Diffuse X-Ray Background

Study of the Diffuse X-Ray Background

The Diffuse X-ray background (DXB) was discovered at the dawn of X-ray astronomy. During the first successful rocket flight to detect extrasolar X-rays, the presence of a residual diffuse emission was also serendipitously revealed. The Wisconsin rocket survey and the ROSAT All Sky Survey (RASS) extensively studied the nature of the diffuse emission below 1 keV and its correlation with the neutral material in the Milky Way have shown that the emission varies considerably over its energy range. It is currently believed that a significant fraction of the X-ray emission in the 1/4 keV band originates in an irregularly shaped cavity in the neutral hydrogen of the Galactic disk which contains the Sun and has been dubbed the Local Hot Bubble (LHB), however recent studies have shown that X-ray emission in this band can also originate within the solar system, via Solar Wind Charge Exchange (SWCX). Above 1 keV, the emission is highly isotropic on large angular scales, has extragalactic origin, and is mostly accounted for by the superposition of unresolved point sources.

The band between 0.4 and 1 keV, often referred to as the 3/4 keV band, is more complicated, as both extragalactic and Galactic sources contribute to the observed flux and is the focus of this investigation. Our current interpretation of the diffuse X-ray emission in the 3/4 keV band uses a combination of 5 components. In addition to solar wind charge exchange, local hot bubble, and unresolved point sources, it is believed that a significant contribution comes from plasma emission from the galactic halo and from intergalactic gas, the so called Warm-Hot Intergalactic medium (WHIM).

Resolving the different components is made difficult by their similar spectral emission, X-ray lines of heavily ionized metals, which are poorly resolved by the energy resolution of CCD cameras onboard current X-ray satellites. Significant progress has been made in recent years toward understanding the individual components of the DXB using data from XMM-Newton, Chandra, and Suzaku. However, due to the overlap between components, a comprehensive investigation of all of them at the same time is necessary to better disentangle them and understand their properties.

Our group is focusing on a systematic, simultaneous study of all components of the DXB below 1 keV to determine the properties of the astrophysical objects responsible for the emission. XMM-Newton, Chandra and Suzaku have a sufficiently mature archives and will be used for this investigation. A multi-wavelength approach will also be used where necessary, in particular for the study of the WHIM. The project is carried on with a combination of many targets available in the public archives of the three missions and a few proprietary observations.

The investigation is organized in three main subjects:

Galactic emission: we analyze the spectra of the soft X-ray diffuse emission to characterize the temperature, pressure, and density of Local Bubble and Galactic Halo and the fluxes in oxygen emission lines as function of position (in particular latitude). We are also monitoring the SWCX using XMM-Newton to characterize the its variability over a 2-year period.

WHIM: our previous work with XMM-Newton has detected the WHIM signature in the angular correlation of soft X-ray images. We are currently moving from detecting to characterizing the WHIM. By significantly increasing the number of targets we will investigate the cosmic variation of the WHIM emission. Moreover, by using the good angular resolution of Chandra we will also be able to investigate the spatial distribution of the WHIM as a function of its distance from point sources.

We are also studying the WHIM properties by looking at the cross correlation between X-ray and Microwave background data from the Atacama Cosmic Telescope (ACT) due to the Sunyaev-Zel'dovich (SZ) effect. We use our simulation of the X-ray emission from the WHIM to investigate the expected cross correlation between ACT and XMM-Newton signals and then use the common fields between XMM-Newton or Chandra and the ACT field to look for the correlation and extract the WHIM properties. In particular, the different dependence on electron density between X-ray emission and SZ effect is an invaluable tool to understand the density distribution of the WHIM.

Point sources: The X-ray emission from unresolved point sources has been studied extensively above 1 keV, however, very little data are available below 1 keV. For example, we recently used 5 XMM-Newton targets and found the presence, in addition to the typical power law spectrum attributed to Active Galacti Nuclei (AGN), of a thermal component around 0.5 keV that we attribute to galaxy clusters and groups, but is also somewhat consistent with stellar emission from the Milky Way. Using a much larger set of data and the higher angular resolution of Chandra will allow a much better characterization of both the AGN contribution and the thermal component, including its origin.

For more details you can read some of our recent publications:

"Probing the mass and anisotropy of the Milky Way gaseous halo: sight-lines toward Mrk 421 and PKS 2155-304", A. Gupta, S. Mathur, M. Galeazzi, Y. Krongold, Astrophys. Space Science, online first, DOI 10.1007/s10509-014-1958-z (2014)

"Observed Limits on Charge Exchange Contributions to the Diffuse X-Ray Background", S. G. Crowder, K. A. Barger, D. E. Brandl, M. E. Eckart, M. Galeazzi, R. L. Kelley, C. A. Kilbourne, D. McCammon, C. G. Pfendner, F. S. Porter, L. Rocks, A. E. Szymkowiak, I. M. Teplin, ApJ 758, 143 (2012)

"A Huge Reservoir of Ionized Gas around the Milky Way: Accounting for the Missing Mass?", A. Gupta, S. Mathur, Y. Krongold, F. Nicastro, and M. Galeazzi, ApJL 756, L8 (2012).

"Search for X-Ray Emission Associated with the Shapley Supercluster with Suzaku", I. Mitsuishi, A. Gupta, N. Y. Yamasaki, Y. Takei, T. Ohashi, K. Sato, M. Galeazzi, J. P. Henry, and R.L. Kelley, Publ. Astron. Soc. Japan 64, 18 (2012).

"Studying the Warm-hot Intergalactic Medium in Emission", Y. Takei, E. Ursino, E. Branchini, T. Ohashi, H. Kawahara, K. Mitsuda, L. Piro, A. Corsi, L. Amati, J. W. den Herder, M. Galeazzi, J. Kaastra, L. Moscardini, F. Nicastro, F. Paerels, M. Roncarelli, and M. Viel, ApJ 734, 91 (2011).

"Study of the Intracluster and Intergalactic Medium in the Sculptor Supercluster with Suzaku", K. Sato, R. L. Kelley, Y. Takei, T. Tamura, N. Y. Yamasaki, T. Ohashi, A. Gupta, and M. Galeazzi, PASJ 62, 1423 (2010).

"Properties of the Diffuse X-ray Background toward MBM20 with Suzaku", A. Gupta, M. Galeazzi, D. Koutroumpa, R. Smith, R. Lallement, ApJ 707, 644 (2009).

"Contribution of Unresolved Point Sources to the Diffuse X-ray Background Below 1 keV", A. Gupta and M. Galeazzi, AspJ 702, 270 (2009).

"Evidence for the missing baryons in the angular correlation of the diffuse x-ray background", M. Galeazzi, A. Gupta and E. Ursino, ApJ 695, 1127 (2009).

"Shadowing Studies in the Moderate (CCD) Resolution Era", M. Galeazzi and A. Gupta, AIP Conf. Proc. 1156, 16 (2009).

"XMM-Newton Observation of the Diffuse X-ray Background", M. Galeazzi, A. Gupta, K. Covey, and E. Ursino, ApJ 658, 1081 (2007).