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Swift J0549.7-6812 is an Be/X-ray binary system (BeXRB) in the Large Magellanic Cloud (LMC) exhibiting a 6s pulse period. Like many such systems the variable X-ray emission is believed to be driven by the underlying behaviour of the mass donor Be star. In this paper we report on X-ray observations of the brightest known outburst from this system which reached a luminosity of 8 x 10^37 erg/s. These observations are supported by contemporaneous optical photometric observations, the first reported optical spectrum, as well as several years of historical data from OGLE and GAIA. The latter strongly suggest a binary period of 46.1d. All the observational data indicate that Swift J0549.7-6812 is a system that spends the vast majority of its time in X-ray quiescence, or even switched off completely. This suggests that occasional observations may easily miss it, and many similar systems, and thereby underestimate the massive star evolution numbers for the LMC., Comment: 7 pages, 11 figures

SXP 15.6 is a recently established Be star X-ray binary system (BeXRB) in the Small Magellanic Cloud (SMC). Like many such systems the variable X-ray emission is driven by the underlying behaviour of the mass donor Be star. It is shown here that the neutron star in this system is exceptionally close to spin equilibrium averaged over several years, with the angular momentum gain from mass transfer being almost exactly balanced by radiative losses. This makes SXP 15.6 exceptional compared to all other known members of its class in the SMC, all of whom exhibit much higher spin period changes. In this paper we report on X-ray observations of the brightest known outburst from this system. These observations are supported by contemporaneous optical and radio observations, as well as several years of historical data., Comment: 9 pages, 10 figures

We report on the discovery of Swift J011511.0-725611, a rare Be X-ray binary system (BeXRB) with a White Dwarf (WD) compact object, in the Small Magellanic Cloud (SMC) by S-CUBED, a weekly X-ray/UV survey of the SMC by the Neil Gehrels Swift Observatory. Observations show an approximately 3 month outburst from Swift J011511.0-725611, the first detected by S-CUBED since it began in 2016 June. Swift J011511.0-725611 shows super-soft X-ray emission, indicative of a White Dwarf compact object, which is further strengthened by the presence of an 0.871 keV edge, commonly attributed to O viii K-edge in the WD atmosphere. Spectroscopy by SALT confirms the Be nature of the companion star, and long term light-curve by OGLE finds both the signature of a circumstellar disk in the system at outburst time, and the presence of a 17.4 day periodicity, likely the orbital period of the system. Swift J011511.0-725611 is suggested to be undergoing a Type-II outburst, similar to the previously reported SMC Be White Dwarf binary (BeWD), Swift J004427.3-734801. It is likely that the rarity of known BeWD is in part due to the difficulty in detecting such outbursts due to both their rarity, and their relative faintness compared to outbursts in Neutron Star BeXRBs., Comment: 8 pages, 10 figures, accepted for publication in MNRAS

RX J0123.4-7321 is a well-established Be star X-ray binary system (BeXRB) in the Small Magellanic Cloud (SMC). Like many such systems the variable X-ray emission is driven by the underlying behaviour of the mass donor Be star. Previous work has shown that the optical and X-ray were characterised by regular outbursts at the proposed binary period of 119 d. However around February 2008 the optical behaviour changed substantially, with the previously regular optical outbursts ending. Reported here are new optical (OGLE) and X-ray (Swift) observations covering the period after 2008 which suggest an almost total circumstellar disc loss followed by a gradual recovery. This indicates the probable transition of a Be star to a B star, and back again. However, at the time of the most recent OGLE data (March 2020) the characteristic periodic outbursts had yet to return to their early state, indicating that the disk still had some re-building yet to complete., Comment: 5 pages, 4 figures

Swift J004929.5-733107 is an X-ray source in the Small Magellanic Cloud (SMC) that has been reported several times, but the optical counterpart has been unclear due to source confusion in a crowded region of the SMC. Previous works proposed [MA93] 302 as the counterpart, however we show here, using data obtained from the S-CUBED project, that the X-ray positio is inconsistent with that object. Instead we propose a previously unclassified object which has all the indications of being a newly identified Be star exhibiting strong HU emission. Evidence for the presence of significant I-band variability strongly suggest that this is, in fact, a Be type star with a large circumstellar disk. Over 18 years worth of optical monitoring by the OGLE project reveal a periodic modulation at a period of 413d, probably the binary period of the system. A SALT optical spectrum shows strong Balmer emission and supports a proposed spectral classification of B1-3 III-IVe. The X-ray data obtained from the S-CUBED project reveal a time-averaged spectrum well fitted by a photon index = 0.93 pm 0.16. Assuming the known distance to the SMC the flux corresponds to a luminosity 10E35 erg/s. All of these observational facts suggest that this is confirmed as a Be star-neutron star X-ray binary (BeXRB) in the SMC, albeit one with an unusually long binary period at the limits of the Corbet Diagram., Comment: 9 pages 12 figures

We report on the discovery of Swift J004516.6-734703, a Be/X-ray binary system by the Swift SMC Survey, S-CUBED. Swift J004516.6-734703, or SXP 146.6, was found to be exhibiting a bright (~10^37 erg/s) X-ray outburst in 2020 June 18. The historical UV and IR light-curves from OGLE and Swift/UVOT showed that after a long period of steady brightness, it experienced a significant brightening beginning around 2019 March. This IR/UV rise is likely the signature of the formation of a circumstellar disc, confirmed by the presence of strong a H{\alpha} line in SALT spectroscopy, that was not previously present. Periodicity analysis of the OGLE data reveals a plausible 426 day binary period, and in the X-ray a pulsation period of 146.6s period is detected. The onset of X-ray emission from Swift J004516.6-734703 is likely the signature of a Type-I outburst from the first periastron passage of the neutron star companion through the newly formed circumstellar disc. We note that the formation of the circumstellar disc began at the predicted time of the previous periastron passage, suggesting its formation was spurred by tidal interaction with the neutron star., Comment: 6 Pages, 7 Figures, 1 Table. Accepted by MNRAS

