22 research outputs found
A rare outburst from the stealthy BeXRB system Swift J0549.7-6812
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 figure
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 years 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 Africa Large Telescope (SALT) where, for the first time, the Hα line is seen in emission, confirming the Be nature of the optical companion. The OGLE colour-magnitude diagrams also exhibit a distinct loop which 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
SRG/eROSITA-triggered XMM-Newton observations of three Be/X-ray binaries in the LMC: Discovery of X-ray pulsations
Using data from eROSITA, the soft X-ray instrument aboard
Spectrum-Roentgen-Gamma (SRG), we report the discovery of two new hard
transients, eRASSU J050810.4-660653 and eRASSt J044811.1-691318, in the Large
Magellanic Cloud. We also report the detection of the Be/X-ray binary RX
J0501.6-7034 in a bright state. We initiated follow-up observations to
investigate the nature of the new transients and to search for X-ray pulsations
coming from RX J0501.6-7034. We analysed the X-ray spectra and light curves
from our XMM-Newton observations, obtained optical spectra using the South
African Large Telescope to look for Balmer emission lines and utilised the
archival data from the Optical Gravitational Lensing Experiment (OGLE) for the
long-term monitoring of the optical counterparts. We find X-ray pulsations for
eRASSU J050810.4-660653, RX J0501.6-7034, and eRASSt J044811.1-691318 of 40.6
s, 17.3 s, and 784 s, respectively. The Halpha emission lines with equivalent
widths of -10.4 A (eRASSU J050810.4-660653) and -43.9 A (eRASSt
J044811.1-691318) were measured, characteristic for a circumstellar disc around
Be stars. The OGLE I- and V-band light curves of all three systems exhibit
strong variability. A regular pattern of deep dips in the light curves of RX
J0501.6-7034 suggests an orbital period of ~451 days. We identify the two new
hard eROSITA transients eRASSU J050810.4-660653 and eRASSt J044811.1-691318 and
the known Be/X-ray binary RX J0501.6-7034 as Be/X-ray binary pulsars.Comment: Accepted for publication in Astronomy & Astrophysic
A soft and transient ultraluminous X-ray source with 6-h modulation in the NGC 300 galaxy
We investigate the nature of CXOU J005440.5-374320 (J0054), a peculiar bright
( erg/s) and soft X-ray transient in the spiral galaxy
NGC 300 with a 6-hour periodic flux modulation that was detected in a 2014
Chandra observation. Subsequent observations with Chandra and XMM-Newton, as
well as a large observational campaign of NGC 300 and its sources performed
with the Swift Neil Gehrels Observatory, showed that this source exhibits
recurrent flaring activity: four other outbursts were detected across 8
years of monitoring. Using data from the Swift/UVOT archive and from the
XMM-Newton/OM and Gaia catalogues, we noted the source is likely associated
with a bright blue optical/ultraviolet counterpart. This prompted us to perform
follow-up observations with the Southern African Large Telescope in December
2019. With the multi-wavelength information at hand, we discuss several
possibilities for the nature of J0054. Although none is able to account for the
full range of the observed peculiar features, we found that the two most
promising scenarios are a stellar-mass compact object in a binary system with a
WolfRayet star companion, or the recurrent tidal stripping of a stellar
object trapped in a system with an intermediate-mass ( )
black hole.Comment: 13 pages, 11 Figures, 3 Tables (the Table in appendix A will be
available in the published version). Accepted for publication in A&
Discovery of a magnetar candidate X-ray pulsar in the Large Magellanic Cloud
During a systematic search for new X-ray pulsators in the XMM-Newton archive,
we discovered a high amplitude () periodic
() modulation in the X-ray flux of 4XMM
J045626.3-694723 (J0456 hereafter), a previously unclassified source in the
Large Magellanic Cloud (LMC). The period of the modulation is strongly
suggestive of a spinning neutron star (NS). The source was detected only during
one out of six observations in 2018-2022. Based on an absorbed power-law
spectral model with photon slope of , we derive a 0.3-10 keV
luminosity of erg cm s for a
distance of 50 kpc. The X-ray properties of J0456 are at variance with those of
variable LMC X-ray pulsars hosted in high-mass X-ray binary systems with a
Be-star companion. Based on SALT spectroscopic observations of the only optical
object that matches the X-ray uncertainty region, we cannot completely rule out
that J0456 is a NS accreting from a late-type (G8-K3) star, an
as-yet-unobserved binary evolutionary outcome in the MCs. We show that the
source properties are in better agreement with those of magnetars. J0456 may
thus be second known magnetar in the LMC after SGR 0526-66.Comment: 9 pages, 6 figures, 1 table. Accepted for publication in MNRA
Optical spectroscopic and photometric classification of the X-ray transient EP240309a (EP J115415.8−501810) as an intermediate polar
