On the Origin of the Absorption Features in SS433

We present high-resolution optical spectroscopy of the X-ray binary system SS433, obtained over a wide range of orbital phases. The spectra display numerous weak absorption features, and include the clearest example seen to date of those features, resembling a mid-A type supergiant spectrum, that have previously been associated with the mass donor star. However, the new data preclude the hypothesis that these features originate solely within the photosphere of the putative mass donor, indicating that there may be more than one region within the system producing an A supergiant-like spectrum, probably an accretion disc wind. Indeed, whilst we cannot confirm the possibility that the companion star is visible at certain phase combinations, it is possible that all supergiant-like features observed thus far are produced solely in a wind. We conclude that great care must be taken when interpreting the behaviour of these weak features.Comment: Accepted for publication in MNRAS, 8 pages, 6 figure

Wheat in Pakistan and other Asian Countries

It seems as if in recent years the development literature has shifted weight towards the agricultural sector, thereby doing more justice to the relative importance of that sector in developing economies. The occurrence of the Green Revolution and, subsequently, the concern for its distribution effects have contributed to this shift. Another cause may have been the accusation of an urban bias in development economics and, particularly, in development policies. Or, more down to earth, the explanation may be simply that in the course of time it was realized that the neglect of wage goods - among which food products are prominent - creates a very serious bottleneck which eventually leads to inflation and balance-of-payments problems, not to mention social discontent and political tension

XMMU J174716.1-281048: a "quasi-persistent" very faint X-ray transient?

The X-ray transient XMMU J174716.1-281048 was serendipitously discovered with XMM-Newton in 2003. It lies about 0.9 degrees off the Galactic Centre and its spectrum shows a high absorption (~8 x 10E22 cm^(-2)). Previous X-ray observations of the source field performed in 2000 and 2001 did not detect the source, indicative of a quiescent emission at least two orders of magnitude fainter. The low luminosity during the outburst (~5 x 10E34 erg/s at 8 kpc) indicates that the source is a member of the ``very faint X-ray transients'' class. On 2005 March 22nd the INTEGRAL satellite caught a possible type-I X-ray burst from the new INTEGRAL source IGR J17464-2811, classified as fast X-ray transient. This source was soon found to be positionally coincident, within the uncertainties, with XMMU J174716.1-281048. Here we report data analysis of the X-ray burst observed with the IBIS and JEM-X telescopes and confirm the type-I burst nature. We also re-analysed XMM-Newton and Chandra archival observations of the source field. We discuss the implications of these new findings, particularly related to the source distance as well as the source classification.Comment: 4 pages, 8 figures, accepted for publication in A&A Letter

Carbon Flashes in the Heavy Element Ocean on Accreting Neutron Stars

We show that burning of a small mass fraction of carbon in a neutron star ocean is thermally unstable at low accumulated masses when the ocean contains heavy ashes from the hydrogen burning rapid proton (rp) process. The key to early unstable ignition is the low thermal conductivity of a heavy element ocean. The instability requires accretion rates in excess of one-tenth the Eddington limit when the carbon mass fraction is 0.1 or less. The unstable flashes release 10^{42} to 10^{43} ergs over hours to days, and are likely the cause of the recently discovered large Type I X-ray bursts (so-called ``superbursts'') from six Galactic low mass X-ray binaries. In addition to explaining the energetics, recurrence times, and durations of the superbursts, these mixed carbon/heavy element flashes have an accretion rate dependence of unstable burning similar to that observed. Though the instability is present at accretion rates near Eddington, there is less contrast with the accretion luminosity there, explaining why most detections are made at accretion rates between 0.1 and 0.3 Eddington. Future comparisons of time dependent calculations with observations will provide new insights into the rp process.Comment: Submitted to Astrophysical Journal Letters (6 pages, 3 figures