Swift J004427.3-734801 is an X-ray source in the Small Magellanic Cloud (SMC) that was first discovered as part of the Swift S-CUBED programme in January 2020. It was not detected in any of the previous 3 years worth of observations. The accurate positional determination from the X-ray data have permitted an optical counterpart to be identified which has the characteristics of an O9V-B2III star. Evidence for the presence of an IR excess and significant I-band variability strongly suggest that this is an OBe type star. Over 17 years worth of optical monitoring by the OGLE project reveal periods of time in which quasi-periodic optical flares occur at intervals of ~21.5d. The X-ray data obtained from the S-CUBED project reveal a very soft spectrum, too soft to be that from accretion on to a neutron star or black hole. It is suggested here that this a rarely identified Be star-white dwarf binary in the SMC., Comment: 6 Pages, 8 Figures. Accepted by MNRAS 2020 June 8. Received 2020 June 5; in original form 2020 April 28

We report the identification from multi-wavelength observations of the Fermi Large Area Telescope (LAT) source 4FGL J1405.1-6119 (= 3FGL J1405.4-6119) as a high-mass gamma-ray binary. Observations with the LAT show that gamma-ray emission from the system is modulated at a period of 13.7135 +/- 0.0019 days, with the presence of two maxima per orbit with different spectral properties. X-ray observations using the Neil Gehrels Swift Observatory X-ray Telescope (XRT) show that X-ray emission is also modulated at this period, but with a single maximum that is closer to the secondary lower-energy gamma-ray maximum. A radio source, coincident with the X-ray source, is also found from Australia Telescope Compact Array (ATCA) observations, and the radio emission is modulated on the gamma-ray period with similar phasing to the X-ray emission. A large degree of interstellar obscuration severely hampers optical observations, but a near-infrared counterpart is found. Near-infrared spectroscopy indicates an O6 III spectral classification. This is the third gamma-ray binary to be discovered with the Fermi LAT from periodic modulation of the gamma-ray emission, the other two sources also have early O star, rather than Be star, counterparts. We consider at what distances we can detect such modulated gamma-ray emission with the LAT, and examine constraints on the gamma-ray binary population of the Milky Way., Comment: Accepted for publication in the Astrophysical Journal

We present X-ray and radio monitoring observations of the gamma-ray binary PSR J2032+4127/MT91 213 during its periastron passage in late 2017. Dedicated Chandra, XMM-Newton,NuSTAR X-ray observations and VLA radio observations of this long orbit (50 years), 143 ms pulsar/Be star system clearly revealed flux and spectral variability during the passage. The X-ray spectrum hardened near periastron, with a significant decrease in the power-law photon index from \Gamma ~ 2 to 1.2 and evidence of an increased absorption column density. We identified a possible spectral break at a few keV in the spectrum that suggests synchrotron cooling. A coincident radio and X-ray flare occurred one week after periastron, which is possibly the result of the pulsar wind interacting with the Be stellar disk and generating synchrotron radiation. However, a multi-wavelength comparison indicate that the X-ray and radio spectra cannot be simply connected by a single power-law component. Hence, the emission in these two energy bands must originate from different particle populations., Comment: Accepted for publication in the ApJ

As the largest population of high mass X-ray binaries, Be/X-ray binaries provide an excellent laboratory to investigate the extreme physics of neutron stars. It is generally accepted that Be stars possess a circumstellar disc, providing an additional source of accretion to the stellar winds present around young hot stars. Interaction between the neutron star and the disc is often the dominant accretion mechanism. A large amount of work has gone into modelling the properties of these circumstellar discs, allowing for the explanation of a number of observable phenomena. In this paper, smoothed particle hydroynamics simulations are performed whilst varying the model parameters (orbital period, eccentricity, the mass ejection rate of the Be star and the viscosity and orientation of the disc). The relationships between the model parameters and the disc's characteristics (base gas density, the accretion rate of the neutron star and the disc's size) are presented. The observational evidence for a dependency of the size of the Be star's circumstellar disc on the orbital period (and semi-major axis) is supported by the simulations., Comment: 9 pages, 10 figures. Accepted by MNRAS

SXP 5.05 is a Be/X-ray binary with a neutron star companion located in the Small Magellanic Cloud. It was first detected in 2013, and later that year, SXP 5.05 underwent a massive optical and X-ray outburst. This outburst dwarfs any other optical event that has been observed for this system during the last 5 years. The large increase in optical brightness of the system implies an increase in the size and density of the Be star's circumstellar disc. The X-ray data show two occultations of the neutron star per orbit and is not consistent with a neutron star passing behind the Be star, and hence the disc is responsible for these occultations. In this paper, we model the outburst of Be/neutron star binary SXP 5.05 as being due to a large increase in mass ejection by the Be star. The neutron star passes directly through the growing disc, and it is shown that the resulting obscuration can qualitatively explain the observed X-ray behaviour of the system. We find the only way to reproduce the timescales of the observed optical behaviour of the system is to increase the mass ejection substantially for a short time (<2 orbits) and to decrease the viscosity during the event. The general behaviour of the observed X-ray and Ha line emission are also reproduced by the model. However, the inferred mass ejection and viscosity needed to produce a sufficiently rapid increase of disc size are both higher than suggested by previous works., Comment: 10 pages, 10 figures. Accepted by MNRAS

X-ray photometry and optical spectra are presented covering the periastron passage of the highly-eccentric, ~50 year binary system PSR J2032+4127 in November 2017. This system consists of a 143 ms pulsar in orbit around a massive OB star, MT 91-213. The data show dramatic changes during the encounter as the pulsar wind collided with the stellar wind. The X-ray flux rose on the approach to periastron, then underwent a major dip in the few days around periastron, and then gradually declined over the next few weeks. The optical spectroscopy revealed a steady decline in the H? line strength on the approach to periastron (from an Equivalent Width of -15A to -7A) implying a truncation of the OB star's circumstellar disk by the approaching neutron star. Smooth Particle Hydrodynamic (SPH) modelling is used here to model the system within the context of the observed behaviour and predict the geometrical configuration of the circumstellar disk with respect to the pulsar's orbit., Comment: 13 pages 14 figures accepted for publication MNRAS

