projection of SWCX contribution to the ROSAT R12 band. The intensity scale shows the SWCX count rate in RU. Note that the sharp edges visible in this map are due to abrupt shifts in vantage point around the Earth's orbit during the ROSAT survey, since the survey comes back to its starting point after six months and there is a missed section that was filled in at a later time.

Fig. 11 .

 11 Fig. 11.-The Aitoff-Hammer projection of ROSAT R12 band in RU after removing the SWCX contribution.

Fig. 12

 12 Fig. 12.-The Aitoff-Hammer projection off SWCX contribution in the ROSAT R45 band. The intensity scale is in ROSAT Units (RU).

Fig. 13

 13 Fig. 13.-ROSAT R45 band map after removing SWCX.

TABLE 1

 1 Summary of the best-fit parameters (described in the text) derived from a fit to DXL and ROSAT data in the R12 band. The table highlights the results from some of the different combination of the parameters α H Summary of the best-fit parameters (described in the text) derived from a fit to DXL and ROSAT data.

	α He	and G.

TABLE 4

 4 Extrapolated SWCX contributions to ROSAT bands averaged across the whole sky.

http://www.atomdb.org/

This work was supported by NASA award numbers NNX11AF04G and NNX09AF09G. We would like to thank the sounding rocket staff at NASA's Wallops Flight Facility and the White Sands Missile Range for their support, technical personnel and undergraduate students at the University of Miami, NASA's Goddard Space Flight Center and the University of Michigan for their support of the instrument's development, and Mark Mulligan at the University of Wisconsin Space Science and Engineering Center (SSEC) for his support with the DXS windows. D.K. and R.L. acknowledge financial support for their activity through the programme 'Soleil Héliosphére Magnétosphére' of the French space agency CNES, and the National Program 'Physique Chimie du Milieu Interstellaire' of the Institut National des Sciences de l'Univers (INSU).


Ioana Georgeta Grosu

Lidia Pop

Maria Miclǎuş

Daniela Niculina

Anamaria Hǎdade

Attila Terec

Crina Bende

Mihail Socaci

Ion Barboiu

Grosu

Ioana Georgeta Grosu

Maria Miclӑuş

Niculina D Hӑdade

Anamaria Terec

Attila Bende

Crina Socaci

Mihail Barboiu

Ion Grosu

-Diiodotetrafluorobenzene Isomers

Keywords: Halogen bonds, N---I contacts, catemers, supramolecular architectures, single crystal X-ray structures

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

The non-covalent self-assembly of molecular building blocks toward complex supramolecular architectures is an exciting process. Their design gives access and investigation to interesting homo-and heteromolecular self-assembled "supramolecular architectures". [1] The main noncovalent forces connecting such organic supramolecular assemblies are hydrogen-bonds, [2]  donor - acceptor contacts, [3] hydrophobic interactions, [4] halogen bonds, [5] but in the large majority of the cases a plethora of contacts are synergistically contributing to the formation of supramolecular architectures. [6] Of special interest, the investigation of halogen bonds [START_REF]Series Editor for Structure and Bonding[END_REF] revealed several types of contacts involving halogen atoms and showed the important contribution of these interactions to stabilize supramolecular structures for which similar strength and directionality like of hydrogen bonds has been revealed. [5e] The X---N contacts (N belongs to a heteroaromatic ring; X is either I or Br) were investigated in finite structures (e.g. supramolecular cages or macrocycles, [8]) or in supramolecular polymers. [9] Difunctional halogen donors and difunctional halogen acceptors (e.g. compounds exhibiting two N heterocycles) prefer to form linear polymeric structures (catemers). [9] Many of these superstructures with unidimensional units were investigated in solid state using single crystal X-ray diffraction. The results of solid-state investigations of X---N contacts were supported by theoretical calculations. [10] Among halogens, iodine gives the strongest contacts and the electron withdrawing groups as F atoms (e.g. in tetrafluorodiiodobenzene isomers versus the non-fluorinated diiodobenzenes) increase significantly the halogen donating ability of the derivatives.

