Soft X-Ray Background Data Sets and Other Resources at the HEASARC and Elsewhere

The last mission to survey the sky in the soft X-ray band was ROSAT, which conducted an all-sky survey from 1990 July 30 to 1991 January 25, and from 1991 August 3 to 13.

The next experiment to carry out a soft X-ray all-sky survey will be eROSITA. eROSITA will be the primary instrument on-board the Russian "Spectrum-Roentgen-Gamma" (SRG) spacecraft that will be launched from Baikonur in 2018 and placed in an L2 orbit. It will perform the first imaging all-sky survey in the soft and medium energy X-ray range (0.3 - 10 keV) with an unprecedented spectral (138 eV at 6 keV) and angular (16 arcseconds on-axis) resolution, and sensitivity, with a grasp (effective area * FOV on sky) that gets as high as 1000 cm2 deg2 at 1.5 keV.

Web Tools at the HEASARC and Elsewhere

All-Sky Soft X-Ray Background Data Sets at the HEASARC

The HEASARC has the following data sets which contain measurements of the X-ray background all-sky intensity and/or spectra obtained with different experiments, listed in inverse chronological order (most recent first):

  • ROSAT All-Sky Survey (RASS)

    These maps of the diffuse X-ray background in the 0.11 - 2.04 keV energy range ('ROSAT Survey Diffuse Background Maps, Paper II', S. L. Snowden, R. Egger, M. J. Freyberg, D. McCammon, P. P. Plucinsky, W. T. Sanders, J. H. M. M. Schmitt, J. Trümper, and W. Voges, ApJ, Vol. 485 (1997), pp. 125-135) are used by the HEASARC's X-Ray Background Tool (XRBT). This tool calculates for a specified astronomical position and either a circle with a specified radius or an annulus with specified inner and outer radii centered on this position the average X-ray background count rate and statistical uncertainty in each of the six standard bands of the ROSAT All-Sky Survey diffuse background maps (R1, R2, R4, R5, R6, R7). In addition, the average count rates are given for the combined 1/4 keV (R1 + R2), 3/4 keV (R4 + R5), and 1.5 keV (R6 + R7) bands. (The count rates for the combined bands are simply the sum of the count rates for each component band, and the uncertainty in the combined band count rate is the square root of the sum of the squares of the uncertainties of each component band). A full description of the use of these maps is given in the XRBT help documentation.

  • Diffuse X-Ray Spectrometer (DXS)

    The DXS experiment was flown on the space shuttle Endeavour from January 13 through 19, 1993. DXS was developed by the University of Wisconsin and its PI was Dr. Wilt Sanders. The HEASARC has, the so-called DXS level 1 reduced data products stored in this directory in the form of gzipped FITS files. The naming convention of the files is of the form "orbitNN_TYPE_SIDE.fits.gz", where NN = 2-digit orbit number in the range 1 to 91, TYPE = 3 or 4 character type of data file, out of the set:

    sci  - science X-ray event data 
    fast - engineering/housekeeping data (1 record per sec) 
    slow - more engineering/housekeeping data (1 record per 4 secs)
    time - time records 
    spoc - spoc (shuttle payload of opportunity carrier) information
    cmd  - commands uploaded during this orbit 
    orb  - orbit information

    and SIDE = 1-character indicator of the DXS instrument that the data are from:

    p  - Port instrument 
    s  - Starboard instrument

    and the final .gz indicates that the file has been gzipped. These files can be read using standard FITS readers such as fdump.

    The HEASARC also has the DXS spectral products in this directory which were derived from a 2004 reprocessing of the raw telemetry. These spectra are trivially different that those presented in Sanders et al. (2001, ApJ, 554, 694) because of minor bug fixes in the processing software. See Sanders et al. (2001) for a detailed discussion of the DXS spectra. Morgenthaler (1998, PhD thesis) has detailed how the spectra and response matrices were derived. The original spectra, response and arf files supplied to the HEASARC were modified in April 2009 in order to be compatible with xspec12 and with the FITS standards for spectra, response and arf.