The Swift Small Magellanic Cloud Survey, S-CUBED, is a high cadence shallow X-ray survey of the SMC. The survey consists of 142 tiled pointings covering the optical extent of the SMC, which is performed weekly by NASA's Neil Gehrels Swift Observatory, with an exposure per tile of 60 seconds. The survey is focused on discovery and monitoring of X-ray outbursts from the large known and unknown population of BeXRBs in the SMC. Given the very low background of Swift's X-ray telescope, even with a short exposure per tile, S- CUBED is typically sensitive to outbursts in the SMC at > 1 - 2% Eddington Luminosity for a typical 1.4M neutron star compact object. This sensitivity, combined with the high cadence, and the fact that the survey can be performed all year round, make it a powerful discovery tool for outbursting accreting X-ray pulsars in the SMC. In this paper describe results from the first year of observations of S-CUBED, which includes the 1SCUBEDX catalog of 265 X-ray sources, 160 of which are not identified with any previously cataloged X-ray source. We report on bulk properties of sources in the 1SCUBEDX catalog. Finally we focus on results of S-CUBED observations of several interesting sources, which includes discovery of three Type II outbursts from BeXRBs, and the detection of Type I outbursts and orbital periods in 6 BeXRB systems., Comment: 27 pages, 16 Figures, 5 Tables. Accepted to ApJS

There are over 100 Be stars that are known to have neutron star companions but only one such system with a black hole. Previous theoretical work suggests this is not due to their formation but due to differences in X-ray luminosity. It has also been proposed that the truncation of the Be star's circumstellar disc is dependent on the mass of the compact object. Hence, Be star discs in black hole binaries are smaller. Since accretion onto the compact object from the Be star's disc is what powers the X-ray luminosity, a smaller disc in black hole systems leads to a lower luminosity. In this paper, simulations are performed with a range of eccentricities and compact object mass. The disc's size and density are shown to be dependent on both quantities. Mass capture and, in turn, X-ray luminosity are heavily dependent on the size and density of the disc. Be/black hole binaries are expected to be up to ~10 times fainter than Be/neutron star binaries when both systems have the same eccentricity and can be 100 times fainter when comparing systems with different eccentricity., Comment: 8 pages, 8 figures

PSR J2032+4127 is a young and rapidly rotating pulsar on a highly eccentric orbit around the high-mass Be star MT91 213. X-ray monitoring of the binary system over a $\sim$4000 day period with Swift has revealed an increase of the X-ray luminosity which we attribute to the synchrotron emission of the shocked pulsar wind. We use Swift X-ray observations to infer a clumpy stellar wind with $r^{-2}$ density profile and constrain the Lorentz factor of the pulsar wind to $10^5<\gamma_w<10^6$. We investigate the effects of an axisymmetric stellar wind with polar gradient on the X-ray emission. Comparison of the X-ray light curve hundreds of days before and after the periastron can be used to explore the polar structure of the wind., Comment: 5 pages, submitted to MNRAS Letters

Multiwavelength monitoring of Be/X-ray binaries is crucial to understand the mechanisms producing their outbursts. H 1145-619 is one of these systems, which has recently displayed X-ray activity. We investigate the correlation between the optical emission and the X-ray activity to predict the occurrence of new X-ray outbursts from the inferred state of the circumstellar disc. We have performed a multiwavelength study of H 1145-619 from 1973 to 2017 and present here a global analysis of its variability over the last 40 years. We have used optical spectra from the SAAO, SMARTS and SALT telescopes and optical photometry from INTEGRAL/OMC and ASAS. We also used X-ray observations from INTEGRAL/JEM-X, and IBIS to generate the light curves and combined them with Swift/XRT to extract the X-ray spectra. In addition, we have compiled archival observations and measurements from the literature to complement these data. Comparing the evolution of the optical continuum emission with the Halpha line variability, we have identified three different patterns of optical variability: Global increases and decreases of the optical brightness (observed from 1982 to 1994 and from 2009 to 2017) that can be explained by the dissipation and replenishment of the circumstellar disc, superorbital variations with a period of Psuperorb~590 days (observed in 2002-2009) which seems to be related with the circumstellar disc, and optical outbursts (observed in 1998-1999 and 2002-2005) that we interpret as mass ejections. We have discovered the presence of a retrograde one-armed density wave, which appeared in 2016 and is still present in the circumstellar disc. We have carried out the most complete long-term optical study of the Be/X-ray binary H 1145-619 in correlation with its X-ray activity. We found for the fist time the presence of a retrograde density perturbation in the circumstellar disc of a Be/X-ray binary., Comment: Accepted on 2 September 2017, 13 pages, 12 figures

We have collected and analyzed the complete archive of {\itshape XMM-Newton\} (116), {\itshape Chandra\} (151), and {\itshape RXTE\} (952) observations of the Small Magellanic Cloud (SMC), spanning 1997-2014. The resulting observational library provides a comprehensive view of the physical, temporal and statistical properties of the SMC pulsar population across the luminosity range of $L_X= 10^{31.2}$--$10^{38}$~erg~s$^{-1}$. From a sample of 67 pulsars we report $\sim$1654 individual pulsar detections, yielding $\sim$1260 pulse period measurements. Our pipeline generates a suite of products for each pulsar detection: spin period, flux, event list, high time-resolution light-curve, pulse-profile, periodogram, and spectrum. Combining all three satellites, we generated complete histories of the spin periods, pulse amplitudes, pulsed fractions and X-ray luminosities. Some pulsars show variations in pulse period due to the combination of orbital motion and accretion torques. Long-term spin-up/down trends are seen in 12/11 pulsars respectively, pointing to sustained transfer of mass and angular momentum to the neutron star on decadal timescales. Of the sample 30 pulsars have relatively very small spin period derivative and may be close to equilibrium spin. The distributions of pulse-detection and flux as functions of spin-period provide interesting findings: mapping boundaries of accretion-driven X-ray luminosity, and showing that fast pulsars ($P<$10 s) are rarely detected, which yet are more prone to giant outbursts. Accompanying this paper is an initial public release of the library so that it can be used by other researchers. We intend the library to be useful in driving improved models of neutron star magnetospheres and accretion physics., Comment: 17 pages, 11 + 58 (appendix) figures. To appear in the Astrophysical Journal Supplement