In this context, we considered of interest to investigate the access, structure and properties of supramolecular architectures obtained from dipyridylarenes (I) as halogen acceptors and diiodobenzene or diiodo-tetrafluorobenzene derivatives (II) as halogen donors (Chart 1) Chart 1. Selected building blocks for the access to halogen bonded supramolecular architectures

The ditopic halogen donors exhibit different binding geometries in correlation with the ortho-, metaor parapositions of the iodine substituents, [11,12] while the acceptor shows a quite linear disposition of the pyridine rings (Chart 2). Considering the geometries of the halogen donors and the ideal linear structure for the acceptor, the expected supramolecular assemblies formed by N---I contacts in the case of p-diiodobenzene ligands are linear polymers, while in the cases of metaand ortho-diiodo derivatives the formation of hexagons or triangles in one side or 1D-zigzag polymer chains in the other side are taken into account (Chart 3). Literature data reveal the higher preference of metaand ortho-diiodotetrafluorobenzene for zigzag polymers. [11][12][13] Chart 2. Binding geometries of the target halogen donors (linear or bent) and halogen acceptor [angular (determined from the single crystal X-ray diffraction structure of 2,7-dipyridyl-fluorene)] Chart 3. Expected supramolecular entities (1D linear, zigzag polymers or triangles and hexagons)

Results and discussion

The supramolecular catemers 6-9 were obtained by the mechano-chemical solvent-drop grinding method (SCD) [14] starting from equimolecular amounts (0.06 mmol) of 2,7-dipyridylfluorene-DPF (1) and p-diiodobenzene (2) and diiodotetrafluorobenzene isomers (3-5) (Scheme 1). Small amounts (50 μL) of chloroform were added as solvent. The formation of the self-assembled products was monitored by powder X-ray diffractometry.

Scheme 1

The self-assembly of 1 with meta-diiodobenzene and ortho-diiodobenzene did not lead to the formation of specific halogen bonds and no supramolecular catemers were obtained.

Samples of 1 and 6-9 were solved in CHCl3 and crystals suitable for single crystal X-ray diffraction were obtained (in each case) by slow evaporation (rt) of the solvent.

In order to verify the compatibility of the bulk X-ray fingerprint with the single crystal structures, bulk powders of 6-9 were investigated by X-ray powder diffraction and they showed the same identity with the separated crystals (see SI).

The structure determination of supramolecular catemers in solid state is confirmed by single crystal X-ray diffractometry and FTIR spectroscopy and these results are in accord with theoretical calculations. Complex NMR investigations were also carried out in order to reveal the formation of halogen bonds in solution.

Single crystal X ray diffraction structures and theoretical calculations for 1, 6-9

The main goal of theoretical calculations was to estimate the contribution of different fragments of the lattice to the stabilization of crystal structures. For these proposes different fragments were extracted from the lattice and their calculated energy was compared with the energy of the system exhibiting the same molecules in isolated behavior.

Intermolecular interaction energies between different crystal fragments were determined at the local second-order Møller-Plesset (LMP2) perturbation theory level [15] together with the density fitting technique [16] implemented in the Molpro quantum chemistry program suite [17,[START_REF] Werner | version 2012.1, a package of ab initio programs[END_REF]. The def2-TZVP [START_REF] Weigend | [END_REF] basis set for all atom cases together with the def2-ecp [20] effective core potential for the iodine atom were used for all calculations. In the framework of the LMP2 theory the effects of the basis set superposition error (BSSE) are substantially diminished [21], therefore their correction in the intermolecular interaction energy values were not taken into account. The theoretical calculations evaluate the diminishing of the energy in different crystal fragments (considered representative) compared to the energies of free molecules and they represent the global stabilization of the considered entities and it is not focused on the evaluation of the energetic stabilization brought individually by the different contacts observed in the lattice. The theoretical results are compared with experimental data and they are commented together with the discussion of X-ray diffraction structures. The details of the theoretical calculations are shown in SI.

The single crystal X ray diffraction structures for 1 and 6-9 reveal the formation of different catemeric structures either by N---H-C(2') for 1 or N---I contacts for 6-9. 2,7-DPF molecules are the halogen acceptors for 6-9 and they show slightly different structures, accordingly to their behavior in the lattice. In compounds 6, 7 and 9 different types of 2,7-DPF molecules were observed and they were denoted with A and B for 6, C and D for 7 and E, F and G for 9 and their characteristic data are shown in Table 1. were observed in the single crystal X-ray diffraction structures of other supramolecular polymers of 1,4-DITFB with some specific bidentated acceptors as trans-1,2-dipyridylethene, [22,23] 1-X,4-Y-cyclohexanes (X = Y or X  Y; X, Y = S, N, O) or DABCO [13,24]. The high ability of p-DITFB to form linear catemers is also observed in their cocrystals with some peculiar acceptors which can form bifurcated halogen bonds (e.g. 1,10-phenantroline-5,6-dione, [25], 2,2'bipyrimidyl derivatives [25] or some acetylacetonates [26] complexes).