We report on optical follow-up observations of an X-ray source initially detected by the Einstein Probe mission. Our investigations categorize the source as an intermediate polar, a class of magnetic cataclysmic variables, exhibiting an orbital period of 3.7614(4) h and a white dwarf spin period of 3.97 min. The orbital period was identified through TESS observations, while our high-speed photometric data, obtained using the 1.9m and Lesedi 1.0m telescopes at the South African Astronomical Observatory, revealed both the spin and beat periods. Additionally, we present orbitally phase-resolved spectroscopic observations using the 1.9m telescope, specifically centred on the H β emission line, which reveal two emission components that exhibit Doppler variations throughout the orbital cycle
Simultaneous X-ray and radio observations of the transitional millisecond pulsar candidate CXOU J110926.4-650224: The discovery of a variable radio counterpart
We present the results of simultaneous observations of the transitional millisecond pulsar (tMSP) candidate CXOU J110926.4-650224 with the XMM-Newton satellite and the MeerKAT telescope. The source was found at an average X-ray luminosity of LX 7 × 1033 erg s-1 over the 0.3-10 keV band (assuming a distance of 4 kpc) and displayed a peculiar variability pattern in the X-ray emission, switching between high, low and flaring modes on timescales of tens of seconds. A radio counterpart was detected at a significance of 7.9σ with an average flux density of 33 μJy at 1.28 GHz. It showed variability over the course of hours and emitted a 10-min long flare just a few minutes after a brief sequence of multiple X-ray flares. No clear evidence for a significant correlated or anticorrelated variability pattern was found between the X-ray and radio emissions over timescales of tens of minutes and longer. CXOU J110926.4-650224 was undetected at higher radio frequencies in subsequent observations performed with the Australia Telescope Compact Array, when the source was still in the same X-ray sub-luminous state observed before, down to a flux density upper limit of 15 μJy at 7.25 GHz (at 3σ). We compare the radio emission properties of CXOU J110926.4-650224 with those observed in known and candidate tMSPs and discuss physical scenarios that may account for its persistent and flaring radio emissions
On the relationship between circumstellar disc size and X-ray outbursts in Be/X-ray binaries
We present long-term Hα monitoring results of five Be/X-ray binaries to study the Be disc size variations and their influence on type II (giant) X-ray outbursts. The work is done in the context of the viscous decretion disc model which predicts that Be discs in binary systems are truncated by resonant torques induced by the neutron star in its orbit. Our observations show that type II outbursts are not correlated (nor anticorrelated) with the disc size, as they are seen to occur both at relatively small and large Be disc radii. We discuss these observations in context of alternate interpretation of Be disc behaviour, such as precession, elongation and density effects, and with cognisance of the limitations of our disc size estimates
The triple-peaked afterglow of GRB 210731A from X-ray to radio frequencies
GRB 210731A was a long-duration gamma-ray burst discovered by the Burst Alert
Telescope (BAT) aboard the Neil Gehrels Swift observatory. Swift triggered the
wide-field, robotic MeerLICHT optical telescope in Sutherland; it began
observing the BAT error circle 286 seconds after the Swift trigger and
discovered the optical afterglow of GRB 210731A in its first 60-second q-band
exposure. Multi-colour observations of the afterglow with MeerLICHT revealed a
light curve that showed three peaks of similar brightness within the first four
hours. We present the results of our follow-up campaign and interpret our
observations in the framework of the synchrotron forward shock model. We
performed temporal and spectral fits to determine the spectral regime and
external medium density profile, and performed detailed multi-wavelength
theoretical modelling of the afterglow following the last optical peak at 0.2
days to determine the intrinsic blast wave parameters. We find a preference for
a stellar wind density profile consistent with a massive star origin, while our
theoretical modelling results in fairly typical shock microphysics parameters.
Based on the energy released in gamma-rays and the kinetic energy in the blast
wave, we determine a low radiative efficiency of ~0.02. The first peak in the
optical light curve is likely the onset of the afterglow. We find that energy
injection into the forward shock offers the simplest explanation for the
subsequent light curve evolution, and that the blast wave kinetic energy
increasing by a factor of ~1000 from the first peak to the last peak is
indicative of substantial energy injection. Our highest-likelihood theoretical
model overpredicts the 1.4 GHz flux by a factor of approximately three with
respect to our upper limits, possibly implying a population of thermal
electrons within the shocked region.Comment: 20 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic