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Astron. Astrophys. 320, 685-695 (1997)


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Search for correlations between COBE DMR and ROSAT PSPC all-sky survey data

Rüdiger Kneissl 1, Roland Egger 2, Günther Hasinger 3, Andrzej M. Sol tan 4 and Joachim Trümper 2

1 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str.1, D-85748 Garching bei München, Germany
2 Max-Planck-Institut für Extraterrestrische Physik, D-85748 Garching bei München, Germany
3 Astrophysikalisches Institut, An der Sternwarte 16, D-14482 Potsdam, Germany
4 Copernicus Astronomical Center, Bartycka 18, PL-00716 Warsaw, Poland

Received 10 July 1996 / Accepted 7 October 1996

Abstract

Results from a cross-correlation analysis between the COBE DMR 4 year, and ROSAT PSPC All-Sky Survey data are presented. Statistical comparisons between microwave and X-ray maps can probe interesting astrophysical environments and processes, such as the warm interstellar medium, the Sunyaev-Zel'dovich effect in clusters of galaxies or gaseous group halos, X-ray luminous radio sources and the Integrated Sachs-Wolfe or the Rees-Sciama effect. In order to test the diffuse, extragalactic X-ray background as probed by ROSAT, against the COBE DMR large-scale CMB structure, our analysis was performed in most detail in a ROSAT selected region of the sky ([FORMULA] b, [FORMULA] l [FORMULA]) in an X-ray energy range with minimal Galactic structure and residual X-ray contamination, and the COBE low noise and least Galactic contribution channels. Comparing to other regions of the sky and neighbouring energies and frequencies, we find indication for a positive Galactic correlation on large scales. This correlation is most prominent in the softest X-ray band and lowest microwave channel, with a [FORMULA] 95 % confidence level detection against COBE noise and CMB cosmic variance, including the high quadrupole value resulting from the power spectrum fit. The spectral dependences are consistent with Galactic thermal X-ray emission, and Galactic synchrotron radiation or free-free (Bremsstrahlung) emission by the warm interstellar medium in the microwave regime. Removing the quadrupole term on a sky map with a Galactic cutout or related gradients in the selected regions leaves no correlations above a 1- [FORMULA] level on smaller angular scales. We conclude that there is no significant extragalactic correlation on scales for which the combined data are sensitive ([FORMULA]) and that Galactic correlation is significant only on large angular scales, of the order of the quadrupole. In the context of removing large angular scale gradients we give results on best fit X-ray dipoles from various ROSAT data and discuss these with respect to the difficulty of finding a cosmological dipole. The lowest correlation upper limits we can place are [FORMULA] 15% of COBE CMB fluctuations and [FORMULA] 10% of the ROSAT extragalactic XRB fluctuations. We discuss these results with respect to the possible correlation mechanisms.

Key words: galaxies: clusters: general – - cosmology: cosmic microwave background – cosmology: observations – cosmology: diffuse radiation – - X-rays: general

Send offprint requests to: R. Kneissl, ruk@mpa-garching.mpg.de

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998

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