Towards 4U 1630-47: a black-hole soft X-ray transient odyssey

4U 1630-47 is a black-hole X-ray transient with one of the shortest recurrence times. Despite its regular outburst behaviour little is known about this source. Only recently has attention to this system increased. I discuss there the basic known (X-ray) properties of 4U 1630-47 and report on X-ray and radio observations obtained during its recent outburst, starting in 1998 February. These observations strengthen some of the similarities seen between 4U 1630-47 and the Galactic superluminal sources GRO J1655-40 and GRS 1915+105, and provide the first detection of 4U 1630-47 in the radio. Using an updated outburst ephemeris I predict the next outburst to occur about a week before Christmas 1999.Comment: 4 pages, 2 figures, updated how to refer to this paper: to appear in: R.N.Ogley, J. Bell Burnell (eds.), 2nd Workshop on Galactic Sources with Relativistic Jets, NewAR, 42, in pres

The first radius-expansion X-ray burst from GX 3+1

During several observations in 1999 August with RXTE of the low-mass X-ray binary GX 3+1, we found a single short and strong X-ray burst. This is the first burst from GX 3+1 which clearly shows evidence for radius expansion of the neutron-star photosphere during the thermo-nuclear runaway. We show that the cooling phase of the neutron star photosphere starts already just before the end of the contraction phase. Considering the fact that the radius expansion is due to the burst luminosity being at the Eddington luminosity, assuming standard burst parameters and accounting for gravitational redshift effects we derive a distance to the source of ~4.5 kpc, although relaxing these assumptions may lead to uncertainties up to ~30%. By comparing the persistent flux with that observed at the peak of the burst we infer that near the time of the X-ray burst the persistent luminosity of GX 3+1 is ~0.17*L_edd, confirming predictions from theoretical modeling of X-ray spectra of bright sources like GX 3+1.Comment: 5 pages, 4 figures, accepted for publication in A&A Letter

RXTE observations of the neutron star low-mass X-ray binary GX 17+2: correlated X-ray spectral and timing behavior

We have analyzed ~600 ks of Rossi X-ray Timing Explorer data of the neutron star low-mass X-ray binary and Z source GX 17+2. A study was performed of the properties of the noise components and quasi-periodic oscillations (QPOs) as a function of the spectral properties, with the main goal to study the relation between the frequencies of the horizontal branch and upper kHz QPOs. It was found that when the upper kHz QPO frequency is below 1030 Hz these frequencies correlate, whereas above 1030 Hz they anti-correlate. GX 17+2 is the first source in which this is observed. We also found that the frequency difference of the high frequency QPOs was not constant and that the quality factors (Q values) of the HBO, its second harmonic, and the kHz QPOs are similar, and vary almost hand in hand by a factor of more than three. Observations of the normal branch oscillations during two type I X-ray bursts showed that their absolute amplitude decreased as the flux from the neutron star became stronger. We discuss these and other findings in terms of models that have been proposed for these phenomena. We also compare the behavior of GX 17+2 and other Z sources with that of black hole sources and consider the possibility that the mass accretion rate might not be driving force behind all spectral and variability changes.Comment: 35 pages, including 14 figures. Accepted for publication in ApJ. Revised discussion, one new figure, and some minor figure changes with respect to old versio

Peculiar spectral and power spectral behaviour of the LMXB GX 13+1

We present results of an analysis of all 480 ks of Rossi X-ray Timing Explorer Proportional Counter Array data obtained from 17 May 1998 to 11 October 1998 on the luminous low mass X-ray binary GX 13+1. We analysed the spectral properties in colour-colour diagrams (CDs) and hardness-intensity diagrams (HIDs) and fitted the power spectra with a multi-Lorentzian model. GX 13+1 traces out a curved track in the CDs on a time scale of hours, which is very reminiscent of a standard atoll track containing an island, and lower and upper banana branch. However, both count rate and power spectral properties vary along this track in a very unusual way, not seen in any other atoll or Z source. The count rate, which varied by a factor of ~1.6, along a given track first decreases and then increases, causing the motion through the HIDs to be in the opposite sense to that in the CD, contrary to all other Z and atoll sources. Along a CD track, the very low frequency noise uniquely decreases in amplitude from ~5 to ~2% (rms). The high frequency noise amplitude decreases from ~4% to less than 1% and its characteristic frequency decreases from ~10 to \~5 Hz. The 57-69 Hz quasi-periodic oscillation (QPO) found earlier is also detected, and no kHz QPOs are found. In addition the entire track shows secular motion on a time scale of about a week. The average count rate as well as the amplitude of the very low frequency noise correlate with this secular motion. We discuss a possible explanation for the peculiar properties of GX 13+1 in terms of an unusual orientation or strength of a relativistic jet.Comment: 15 pages, 13 figures. Accepted for publication in A&