Using hundreds of XMM-Newton and Chandra archival observations and nearly a thousand RXTE observations, we have generated a comprehensive library of the known pulsars in the Small and Large Magellanic Clouds (SMC, LMC). The pulsars are detected multiple times across the full parameter spaces of X-ray luminosity ($L_X= 10^{31-38}$~erg/s) and spin period ( P$<$1s -- P$>$1000s) and the library enables time-domain studies at a range of energy scales. The high time-resolution and sensitivity of the EPIC cameras are complemented by the angular resolution of Chandra and the regular monitoring of RXTE. Our processing %$\sim$15 year pipeline uses the latest calibration files and software to generate a suite of useful products for each pulsar detection: event lists, high time-resolution light curves, periodograms, spectra, and complete histories of $\dot{P}$, the pulsed fraction, etc., in the broad (0.2-12 keV), soft (0.2-2 keV), and hard (2-12 keV) energy bands. After combining the observations from these telescopes, we found that 28 pulsars show long-term spin up and 25 long-term spin down. We also used the faintest and brightest sources to map out the lower and upper boundaries of accretion-powered X-ray emission: the propeller line and the Eddington line, respectively. We are in the process of comparing the observed pulse profiles to geometric models of X-ray emission in order to constrain the physical parameters of the pulsars. Finally we are preparing a public release of the library so that it can be used by others in the astronomical community., Comment: 6 pages, 6 figures

The X-ray binary population of the Small Magellanic Cloud (SMC) contains a large number of massive X-ray binaries and the recent survey of the SMC by XMM-Newton has resulted in almost 50 more tentative high mass X-ray binary candidates. Using probability parameters from Haberl & Sturm (2016) together with the optical spectra and timing in this work, we confirm six new massive X-ray binaries in the SMC. We also report two very probable binary periods; of 36.4d in XMM 1859 and of 72.2 d in XMM 2300. These Be X-ray binaries are likely part of the general SMC population which rarely undergoes an X-ray outburst., Comment: 6 pages, 3 figures, accepted for publication in MNRAS

On 2016 July 30 (MJD 57599), observations of the Small Magellanic Cloud by Swift/XRT found an increase in X-ray counts coming from a position consistent with the Be/X-ray binary pulsar SMC X-3. Follow-up observations on 2016 August 3 (MJD 57603) and 2016 August 10 (MJD 57610) revealed a rapidly increasing count rate and confirmed the onset of a new X-ray outburst from the system. Further monitoring by Swift began to uncover the enormity of the outburst, which peaked at 1.2 x 10^39 erg/s on 2016 August 25 (MJD 57625). The system then began a gradual decline in flux that was still continuing over 5 months after the initial detection. We explore the X-ray and optical behaviour of SMC X-3 between 2016 July 30 and 2016 December 18 during this super-Eddington outburst. We apply a binary model to the spin-period evolution that takes into account the complex accretion changes over the outburst, to solve for the orbital parameters. Our results show SMC X-3 to be a system with a moderately low eccentricity amongst the Be/X-ray binary systems and to have a dynamically determined orbital period statistically consistent with the prominent period measured in the OGLE optical light curve. Our optical and X-ray derived ephemerides show that the peak in optical flux occurs roughly 6 days after periastron. The measured increase in I-band flux from the counterpart during the outburst is reflected in the measured equivalent width of the H-alpha line emission, though the H-alpha emission itself seems variable on sub-day time-scales, possibly due to the NS interacting with an inhomogeneous disc., Comment: 10 pages, 8 figures, accepted for publication in MNRAS. This article replaces a previously submitted version

We present a detailed optical and X-ray study of the 2013 outburst of the Small Magellanic Cloud Be/X-ray binary SXP7.92, as well as an overview of the last 18 years of observations from OGLE, RXTE, Chandra and XMM-Newton. We revise the position of this source to RA(J2000)=00:57:58.4, Dec(J2000)=-72:22:29.5 with a $1\sigma$ uncertainty of 1.5 arcsec, correcting the previously reported position by Coe et al. (2009) by more than 20 arcminutes. We identify and spectrally classify the correct counterpart as a B1Ve star. The optical spectrum is distinguished by an uncharacteristically deep narrow Balmer series, with the H$\alpha$ line in particular having a distinctive shell profile, i.e. a deep absorption core embedded in an emission line. We interpret this as evidence that we are viewing the system edge on and are seeing self obscuration of the circumstellar disc. We derive an optical period for the system of 40.0$\pm0.3$ days, which we interpret as the orbital period, and present several mechanisms to describe the X-ray/Optical behaviour in the recent outburst, in particular the "flares" and "dips" seen in the optical light curve, including a transient accretion disc and an elongated precessing disc., Comment: 15 pages, 7 figures, 3 tables, accepted for publication in MNRAS

We present simultaneous, multi-wavelength observations of the Small Magellanic Cloud Be/XRB IGR J01217-7257 (=SXP 2.16) during outbursts in 2014, 2015 and 2016. We also present the results of RXTE observations of the Small Magellanic Cloud during which the source was initially discovered with a periodicity of 2.1652$\pm$0.0001 seconds which we associate with the spin period of the neutron star. A systematic temporal analysis of long term Swift/BAT data reveals a periodic signal of 82.5$\pm$0.7 days, in contrast with a similar analysis of long base line OGLE I-band light curves which reveals an 83.67$\pm$0.05 days also found in this work. Interpreting the longer X-ray periodicity as indicative of binary motion of the neutron star, we find that outbursts detected by INTEGRAL and Swift between 2014 and 2016 are consistent with Type I outbursts seen in Be/XRBs, occurring around periastron. Comparing these outbursts with the OGLE data, we see a clear correlation between outburst occurrence and increasing I-band flux. A periodic analysis of subdivisions of OGLE data reveals three epochs during which short periodicities of $\sim$1 day are significantly detected which we suggest are non-radial pulsations (NRPs) of the companion star. These seasons immediately precede those exhibiting clear outburst behaviour, supporting the suggested association between the NRPs, decretion disk growth and the onset of Type I outbursts., Comment: 12 pages, 13 figures, 3 tables. Accepted for publication in MNRAS

Gamma-ray binaries consist of a neutron star or a black hole interacting with a normal star to produce gamma-ray emission that dominates the radiative output of the system. Only a handful of such systems have been previously discovered, all within our Galaxy. Here we report the discovery with the Fermi Large Area Telescope (LAT) of a luminous gamma-ray binary in the Large Magellanic Cloud from a search for periodic modulation in all sources in the third Fermi LAT catalog. This is the first such system to be found outside the Milky Way. The system has an orbital period of 10.3 days and is associated with a massive O5III star located in the supernova remnant DEM L241, previously identified as the candidate high-mass X-ray binary (HMXB) CXOU J053600.0-673507. X-ray and radio emission are also modulated on the 10.3 day period, but are in anti-phase with the gamma-ray modulation. Optical radial velocity measurements suggest that the system contains a neutron star. The source is significantly more luminous than similar sources in the Milky Way at radio, optical, X-ray and gamma-ray wavelengths. The detection of this extra-galactic system, but no new Galactic systems raises the possibility that the predicted number of gamma-ray binaries in our Galaxy has been overestimated, and that HMXBs may be born containing relatively slowly rotating neutron stars., Comment: 11 pages, 6 figures. Accepted for publication in The Astrophysical Journal

We present the results of our monitoring program to study the long-term variability of the Halpha line in high-mass X-ray binaries. We have carried out the most complete optical spectroscopic study of the global properties of high-mass X-ray binaries so far with the analysis of more than 1100 spectra of 20 sources. Our aim is to characterise the optical variability timescales and study the interaction between the neutron star and the accreting material. Our results can be summarised as follows: i) we find that Be/X-ray binaries with narrow orbits are more variable than systems with long orbital periods, ii) we show that a Keplerian distribution of the gas particles provides a good description of the disks in Be/X-ray binaries, as it does in classical Be stars, iii) a decrease in the Halpha equivalent width is generally observed after major X-ray outbursts, iv) we confirm that the Halpha equivalent width correlates with disk radius, v) while systems with supergiant companions display, multi-structured profiles, most of the Be/X-ray binaries show at some epoch double-peak asymmetric profiles, indicating that density inhomogeneities is a common property in the disk of Be/X-ray binaries, vi) the profile variability (V/R ratio) timescales are shorter and the Halpha equivalent width are smaller in Be/X-ray binaries than in isolated Be stars, and vii) we provide new evidence that the disk in Be/X-ray binaries is on average denser than in classical Be stars., Comment: new version after language editting and proofs. Reference updated. Accepted for publication in A&A

This is a catalogue of approximately 70 X-ray emitting binary systems in the Small Magellanic Cloud (SMC) that contain a Be star as the mass donor in the system and a clear X-ray pulse signature from a neutron star. The systems are generally referred to as Be/Xray binaries. It lists all their known binary characteristics (orbital period, eccentricity), the measured spin period of the compact object, plus the characteristics of the Be star (spectral type, size of the circumstellar disk, evidence for NRP behaviour). For the first time data from the Spitzer Observatory are combined with ground-based data to provide a view of these systems out into the far-IR. Many of the observational parameters are presented as statistical distributions and compared to other similar similar populations (eg isolated Be & B stars) in the SMC, and to other Be/X-ray systems in the Milky Way. In addition previous important results are re-investigated using this excellently homogeneous sample. In particular, the evidence for a bi-modality in the spin period distribution is shown to be even stronger than first proposed, and the correlation between orbital period and circumstellar disk size seen in galactic sources is shown to be clearly present in the SMC systems and quantised for the first time., Comment: 10 pages, 10 figures, 4 tables. Accepted for publication in MNRAS

On MJD 56590-1 (2013 Oct 25-26) observations of the Magellanic Clouds by the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) observatory discovered a previously-unreported bright, flaring X-ray source. This source was initially given the identification IGR J00569-7226. Subsequent multi-wavelength observations identified the system as new Be/X-ray binary system in the Small Magellanic Cloud. Follow-up X-ray observations by Swift and XMM-Newton revealed an X-ray pulse period of 5.05s and that the system underwent regular occulation/eclipse behaviour every 17d. This is the first reported eclipsing Be/X-ray binary system in the SMC, and only the second such system known to date. Furthermore, the nature of the occultation makes it possible to use the neutron star to X-ray the circumstellar disk, thereby, for the first time, revealing direct observational evidence for its size and clumpy structure. Swift timing measurements allowed for the binary solution to be calculated from the Doppler shifted X-ray pulsations. This solution suggests this is a low eccentricity binary relative to others measured in the SMC. Finally it is interesting to note that the mass determined from this dynamical method for the Be star (approx 13 solar masses) is significantly different from that inferred from the spectroscopic classification of B0.2Ve (approx 16 solar masses) - an effect that has been noted for some other high mass X-ray binary (HMXB) systems., Comment: 19 pages, accepted by MNRAS

An exceptionally bright new X-ray source in the Large Magellanic Cloud was discovered by the Swift/BAT telescope on MJD 54923 (2 April 2009), and shown to have a pulse period of 27s using follow-up observations by RXTE/PCA. We report here on detailed timing observations taken over the following weeks using Fermi/GBM which reveal an excellent orbital solution and indicate that the source flux peaked at $\sim10^{38}$ erg s$^{-1}$. In addition, we report on follow-up optical observations (spectroscopic and photometric) which permit a classification of the mass donor star as B1Ve, and furthermore reveal a strong optical modulation at a period consistent with the binary period found from the Fermi/GBM data - 27.4 d. The dynamical mass estimate for the Be star is not in agreement with that expected for a B1V star - a mismatch similar to this has been previously reported for the few other Magellanic Cloud stars that also have dynamical mass estimates. In addition, the neutron star magnetic field determination from the X-ray data ($\sim10^{13}$ G) adds further evidence to the possibility that many such stars in High Mass X-ray Binary systems may have magnetic fields greater than previously expected ($\sim10^{12}$ G)., Comment: 9 pages, 12 figures; accepted for publication in MNRAS

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 these 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 s, 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, of 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., Comment: 22 pages, 50 figures; to appear in MNRAS

