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

RXTE observations of Galactic microquasar XTE J1748--288 during its 1998 outburst

We present an analysis of the RXTE observations of the recently discovered Galactic microquasar XTE J1748--288 during its 1998 outburst. The spectral evolution of the source during the outburst can be considered a sequence of qualitatively distinct states. During the first observations, corresponding to the maximum of X-ray flux, the spectrum of the source consisted of a dominating hard power law component and a soft thermal component, which can be described by the model of multicolor disk emission. The hard component contributed >80% to the X-ray luminosity in the 3-25 keV energy band. Overall two-component spectral shape is an attribute of Very High state (VHS) observed previously in BHC, but the domination of hard component is unusual. Later on, as the X-ray source faded, its energy spectrum qualitatively changed, showing High (HS) and then Low (LS) states, both typical for black hole binaries. As the energy spectrum changed, the fast variability also evolved dramatically. Initially the power density spectrum was formed by a dominating band-limited noise component, QPO features at 20-30 Hz and at ~0.5 Hz, and a very low frequency noise component. After a significant decrease of the contribution of the hard spectral component the amplitude of the fractional variability decreased by an order of magnitude and the PDS spectrum adopted a power-law shape with a broad QPO peak around 0.03 Hz. When the system switched to the LS, the PDS shape changed again and the QPOs have not been detected since. When the source was observed in VHS, a clear correlation between QPO parameters and X-ray flux was seen.Comment: 9 pages, 7 figures, accepted for publication in MNRA

X-ray Spectrum of the Black Hole Candidate X1755-338

We report the first detection of a hard power-law tail in the X-ray spectrum of the black hole candidate (BHC) binary X1755-338, which was observed in 1989 March-September during the TTM Galactic Centre survey. In addition, an ultrasoft thermal component with a temperature of ~1.1-1.4 keV was also detected. We demonstrate that the soft and hard X-ray components of X1755-338 vary independently, as in the spectra of the well known BHCs LMC X-1, LMC X-3 and GX339-4 in their high (intensity) state. If the hard tail observed from X1755-338 is generated near the black hole by energetic electrons up-scattering low energy photons, the un-correlated variations imply that the soft X-rays from accretion disc may not be the main photon seeds needed for inverse Comptonization. The TTM observations strongly suggest that X1755-338 does indeed belong to the family of BHCs.Comment: 4 pages, compressed and uuencoded postscript file, to be published in MNRAS (Letters

SkyDOT (Sky Database for Objects in the Time Domain): A Virtual Observatory for Variability Studies at LANL

The mining of Virtual Observatories (VOs) is becoming a powerful new method for discovery in astronomy. Here we report on the development of SkyDOT (Sky Database for Objects in the Time domain), a new Virtual Observatory, which is dedicated to the study of sky variability. The site will confederate a number of massive variability surveys and enable exploration of the time domain in astronomy. We discuss the architecture of the database and the functionality of the user interface. An important aspect of SkyDOT is that it is continuously updated in near real time so that users can access new observations in a timely manner. The site will also utilize high level machine learning tools that will allow sophisticated mining of the archive. Another key feature is the real time data stream provided by RAPTOR (RAPid Telescopes for Optical Response), a new sky monitoring experiment under construction at Los Alamos National Laboratory (LANL).Comment: to appear in SPIE proceedings vol. 4846, 11 pages, 5 figure

RX J0042.3+4115: a stellar mass black hole binary identified in M31

Four XMM-Newton X-ray observations of the central region of the Andromeda Galaxy (M31) have revealed an X-ray source that varies in luminosity over \~1--3, 10^{38} erg s^{-1} between observations and also displays significant variability over time-scales of a few hundred seconds. The power density spectra of lightcurves obtained in the 0.3--10 keV energy band from the three EPIC instruments on board XMM-Newton are typical of disc-accreting X-ray binaries at low accretion rates, observed in neutron star binaries only at much lower luminosities (~10^{36} erg s^{-1}). However X-ray binaries with massive black hole primaries have exhibited such power spectra for luminosities >10^{38} erg s^{-1}. We discuss alternative possibilities where RX J0042.3+4115 may be a background AGN or foreground object in the field of view, but conclude that it is located within M31 and hence use the observed power spectra and X-ray luminosities to identify the primary as a black hole candidate.Comment: 7 pages, 4 figures, A&A accepte