The IR counterpart of the black-hole candidate 4U 1630-47

We present K band photometry of the region including the radio error box of the soft X-ray transient 4U 1630-47 during its 1998 outburst. We detect a variable source at K=16.1 mag located inside the radio error circle which we identify as the counterpart to the X-ray source. We discuss the properties of the source, and conclude that it is most likely a black-hole X-ray binary similar to 4U 1543-47, GRO J1655-40 or SAX J1819.3-2525, containing a relatively early-type secondary.Comment: 7 pages, 3 figures, A&A, in pres

Normal Branch Quasi-Periodic Oscillations in Sco X-1: Viscous Oscillations of a Spherical Shell Near the Neutron Star

We present a comprehensive classification of all observed QPOs within the framework of the transition layer model using a large set of Rossi X-ray Timing Explorer (RXTE) data for Sco X-1. The model assumes an optically thin material along the observer's line of sight in the horizontal branch and an increasingly optically thick material while in the other two branches that is consistent with X-ray and radio observations and the disk transition layer model of QPOs. We identify the ~ 6 Hz frequencies in the normal branch as acoustic oscillations of a spherical shell around the neutron star (NS) that is formed after radiation pressure near the Eddington accretion rate destroys the disk. The size of the shell is on the order of one NS radii from the NS. We also estimate the upper limit of Sco X-1's magnetic field to be 0.7 x 10^6 G at about one NS radii above the NS surface while in the horizontal X-ray branch.Comment: 12 pages and 2 figures, to be published in the Astrophysical Journal Letter

The Optical Light Curves of Cygnus X-2 (V1341 Cyg) and the Mass of its Neutron Star

We present U, B and V light curves (taken from the literature) of the low mass X-ray binary Cygnus X-2. The ``lower envelope'' of the light curves folded on the orbital period are ellipsoidal. We fit an ellipsoidal model to the lower envelopes of the B and V light curves to derive inclination constraints. If we assume the accretion disc is steady-state where its radial temperature profile goes as T(r) \propto r^{-3/4}, we find an inclination of i = 62.5 +/- 4 deg. However, the predicted ratio of the disc flux to the total flux in B (the ``disc fraction'') is larger than what is observed (about 0.55 compared to < 0.3). If we use a flatter radial temperature profile of the disc expected for strongly irradiated discs (T(r) \propto r^{-3/7}), then we find an inclination of i = 54.6 deg and a disc fraction in B of approximately 0.30. However, in this case the value of chi^2 is much larger (48.4 with 36 degrees of freedom compared to 40.9 for the steady-state case). Adopting i = 62.5 +/- 4 deg and using a previous determination of the mass ratio (q = M_c/M_x = 0.34 +/- 0.04) and the optical mass function (f(M) = 0.69 +/- 0.03 solar masses), we find that the mass of the neutron star is M_x = 1.78 +/- 0.23 solar masses and the mass of the secondary star is M_c = 0.60 +/- 0.13 solar masses. We derive a distance of d = 7.2 +/- 1.1 kpc, which is significantly smaller than a recent distance measurement of d = 11.6 +/- 0.3 kpc derived from an observation of a type I radius-expansion X-ray burst, but consistent with earlier distance estimates. (abridged)Comment: 12 pages, 9 figures, to appear in MNRAS, LaTeX, uses mn.st

Correlations between X-ray Spectral and Timing Characteristics in Cyg X-2

Correlations between the quasi-periodic oscillations (QPOs) and the spectral power-law index have been reported for a number of black hole candidate sources and for four neutron star (NS) sources, 4U 0614+09, 4U 1608-52, 4U 1728-34 and Sco X-1. An examination of QPO frequencies and index relationship in Cyg X-2 is reported herein. The RXTE spectrum of Cyg X-2 can be adequately represented by a simple two-component model of Compton up-scattering with a soft photon electron temperature of about 0.7 keV and an iron K-line. Inferred spectral power-law index shows correlation with the low QPO frequencies. We find that the Thomson optical depth of the Compton cloud (CC) tau, in framework of spherical geometry, is in the range of ~4-6, which is consistent with the neutron star's surface being obscured. The NS high frequency pulsations are presumably suppressed as a result of photon scattering off CC electrons because of such high values of tau. We also point out a number of similarities in terms timing (presence of low and high frequency QPOs) and spectral (high CC optical depth and low CC plasma temperature) appearances between Cyg X-2 and Sco X-1.Comment: 7 pages, 4 figures, accepted for publication in ApJ (October 1, 2007, v667n2 issue