L. J. Townsend

Spin period change and the magnetic fields of neutron stars in Be X-ray binaries in the Small Magellanic Cloud

We report on the long term average spin period, rate of change of spin period and X-ray luminosity during outbursts for 42 Be X-ray binary systems in the Small Magellanic Cloud. We also collect and calculate parameters of each system and use this data to determine that all systems contain a neutron star which is accreting via a disc, rather than a wind, and that if these neutron stars are near spin equilibrium, then over half of them, including all with spin periods over about 100 seconds, have magnetic fields over the quantum critical level of 4.4x10^13 G. If these neutron stars are not close to spin equilibrium, then their magnetic fields are inferred to be much lower, on the order of 10^6-10^10 G, comparable to the fields of neutron stars in low mass X-ray binaries. Both results are unexpected and have implications for the rate of magnetic field decay and the isolated neutron star population.

On the orbital parameters of Be/X-ray binaries in the Small Magellanic Cloud

The orbital motion of a neutron star about its optical companion presents a window through which we can study the orbital parameters of that binary system. This has been used extensively in the Milky Way to calculate these parameters for several high-mass X-ray binaries. Using several years of Rossi X-ray Timing Explorer Proportional Counter Array data, we derive the orbital parameters of four Be/X-ray binary systems in the Small Magellanic Cloud (SMC), increasing the number of systems with orbital solutions by a factor of 3. We find one new orbital period, confirm a second and discuss the parameters with comparison to the Galactic systems. Despite the low metallicity in the SMC, these binary systems sit amongst the Galactic distribution of orbital periods and eccentricities, suggesting that metallicity may not play an important role in the evolution of high-mass X-ray binary systems. A plot of orbital period against eccentricity shows that the supergiant, Be and low-eccentricity OB transient systems occupy separate regions of the parameter space; akin to the separated regions on the Corbet diagram. Using a Spearman’s rank correlation test, we also find a possible correlation between the two parameters. The mass functions, inclinations and orbital semimajor axes are derived for the SMC systems based on the binary parameters and the spectral classification of the optical counterpart. As a by-product of our work, we present a catalogue of the orbital parameters for every high-mass X-ray binary in the Galaxy and Magellanic Clouds for which they are known.

X-ray pulsations from the region of the supergiant fast X-ray transient IGR J17544−2619

Phase-targeted RXTE observations have allowed us to detect a transient 71.49 0.02 s signal that is most likely to be originating from the supergiant fast X-ray transient IGR J17544 2619. The phase-folded light curve shows a possible double-peaked structure with a pulsed flux of 4.8 10 12 erg cm 2 s 1 (3 10 keV). Assuming the signal to indicate the spin period of the neutron star in the system, the provisional location of IGR J17544 2619 on the Corbet diagram places the system within the classical wind-fed supergiant XRB region. Such a result illustrates the growing trend of supergiant fast X-ray transients to span across both of the original classes of HMXB in Porb Pspin space.

INTEGRAL deep observations of the Small Magellanic Cloud

Deep observations of the Small Magellanic Cloud (SMC) and region were carried out in the hard X-ray band by the INTEGRAL observatory in 2008-2009. The field of view of the instrument permitted simultaneous coverage of the entire SMC and the eastern end of the Magellanic Bridge. In total, INTEGRAL detected seven sources in the SMC and five in the Magellanic Bridge; the majority of the sources were previously unknown systems. Several of the new sources were detected undergoing bright X- ray outbursts and all the sources exhibited transient behaviour except the supergiant system SMC X-1. They are all thought to be High Mass X-ray Binary (HMXB) systems in which the compact object is a neutron star.

The XMM–Newton survey of the Small Magellanic Cloud: XMMU J005011.2−730026 = SXP 214, a Be/X-ray binary pulsar★

In the course of the XMM―Newton survey of the Small Magellanic Cloud, a region to the east of the emission nebula N19 was observed in 2009 November. To search for new candidates for high-mass X-ray binaries, the EPIC-pn and EPIC-MOS data of the detected point sources were investigated and their spectral and temporal characteristics identified. A new transient (XMMU J005011.2-730026 = SXP 214) with a pulse period of 214.05 s was discovered; the source had a hard X-ray spectrum with a power-law index of ~0.65. The accurate X-ray source location permits the identification of the X-ray source with an ~15th magnitude Be star, thereby confirming this system as a new Be/X-ray binary.

The XMM-Newton survey of the Small Magellanic Cloud: discovery of the 11.866 s Be/X-ray binary pulsar XMMU J004814.0-732204(SXP11.87)

Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warsaw, PolandReceived 21 September 2010 / Accepted 6 November 2010ABSTRACTAims.One of the goals of the XMM-Newton survey of the Small Magellanic Cloud is the study of the Be/X-ray binary population.During one of our first survey observations a bright new trans ient − XMMUJ004814.0-732204− was discovered.Methods.We present the analysis of the EPIC X-ray data together with optical observations, to investigate the spectral and temporalcharacteristics of XMMUJ004814.0-732204.Results.We found coherent X-ray pulsations in the EPIC data with a period of (11.86642 ± 0.00017) s. The X-ray spectrum can bemodelled by an absorbed power-law with indication for a softexcess. Depending on the modelling of the soft X-ray spectrum, thephoton index ranges between 0.53 and 0.66. We identify the optical counterpart as a B = 14.9mag star which was monitored duringthe MACHO and OGLE-III projects. The optical light curves show regular outbursts by ∼0.5 mag in B and R and up to 0.9 mag in Iwhich repeat with a time scale of about 1000 days. The OGLE-III optical colours of the star are consistent with an early B spectraltype. An optical spectrum obtained at the 1.9m telescope of the South African Astronomical Observatory in December 2009 showsHα emission with an equivalent width of 3.5 ± 0.6A.Conclusions.The X-ray spectrum and the detection of pulsations suggest that XMMUJ004814.0-732204 is a new high mass X-raybinary pulsar in the SMC. The long term variability and the Hα emission line in the spectrum of the optical counterpart identify it asa Be/X-ray binary system.Key words. galaxies: individual: Small Magellanic Cloud – galaxies: stellar content – stars: emission-line, Be – stars: neutron –X-rays: binaries

High-mass X-ray binary SXP18.3 undergoes the longest type II outburst ever seen in the Small Magellanic Cloud

On 2006 August 30, SXP18.3 a high-mass X-ray binary (HMXB) in the Small Magellanic Cloud (SMC) with an 18.3 s pulse period was observed by Rossi X-ray Timing Explorer (RXTE). The source was seen continuously for the following 36 weeks. This is the longest type II outburst ever seen from a HMXB in the SMC. During the outburst, SXP18.3 was located from serendipitous XMM–Newton observations. The identification of the optical counterpart has allowed SXP18.3 to be classified as a Be/X-ray binary. This paper will report on the analysis of the optical and weekly RXTE X-ray data that span the last 10 yr. The extreme length of this outburst has for the first time enabled us to perform an extensive study of the pulse timing of a SMC Be/X-ray binary. We present a possible full orbital solution from the pulse timing data. An orbital period of 17.79 d is proposed from the analysis of the Optical Gravitational Lensing Experiment (OGLE) III light curve placing SXP18.3 on the boundary of known sources in the Corbet diagram.

The Magellanic Bridge: evidence for a population of X-ray binaries

INTEGRAL observations of the Small Magellanic Cloud region have resulted in the serendipitous detection of two transient hard X-ray sources in the Magellanic Bridge. In this paper, we present the timing and spectral characteristics of these sources across the 2-100?keV energy range, which, in conjunction with their optical counterparts, demonstrate that they are high-mass X-ray binaries (HMXBs) in the Magellanic Bridge. Together with one previously known HMXB system, and three candidates, these sources represent an emerging population of X-ray binaries in the Bridge, probably initiated by tidally induced star formation as a result of the gravitational interaction between the Large and Small Magellanic Clouds.

Anisotropic inverse Compton scattering of photons from the circumstellar disc in PSR B1259–63

The gamma-ray binary system PSR B1259-63 consists of a 48 ms pulsar orbiting a Be star. The system is particularly interesting because it is the only gamma-ray binary system where the nature of the compact object is known. The non-thermal radiation from the system is powered by the spin-down luminosity of the pulsar and the unpulsed radiation originates from the stand-off shock front which forms between the pulsar and stellar wind. The Be star/optical companion in the system produces an excess infrared flux from the associated circumstellar disc. This infrared excess provides an additional photon source for inverse Compton scattering. We discuss the effects of the IR excess near periastron, for anisotropic inverse Compton scattering and associated gamma-ray production. We determine the infrared excess from the circumstellar disc using a modified version of a curve of growth method, which takes into account the changing optical depth through the circumstellar disc during the orbit. The model is constrained using archive data and additional mid-IR observations obtained with the VLT during January 2011. The inverse Compton scattering rate was calculated for three orientations of the circumstellar disc. The predicted gamma-ray light curves show that the disc contribution is a maximum around periastron and not around the disc crossing epoch. This is a result of the disc being brightest near the stellar surface. Additional spectroscopic and near-infrared observations were obtained of the system and these are discussed in relation to the possibility of shock heating during the disc crossing epoch.

Orbital period determinations for four SMC Be/X-ray binaries

We present an optical and X-ray study of four Be/X-ray binaries located in the Small Magellanic Cloud (SMC). OGLE I-band data of up to 11 years of semicontinuous monitoring has been analysed for SMC X-2, SXP172 and SXP202B, providing both a measurement of the orbital period (P orb = 18.62, 68.90 and 229.9 d for the pulsars, respectively) and a detailed optical orbital profile for each pulsar. For SXP 172 this has allowed a direct comparison of the optical and X-ray emission seen through regular RXTE monitoring, revealing that the X-ray outbursts precede the optical by around 7 d. Recent X-ray studies by XMM-Newton have identified a new source in the vicinity of SXP15.3 raising doubt on the identification of the optical counterpart to this X-ray pulsar. Here we present a discussion of the observations that led to the proposal of the original counterpart and a detailed optical analysis of the counterpart to the new X-ray source, identifying a 21.7 d periodicity in the OGLE I-band data. The optical characteristics of this star are consistent with that of a SMC Be/X-ray binary. However, this star was rejected as the counterpart to SXP15.3 in previous studies due to the lack of Hα emission.