ABSTRACT
We present high-cadence multifrequency radio observations of the long gamma-ray burst (GRB) 190829A, which was detected at photon energies above 100 GeV by the High Energy Stereoscopic ...System (H.E.S.S.). Observations with the Meer Karoo Array Telescope (MeerKAT, 1.3 GHz) and Arcminute Microkelvin Imager – Large Array (AMI-LA, 15.5 GHz) began one day post-burst and lasted nearly 200 d. We used complementary data from Swift X-Ray Telescope (XRT), which ran to 100 d post-burst. We detected a likely forward shock component with both MeerKAT and XRT up to over 100 d post-burst. Conversely, the AMI-LA light curve appears to be dominated by reverse shock emission until around 70 d post-burst when the afterglow flux drops below the level of the host galaxy. We also present previously unpublished observations of the other H.E.S.S.-detected GRB, GRB 180720B from AMI-LA, which shows likely forward shock emission that fades in less than 10 d. We present a comparison between the radio emission from the three GRBs with detected very high energy (VHE) gamma-ray emission and a sensitivity-limited radio afterglow sample. GRB 190829A has the lowest isotropic radio luminosity of any GRB in our sample, but the distribution of luminosities is otherwise consistent, as expected, with the VHE GRBs being drawn from the same parent distribution as the other radio-detected long GRBs.
Type Ia supernovae are cosmic distance indicators
, and the main source of iron in the Universe
, but their formation paths are still debated. Several dozen supersoft X-ray sources, in which a white ...dwarf accretes hydrogen-rich matter from a non-degenerate donor star, have been observed
and suggested as Type Ia supernovae progenitors
. However, observational evidence for hydrogen, which is expected to be stripped off the donor star during the supernova explosion
, is lacking. Helium-accreting white dwarfs, which would circumvent this problem, have been predicted for more than 30 years (refs.
), including their appearance as supersoft X-ray sources, but have so far escaped detection. Here we report a supersoft X-ray source with an accretion disk whose optical spectrum is completely dominated by helium, suggesting that the donor star is hydrogen-free. We interpret the luminous and supersoft X-rays as resulting from helium burning near the surface of the accreting white dwarf. The properties of our system provide evidence for extended pathways towards Chandrasekhar-mass explosions based on helium accretion, in particular for stable burning in white dwarfs at lower accretion rates than expected so far. This may allow us to recover the population of the sub-energetic so-called Type Iax supernovae, up to 30% of all Type Ia supernovae
, within this scenario.
We investigate the nature of CXOU\,J005440.5-374320 ( a peculiar bright ($ and soft X-ray transient in the spiral galaxy NGC\,300 with a six-hour periodic flux modulation that was detected in a 2014 ...observation. Subsequent observations with and 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 sim 8 years of monitoring. Using data from the archive and from the and catalogues, we determined that 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 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 Wolf--Rayet star companion, or the recurrent tidal stripping of a stellar object trapped in a system with an intermediate-mass (sim 1000\,M$_ black hole.
Abstract We report on the discovery of Swift J010902.6-723710, a rare eclipsing Be/X-ray Binary system by the Swift SMC Survey (S-CUBED). Swift J010902.6-723710 was discovered via weekly S-CUBED ...monitoring observations when it was observed to enter a state of X-ray outburst on 2023 October 10. X-ray emission was found to be modulated by a 182 s period. Optical spectroscopy is used to confirm the presence of a highly inclined circumstellar disk surrounding a B0-0.5Ve optical companion. Historical UV and IR photometry is then used to identify strong eclipse-like features reoccurring in both light curves with a 60.623 day period, which is adopted as the orbital period of the system. Eclipsing behavior is found to be the result of a large accretion disk surrounding the neutron star. Eclipses are produced when the disk passes in front of the OBe companion, blocking light from both the stellar surface and circumstellar disk. This is only the third Be/X-ray binary to have confirmed eclipses. We note that this rare behavior provides an important opportunity to constrain the physical parameters of a Be/X-ray binary with greater accuracy than is possible in noneclipsing systems.
