Three new high-mass X-ray binaries in the Large Magellanic Cloud

The Magellanic Clouds host a large population of high-mass X-ray binary (HMXB) systems, but although the Large Magellanic Cloud (LMC) is an order of magnitude more massive than the Small Magellanic Cloud, significantly fewer HMXBs are known. We conducted a search for new HMXBs in XMM-Newton observations, which we performed to investigate supernova remnant candidates in the supergiant shells LMC5 and LMC7. The three observed fields are located in regions, which were little explored in X-rays before. We analysed the XMM-Newton data to look for sources with hard X-ray spectrum and counterparts with optical colours and brightness typical for HMXBs. We report the discovery of three new Be/X-ray binaries, two of them showing pulsations in their X-ray flux. With a luminosity of 6.5e34 erg/s, XMMU J045315.1-693242 in LMC7 was relatively X-ray faint. The long-term OGLE I-band light curve of the V = 15.5 mag counterpart suggests a 49.6 day or 24.8 day orbital period for the binary system. XMMU J045736.9-692727, also located in LMC7 was brighter with a luminosity of 5.6e35 erg/s and hard spectrum with a power-law photon index of 0.63. The X-ray flux revealed clear pulsations with a period of 317.7 s. We obtained optical high resolution spectra from the V = 14.2 mag counterpart using the SALT-HRS spectrograph. Halpha and Hbeta were observed in emission with complex line profiles and equivalent widths of -8.0 A and -1.3 A, respectively. The I-band light curve obtained from OGLE shows a series of four strong outbursts followed by a sudden drop in brightness by more than 1 mag within 73-165 days and a recovery to the level before the outbursts. RX J0524.2-6620, previously classified as X-ray binary candidate, is located at the eastern part of LMC5. We report the discovery of 360.7 s pulsations. (abridged)
Comment: 14 pages, 22 figures, accepted for publication in Astronomy & Astrophysics