Many pulsars are formed with a binary companion from which they can accrete matter. Torque exerted by accreting matter can cause the pulsar spin to increase or decrease, and over long times, an equilibrium spin rate is achieved. Application of accretion theory to these systems provides a probe of the pulsar magnetic field. We compare the large number of recent torque measurements of accreting pulsars with a high-mass companion to the standard model for how accretion affects the pulsar spin period. We find that many long spin period (P > 100 s) pulsars must possess either extremely weak (B < 10^10 G) or extremely strong (B > 10^14 G) magnetic fields. We argue that the strong-field solution is more compelling, in which case these pulsars are near spin equilibrium. Our results provide evidence for a fundamental link between pulsars with the slowest spin periods and strong magnetic fields around high-mass companions and pulsars with the fastest spin periods and weak fields around low-mass companions. The strong magnetic fields also connect our pulsars to magnetars and strong-field isolated radio/X-ray pulsars. The strong field and old age of our sources suggests their magnetic field penetrates into the superconducting core of the neutron star., Comment: 6 pages, 4 figures; to appear in MNRAS

Several persistent, low luminosity (L_X 10^{34} erg s^{-1}), long spin period (P>100 s) High Mass X-ray Binaries have been reported with blackbody components with temperatures >1 keV. These hot thermal excesses have correspondingly small emitting regions (<1 km^2) and are attributed to the neutron star polar caps. We present a recent XMM-Newton target of opportunity observation of the newest member of this class, Swift J045106.8-694803. The period was determined to be 168.5+/-0.2 s as of 17 July 2012 (MJD = 56125.0). At $L_X 10^{36} erg s^{-1}, Swift J045106.8-694803 is the brightest member of this new class, as well as the one with the shortest period. The spectral analysis reveals for the first time the presence of a blackbody with temperature kT_{BB}=1.8^{+0.2}_{-0.3} keV and radius R_{BB}=0.5+/-0.2 km. The pulsed fraction decreases with increasing energy and the ratio between the hard (>2 keV) and soft (<2 keV) light curves is anticorrelated with the pulse profile. Simulations of the spectrum suggest that this is caused by the pulsations of the blackbody being pi out of phase with those of the power law component. Using a simple model for emission from hot spots on the neutron star surface, we fit the pulse profile of the blackbody component to obtain an indication of the geometry of the system., Comment: 10 pages, 9 figures, 4 tables. Accepted for publication in MNRAS

We observed the newly discovered X-ray source Swift J053041.9-665426 in the X-ray and optical regime to confirm its proposed nature as a high mass X-ray binary. We obtained XMM-Newton and Swift X-ray data, along with optical observations with the ESO Faint Object Spectrograph, to investigate the spectral and temporal characteristics of Swift J053041.9-665426. The XMM-Newton data show coherent X-ray pulsations with a period of 28.77521(10) s (1 sigma). The X-ray spectrum can be modelled by an absorbed power law with photon index within the range 0.76 to 0.87. The addition of a black body component increases the quality of the fit but also leads to strong dependences of the photon index, black-body temperature and absorption column density. We identified the only optical counterpart within the error circle of XMM-Newton at an angular distance of ~0.8 arcsec, which is 2MASS J05304215-6654303. We performed optical spectroscopy from which we classify the companion as a B0-1.5Ve star. The X-ray pulsations and long-term variability, as well as the properties of the optical counterpart, confirm that Swift J053041.9-665426 is a new Be/X-ray binary pulsar in the Large Magellanic Cloud., Comment: 10 pages, 8 figures, accepted for publication in A&A

Local-Group galaxies provide access to samples of X-ray source populations of whole galaxies. The XMM-Newton survey of the Small Magellanic Cloud (SMC) completely covers the bar and eastern wing with a 5.6 deg^2 area in the (0.2-12.0) keV band. To characterise the X-ray sources in the SMC field, we created a catalogue of point sources and sources with moderate extent. Sources with high extent (>40") have been presented in a companion paper. We searched for point sources in the EPIC images using sliding-box and maximum-likelihood techniques and classified the sources using hardness ratios, X-ray variability, and their multi-wavelength properties. The catalogue comprises 3053 unique X-ray sources with a median position uncertainty of 1.3" down to a flux limit for point sources of ~10^-14 erg cm^-2 s^-1 in the (0.2-4.5) keV band, corresponding to 5x10^33 erg s^-1 for sources in the SMC. We discuss statistical properties, like the spatial distribution, X-ray colour diagrams, luminosity functions, and time variability. We identified 49 SMC high-mass X-ray binaries (HMXB), four super-soft X-ray sources (SSS), 34 foreground stars, and 72 active galactic nuclei (AGN) behind the SMC. In addition, we found candidates for SMC HMXBs (45) and faint SSSs (8) as well as AGN (2092) and galaxy clusters (13). We present the most up-to-date catalogue of the X-ray source population in the SMC field. In particular, the known population of X-ray binaries is greatly increased. We find that the bright-end slope of the luminosity function of Be/X-ray binaries significantly deviates from the expected universal high-mass X-ray binary luminosity function., Comment: 32 pages, 18 figures, accepted for publication in A&A, catalog will be available at CDS

The mechanism by which supergiant (sg)B[e] stars support cool, dense dusty discs/tori and their physical relationship with other evolved, massive stars such as luminous blue variables is uncertain. In order to investigate both issues we have analysed the long term behaviour of the canonical sgB[e] star LHA 115-S 18. We employed the OGLE II-IV lightcurve to search for (a-)periodic variability and supplemented these data with new and historic spectroscopy. In contrast to historical expectations for sgB[e] stars, S18 is both photometrically and spectroscopically highly variable. The lightcurve is characterised by rapid aperiodic `flaring' throughout the 16 years of observations. Changes in the high excitation emission line component of the spectrum imply evolution in the stellar temperature - as expected for luminous blue variables - although somewhat surprisingly, spectroscopic and photometric variability appears not to be correlated. Characterised by emission in low excitation metallic species, the cool circumstellar torus appears largely unaffected by this behaviour. Finally, in conjunction with intense, highly variable HeII emission, X-ray emission implies the presence of an unseen binary companion. S18 provides observational support for the putative physical association of (a subset of) sgB[e] stars and luminous blue variables. Given the nature of the circumstellar environment of S18 and that luminous blue variables have been suggested as SN progenitors, it is tempting to draw a parallel to the progenitors of SN1987A and the unusual transient SN2009ip. Moreover the likely binary nature of S18 strengthens the possibility that the dusty discs/tori that characterise sgB[e] stars are the result of binary-driven mass-loss; consequently such stars may provide a window on the short lived phase of mass-transfer in massive compact binaries., Comment: 15 pages, 10 figures; accepted for publication in Astronomy and Astrophysics