    In brief, the clean* spectra represent spectra from different regions along the galactic equator, as indicated by the GAL_LON bin in the FITS headers (see also Sanders et al. figs. 8 and 10). The file clean_Allsky_p.flat contains the summation of all of the sky spectra and clean_HISM_p.flat contains the summation of the regions with the lowest count rate (Crux, Pup, Aur). The file clean_back_p.flat contains the spectrum of the instrument background and the dxsmodel* files contain the instrument response matrix in standard OGIP form.

    The total instrument count rate varied in time over the duration of the flight, with the first half having a higher count rate than the second half (see Sanders et al. fig. 9). The files prepended with "1_" and "2_" contain the spectra from the first and second half of the flight, respectively.

  • The SAS-3 low-energy detector

    This directory contains the all-sky image from the SAS-3 low-energy X-ray detector in the C band (0.10-0.28 keV in FITS format. These data were discussed in the paper by Marshall and Clark (1984, ApJ, 287, 633-652: see the ADS abstract appended below). The file sas.fits contains the all-sky intensity map in Galactic coordinates in an Aitoff projection centered on the Galactic Center.

    Title: SAS 3 survey of the soft X-ray background

    Authors: Marshall, F. J.; Clark, G. W.

    Publication: Astrophysical Journal, Part 1, vol. 287, Dec. 15, 1984, p. 633-652.

    Abstract: The results of a survey of the soft X-ray sky in the C band (0.10 - 0.28 keV) are reported. The observations were carried out using two independent flow proportional counters on board the SAS 3 X-ray satellite which had a total angular resolution of 2.9 deg FWHM, and a total exposure of 22,000 cm^2 s sr. It is found that C band counting rates were generally inversely correlated with the column density of the neutral hydrogen on all angular scales down to the lowest angular resolution of the detectors. In the region 90-180 degrees Galactic longitude and 0-90 degrees Galactic latitude, the relation between C-band rates and the column densities of neutral hydrogen was fitted with a residual rms deviation of less than 13% by a two-component numerical model of the X-ray background. For the apparent attenuation column density, a value of 2.7 x 10^20 per sq cm was obtained. On the basis of a computer simulation of the SAS 3 data, it is shown that the observed clumping of interstellar matter was consistent with the magnitude of spatial fluctuations in the C-band map. When the background rates were subtracted from the survey map, the subsequent map showed foreground emission and absorption features with improved sensitivity and clarity. A series of computer-generated maps incorporating the SAS 3 data is given in an appendix.

  • The HEAO 1-A2 experiment

    This directory contains the spectra (and response matrices) of the X-ray background obtained with the HEAO1 A2 experiment (Marshall et al., 1980, ApJ, 235, 4: see below) in mid-1977. The spectra and responses are all in FITS formats:

     hed11c.pha     h111095c.rsp   HED1 M1
     hed12c.pha     h121095c.rsp   HED1 M2
     hed31c.pha     h31257c.rsp    HED3 M1 
     hed32c.pha     h321095c.rsp   HED3 M2
     med1c.pha      m11n.rsp       MED  M1
     med2c.pha      m12n.rsp       MED  M2 

    Title: The diffuse X-ray background spectrum from 3 to 50 keV

    Authors: Marshall, F. E.; Boldt, E. A.; Holt, S. S.; Miller, R. B.; Mushotzky, R. F.; Rose, L. A.; Rothschild, R. E.; Serlemitsos, P. J.

    Publication: Astrophysical Journal, Part 1, vol. 235, Jan. 1, 1980, p. 4-10.

    Abstract: The spectrum of the extragalactic diffuse X-ray background has been measured with the GSFC Cosmic X-ray Experiment on HEAO 1 for regions of the sky away from known point sources and more than 20 degrees from the galactic plane. A total exposure of 80 m^2-s-sr is available at present. Free-free emission from an optically thin plasma of 40 +/- 5 keV provides an excellent description of the observed spectrum from 3 to 50 keV. This spectral shape is confirmed by measurements from five separate layers of three independent detectors. With an estimated absolute precision of about 10%, the intensity of the emission at 10 keV is 3.2 keV/keV/cm^2/s/sr, a value consistent with the average of previously reported spectra. A uniform hot intergalactic medium of approximately 36% of the closure density of the universe would produce such a flux, although nonuniform models indicating less total matter are probably more realistic.