In the case of catemer 8, the 2,7-DPF molecules exhibit planar fluorene moieties (Figure 6a) and slight torsions of pyridine rings (β, γ = 0.5 and 7.3°). Zigzag shaped chains (Figure 6b According to theoretical calculations, the contributions of N---I halogen bonds for 9 (-4.80 kcal/mol) are lower than for 7 and 8 (7: -7.01, 8: -5.69 kcal/mol) but the contacts observed in other molecular arrangements of the lattice (Figures 9a-e) participate to the stabilization of the structure.

The stacked and the T-shaped dimeric structures of 2,7-DPF molecules (Figure 9a Zigzag type catemers, similar to those of 8 and 9, were also reported in the cocrystals of 1,3and/or 1,2-DITFB with other halogen acceptors as [Cl -, [11,12], aromatic heterocycles with two nitrogen atoms (e.g. 1,4-diazine, 1,4-diazanaphthalene or 9,10-diazaanthracene) [13]. The zigzag shape of the catemers could be also induced by the structure of the halogen acceptors and in this case such structures are obtained even when linear 1,4-DITFB is used as halogen donor. Relevant examples of zigzag chains were reported for the crystals of 1,4-DITFB with 1,4-bis(pyridine-3'yl)-, 1,4-bis(quinoline-4'-yl)-and 1,4-bis(isoquinoline-4'-yl)-1,3-butandiyne, [22] and terpyridine (in trans-trans configuration) derivatives. [27] Tetraiodoethene and 2,3,5,6-tetrapyridylbenzene are leading to formally linear catemers by the formation of cyclic entities by N---I contacts. [28] Solid state FTIR and solution multinuclear NMR investigations

The FT-IR spectra of halogen acceptor 1 and donors (2, 3 and 5) were compared with the corresponding spectra of the co-crystals (6-9) in order to evaluate the presence of halogen bonding interactions. The observed shifts between the free and bound molecules, for some IR vibrational bands, are presented in Table 2 andFigure 10. Similar to the literature [29], there is a shift of the band corresponding to the C=N bond (around 1600 cm -1 ) of 1, but it is small for all investigated co-crystals. According to the literature [30], the band around 1300 cm -1 was assigned to symmetric ring contraction of the halogen bonding donors. We have observed small shifts of the IR bands only for the two para-positioned donors (2 and 3). Some changes were also measured for vibrational bands between 1400 -1500 cm -1 , previously [30] associated with the C-C stretch vibration. The formation of halogen bonding generated some other small shifts in the band at 802 cm -1 of the acceptor 1, while in case of donor 3 we noticed two bands moving after the cocrystalization with the acceptor. The characterization of N---I halogen bonding by solving the corresponding cocrystals in chloroform solutions was attempt using NMR experiments. Thus, 1 H-NMR, 19 F-NMR and 1 H, 15 N-HMBC spectra of ligand 1, of diiodotetrafluorobenzene isomers 3-5 and of cocrystals 7-9 were recorded in order to observe 1 H, 19 F and 15 N NMR chemical shift changes as result of the halogen bonding associations (spectra are presented in SI). For the measurements of the NMR behavior of the associated molecules, the investigated solutions were obtained the NMR solvent.

The comparison of the spectra of free ligands and cocrystals in both 1 H and 19 atom in ligand 1 could not be determined due to the low solubility of quinoline. However, the small changes in 15 N chemical shifts of pyridine nitrogen in the spectra of 7-9, together with the relevant shifting of the signals in 1 H and 19 F NMR spectra in cocrystals can be considered as evidences for the presence of I---N halogen bond associations in solution, too.

Conclusions

The

#Figure 1 .Figure 2 .Figure 4 .

 124 Figure 1. Single crystal X-ray diffraction results for 1: molecular structure (a) and molecule

Figure 7 .Figure 8 .

 78 Figure 7. Results of theoretical calculations for 8 (G°)The repetitive unit of catemer 9 exhibits three types of 2,7-DPF and ortho-DITFB molecules

Figure 9 .

 9 Figure 9. Results of theoretical calculations for 9 (G°)

Figure 10 .

