Diffuse 0.5-1 keV X-Rays and Nuclear Gamma-Rays from Fast Particles in the Local Hot Bubble

We show that interactions of fast particles with the boundary shell of the local hot bubble could make an important contribution to the 0.5-1 keV diffuse X-ray background observed with ROSAT. The bulk of these nonthermal X-rays are due to line emission from fast O ions of energies around 1 MeV/nucleon. This is the typical energy per particle in the ejecta of the supernova which is thought to have energized the bubble. We find that there is sufficient total energy in the ejecta to produce X-rays of the required intensity, subject to the details of the evolution of the fast particle population since the supernova explosion (about 3 10$^5$ years ago based on the age of the Geminga pulsar). The unequivocal signature of lines from deexcitations in fast ions is their large width ($\delta E/E$~0.1 for O lines), which will clearly distinguishes them from X-ray lines produced in a hot plasma. If a small fraction of the total ejecta energy is converted into accelerated particle kinetic energy (>~30 MeV/nucleon), the gamma-ray line emission produced in the boundary shell of the local hot bubble could account for the recently reported COMPTEL observations of nuclear gamma-ray lines from a broad region towards the Galactic center.Comment: 13 pages, 4 figures, submitted to Ap

The large-scale bias of the hard X-ray background

Recent deep X-ray surveys combined with spectroscopic identification of the sources have allowed the determination of the rest-frame 2-8 keV luminosity as a function of redshift. In addition, an analysis of the HEAO1 A2 2-10 keV full-sky map of the X-ray background (XRB) reveals clustering on the scale of several degrees. Combining these two results in the context of the currently favored Lambda-CDM cosmological model implies an average X-ray bias factor, b_x, of b_x^2 = 1.12 +- 0.33, i.e., b_x = 1.06 +- 0.16. These error estimates include only statistical error; the systematic error sources, while comparable, appear to be sub-dominant. This result is in contrast to the large biases of some previous estimates and is more in line with current estimates of the optical bias of L* galaxies.Comment: 6 pages, 3 eps figures, accepted for ApJ, vol. 612, 10 September 200

Absolute measurement of the unresolved cosmic X-ray background in the 0.5-8 keV band with Chandra

We present the absolute measurement of the unresolved 0.5-8 keV cosmic X-ray background (CXB) in the Chandra Deep Fields (CDFs) North and South, the longest observations with Chandra (2 Ms and 1 Ms, respectively). We measure the unresolved CXB intensity by extracting spectra of the sky, removing all point and extended sources detected in the CDF. To model and subtract the instrumental background, we use observations obtained with ACIS in stowed position, not exposed to the sky. The unresolved signal in the 0.5-1 keV band is dominated by diffuse Galactic and local thermal-like emission. In the 1-8 keV band, the unresolved spectrum is adequately described by a power law with a photon index 1.5. We find unresolved CXB intensities of (1.04+/-0.14)x10^-12 ergs cm^-2 s^-1 deg^-2 for the 1-2 keV band and (3.4+/-1.7)x10^-12 ergs cm^-2 s^-1 deg^-2 for the 2-8 keV band. Our detected unresolved intensities in these bands significantly exceed the expected flux from sources below the CDF detection limits, if one extrapolates the logN/logS curve to zero flux. Thus these background intensities imply either a genuine diffuse component, or a steepening of the logN/logS curve at low fluxes, most significantly for energies <2 keV. Adding the unresolved intensity to the total contribution from sources detected in these fields and wider-field surveys, we obtain a total intensity of the extragalactic CXB of (4.6+/-0.3)x10^-12 ergs cm^-2 s^-1 deg^-2 for 1-2 keV and (1.7+/-0.2)x10^-11 ergs cm^-2 s^-1 deg^-2 for 2-8 keV. These totals correspond to a CXB power law normalization (for photon index 1.4) of 10.9 photons cm^-2 s^-1 keV^-1 sr^-1 at 1 keV. This corresponds to resolved fracations of 77+/-3% and 80+/-8% for 1-2 and 2-8 keV, respectively.Comment: 23 emulateapj pages, accepted for publication in ApJ. Minor revisions, most notably a new summary of the error analysi

