RXTE Observations of the Be star X-ray Transient X0726-260 (4U0728-25) - Orbital and Pulse Periods

Rossi X-ray Timing Explorer (RXTE) All Sky Monitor observations of the transient Be star X-ray source X0726-260 suggest a 34.5 day period. This is apparently confirmed by a serendipitous RXTE Proportional Counter Array (PCA) slew detection of the source on 1997 May 5, near the time of a predicted flux maximum. A subsequent 5000 second pointed observation of X0726-260 with the RXTE PCA detector was carried out on 1997 June 7, when X0726-260 was predicted to be bright again, and this revealed pulsations at a period of 103.2 seconds. If the 34.5 day period is orbital, then the pulse period is surprisingly long compared to that predicted by the correlation between orbital period and spin period observed for other Be/neutron star systems. A possible similarity with GROJ2058+42 is briefly discussed.Comment: 7 pages LateX, 7 figures. To be published in Astrophysical Journal Letter

A Second Intense Burst with Photospheric Radius Expansion from X2127+119 in M15

In 2000 September we observed a bright X-ray burst from X2127+119 with the Rossi X-ray Timing Explorer. This burst has a multi-peaked profile, a peak luminosity of ~6.5x10^{38} erg s^{-1}, a total integrated energy of ~2x10^{40} ergs, and significant photospheric radius expansion to a maximum extent of R=118+-5 km. From the luminosity-temperature relation during the expansion phase we derive estimates for the gravitational redshift at the neutron star surface, the corrected Eddington luminosity, and the neutron star mass. We detect no slow (~30s) radial oscillations or fast (100-1200 Hz) coherent oscillations or QPO during the burst. The 2000 September event is only the second burst ever observed from this globular cluster binary (in M15 = NGC 7078), and it shares many characteristics with the event detected by Ginga in 1988 October (Dotani et al. 1990; van Paradijs et al. 1990), the key difference probably being the total amount of material consumed in the thermonuclear flash.Comment: 8 pages, 5 figures; accepted for publication in Ap

The rms-flux relations in different branches in Cyg X-2

In this paper, the rms-flux (root mean square-flux) relation along the Z-track of the bright Z-Source Cyg X-2 is analyzed using the observational data of Rossi X-ray Timing Explorer (RXTE). Three types of rms-flux relations, i.e. positive, negative, and 'arch'-like correlations are found in different branches. The rms is positively correlated with flux in normal branch (NB), but anti-correlated in the vertical horizontal branch (VHB). The rms-flux relation shows an 'arch'-like shape in the horizontal branch (HB). We also try to explain this phenomenon using existing models.Comment: Accepted for publication in Astrophysics & Space Scienc

The first multi-wavelength campaign of AXP 4U 0142+61 from radio to hard X-rays

For the first time a quasi-simultaneous multi-wavelength campaign has been performed on an Anomalous X-ray Pulsar from the radio to the hard X-ray band. 4U 0142+61 was an INTEGRAL target for 1 Ms in July 2005. During these observations it was also observed in the X-ray band with Swift and RXTE, in the optical and NIR with Gemini North and in the radio with the WSRT. In this paper we present the source-energy distribution. The spectral results obtained in the individual wave bands do not connect smoothly; apparently components of different origin contribute to the total spectrum. Remarkable is that the INTEGRAL hard X-ray spectrum (power-law index 0.79 +/- 0.10) is now measured up to an energy of ~230 keV with no indication of a spectral break. Extrapolation of the INTEGRAL power-law spectrum to lower energies passes orders of magnitude underneath the NIR and optical fluxes, as well as the low ~30 microJy (2 sigma) upper limit in the radio band.Comment: 6 pages, 1 figure. To be published in the proceedings of the conference "Isolated Neutron Stars: from the Interior to the Surface" (April 24-28, 2006, London, UK), eds. S. Zane, R. Turolla and D. Pag

States and transitions in black-hole binaries

With the availability of the large database of black-hole transients from the Rossi X-Ray Timing Explorer, the observed phenomenology has become very complex. The original classification of the properties of these systems in a series of static states sorted by mass accretion rate proved not to be able to encompass the new picture. I outline here a summary of the current situation and show that a coherent picture emerges when simple properties such as X-ray spectral hardness and fractional variability are considered. In particular, fast transition in the properties of the fast time variability appear to be crucial to describe the evolution of black-hole transients. Based on this picture, I present a state-classification which takes into account the observed transitions. I show that, in addition to transients systems, other black-hole binaries and Active Galactic Nuclei can be interpreted within this framework. The association between these states and the physics of the accretion flow around black holes will be possible only through modeling of the full time evolution of galactic transient systems.Comment: 30 pages, 11 figures, To appear in Belloni, T. (ed.): The Jet Paradigm - From Microquasars to Quasars, Lect. Notes Phys. 794 (2009