High-Mass X-ray Binaries and the Spiral Structure of the Host Galaxy

We investigate the manifestation of the spiral structure in the distribution of high-mass X-ray binaries (HMXBs) over the host galaxy. We construct the simple kinematic model. It shows that the HMXBs should be displaced relative to the spiral structure observed in such traditional star formation rate indicators as the Halpha and FIR emissions because of their finite lifetimes. Using Chandra observations of M51, we have studied the distribution of X-ray sources relative to the spiral arms of this galaxy observed in Halpha. Based on K-band data and background source number counts, we have separated the contributions from high-mass and low-mass X-ray binaries and active galactic nuclei. In agreement with model predictions, the distribution of HMXBs is wider than that of bright HII regions concentrated in the region of ongoing star formation. However, the statistical significance of this result is low, as is the significance of the concentration of the total population of X-ray sources to the spiral arms. We also predict the distribution of HMXBs in our Galaxy in Galactic longitude. The distribution depends on the mean HMXB age and can differ significantly from the distributions of such young objects as ultracompact HII regions.Comment: 18 pages, 6 figures; Astronomy Letters, Vol. 33, No. 5, 2007, pp. 299-30

Luminosity Function of High-Mass X-ray Binaries and Anisotropy in the Distribution of Active Galactic Nuclei toward the Large Magellanic Cloud

In 2003-2012, the INTEGRAL observatory has performed long-term observations of the Large Magellanic Cloud (LMC). At present, this is one of the deepest hard X-ray (20-60 keV) surveys of extragalactic fields in which more than 20 sources of different natures have been detected. We present the results of a statistical analysis of the population of high-mass X-ray binaries in the LMC and active galactic nuclei (AGNs) observed in its direction. The hard X-ray luminosity function of high-mass X-ray binaries is shown to be described by a power law with a slope alpha~1.8, that in agreement with the luminosity function measurements both in the LMC itself, but made in the soft X-ray energy band, and in other galaxies. At the same time, the number of detected AGNs toward the LMC turns out to be considerably smaller than the number of AGNs registered in other directions, in particular, toward the source 3C 273. The latter confirms the previously made assumption that the distribution of matter in the local Universe is nonuniform.Comment: 5 pages, 5 figures, will be published in Astronomy Letters, 2012, Vol. 38, No. 8, p. 492--49

Constraints on the luminosity of the stellar remnant in SNR1987A

We obtain photometric constraints on the luminosity of the stellar remnant in SNR1987A using XMM-Newton and INTEGRAL data. The upper limit in the 2--10 keV band based on the XMM-Newton data is L<5*10^{34}erg/s. We note, however, that the optical depth of the envelope is still high in the XMM-Newton band, therefore, this upper limit does not constrain the true unabsorbed luminosity of the central source. The optical depth is expected to be small in the hard X-ray band of the IBIS telescope aboard the INTEGRAL observatory, therefore it provides an unobscured look at the stellar remnant. We did not detect statistically significant emission from SN1987A in the 20-60 keV band with the upper limit of L<1.1*10^{36}erg/s. We also obtained an upper limit on the mass of radioactive 44Ti M(44Ti)<10^{-3}Msun.Comment: 5 pages, 3 figures, accepted for publication in Astronomy Letter

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 semi-continuous monitoring has been analysed for SMC X-2, SXP172 and SXP202B, providing both a measurement of the orbital period (Porb = 18.62, 68.90, and 229.9 days for the pulsars respectively) and a detailed optical orbital profile for each pulsar. For SXP172 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 days. 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 H{\alpha} emission.Comment: Accepted for publication in MNRAS, 11 pages, 17 figure

The Orbital Solution and Spectral Classification of the High-Mass X-Ray Binary IGR J01054-7253 in the Small Magellanic Cloud

We present X-ray and optical data on the Be/X-ray binary (BeXRB) pulsar IGR J01054-7253 = SXP11.5 in the Small Magellanic Cloud (SMC). Rossi X-ray Timing Explorer (RXTE) observations of this source in a large X-ray outburst reveal an 11.483 +/- 0.002s pulse period and show both the accretion driven spin-up of the neutron star and the motion of the neutron star around the companion through Doppler shifting of the spin period. Model fits to these data suggest an orbital period of 36.3 +/- 0.4d and Pdot of (4.7 +/- 0.3) x 10^{-10} ss^{-1}. We present an orbital solution for this system, making it one of the best described BeXRB systems in the SMC. The observed pulse period, spin-up and X-ray luminosity of SXP11.5 in this outburst are found to agree with the predictions of neutron star accretion theory. Timing analysis of the long-term optical light curve reveals a periodicity of 36.70 +/- 0.03d, in agreement with the orbital period found from the model fit to the X-ray data. Using blue-end spectroscopic observations we determine the spectral type of the counterpart to be O9.5-B0 IV-V. This luminosity class is supported by the observed V-band magnitude. Using optical and near-infrared photometry and spectroscopy, we study the circumstellar environment of the counterpart in the months after the X-ray outburst.Comment: 12 pages, 13 figures and 3 tables. This paper has been accepted for publication in MNRA

