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Register a new userA High Spectral Resolution Study of the Soft X-ray Background with the X-ray Quantum Calorimeter
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We present here a combined analysis of four high spectral resolution observations of the Diffuse X-ray Background (DXRB), made using the University of Wisconsin-Madison/Goddard Space Flight Center X-ray Quantum Calorimeter (XQC) sounding rocket payload. The observed spectra support the existence of a $sim0.1~$keV Local Hot Bubble and a $sim0.2~$keV Hot Halo, with discrepancies between repeated observations compatible with expected contributions of time-variable emission from Solar Wind Charge Exchange (SWCX). An additional component of $sim0.9~$keV emission observed only at low galactic latitudes can be consistently explained by unresolved dM stars.
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We present measurements of the Galactic halos X-ray emission for 110 XMM-Newton sight lines, selected to minimize contamination from solar wind charge exchange emission. We detect emission from few million degree gas on ~4/5 of our sight lines. The temperature is fairly uniform (median = 2.22e6 K, interquartile range = 0.63e6 K), while the emission measure and intrinsic 0.5--2.0 keV surface brightness vary by over an order of magnitude (~(0.4-7)e-3 cm^-6 pc and ~(0.5-7)e-12 erg cm^-2 s^-1 deg^-2, respectively, with median detections of 1.9e-3 cm^-6 pc and 1.5e-12 erg cm^-2 s^-1 deg^-2, respectively). The high-latitude sky contains a patchy distribution of few million degree gas. This gas exhibits a general increase in emission measure toward the inner Galaxy in the southern Galactic hemisphere. However, there is no tendency for our observed emission measures to decrease with increasing Galactic latitude, contrary to what is expected for a disk-like halo morphology. The measured temperatures, brightnesses, and spatial distributions of the gas can be used to place constraints on models for the dominant heating sources of the halo. We provide some discussion of such heating sources, but defer comparisons between the observations and detailed models to a later paper.
RS Cae is the third target in our series of XMM-Newton observations of soft X-ray-dominated polars. Our observational campaign aims to better understand and describe the multiwavelength data, the physical properties of the system components, and the short- and long-term behavior of the component fluxes in RS Cae. We employ stellar atmosphere, stratified accretion-column, and widely used X-ray spectral models. We fit the XMM-Newton spectra, model the multiband light curves, and opt for a mostly consistent description of the spectral energy distribution. Results. Our XMM-Newton data of RS Cae are clearly dominated by soft X-ray emission. The X-ray light curves are shaped by emission from the main accretion region, which is visible over the whole orbital cycle, interrupted only by a stream eclipse. The optical light curves are formed by cyclotron and stream emission. The XMM-Newton X-ray spectra comprise a black-body-like and a plasma component at mean temperatures of 36eV and 7keV. The spectral fits give evidence of a partially absorbing and a reflection component. Multitemperature models, covering a broader temperature range in the X-ray emitting accretion regions, reproduce the spectra appropriately well. Including archival data, we describe the spectral energy distribution with a combination of models based on a consistent set of parameters and derive a lower limit estimate of the distance d > 750pc. Conclusions. The high bolometric soft-to-hard flux ratios and short-term variability of the (X-ray) light curves are characteristic of inhomogeneous accretion. RS Cae clearly belongs in the group of polars that show a very strong soft X-ray flux compared to their hard X-ray flux. The different black-body fluxes and similar hard X-ray and optical fluxes during the XMM-Newton and ROSAT observations show that soft and hard X-ray emission are not directly correlated.
The cosmic X-ray background (CXB), which peaks at an energy of ~30 keV, is produced primarily by emission from accreting supermassive black holes (SMBHs). The CXB therefore serves as a constraint on the integrated SMBH growth in the Universe and the accretion physics and obscuration in active galactic nuclei (AGNs). This paper gives an overview of recent progress in understanding the high-energy (>~10 keV) X-ray emission from AGNs and the synthesis of the CXB, with an emphasis on results from NASAs NuSTAR hard X-ray mission. We then discuss remaining challenges and open questions regarding the nature of AGN obscuration and AGN physics. Finally, we highlight the exciting opportunities for a next-generation, high-resolution hard X-ray mission to achieve the long-standing goal of resolving and characterizing the vast majority of the accreting SMBHs that produce the CXB.
X-ray calorimeters routinely achieve very high spectral resolution, typically a few eV full width at half maximum (FWHM). Measurements of calorimeter line shapes are usually dominated by the natural linewidth of most laboratory calibration sources. This compounds the data acquisition time necessary to statistically sample the instrumental line broadening, and can add systematic uncertainty if the intrinsic line shape of the source is not well known. To address these issues, we have built a simple, compact monochromatic x-ray source using channel cut crystals. A commercial x-ray tube illuminates a pair of channel cut crystals which are aligned in a dispersive configuration to select the kaone line of the x-ray tube anode material. The entire device, including x-ray tube, can be easily hand carried by one person and may be positioned manually or using a mechanical translation stage. The output monochromatic beam provides a collimated image of the anode spot with magnification of unity in the dispersion direction (typically 100-200 $mu$m for the x-ray tubes used here), and is unfocused in the cross-dispersion direction, so that the source image in the detector plane appears as a line. We measured output count rates as high as 10 count/s/pixel for the Hitomi Soft X-ray Spectrometer, which had 819 $mu$m square pixels. We implemented different monochromator designs for energies of 5.4 keV (one design) and 8.0 keV (two designs) which have effective theoretical FWHM energy resolution of 0.125, 0.197, and 0.086 eV, respectively; these are well-suited for optimal calibration measurements of state-of-the art x-ray calorimeters. We measured an upper limit for the energy resolution of our crkaone monochromator of 0.7 eV FWHM at 5.4 keV, consistent with the theoretical prediction of 0.125 eV.
We investigate the X-ray variability characteristics of hard X-ray selected AGNs (based on Swift/BAT data) in the soft X-ray band using the RXTE/ASM data. The uncertainties involved in the individual dwell measurements of ASM are critically examined and a method is developed to combine a large number of dwells with appropriate error propagation to derive long duration flux measurements (greater than 10 days). We also provide a general prescription to estimate the errors in variability derived from rms values from unequally spaced data. Though the derived variability for individual sources are not of very high significance, we find that, in general, the soft X-ray variability is higher than those in hard X-rays and the variability strengths decrease with energy for the diverse classes of AGN. We also examine the strength of variability as a function of the break time scale in the power density spectrum (derived from the estimated mass and bolometric luminosity of the sources) and find that the data are consistent with the idea of higher variability at time scales longer than the break time scale.
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