Eight years of optical photometry from OGLE-III are presented of the optical counterpart to the High Mass X-ray Binary system, SMC X-1. The optical data provide the best view to date of the modulation of the light from the system at the binary period of 3.9d. In addition, it is shown for the first time that the light is also modulated at the superorbital period - a period associated with the precessing of a warped accretion disk around the neutron star partner. The implications for the sources of the various components of the optical light in this system are discussed., Comment: 6 pages, accepted by MNRAS

In this paper we present the identification of two periodic X-ray signals coming from the direction of the Small Magellanic Cloud (SMC). On detection with the Rossi X-ray Timing Explorer (RXTE), the 175.4s and 85.4s pulsations were considered to originate from new Be/X-ray binary (BeXRB) pulsars with unknown locations. Using rapid follow-up INTEGRAL and XMM-Newton observations, we show the first pulsar (designated SXP175) to be coincident with a candidate high-mass X-ray binary (HMXB) in the northern bar region of the SMC undergoing a small Type II outburst. The orbital period (87d) and spectral class (B0-B0.5IIIe) of this system are determined and presented here for the first time. The second pulsar is shown not to be new at all, but is consistent with being SXP91.1 - a pulsar discovered at the very beginning of the 13 year long RXTE key monitoring programme of the SMC. Whilst it is theoretically possible for accreting neutron stars to change spin period so dramatically over such a short time, the X-ray and optical data available for this source suggest this spin-up is continuous during long phases of X-ray quiescence, where accretion driven spin-up of the neutron star should be minimal., Comment: 13 pages, 16 figures, accepted for publication in MNRAS

We present a detailed X-ray study of the 2003 XMM-Newton observation of the High Mass X-ray Binary XTE J0421+560/CI Cam. The continuum of the X-ray spectrum is well described by a flat power law (Gamma=1.0+/-0.2) with a large intrinsic absorbing column (N_H=(4.4+/-0.5)\times10^{23} cm^{-2}). We have decomposed the broad iron line into 3 separate components: Fe\textsc{i}-K$\alpha$, Fe\textsc{i}-K$\beta$ and Fe\textsc{xxiv-xxv}K$\alpha$. It is unclear how both neutral and almost fully ionised iron can exist simultaneously, however we suggest this could be evidence that the compact object is embedded in the circumstellar material. This doesn't appear to be consistent with the X-ray flux and spectrum of the source, which has remained essentially unchanged since the initial outburst. The iron abundance implied by the ratio of the neutral Fe-K$\alpha$ and Fe-K$\beta$ is compatible with solar. We search for lags in the neutral Fe-K$\alpha$ with respect to the continuum and find marginal evidence for a lag at ~10 ks. We interpret this as the light crossing time of the torus which would suggest that the neutral iron is located at a radius of 10 AU. This result depends on several assumptions including the distance to the system, the inclination, the mass of system and the orbital period none of which are known with any great certainty. Better constraints on these system parameters and further observations of this system are required to confirm this result. We discuss the nature of this system in light of our results and place it in context with other binary B[e] stars., Comment: 9 pages, 8 figures, 3 tables accepted for publication in MNRAS

Although numerous archival XMM-Newton observations existed towards the Small Magellanic Cloud (SMC) before 2009, only a fraction of the whole galaxy was covered. Between May 2009 and March 2010 we carried out an XMM-Newton survey of the SMC, in order to obtain a complete overage of both its bar and wing. Thirty-three observations of 30 different fields with a total exposure of about ne Ms filled the missing parts. We systematically processed all available SMC data from the European Photon Imaging Camera. After rejecting observations with very high background we included 53 archival and the 33 survey observations. We produced images in five different energy bands. We applied astrometric boresight corrections using secure identifications of X-ray sources and combine all the images to produce a mosaic, which covers the main body of the SMC. We present an overview of the XMM-Newton observations, describe their analysis and summarise first results which will be presented in follow-up papers in detail. Here, we mainly focus on extended X-ray sources like supernova remnants (SNRs) and clusters of galaxies which are seen in our X-ray images. The XMM-Newton survey represents the deepest complete survey of the SMC in the 0.15-12.0 keV X-ray band. We propose three new SNRs with low surface brightness of a few 10^-14 erg s^-1 cm^-2 arcmin^-2 and large extent. Also several known remnants appear larger than previously measured from X-rays or other wavelengths extending the size distribution of SMC SNRs to larger values., Comment: 11 pages, 10 figures (images in low resolution), Astronomy & Astrophysics in press

We present a multi-wavelength analysis of the very fast X-ray transient MAXI J0158-744, which was detected by MAXI/GSC on 2011 November 11. The subsequent exponential decline of the X-ray flux was followed with Swift observations, all of which revealed spectra with low temperatures (~100eV) indicating that MAXI J0158-744 is a new Supersoft Source (SSS). The Swift X-ray spectra near maximum show features around 0.8 keV that we interpret as possible absorption from OVIII, and emission from O, Fe, and Ne lines. We obtained SAAO and ESO optical spectra of the counterpart early in the outburst and several weeks later. The early spectrum is dominated by strong Balmer and HeI emission, together with weaker HeII emission. The later spectrum reveals absorption features that indicate a B1/2IIIe spectral type, and all spectral features are at velocities consistent with the Small Magellanic Cloud. At this distance, it is a luminous SSS (>10^37 erg/s) but whose brief peak luminosity of >10^39 erg/s in the 2-4 keV band makes it the brightest SSS yet seen at "hard" X-rays. We propose that MAXI J0158-744 is a Be-WD binary, and the first example to possibly enter ULX territory. The brief hard X-ray flash could possibly be a result of the interaction of the ejected nova shell with the B star wind in which the white dwarf (WD) is embedded. This makes MAXI J0158-744 only the third Be/WD system in the Magellanic Clouds, but it is by far the most luminous. The properties of MAXI J0158-744 give weight to previous suggestions that SSS in nearby galaxies are associated with early-type stellar systems., Comment: 15 pages, 8 figures; ApJ accepted