The 3-53 keV Spectrum of the Quasar 1508+5714: X-rays from z = 4.3

We present a high-quality X-ray spectrum in the 3--53 keV rest-frame band of the radio-loud quasar 1508+5714, by far the brightest known X-ray source at z > 4. A simple power-law model with an absorption column density equal to the Galactic value in the direction of the source provides an excellent and fully adequate fit to the data; the measured power-law photon index Gamma = 1.42 (+0.13,-0.10). Upper limits to Fe K alpha line emission and Compton-reflection components are derived. We offer evidence for both X-ray and radio variability in this object and provide the first contemporaneous radio spectrum (alpha = -0.25). The data are all consistent with a picture in which the emission from this source is dominated by a relativistically beamed component in both the X-ray and radio bands.Comment: 8 pages, TeX, 2 postscript figures; to appear in ApJ Letter

A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608−52

It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below ∼10⁻² of the Eddington limit (L_(Edd)). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608−52, to study changes in the reflection spectrum. We find that the broad Fe–Kα line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is ∼10³⁷ erg s⁻¹ (∼0.05 L_(Edd)), disappear during the decay of the outburst when the source luminosity drops to ∼4.5 × 10³⁵ erg s⁻¹ (∼0.002 L_(Edd)). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise ratio at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disc radius, disc ionization, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disc ionization (log ξ ≳ 4.1) or a much decreased reflection fraction. A changing disc truncation alone, however, cannot account for the lack of reprocessed Fe–Kα emission. The required increase in ionization parameter could occur if the inner accretion flow evaporates from a thin disc into a geometrically thicker flow, such as the commonly assumed formation of a radiatively inefficient accretion flow at lower mass accretion rates

The mass density in black holes inferred from the X-ray background

The X-ray Background (XRB) probably originates from the integrated X-ray emission of active galactic nuclei (AGN). Modelling of its flat spectrum implies considerable absorption in most AGN. Compton down-scattering means that sources in which the absorption is Compton thick are unlikely to be major contributors to the background intensity so the observed spectral intensity at about 30 keV is little affected by photoelectric absorption. Assuming that the intrinsic photon index of AGN is 2, we then use the 30 keV intensity of the XRB to infer the absorption-corrected energy density of the background. Soltan's argument then enables us to convert this to a mean local density in black holes, assuming an accretion efficiency of 0.1 and a mean AGN redshift of 2. The result is within a factor of two of that estimated by Haehnelt et al from the optically-determined black hole masses of Magorrian et al. We conclude that there is no strong need for any radiatively inefficient mode of accretion for building the masses of black holes. Furthermore we show that the absorption model for the XRB implies that about 85 per cent of accretion power in the Universe is absorbed. This power probably emerges in the infrared bands where it can be several tens per cent of the recently inferred backgrounds there. The total power emitted by accretion is then about one fifth that of stars.Comment: 4 pages, accepted for publication in MNRA

Could a Local Group X-Ray Halo Affect the X-Ray and Microwave Backgrounds?

Suto et al. (1996, astro-ph/9602061 and ApJ, 461, L33) have suggested that an X-ray halo in the Local Group might explain both the observed low-energy excess in the X-ray background and the quadrupole anisotropy in the cosmic microwave background. Recent observations of poor groups of galaxies by the ROSAT PSPC set reasonable limits on how extensive and dense such a halo could be. The poor groups most similar to the Local Group do not have a detectable halo, and the upper limits of these observations suggest that any Local Group halo would be nearly two orders of magnitude too tenuous to produce the effects Suto et al. (1996) discuss. In particular, the Sunyaev-Zel'dovich effect cannot contribute significantly to the quadrupole anisotropy measured by COBE.Comment: 8 pages, to appear in The Astrophysical Journal Letters, paper can also be obtained at http://hea-www.harvard.edu/~pildis/smio.htm