Recent observations with the Chandra X-ray telescope continue to detect X-ray emission from the transient GW170817. In a total exposure of 96.6 ks, performed between 2020 March 9 and 16 (935–942 d ...after the merger), a total of 8 photons are measured at the source position, corresponding to a significance of ≈5σ. Radio monitoring with the Australian Telescope Compact Array (ATCA) shows instead that the source has faded below our detection threshold (<33 μJy, 3σ). By assuming a constant spectral index of β = 0.585, we derive an unabsorbed X-ray flux of ≈1.4 × 10−15 erg cm−2 s−1, higher than earlier predictions, yet still consistent with a simple structured jet model. We discuss possible scenarios that could account for prolonged emission in X-rays. The current data set appears consistent both with energy injection by a long-lived central engine and with the onset of a kilonova afterglow, arising from the interaction of the sub-relativistic merger ejecta with the surrounding medium. Long-term monitoring of this source will be essential to test these different models.
Abstract
We present the results of our year-long afterglow monitoring of GW170817, the first binary neutron star (NS) merger detected by advanced LIGO and advanced Virgo. New observations with the ...Australian Telescope Compact Array (ATCA) and the Chandra X-ray Telescope were used to constrain its late-time behavior. The broadband emission, from radio to X-rays, is well-described by a simple power-law spectrum with index β ∼0.585 at all epochs. After an initial shallow rise ∝ t0.9, the afterglow displayed a smooth turn-over, reaching a peak X-ray luminosity of LX≈5 ×1039 erg s−1 at 160 d, and has now entered a phase of rapid decline, approximately ∝ t−2. The latest temporal trend challenges most models of choked jet/cocoon systems, and is instead consistent with the emergence of a relativistic structured jet seen at an angle of ≈22○ from its axis. Within such model, the properties of the explosion (such as its blastwave energy EK ≈ 2 × 1050 erg, jet width θc ≈4○, and ambient density n ≈3 × 10−3 cm−3) fit well within the range of properties of cosmological short GRBs.
ABSTRACT
We present our broad-band study of GW170817 from radio to hard X-rays, including NuSTAR and Chandra observations up to 165 d after the merger, and a multimessenger analysis including LIGO ...constraints. The data are compared with predictions from a wide range of models, providing the first detailed comparison between non-trivial cocoon and jet models. Homogeneous and power-law shaped jets, as well as simple cocoon models are ruled out by the data, while both a Gaussian shaped jet and a cocoon with energy injection can describe the current data set for a reasonable range of physical parameters, consistent with the typical values derived from short GRB afterglows. We propose that these models can be unambiguously discriminated by future observations measuring the post-peak behaviour, with Fν ∝ t∼−1.0 for the cocoon and Fν∝ t∼−2.5 for the jet model.
A long-standing paradigm in astrophysics is that collisions- or mergers-of two neutron stars form highly relativistic and collimated outflows (jets) that power Y-ray bursts of short (less than two ...seconds) duration. The observational support for this model, however, is only indirect. A hitherto outstanding prediction is that gravitational-wave events from such mergers should be associated with Y-ray bursts, and that a majority of these bursts should be seen off-axis, that is, they should point away from Earth. Here we report the discovery observations of the X-ray counterpart associated with the gravitational-wave event GW170817. Although the electromagnetic counterpart at optical and infrared frequencies is dominated by the radioactive glow (known as a 'kilonova') from freshly synthesized rapid neutron capture (r-process) material in the merger ejecta, observations at X-ray and, later, radio frequencies are consistent with a short Y-ray burst viewed off-axis. Our detection of X-ray emission at a location coincident with the kilonova transient provides the missing observational link between short Y-ray bursts and gravitational waves from neutron-star mergers, and gives independent confirmation of the collimated nature of the Y-ray-burst emission.
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IJS, KISLJ, NUK, SBMB, UL, UM, UPUK
It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods. During the accretion ...stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar whose emission is powered by the neutron star's rotating magnetic field. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars and also by the evidence for a past accretion disc in a rotation-powered millisecond pulsar. It has been proposed that a rotation-powered pulsar may temporarily switch on during periods of low mass inflow in some such systems. Only indirect evidence for this transition has hitherto been observed. Here we report observations of accretion-powered, millisecond X-ray pulsations from a neutron star previously seen as a rotation-powered radio pulsar. Within a few days after a month-long X-ray outburst, radio pulses were again detected. This not only shows the evolutionary link between accretion and rotation-powered millisecond pulsars, but also that some systems can swing between the two states on very short timescales.