ABSTRACT
We report on results of spectropolarimetry of the afterglow of the long gamma-ray burst GRB 191221B, obtained with SALT/RSS and VLT/FORS2, as well as photometry from two telescopes in the ...MASTER Global Robotic Network, at the MASTER-SAAO (South Africa) and MASTER-OAFA (Argentina) stations. Prompt optical emission was detected by MASTER-SAAO 38 s after the alert, which dimmed from a magnitude (white-light) of ∼10–16.2 mag over a period of ∼10 ks, followed by a plateau phase lasting ∼10 ks and then a decline to ∼18 mag after 80 ks. The light curve shows complex structure, with four or five distinct breaks in the power-law decline rate. SALT/RSS linear spectropolarimetry of the afterglow began ∼2.9 h after the burst, during the early part of the plateau phase of the light curve. Absorption lines seen at ∼6010 and 5490 Å are identified with the Mg ii 2799 Å line from the host galaxy at z = 1.15 and an intervening system located at z = 0.96. The mean linear polarization measured over 3400–8000 Å was ∼1.5 per cent and the mean equatorial position angle (θ) was ∼65°. VLT/FORS2 spectropolarimetry was obtained ∼10 h post-burst, during a period of slow decline (α = −0.44), and the polarization was measured to be p = 1.2 per cent and θ = 60°. Two observations with the MeerKAT radio telescope, taken 30 and 444 d after the GRB trigger, detected radio emission from the host galaxy only. We interpret the light curve and polarization of this long GRB in terms of a slow-cooling forward shock.
We present long-term Ha 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.
ABSTRACT
Hot subdwarfs are early-type low-mass stars lying between the main sequence and the white dwarf sequence. The majority have helium-burning cores and helium-poor surfaces. A minority have ...helium-rich surfaces. A few, including Ton S 415, have surfaces with between 10 per cent and 90 per cent helium (by number). Their properties are diverse and their origins mostly unknown. Ton S 415 was identified as a suspected binary in a Southern African Large Telescope (SALT) survey of hot subdwarfs from its large radial velocity. Using SALT spectroscopy and Transiting Exoplanet Survey Satellite light curves, we confirm that Ton S 415 is a close binary with an orbital period $p=84.6460\pm 0.0004\, {\rm min}$ and velocity semi-amplitude $K=175.5\pm 1.0\, {\rm km\, s^{-1}}$. Analysis of the SALT spectrum and broad-band spectral energy distribution shows the visible star to have an effective temperature $T_{\rm eff}=43\, 300\pm 1000\, {\rm K}$, surface gravity $\log g/{\rm cm\, s^{-2}}=5.89\pm 0.10$, surface helium-to-hydrogen ratio log y = −0.62 ± 0.10, radius $R=0.1074\pm 0.0025\, R_{\odot }$, and mass $M=0.33\pm 0.09\, {\rm {\rm M}_{\odot }}$. By modelling the light curve, we estimate the binary mass ratio to be q = 0.7 ± 0.3 and infer an unseen white dwarf companion with a mass of $M_{\rm WD}=0.47\pm 0.24\, {\rm {\rm M}_{\odot }}$. The results are consistent with a post-common-envelope binary that lost its hydrogen envelope before reaching the peak of the red giant branch, becoming a hot subdwarf with a non-canonical mass. We predict that the system will evolve into a double white dwarf binary before eventually either forming a stably accreting AM CVn system, or merging to form an R CrB star.
ABSTRACT
We present the results of a weekly monitoring of the new black hole candidate X-ray binary MAXI J1631-472 carried out with the MeerKAT radio interferometer, the Neil Gehrels Swift ...Observatory, and the Monitor of All-sky X-ray Image (MAXI) instrument, during its 2018–2019 outburst. The source exhibits a number of X-ray states, in particular both high- and low-luminosity hard states bracketed by extended soft states. Radio flaring is observed shortly after a transition from hard/intermediate states to the soft state. This is broadly in agreement with existing empirical models, but its extended duration hints at multiple unresolved flares and/or jet–ISM interactions. In the hard state radio:X-ray plane, the source is revealed to be ‘radio quiet’ at high luminosities, but to rejoin the ‘standard’ track at lower luminosities, an increasingly common observed pattern of behaviour.
ABSTRACT
RX J0209.6–7427 is an X-ray source in the Magellanic Bridge that was first detected in 1993, but not seen again till 2019. It has been identified as a member of the Be/X-ray binary class, a ...category of objects that are well established as bright, often-unpredictable transients. Such systems are rarely known in the Bridge, possibly because they lie outside the area most commonly studied by X-ray telescopes. Whatever be the reason for the sparse number of such systems in the Bridge, they can provide useful tools for trying to understand the result of the tidal dynamics of the two Magellanic Clouds. In this paper, the nature of the object is explored with the help of new data obtained during the latest outburst. In particular, the first optical spectrum of the counterpart is presented to help classify the star, plus measurements of the Balmer emission lines over several years are used to investigate changes in the size and structure of the circumstellar disc.
ABSTRACT
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.
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