 10 Figure 10. Comparison of FTIR spectra of 1, 3 and 7

  F NMR experiments showed relevant shifting of the signals. Thus, the signals of the protons at positions 2' (close to N atom of pyridine rings) and 3' exhibiting δ2' = 8.705 and δ3' = 7.698 ppm in 2,7-DPF (1) were shifted in the spectra of 7-9 at δ2' = 8.691, 8.694 and 8.693 ppm and δ3' = 7.601,

  single crystal X-ray diffractionstructuresof catemericsupramolecular architectures obtained by the cocrystalization of 2,7-dipyridylfluorene with the isomers of diiodotetrafluorobenzene and with para-diiodobenzene revealed the formation of catemeric structures via N---I halogen bondsand furtherorganization of the catemers into 3D networks reinforced by other contacts (C-H---aromatic, C-H---F, F---I). The theoretical calculations revealed the important role of the contacts between the aromatic units of 2,7-dipyridylfluorene molecules for the crystal cohesive energy. The shape of the structures is strongly correlated with para-, metaor orthoposition of the iodine atoms and the N---I halogen bondingare considerably stronger in the tetrafluoro derivatives as compared to the simple para-diiodobenzene as supported by observed bonding distances and confirmed by theoretical calculationsThe single crystal X-ray diffraction structures of supramolecular architectures obtained by the cocrystalization of 2,7-dipyridylfluorene with the isomers of diiodotetrafluorobenzene and with para-diiodobenzene revealed the formation, via N---I halogen bonds, of linear catemers in the cases of para-diiodobenzene compounds and zigzag catemers for metaand orthoisomers of DITFB. The N---I contacts are weaker for the non-fluorinated diiodobenzene halogen donor and considerably stronger for the diiodotetrafluorinated halogen donors, decreasing in the series para > meta > ortho as it can be deduced from bonding distances and theoretical calculations. The organization of the catemers into 3D networks is reinforced by other contacts (C-H---aromatic, C-H---F, F---I). The contacts in dimer 2,7-DPF entities, either in stacked (6, 7 and 8) or in T shaped (8 and 9) arrangements play an important role for the crystal cohesive energy, while the stacking among DITFB molecules has an important role for 9.

Table 1 .

 1 Structural characteristics of 2,7-DPF molecules encountered in the single crystal X-

	ray diffraction structures of 1 and 6-9
	Compound symbols	α () #	β () #	γ () #	Drawings
	1	-	6	2.3	46	Nu cred ca are rost
	6	A	6.7	30.4	39.1	sa pun deseneleaici
		B	10.5	27.8	35.0	ca o sa para toate la fel
	7	C	0	22.2	24.9	Ce zici?????

Nu prea e clar ce e ecu A B C direct din tabel Dar formulele for fi mai complicat de pus

		D	0	20.9	23.2
	8	-	0	0.5	7.3
	9	E	0	16.2	24.2
		F	0	14.9	24.5
		G	0	24.1	27.5

Table 2 .

 2 Relevant data concerning the modification of IR bands in cocrystals 6-9 in comparison with the starting compounds 1-3 and 5

	Cmpd			IR vibration bands (cm -1 )
		C=N		C-C strech	Ring	Other vibrations
					contraction
	1	1590	1411		-	802
	2				1370
	3			1467	1355	759 945
	5			1442	1493
	6	1592	1409		1373	808
	7	1593	no shift	1455	1360	751 940
	8	1591	no shift			807
	9	1592	no shift	1438	1485

Acknowledgements

We are grateful for the financial support of this work by CNCS-UEFISCDI, project PN-III-P4-ID-PCCF-2016-0088 and by the Ministry of Research and Innovation-MCI, Core Programme, Projects PN19 35 02 01 and PNCDI III 32 PFE.

7.622 and 7.616 ppm. The shifting (upfield) of the signals (δ2') for the protons at positions 2' are -0.014 [START_REF]Series Editor for Structure and Bonding[END_REF], -0.011 (8) and -0.010 ppm (9), while for the signals belonging to the protons at positions 3' considerably higher shifts (upfield, δ3') were observed [0.097 (7), 0.076 (8) and0 Direct evidences for the formation of X---N halogen-bond in solution were obtained by observing the 15 N chemical shifts changes in the spectra of 7-9 and 1. 15 N-chemical shifts were indirectly determined from 1 H, 15 N HMBC spectra through the correlation between pyridine nitrogen and the proton located at the carbon in β-position. In order to increase the resolution and accuracy, 15 N was referenced to quinoline used as external standard, added in a closed capillary in the NMR tube (i.e. the chemical shift of quinoline nitrogen was determined at -68.18 ppm using nitromethane as internal standard), and the spectral window for 15 N spectrum was set to 60 ppm, following a previously reported protocol. [31] Small variations in the chemical shifts of 15 N for the cocrystals of 1 with ortho-, metaand para-diiodotetrafluorobenzene at δ(