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 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

An absorbed view of a new class of INTEGRAL sources

The European gamma-ray observatory INTEGRAL has found a group of hard X-ray sources which are highly absorbed, i.e., with column densities higher than about 10^{23}/cm^2. Here I give an overview of this class of INTEGRAL sources. The X-ray, as well as the optical/IR, properties of these sources and their location in the sky suggest that they belong to the class of high-mass X-ray binaries, some of them possibly long-period X-ray pulsars. The donors in these binaries are most probably giant or supergiant stars. I suggest that the soft X-ray spectrum below ~5 keV of IGR J16318-4848, as well as in several other X-ray binaries (e.g., XTE J0421+56), can be described by emission from a compact object which is strongly absorbed by a partionally ionised dense envelope.Comment: 8 pages, 2 figures, in "Interacting Binaries: Accretion, Evolution and Outcomes", Eds. L.A. Antonelli, et al., Proc. of the Interacting Binaries Meeting of Cefalu, Italy, July 2004, AIP, 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

GX 349+2 (Sco X-2): An odd-ball among the Z sources

We report on about 4 hrs of observations made with the Rossi X-ray Timing Explorer on 1997 May 2 of the low-mass X-ray binary and Z source GX 349+2 (Sco X-2). Initially the source was in the normal branch (NB), later it moved to the flaring branch (FB). In the NB the power spectra reveal a broad (FWHM ~16 Hz) noise component peaking near 9 Hz, with a fractional rms amplitude of ~3% (2-30 keV). This noise component does not resemble the strong quasi-periodic oscillations (QPO) usually seen in the NB of other Z sources. We set 95% confidence upper limits on the fractional rms of ~0.9% (2-30 keV) on any such QPO. In the FB the power spectrum showed a somewhat less broad noise component (FWHM ~11 Hz) peaking near 6 Hz with a fractional rms of ~4% (2-30 keV). We compare our results with previous reports, and find that the fast timing behaviour changes not only with position in the Z, but also as a function of the position of the Z track in the hardness-intensity diagram. By comparing GX 349+2 with the other Z and atoll sources, we conclude that GX 349+2 differs from the Z sources in various aspects and shows similarities to the behaviour seen in the bright atoll sources, such as GX 13+1 and GX 3+1. We also searched for kilo-Hertz QPO, similar to those present in other Z sources. We only found weak evidence (2.6 sigma confidence) for a QPO near 1020 Hz with FWHM ~40 Hz and fractional rms of ~1%. We note that this frequency and fractional rms are consistent with those expected and observed in the lower NB/FB of the Z sources Sco X-1, GX 17+2 and Cyg X-2.Comment: 8 pages, including 3 Postscript figures. Accepted by A&A Main Journa

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