Number Of High-mass X-ray Binaries Research Articles

Hα imaging for BeXRBs in the Small Magellanic Cloud

AbstractThe Small Magellanic Cloud (SMC) hosts a large number of high-mass X-ray binaries, and in particular of Be/X-ray Binaries (BeXRBs; neutron stars orbiting OBe-type stars), offering a unique laboratory to address the effect of metalicity. One key property of their optical companion is Hα in emission, which makes them bright sources when observed through a narrow-band Hα filter. We performed a survey of the SMC Bar and Wing regions using wide-field cameras (WFI@MPG/ESO and MOSAIC@CTIO/Blanco) in order to identify the counterparts of the sources detected in our XMM-Newton survey of the same area. We obtained broad-band R and narrow-band Hα photometry, and identified ~10000 Hα emission sources down to a sensitivity limit of 18.7 mag (equivalent to ~B8 type Main Sequence stars). We find the fraction of OBe/OB stars to be 13% down to this limit, and by investigating this fraction as a function of the brightness of the stars we deduce that Hα excess peaks at the O9-B2 spectral range. Using the most up-to-date numbers of SMC BeXRBs we find their fraction over their parent population to be ~0.002 − 0.025 BeXRBs/OBe, a direct measurement of their formation rate.

Metallicity dependence of high-mass X-ray binary populations

High-mass X-ray binaries (HMXBs) might have contributed a non-negligible fraction of the energy feedback to the interstellar and intergalactic media at high redshift, becoming important sources for the heating and ionization history of the Universe. However, the importance of this contribution depends on the hypothesized increase in the number of HMXBs formed in low-metallicity galaxies and in their luminosities. In this work we test the aforementioned hypothesis, and quantify the metallicity dependence of HMXB population properties. We compile from the literature a large set of data on the sizes and X-ray luminosities of HMXB populations in nearby galaxies with known metallicities and star formation rates. We use Bayesian inference to fit simple Monte Carlo models that describe the metallicity dependence of the size and luminosity of the HMXB populations. We find that HMXBs are typically ten times more numerous per unit star formation rate in low-metallicity galaxies (12 + log(O/H) < 8, namely < 20% solar) than in solar-metallicity galaxies. The metallicity dependence of the luminosity of HMXBs is small compared to that of the population size. Our results support the hypothesis that HMXBs are more numerous in low-metallicity galaxies, implying the need to investigate the feedback in the form of X-rays and energetic mass outflows of these high-energy sources during cosmic dawn.

XMM-Newtonobservation of the interacting galaxies NGC 1512 and NGC 1510

The galaxy NGC1512 is interacting with the smaller galaxy NGC1510 and shows a peculiar morphology, characterised by two extended arms immersed in an HI disc whose size is about four times larger than the optical diameter of NGC1512. For the first time we performed a deep X-ray observation of the galaxies NGC1512 and NGC1510 with XMM-Newton to gain information on the population of X-ray sources and diffuse emission in a system of interacting galaxies. We identified and classified the sources detected in the XMM-Newton field of view by means of spectral analysis, hardness-ratios calculated with a Bayesian method, X-ray variability, and cross-correlations with catalogues in optical, infrared, and radio wavelengths. We also made use of archival Swift (X-ray) and Australia Telescope Compact Array (radio) data to better constrain the nature of the sources detected with XMM-Newton. We detected 106 sources in the energy range of 0.2-12 keV, out of which 15 are located within the D_25 regions of NGC1512 and NGC1510 and at least six sources coincide with the extended arms. We identified and classified six background objects and six foreground stars. We discussed the nature of a source within the D_25 ellipse of NGC1512, whose properties indicate a quasi-stellar object or an intermediate ultra-luminous X-ray source. Taking into account the contribution of low-mass X-ray binaries and active galactic nuclei, the number of high-mass X-ray binaries detected within the D_25 region of NGC1512 is consistent with the star formation rate obtained in previous works based on radio, infrared optical, and UV wavelengths. We detected diffuse X-ray emission from the interior region of NGC1512 with a plasma temperature of kT=0.68(0.31-0.87) keV and a 0.3-10 keV X-ray luminosity of 1.3E38 erg/s, after correcting for unresolved discrete sources.

Optical spectra of five new Be/X-ray binaries in the Small Magellanic Cloud and the link of the supergiant B[e] star LHA 115-S 18 with an X-ray source

The Small Magellanic Cloud (SMC) is well known to harbor a large number of High-Mass X-ray Binaries (HMXBs). The identification of their optical counterparts provides information on the nature of the donor stars and can help to constrain the parameters of these systems and their evolution. We obtained optical spectra for a number of HMXBs identified in previous Chandra and XMM-Newton surveys of the SMC using the AAOmega/2dF fiber-fed spectrograph at the Anglo-Australian Telescope. We find 5 new Be/X-ray binaries (BeXRBs; including a tentative one), by identifying the spectral type of their optical counterparts, and we confirm the spectral classification of an additional 15 known BeXRBs. We compared the spectral types, orbital periods, and eccentricities of the BeXRB populations in the SMC and the Milky Way and we find marginal evidence for difference between the spectral type distributions, but no statistically significant differences for the orbital periods and the eccentricities. Moreover, our search revealed that the well known supergiant B[e] star LHA 115-S 18 (or AzV 154) is associated with the weak X-ray source CXOU J005409.57-724143.5. We provide evidence that the supergiant star LHA 115-S 18 is the optical counterpart of the X-ray source, and we discuss different possibilities of the origin of its low X-ray luminosity (Lx ~ 4 x 10^33 erg/s).

Population synthesis of high mass X-ray binaries

By simulating the evolution of spin periods of magnetized neutron stars which interact with their environment in binary systems, we investigate the Galactic population of high mass X-ray binaries (HMXBs). The number of HMXBs in the Galaxy is between 190 and 240, and their birthrate is from 5.9 × 10−5 yr−1 to 6.3 × 10−5 yr−1. Comparing the Corbet diagram (the positions of the spin periods vs. the orbital periods of HMXBs) in our model with the associated observations, we find that the stellar wind structure and the process of matter transfer are very important for understanding HMXBs.

Spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud★

The spectral distributions of Be/X-ray binaries in the Large Magellanic Cloud and Galaxy have been shown to differ significantly from the distribution of isolated Be stars in the Galaxy. Population synthesis models can explain this difference in spectral distributions through substantial angular momentum loss from the binary system. In this work, we explore the spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud (SMC) using high signal-to-noise ratio spectroscopy of a sample of 37 optical counterparts to known X-ray pulsars. Our results show that the spectral distribution of Be/X-ray binaries in the SMC is consistent with that of the Galaxy, despite the lower metallicity environment of the SMC. This may indicate that, although the metallicity of the SMC is conducive to the formation of a large number of high-mass X-ray binaries, the spectral distribution of these systems is likely to be most strongly influenced by angular momentum losses during binary evolution, which are not particularly dependent on the local metallicity.