RX J0123.4-7321-the story continues: major circumstellar disc loss and recovery

ABSTRACT RX J0123.4-7321 is a well-established Be star X-ray binary system in the Small Magellanic Cloud. 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 characterized by regular outbursts at the proposed binary period of 119 d. However, around 2008 February 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 that 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 (2020 March) the characteristic periodic outbursts had yet to return to their early state, indicating that the disc still had some re-building yet to complete

Swift J004427.3-734801-a probable Be/white dwarf system in the Small Magellanic Cloud

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 2020 January. It was not detected in any of the previous 3 yr worth of observations. The accurate positional determination from the X-ray data has permitted an optical counterpart to be identified that has the characteristics of an O9V−B2III star. Evidence for the presence of an infrared excess and significant Iband variability strongly suggests that this is an OBe-type star. Over 17 yr worth of optical monitoring by the OGLE (Optical Gravitational Lensing Experiment) project reveals periods of time in which quasi-periodic optical flares occur at intervals of ∼21.5 d. 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 is a rarely identified Be star-white dwarf binary in the SMC

Swift J011511.0-725611: Discovery of a rare Be star/white dwarf binary system in the SMC

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 supersoft X-ray emission, indicative of a WD 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 South African Large Telescope confirms the Be nature of the companion star, and long term light curve by OGLE finds both the signature of a circumstellar disc 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

A rare outburst from the stealthy BeXRB system Swift J0549.7-6812

Swift J0549.7 -6812 is a Be/X-ray binary system (BeXRB) in the Large Magellanic Cloud (LMC) exhibiting an ~6 s 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 × 1037 erg s-1. These observations are supported by contemporaneous optical photometric observations, the first reported optical spectrum, as well as several years of historical data from Optical Gravitational Lens Explorer (OGLE) and Gaia . The latter strongly suggest a binary period of 46.1 d. 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

The Be/neutron star system Swift J004929.5-733107 in the Small Magellanic Cloud-X-ray characteristics and optical counterpart candidates

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 position 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 H α 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 disc. Over 18 yr worth of optical monitoring by the OGLE project reveal a periodic modulation at a period of 413 d, 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 Γ = 0.93 ± 0.16. Assuming the known distance to the SMC, the flux corresponds to a luminosity ∼1035 erg s-1. 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

A real-time transient detector and the living Swift-XRT point source catalogue

We present the Living Swift-XRT Point Source (LSXPS) catalogue and real-time transient detector. This system allows us for the first time to carry out low-latency searches for new transient X-ray events fainter than those available to the current generation of wide-field imagers, and report their detection in near real time. Previously, such events could only be found in delayed searches, e.g. of archival data; our low-latency analysis now enables rapid and ongoing follow-up of these events, enabling the probing of time-scales previously inaccessible. The LSXPS is, uniquely among X-ray catalogues, updated in near real time, making this the first up-to-date record of the point sources detected by a sensitive X-ray telescope: the SwiftX-ray Telescope. The associated upper limit calculator likewise makes use of all available data allowing contemporary upper limits to be rapidly produced on demand. These facilities, which enable the low-latency transient system, are also fully available to the community, providing a powerful resource for time-domain and multimessenger astrophysics

Optical and X-ray study of the peculiar high-mass X-ray binary XMMU J010331.7-730144