In the course of the XMM-Newton survey of the Small Magellanic Cloud (SMC), two new bright X-ray sources were discovered exhibiting the spectral characteris- tics of High Mass X-ray Binaries - but revealing only weak evidence for pulsations in just one of the objects(at 153s in XMMUJ010743.1-715953). The accurate X- ray source locations permit the identification of these X-ray source with Be stars, thereby strongly suggesting these systems are new Be/X-ray binaries. From blue spectra the proposed classification for XMMUJ010633.1-731543 is B0.5-1Ve and for XMMUJ010743.1-715953 it is B2IV-Ve., Comment: MNRAS (accepted), 12 pages, 17 figures, 4 tables

We present a comprehensive study of the periodic variations observed in OGLE I-band light curves of SMC Be/X-ray binaries, discovering new optical periodicities in 9 systems. We find that these periodicities derive from a number of mechanisms, notably disturbance of the decretion disk on the orbital period of the system, and aliased non-radial pulsations. We develop metrics that allow these mechanisms to be distinguished on the basis of the shape of the folded optical light curve, and use these metrics to categorise the periodicities present in \sim 50 SMC binary systems. We conclude that extreme care must be taken in the interpretation of the OGLE light curves since only around 30% of the periodicities present can be unambiguously attributed to orbital periods., Comment: 16 pages, 17 figures, accepted for publication in MNRAS

The number of known Be/X-ray binaries in the Large Magellanic Cloud is small compared to the observed population of the Galaxy or the Small Magellanic Cloud. The discovery of a system in outburst provides the rare opportunity to measure its X-ray properties in detail. IGR J05414-6858 was discovered in 2010 by INTEGRAL and found in another outburst with the Swift satellite in 2011. In order to characterise the system, we analysed the data from a follow-up XMM-Newton target of opportunity observation of the 2011 outburst and investigate the stellar counterpart with photometry and spectroscopy. We modelled the X-ray spectra from the EPIC instruments on XMM-Newton and compared them with Swift archival data. In the X-ray and optical light curves, we searched for periodicities and variability. The optical counterpart was classified using spectroscopy obtained with ESO's Faint Object Spectrograph at NTT. The X-ray spectra as seen in 2011 are relatively hard with a photon index of ~0.3-0.4 and show only low absorption. They deviate significantly from earlier spectra of a probable type II outburst in 2010. The neutron star spin period of P_s = 4.4208 s was discovered with EPIC-pn. The I-band light curve revealed a transition from a low to a high state around MJD 54500. The optical counterpart is classified to B0-1IIIe and shows Halpha emission and a variable NIR excess, vanishing during the 2010 outburst. In the optical high state, we found a periodicity at 19.9 days, probably caused by binarity and indicating the orbital period., Comment: Accepted for publication in A&A, 10 pages, 12 figures

We report on the first X-ray images of the Phoenix dwarf galaxy, taken with \emph{XMM-Newton} in July 2009. This local group dwarf galaxy shares similarities with the Small Magellanic Cloud (SMC) including a burst of star formation $\sim$50 Myr ago. The SMC has an abundance of High Mass X-ray Binaries (HMXBs) and so we have investigated the possibility of an HMXB population in Phoenix with the intention of furthering the understanding of the HMXB-star formation rate relation. The data from the combined European Photon Imaging Cameras (EPIC) were used to distinguish between different source classes (foreground stars, background galaxies, AGN and supernova remnants) using EPIC hardness ratios and correlations with optical and radio catalogues. Of the 81 X-ray sources in the field of view, six are foreground stars, four are galaxies and one is an AGN. The remaining sources with optical counterparts have log($\frac{f_X}{f_{opt}}$) consistent with AGN in the local universe. Further investigation of five sources in the field of view suggests they are all background AGN. Their position behind the gas cloud associated with Phoenix makes them a possible tool for further probing the metallicity of this region. We find no evidence for any HMXBs in Phoenix at this time. This rules out the existence of the X-ray persistent supergiant X-ray binary systems. However the transient nature of the Be/X-ray binaries means we cannot rule out a population of these sources but can conclude that it is not extensive., Comment: 13 pages, 4 figures, 4 tables, Accepted for publication in MNRAS

Gamma-ray binaries are stellar systems containing a neutron star or black hole with gamma-ray emission produced by an interaction between the components. These systems are rare, even though binary evolution models predict dozens in our Galaxy. A search for gamma-ray binaries with the Fermi Large Area Telescope (LAT) shows that 1FGL J1018.6-5856 exhibits intensity and spectral modulation with a 16.6 day period. We identified a variable X-ray counterpart, which shows a sharp maximum coinciding with maximum gamma-ray emission, as well as an O6V((f)) star optical counterpart and a radio counterpart that is also apparently modulated on the orbital period. 1FGL J1018.6-5856 is thus a gamma-ray binary, and its detection suggests the presence of other fainter binaries in the Galaxy., Comment: Contact authors: R.H.D. Corbet, M. Kerr, C.C. Cheung

The Small Magellanic Cloud (SMC) hosts a large number of Be/X-ray binaries, however no Be/white dwarf system is known so far, although population synthesis calculations predict that they might be more frequent than Be/neutron star systems. XMMUJ010147.5-715550 was found as a new faint super-soft X-ray source (SSS) with a likely Be star optical counterpart. We investigate the nature of this system and search for further high-absorbed candidates in the SMC. We analysed the XMM-Newton X-ray spectrum and light curve, optical photometry, and the I-band OGLE III light curve. The X-ray spectrum is well represented by black-body and white dwarf atmosphere models with highly model-dependent temperature between 20 and 100 eV. The likely optical counterpart AzV 281 showed low near infrared emission during X-ray activity, followed by a brightening in the I-band afterwards. We find further candidates for high-absorbed SSSs with a blue star as counterpart. We discuss XMMUJ010147.5-715550 as the first candidate for a Be/white dwarf binary system in the SMC., Comment: 6 pages, 4 figures, accepted by A&A