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ROSAT Soft X-ray Background HEALPix maps

RASS SXRB R4+R5 map
ROSAT All-Sky Survey SXRB, Mollweide projection, R4+R5 band, E ~ 0.8 keV.

The ROSAT Soft X-ray Diffuse background (SXRB) maps made by Snowden et al. (1997) are one of the most useful all-sky surveys for study of the interstellar medium. They consist of maps of almost the entire sky in six energy bands covering 0.1–2 keV.

The maps as originally made are available at this web page of the Max-Planck-Institut für extraterrestrische Physik (MPE). They consist of 72 fits files, containing count rate and error images for each band, for each of six panels which together cover the sky, with slight overlaps between the panels. These files are derived from the 1990–1991 ROSAT survey observations, and do not include the make-up data collected in 1997 and presented by Freyberg & Egger (1999); consequently they show narrow strips of missing data.

Recently, many all-sky surveys have been produced in HEALPix format, including the WMAP and Planck surveys of the microwave sky. A major advantage of HEALPix is that the entire sky is consistently covered by an equal-area pixelization. In Planck Collaboration XXV (2015) we transformed the original SXRB maps into HEALPix format, to allow detailed comparison with WMAP and Planck results. We provide the resulting images on this page in the form of six fits files, one for each band. Each file contains the count rate and uncertainty data as separate table columns. The files can be visualised by standard HEALPix utilities.

Description File Info File Name PI Channels Energya
(keV)
ROSAT All-Sky Survey SXRB band R1 map
Nside = 256
25 MB
FITS HDR
RASS_SXRB_R1.fits 8–190.11–0.284
ROSAT All-Sky Survey SXRB band R2 map
Nside = 256
25 MB
FITS HDR
RASS_SXRB_R2.fits 20–410.14–0.284
ROSAT All-Sky Survey SXRB band R4 map
Nside = 256
25 MB
FITS HDR
RASS_SXRB_R4.fits 52–690.44–1.01
ROSAT All-Sky Survey SXRB band R5 map
Nside = 256
25 MB
FITS HDR
RASS_SXRB_R5.fits 70–900.56–1.21
ROSAT All-Sky Survey SXRB band R6 map
Nside = 256
25 MB
FITS HDR
RASS_SXRB_R6.fits 91–1310.73–1.56
ROSAT All-Sky Survey SXRB band R7 map
Nside = 256
25 MB
FITS HDR
RASS_SXRB_R7.fits 132–2011.05–2.05
a Reproduced from Table 1 of Snowden et al. (1997): Energy range is 10% of peak response. The R1 and R2 bands share the same upper energy limit because they are bounded by the carbon Kα absorption edge of the PSPC entrance window.

Usage notes

These files are provided to the community as an interim data product, pending a more accurate reduction of the ROSAT SXRB data into HEALPix format by the MPE team. In our analysis, because each output pixel intersects several input pixels, the derived count rate is an average, which somewhat degrades the angular resolution, and introduces correlations between neighbouring pixels. Since we do not have access to the exposure maps, we cannot distinguish pixels with zero detected photons from unobserved pixels. Pixels on the edge of the unobserved strips in the sky coverage have short exposure times and correspondingly large count rate uncertainty, and also are often significantly negative, presumably because the background correction over-compensates for pixels with only partial coverage. Both these effects are propagated into neighbouring pixels by our reduction.

A definitive reduction would:

Description of processing

All the MPE FITS headers list the field centre as the North Galactic pole, which is correct for only one of the six panels. Fortunately the actual field centre is encoded in the file name. After correcting this, we used nearest-neighbour interpolation to put the original pixel data (count rate and error) onto a fine-grained HEALPix map with Nside = 2048. We used the IDL astrolib WCS software to account for the different map projections. The fine HEALPix pixels have size 1.7 arcmin, so that the original 12 arcmin pixels are highly oversampled. We then averaged the data and uncertainties onto an Nside = 256 pixel grid (pixel size 13.7 arcmin), weighting each original pixel by the number of fine-grain pixels it contributes to the output HEALPix pixel. We then merged the HEALPix maps of the six original panels into a single all-sky map, using simple averaging for pixels covered by two or more panels.