For a long time XMMU J010331.7−730144 was proposed as a high-mass X-ray binary candidate based on its X-ray properties, however, its optical behaviour was unclear – in particular previous observations did not reveal key Balmer emission lines. In this paper, we report on optical and X-ray variability of the system. XMMU J010331.7–730144 has been monitored with the Optical Gravitational Lensing Experiment (OGLE) in the I and V bands for the past 9 yr where it has shown extremely large amplitude outbursts separated by long periods of low-level flux. During its most recent optical outburst we obtained spectra with the Southern African Large Telescope (SALT) where, for the first time, the H α line is seen in emission, confirming the Be nature of the optical companion. The OGLE colour–magnitude diagrams also exhibit a distinct loop that is explained by changes in mass-loss from the Be star and mass outflow in its disc. In the X-rays, XMMU J010331.7−730144 has been monitored by the Neil Gehrels Swift Observatory through the S-CUBED programme. The X-ray flux throughout the monitoring campaign shows relatively low values for a typical Be/X-ray binary system. We show, from the analysis of the optical data, that the variability is due to the Be disc density and opacity changing rather than its physical extent as a result of efficient truncation by the NS. The relatively low X-ray flux can then be explained by the neutron star normally accreting matter at a low rate due to the small radial extent of the Be disc

The SMC X-ray binary SXP4.78: A new Type II outburst and the identification and study of the optical counterpart

SXP4.78 was originally discovered in 2000 as a pulsar in the Small Magellanic Cloud by the Rossi X-ray Timing Explorer, but it was not spatially located at that time. A new detection in 2018 with the Neil Gehrels Swift Observatory during a Type II outburst permitted its position to be accurately located and its optical counterpart to be identified.We report X-ray and optical monitoring covering epochs before and during the outburst. Using photometric data, we show the long-term variability of the Be disc where we present flux and colour changes associated with the disc growth and decay over a period of ∼ 6000 d. We show evidence of disc growth during the recent outburst through an increase in the HÁ equivalent width and photometric flux. Period analysis was performed using both optical photometric and spectroscopic data, but with no significant detection of an orbital period. A modest periodic signature of 2.65 d was detected from the Optical Gravitational Lensing Experiment (OGLE) I-band data, but we attribute that to the non-radial pulsations of the Be star.We also obtained a blue spectrum from the Southern African Large Telescope, which permits us to classify the spectral type as B0.5 IVCV

Disentangling the neighbouring pulsars SXP 15.3 and SXP 305

SXP 15.3 and SXP 305 are two Be X-ray binaries in the Small Magellanic Cloud that are spatially separated by ∼7 arcsec. The small separation between these sources has, in the past, resulted in confusion about the origin of the emission from the combined region. We present long-term optical and X-ray monitoring results of both sources, where we study the historic and recent behaviour. In particular, from data collected as part of the S-CUBED project we see repeating X-ray outbursts from the combined region of the two sources in the recent light curve from the Neil Gehrels Swift Observatory, and we investigate the origin of this emission. Using the Hα emission line from the Southern African Large Telescope and photometric flux from the Optical Gravitational Lensing Experiment to study the changes in the size and structure of the Be disc, we demonstrate that the X-ray emission likely originates from SXP 15.3. Timing analysis reveals unusual behaviour, where the optical outburst profile shows modulation at approximately twice the frequency of the X-ray outbursts. We consider either of these periodicities being the true orbital period in SXP 15.3 and propose models based on the geometric orientations of the Be disc and neutron star to explain the physical origin of the outbursts

2SXPS: An improved and expanded Swift X-ray telescope point source catalog

We present the 2SXPS (Swift-XRT Point Source) catalog, containing 206,335 point sources detected by the Swift X-ray Telescope (XRT) in the 0.3--10 keV energy range. This catalog represents a significant improvement over 1SXPS, with double the sky coverage (now 3,790 deg$^2$), and several significant developments in source detection and classification. In particular, we present for the first time techniques to model the effect of stray light -- significantly reducing the number of spurious sources detected. These techniques will be very important for future, large effective area X-ray mission such as the forthcoming Athena X-ray observatory. We also present a new model of the XRT point spread function, and a method for correctly localising and characterising piled up sources. We provide light curves -- in four energy bands, two hardness ratios and two binning timescales -- for every source, and from these deduce that over 80,000 of the sources in 2SXPS are variable in at least one band or hardness ratio. The catalog data can be queried or downloaded via a bespoke web interface at https://www.swift.ac.uk/2SXPS, via HEASARC, or in Vizier (IX/58)