Detection of Low-Hard State Spectral and Timing Signatures from the Black Hole X-Ray Transient XTE J1650-500 at Low X-Ray Luminosities

Using the Chandra X-ray Observatory and the Rossi X-ray Timing Explorer, we have studied the black hole candidate (BHC) X-ray transient XTE J1650-500 near the end of its 2001-2002 outburst after its transition to the low-hard state at X-ray luminosities down to L = 1.5E34 erg/s (1-9 keV, assuming a source distance of 4 kpc). Our results include a characterization of the spectral and timing properties. At the lowest sampled luminosity, we used an 18 ks Chandra observation to measure the power spectrum at low frequencies. For the 3 epochs at which we obtained Chandra/RXTE observations, the 0.5-20 keV energy spectrum is consistent with a spectral model consisting of a power-law with interstellar absorption. We detect evolution in the power-law photon index from 1.66 +/- 0.05 to 1.93 +/- 0.13 (90% confidence errors), indicating that the source softens at low luminosities. The power spectra are characterized by strong (20-35% fractional rms) band-limited noise, which we model as a zero-centered Lorentzian. Including results from an RXTE study of XTE J1650-500 near the transition to the low-hard state by Kalemci et al. (2003), the half-width of the zero-centered Lorentzian (roughly where the band-limited noise cuts off) drops from 4 Hz at L = 7E36 erg/s (1-9 keV, absorbed) to 0.067 +/- 0.007 Hz at L = 9E34 erg/s to 0.0035 +/- 0.0010 Hz at the lowest luminosity. While the spectral and timing parameters evolve with luminosity, it is notable that the general shapes of the energy and power spectra remain the same, indicating that the source stays in the low-hard state. This implies that the X-ray emitting region of the system likely keeps the same overall structure, while the luminosity changes by a factor of 470. We discuss how these results may constrain theoretical black hole accretion models.Comment: 11 pages, accepted by ApJ after minor revision

Constraints on the luminosity of the stellar remnant in SNR1987A

We obtain photometric constraints on the luminosity of the stellar remnant in SNR1987A using XMM-Newton and INTEGRAL data. The upper limit in the 2--10 keV band based on the XMM-Newton data is L<5*10^{34}erg/s. We note, however, that the optical depth of the envelope is still high in the XMM-Newton band, therefore, this upper limit does not constrain the true unabsorbed luminosity of the central source. The optical depth is expected to be small in the hard X-ray band of the IBIS telescope aboard the INTEGRAL observatory, therefore it provides an unobscured look at the stellar remnant. We did not detect statistically significant emission from SN1987A in the 20-60 keV band with the upper limit of L<1.1*10^{36}erg/s. We also obtained an upper limit on the mass of radioactive 44Ti M(44Ti)<10^{-3}Msun.Comment: 5 pages, 3 figures, accepted for publication in Astronomy Letter

Detection of a 5-Hz QPO from X-ray Nova GRS 1739-278

The X-ray nova GRS1739-278 flared up near the Galactic center in the spring of 1996. Here we report on the discovery of a ~5-Hz quasi-periodic oscillations (QPO) in RXTE/PCA observations of GRS1739-278. The QPO were only present when the source was in its very high state, and disappeared later, when it made a transition down into the high state. We present the energy spectra of this black hole candidate measured in both high and very high states, and discuss the similarities between this system and other X-ray transients.Comment: 12 pages, 4 figures, accepted for publication in ApJ Letter

A Hot Helium Plasma in the Galactic Center Region

Recent X-ray observations by the space mission Chandra confirmed the astonishing evidence for a diffuse, hot, thermal plasma at a temperature of 9. $10^7$ K (8 keV) found by previous surveys to extend over a few hundred parsecs in the Galactic Centre region. This plasma coexists with the usual components of the interstellar medium such as cold molecular clouds and a soft (~0.8 keV) component produced by supernova remnants, and its origin remains uncertain. First, simple calculations using a mean sound speed for a hydrogen-dominated plasma have suggested that it should not be gravitationally bound, and thus requires a huge energy source to heat it in less than the escape time. Second, an astrophysical mechanism must be found to generate such a high temperature. No known source has been identified to fulfill both requirements. Here we address the energetics problem and show that the hot component could actually be a gravitationally confined helium plasma. We illustrate the new prospects this opens by discussing the origin of this gas, and by suggesting possible heating mechanisms.Comment: 9 pages, accepted for publication in APJ

X-ray variability of AGNs in the soft and the hard X-ray bands

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.Comment: 17 pages, 15 Postscript figures, 3 tables, accepted for publication in Ap

Turbulence in Clusters of Galaxies and X-Ray Line Profiles

Large-scale bulk motions and hydrodynamic turbulence in the intergalactic gas inside clusters of galaxies significantly broaden X-ray emission lines. For lines of heavy ions (primarily helium-like and hydrogen-like iron ions), the hydrodynamic broadening is significantly larger than the thermal broadening. Since cluster of galaxies have a negligible optical depth for resonant scattering in forbidden and intercombination lines of these ions, these lines are not additionally broadened. At the same time, they are very intense, which allows deviations of the spectrum from the Gaussian spectrum in the line wings to be investigated. The line shape becomes an important indicator of bulk hydrodynamic processes because the cryogenic detectors of new generation of X-ray observatories will have a high energy resolution (from 5 eV for ASTRO-E2 to 1-2 eV for Constellation-X and XEUS). We use the spectral representation of a Kolmogorov cascade in the inertial range to calculate the characteristic shapes of X-ray lines. Significant deviations in the line profiles from the Gaussian profile (shape asymmetry, additional peaks, sharp breaks in the exponential tails) are expected for large-scale turbulence. The kinematic SZ effect and the X-ray line profile carry different information about the hydrodynamic velocity distribution in clusters of galaxies and complement each other, allowing the redshift, the peculiar velocity of the cluster, and the bulk velocity dispersion to be measured and separatedComment: 29 pages, 12 figures, Astronomy Letters 2003, v.29, p.79