Burst-properties as a function of mass accretion rate in GX 3+1

GX 3+1 is a low-mass X-ray binary that is persistently bright since its discovery in 1964. It was found to be an X-ray burster twenty years ago proving that the compact object in this system is a neutron star. The burst rate is so low that only 18 bursts were reported prior to 1996. The Wide Field Cameras on BeppoSAX have, through a dedicated monitoring program on the Galactic center region, increased the number of X-ray bursts from GX 3+1 by 61. Since GX 3+1 exhibits a slow (order of years) modulation in the persistent flux of about 50%, these observations opens up the unique possibility to study burst properties as a function of mass accretion rate for very low burst rates. This is the first time that bursts are detected from GX 3+1 in the high state. From the analysis we learn that all bursts are short with e-folding decay times smaller than 10 s. Therefore, all bursts are due to unstable helium burning. Furthermore, the burst rate drops sixfold in a fairly narrow range of 2-20 keV flux; we discuss possible origins for this.Comment: 9 pages and 7 figures. Accepted for publication in Astronomy & Astrophysic

Correlated Optical/X-ray Long-term Variability in LMXB 4U1636-536

We have conducted a 3-month program of simultaneous optical, soft and hard X-ray monitoring of the LMXB 4U1636-536/V801 Ara using the SMARTS 1.3m telescope and archival RXTE/ASM and Swift/XRT data. 4U1636-536 has been exhibiting a large amplitude, quasi-periodic variability since 2002 when its X-ray flux dramatically declined by roughly an order of magnitude. We confirmed that the anti-correlation between soft (2-12 keV) and hard (> 20 keV) X-rays, first investigated by Shih et al. (2005), is not an isolated event but a fundamental characteristic of this source's variability properties. However, the variability itself is neither strictly stable nor changing on an even longer characteristic timescale. We also demonstrate that the optical counterpart varies on the same timescale, and is correlated with the soft, and not the hard, X-rays. This clearly shows that X-ray reprocessing in LMXB discs is mainly driven by soft X-rays. The X-ray spectra in different epochs of the variability revealed a change of spectral characteristics which resemble the state change of black hole X-ray binaries. All the evidence suggests that 4U1636-536 is frequently (~monthly) undergoing X-ray state transitions, a characteristic feature of X-ray novae with their wide range of luminosities associated with outburst events. In its current behavioural mode, this makes 4U1636-536 an ideal target for investigating the details of state changes in luminous X-ray binaries.Comment: 7 pages, 6 figures, accepted for publication in MNRA

Theoretical Models of Superbursts on Accreting Neutron Stars

We carry out a general-relativistic global linear stability analysis of the amassed carbon fuel on the surface of an accreting neutron star to determine the conditions under which superbursts occur. We reproduce the general observational characteristics of superbursts, including burst fluences, recurrence times, and the absence of superbursts on stars with accretion rates below 10% of the Eddington limit. By comparing our results with observations, we are able to set constraints on neutron star parameters such as the stellar radius and neutrino cooling mechanism in the core. Specifically, we find that accreting neutron stars with ordered crusts and highly efficient neutrino emission in their cores (due to direct URCA or pionic reactions, for example) produce extremely energetic (> 10^44 ergs) superbursts which are inconsistent with observations, in agreement with previous investigations. Also, because of pycnonuclear burning of carbon, they do not have superbursts in the range of accretion rates at which superbursts are actually observed unless the crust is very impure. Stars with less efficient neutrino emission (due to modified URCA reactions, for example) produce bursts that agree better with observations. Stars with highly inefficient neutrino emission in their cores produce bursts that agree best with observations. All systems that accrete primarily hydrogen and in which superbursts are observed show evidence of H- and He-burning delayed mixed bursts. We speculate that delayed mixed bursts provide sufficient amounts of carbon fuel for superbursts and are thus a prerequisite for having superbursts. We compare our global stability analysis to approximate one-zone criteria used by other authors and identify a particular set of approximations that give accurate results for most choices of parameters. (abstract truncated)Comment: 43 pages, 18 figures, accepted by Ap