The VLT-FLAMES Tarantula survey XX. The nature of the X-ray bright emission-line star VFTS 399

Context. The stellar population of the 30 Doradus star-forming region in the Large Magellanic Cloud contains a subset of apparently single, rapidly rotating O-type stars. The physical processes leading to the formation of this cohort are currently uncertain. Aims. One member of this group, the late O-type star VFTS 399, is found to be unexpectedly X-ray bright for its bolometric luminosity − in this study we aim to determine its physical nature and the cause of this behaviour. Methods. To accomplish this we performed a time-resolved analysis of optical, infrared and X-ray observations. Results. We found VFTS 399 to be an aperiodic photometric variable with an apparent near-IR excess. Its optical spectrum demonstrates complex emission profiles in the lower Balmer series and select He i lines − taken together these suggest an OeBe classification. The highly variable X-ray luminosity is too great to be produced by a single star, while the hard, non-thermal nature suggests the presence of an accreting relativistic companion. Finally, the detection of periodic modulation of the X-ray lightcurve is most naturally explained under the assumption that the accretor is a neutron star. Conclusions. VFTS 399 appears to be the first high-mass X-ray binary identified within 30 Dor, sharing many observational characteristics with classical Be X-ray binaries. Comparison of the current properties of VFTS 399 to binary-evolution models suggests a progenitor mass ≳25 M⊙ for the putative neutron star, which may host a magnetic field comparable in strength to those of magnetars. VFTS 399 is now the second member of the cohort of rapidly rotating “single” O-type stars in 30 Dor to show evidence of binary interaction resulting in spin-up, suggesting that this may be a viable evolutionary pathway for the formation of a subset of this stellar population

Evolution of the X-ray Luminosity in Young HII Galaxies

In an effort to understand the correlation between X-ray emission and present star formation rate (SFR), we obtained XMM-Newton data to estimate the X-ray luminosities of a sample of actively starforming HII galaxies. The obtained X-ray luminosities are compared to other well known tracers of star formation activity such as the far infrared and the ultraviolet luminosities. We also compare the obtained results with empirical laws from the literature and with recently published analysis applying synthesis models. We use the time delay between the formation of the stellar cluster and that of the first X-ray binaries, in order to put limits on the age of a given stellar burst. We conclude that the generation of soft X-rays, as well as the Ha or infrared luminosities is instantaneous. The relation between the observed radio and hard X-ray luminosities, on the other hand, points to the existence of a time delay between the formation of the stellar cluster and the explosion of the first massive stars and the consequent formation of supernova remnants and high mass X-ray binaries (HMXB) which originate the radio and hard X-ray fluxes respectively. When comparing hard X-rays with a star formation indicator that traces the first million years of evolution (e.g. Ha luminosities) we found a deficit in the expected X-ray luminosity. This deficit is not found when the X-ray luminosities are compared with infrared luminosities, a star formation tracer that represents an average over the last 10^8 years. The results support the hypothesis that hard X-rays are originated in X-ray binaries which, as supernova remnants, have a formation time delay of a few mega years after the starforming burst.Comment: Accepted by MNRA

Long-Term INTEGRAL and RXTE Observations of the X-Ray Pulsar LMC X-4

We analyze the observations of the X-ray pulsar LMC X-4 performed by the INTEGRAL observatory and the All-Sky Monitor (ASM) of the RXTE observatory over a wide energy range. The observed hard X-ray flux from the source is shown to change by more than a factor of 50 (from ~70 mCrab in the high state to ~1.3 mCrab in the low state) on the time scale of the accretion-disk precession period, whose mean value for 1996-2004 was determined with a high accuracy, Pprec=30.275+/-0.004 days. In the low state, a flare about 10 h in duration was detected from the source; the flux from the source increased by more than a factor of 4 during this flare. The shape of the pulsar's broadband spectrum is essentially invariable with its intensity; no statistically significant features associated with the possible resonance cyclotron absorption line were found in the spectrum of the source.Comment: 16 pages, 5 figures, Astronomy Letters, 31, 380-387 (2005