    The led sub-directory contains the A2 LED C-band (0.25 keV) All-Sky map of the soft X-ray diffuse background (Garmire et al. 1992, ApJ, 399, 694) in FITS format.

    Title: The soft X-ray diffuse background observed with the HEAO 1 low-energy detectors

    Authors: Garmire, G. P.; Nousek, J. A.; Apparao, K. M. V.; Burrows, D. N.; Fink, R. L.; Kraft, R. P.

    Publication: Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 399, no. 2, p. 694-703.

    Abstract: Results of a study of the diffuse soft-X-ray background as observed by the low-energy detectors of the A-2 experiment aboard the HEAO 1 satellite are reported. The observed sky intensities are presented as maps of the diffuse X-ray background sky in several energy bands covering the energy range 0.15-2.8 keV. It is found that the soft X-ray diffuse background (SXDB) between 1.5 and 2.8 keV, assuming a power law form with photon number index 1.4, has a normalization constant of 10.5 +/- 1.0 photons/cm^2/s/sr/keV. Below 1.5 keV, the spectrum of the SXDB exceeds the extrapolation of this power law. The low-energy excess for the north ecliptic pole (NEP) can be fitted with emission from a two-temperature equilibrium plasma model with the temperatures given by log T1 = 6.16 and log T2 = 6.33. It is found that this model is able to account for the spectrum below 1 keV, but fails to yield the observed Galactic latitude variation.

  • The University of Wisconsin experiments flown on 10 sounding rockets in the period from 1972 to 1980

    A compendium of documentation and data from the University of Wisconsin experiments flown on 10 sounding rockets over the period from 1972 to 1980 is available in the CDS Archive.

    In this HEASARC directory, we have the all-sky images created from these data. All of the files are in FITS format. These data were discussed in the paper by McCammon et al. (1983, ApJ, 269, 107-135: see the ADS abstract appended below). The 52 files comprise 26 pairs of different visualizations of the soft X-ray diffuse background, corresponding to different energy bands and projections, the rat.fits files containing the intensity maps, and the sig.fits files containing the uncertainty maps. The first part of the file names codes the X-ray band to which it corresponds as follows:

    Wisconsin Band    Energy Range in keV
    B                  0.130 - 0.188
    C                  0.160 - 0.284
    M1                 0.440 - 0.930
    M2                 0.600 - 1.100
    I                  0.770 - 1.500
    J                  1.100 - 2.200
    2-6                1.800 - 6.300

    The second part of the file names codes the all-sky projection used as follows (all maps are in Galactic coordinates):

    Code          Projection
    0         Aitoff centered on Galactic Longitude = 0 degrees
    180       Aitoff centered on Galactic Longitude = 180 degrees
    npol      Polar equal-area centered on North Galactic Pole
    spol      Polar equal-area centered on South Galactic Pole

    Title: The soft X-ray diffuse background

    Authors: McCammon, D.; Burrows, D. N.; Sanders, W. T.; Kraushaar, W. L.

    Publication: Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 269, June 1, 1983, p. 107-135.

    Abstract: Maps of the diffuse X-ray background intensity covering essentially the entire sky with approximately 7 degree spatial resolution are presented for seven energy bands. The data were obtained on a series of ten sounding rocket flights conducted over a seven-year period. The different nature of the spatial distributions in different bands implies at least three distinct origins for the diffuse X-rays, none of which is well-understood. At energies of approximately 2000 eV, an isotropic and presumably extragalactic 500 and 1000 eV, an origin which is at least partially galactic seems called for. At energies 284 eV, the observed intensity is anticorrelated with neutral hydrogen column density, but we find it unlikely that this anticorrelation is simply due to absorption of an extragalactic or halo source. Previously announced in STAR as N83-20892.