XMM-NEWTONMEASUREMENT OF THE GALACTIC HALO X-RAY EMISSION USING A COMPACT SHADOWING CLOUD

Abstract

Observations of interstellar clouds that cast shadows in the soft X-ray background can be used to separate the background Galactic halo emission from the local emission due to solar wind charge exchange (SWCX) and/or the Local Bubble (LB). We present an XMM-Newton observation of a shadowing cloud, G225.60−66.40, that is sufficiently compact that the on-and off-shadow spectra can be extracted from a single field of view (unlike previous shadowing observations of the halo with CCD-resolution spectrometers, which consisted of separate onand off-shadow pointings). We analyzed the spectra using a variety of foreground models: one representing LB emission, and two representing SWCX emission. We found that the resulting halo model parameters (temperature T h ≈ 2 × 10 6 K, emission measure E h ≈ 4 × 10 −3 cm −6 pc) were not sensitive to the foreground model used. This is likely due to the relative faintness of the foreground emission in this observation. However, the data do favor the existence of a foreground. The halo parameters derived from this observation are in good agreement with those from previous shadowing observations, and from an XMM-Newton survey of the Galactic halo emission. This supports the conclusion that the latter results are not subject to systematic errors, and can confidently be used to test models of the halo emission.

XMM-NEWTONANDSUZAKUX-RAY SHADOWING MEASUREMENTS OF THE SOLAR WIND CHARGE EXCHANGE, LOCAL BUBBLE, AND GALACTIC HALO EMISSION

“…Figure 1 shows the locations of these shadows on the sky. This figure also shows the location of shadow G225.60−66.40 (G225−66 hereafter), analyzed by Henley et al (2015a). Unlike the shadows analyzed here, the on-and off-shadow spectra for G225−66 were extracted from a single XMM-Newton field.…”

“…In order to separate the foreground and halo emission, we used XSPEC version 12.8.1l (Arnaud 1996) to fit an SXRB spectral model to the on-and off-shadow spectra from each pair of observations in Table 1. Our spectral model is essentially the same as that used by Henley et al (2015a), and consists of components representing (1) the foreground emission, (2) the Galactic halo emission, (3) the extragalactic background emission, and (4) (for XMM-Newton spectra only) components of the particle background not removed by the filtering and QPB subtraction described in Section 2.1. In most regards, the on-and off-shadow models for each pair of observations were the same, apart from the absorbing column used to attenuate the halo and extragalactic components-this difference between the on-and offshadow models is the key to separating the foreground and halo emission.…”

“…More recently, the CCD spectrometers on board XMM-Newton and Suzaku have been used for such observations (Smith et al 2007;Galeazzi et al 2007;Henley et al 2007;Henley & Shelton 2008;Gupta et al 2009;Lei et al 2009;Henley et al 2015a). These instruments have higher spectral resolution, allowing some line emission features (e.g., O VII and O VIII) to be resolved in the spectra.…”

“…The shadowing clouds that have been observed with XMM-Newton and/or Suzaku have typically yielded halo temperatures and emission measures of ∼2 × 10 6 K and a few times 10 −3 cm −6 pc, respectively (Smith et al 2007;Galeazzi et al 2007;Gupta et al 2009;Lei et al 2009;Henley et al 2015a). However, these observations were not analyzed in a homogeneous fashion.…”

“…However, the relative contributions of the LB and of SWCX to the foreground in an arbitrary XMM-Newton or Suzaku observation are not known. Therefore, we follow Henley et al (2015a), and consider limits in which either LB or SWCX emission dominate the foreground. (Note that in most previous shadowing studies, an LBlike spectral model was used for the foreground.)…”