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IJS, KISLJ, NUK, UL, UM, UPUK
We present radio observations of the tidal disruption event candidate (TDE) XMMSL1 J0740−85 spanning 592 to 875 days post X-ray discovery. We detect radio emission that fades from an initial peak ...flux density at 1.6 GHz of 1.19 0.06 mJy to 0.65 0.06 mJy, suggesting an association with the TDE. This makes XMMSL1 J0740−85 at d = 75 Mpc the nearest TDE with detected radio emission to date and only the fifth TDE with radio emission overall. The observed radio luminosity rules out a powerful relativistic jet like that seen in the relativistic TDE Swift J1644+57. Instead, we infer from an equipartition analysis that the radio emission most likely arises from a non-relativistic outflow similar to that seen in the nearby TDE ASASSN-14li, with a velocity of about 104 km s−1 and a kinetic energy of about 1048 erg, expanding into a medium with a density of about 102 cm−3. Alternatively, the radio emission could arise from a weak initially relativistic but decelerated jet with an energy of erg, or (for an extreme disruption geometry) from the unbound debris. The radio data for XMMSL1 J0740−85 continues to support the previous suggestion of a bimodal distribution of common non-relativistic isotropic outflows and rare relativistic jets in TDEs (in analogy with the relation between Type Ib/c supernovae and long-duration gamma-ray bursts). The radio data also provide a new measurement of the circumnuclear density on a sub-parsec scale around an extragalactic supermassive black hole.
ABSTRACT
Neutron star mergers produce a substantial amount of fast-moving ejecta, expanding outwardly for years after the merger. The interaction of these ejecta with the surrounding medium may ...produce a weak isotropic radio remnant, detectable in relatively nearby events. We use late-time radio observations of short duration gamma-ray bursts (sGRBs) to constrain this model. Two samples of events were studied: four sGRBs that are possibly in the local (<200 Mpc) Universe were selected to constrain the remnant non-thermal emission from the sub-relativistic ejecta, whereas 17 sGRBs at cosmological distances were used to constrain the presence of a proto-magnetar central engine, possibly re-energizing the merger ejecta. We consider the case of GRB 170817A/GW170817 and find that in this case the early radio emission may be quenched by the jet blast-wave. In all cases, for ejecta mass range of ${M}_{\rm {ej}}\lesssim 10^{-2}\, (5\times 10^{-2})\, \mathrm{M}_\odot$, we can rule out very energetic merger ejecta ${E}_{\rm {ej}}\gtrsim 5\times 10^{52}\, (10^{53})\, \rm erg$, thus excluding the presence of a powerful magnetar as a merger remnant.
We present the results of our year-long afterglow monitoring of GW 170817, the first binary neutron star merger detected by Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and ...Advanced Virgo. New observations with the Australian Telescope Compact Array and the Chandra X-ray Telescope were used to constrain its late-time behaviour. The broad-band emission, from radio to X-rays, is well-described by a simple power-law spectrum with index β ∼ 0.585 at all epochs. After an initial shallow rise ∝t0.9, the afterglow displayed a smooth turnover, reaching a peak X-ray luminosity of LX ≈ 5 × 1039 erg s−1 at 160 d, and has now entered a phase of rapid decline, approximately ∝t−2. The latest temporal trend challenges most models of choked jet/cocoon systems, and is instead consistent with the emergence of a relativistic structured jet seen at an angle of ≈22◦ from its axis. Within such model, the properties of the explosion (such as its blast wave energy EK ≈ 2 × 1050 erg, jet width θc ≈ 4◦, and ambient density n ≈ 3 × 10−3 cm−3) fit well within the range of properties of cosmological short gamma-ray bursts.
ABSTRACT
We present radio and optical afterglow observations of the TeV-bright long gamma-ray burst 190114C at a redshift of z = 0.425, which was detected by the Major Atmospheric Gamma Imaging ...Cherenkov telescope. Our observations with Atacama Large Millimeter/submillitmeter Array, Australia Telescope Compact Array, and upgraded Giant Metre-wave Radio Telescope were obtained by our low frequency observing campaign and range from ∼1 to ∼140 d after the burst and the optical observations were done with three optical telescopes spanning up to ∼25 d after the burst. Long-term radio/mm observations reveal the complex nature of the afterglow, which does not follow the spectral and temporal closure relations expected from the standard afterglow model. We find that the microphysical parameters of the external forward shock, representing the share of shock-created energy in the non-thermal electron population and magnetic field, are evolving with time. The inferred kinetic energy in the blast-wave depends strongly on the assumed ambient medium density profile, with a constant density medium demanding almost an order of magnitude higher energy than in the prompt emission, while a stellar wind-driven medium requires approximately the same amount energy as in prompt emission.
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