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Register a new userFallback Accretion Model for the Years-to-Decades X-ray Counterpart to GW170817
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A new component was reported in the X-ray counterpart to the binary neutron-star merger and gravitational wave event GW170817, exceeding the afterglow emission from an off-axis structured jet. The afterglow emission from the kilonova/macronova ejecta may explain the X-ray excess but exceeds the radio observations if the spectrum is the same. We propose a fallback accretion model that a part of ejecta from the neutron star merger falls back and forms a disk around the central compact object. In the super-Eddington accretion phase, the X-ray luminosity stays near the Eddington limit of a few solar masses and the radio is weak, as observed. This will be followed by a power law decay. The duration of the constant luminosity phase conveys the initial fallback timescale $t_0$ in the past. The current multi-year duration requires $t_0 > 3$--$30$ sec, suggesting that the disk wind rather than the dynamical ejecta falls back after the jet launch. Future observations in the next decades will probe the timescale of $t_0 sim 10$--$10^4$ sec, around the time of extended emission in short gamma-ray bursts. The fallback accretion has not been halted by the $r$-process heating, implying that fission is weak on the year scale. We predict that the X-ray counterpart will disappear in a few decades due to the $r$-process halting or the depletion of fallback matter.
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X-ray emission from the gravitational wave transient GW170817 is well described as non-thermal afterglow radiation produced by a structured relativistic jet viewed off-axis. We show that the X-ray counterpart continues to be detected at 3.3 years after the merger. Such long-lasting signal is not a prediction of the earlier jet models characterized by a narrow jet core and a viewing angle of about 20 deg, and is spurring a renewed interest in the origin of the X-ray emission. We present a comprehensive analysis of the X-ray dataset aimed at clarifying existing discrepancies in the literature, and in particular the presence of an X-ray rebrightening at late times. Our analysis does not find evidence for an increase in the X-ray flux, but confirms a growing tension between the observations and the jet model. Further observations at radio and X-ray wavelengths would be critical to break the degeneracy between models.
The hard to soft state transition of the outbursts in X-ray binaries (XRBs) is triggered by the rising of the mass accretion rate due to the disk instability. In order to explain the observed correlation between the hard X-ray transition luminosity and the soft X-ray peak luminosity in the soft state, we construct a magnetic disk-outflow model for the state transition in XRBs. We assume that the large-scale magnetic field in the outer thin disk is formed through inverse cascade of small-scale dynamo generated field, and it is then advected by the inner advection dominated accretion flow (ADAF), which accelerates a fraction of the gas into the outflows. During the outbursts, the heating front moves inwards, and the field strength at the heating front of the outer disk is proportional to the accretion rate of the disk. Much angular momentum of the inner ADAF is carried away by the outflows for a stronger magnetic field, which leads to a high radial velocity of the ADAF. This makes the critical mass accretion rate of the ADAF increases with the field strength, and it therefore leads to a correlation between transition luminosity and the peak luminosity in the thermal state. We found that the values of the viscosity parameter $alpha$ of the neutron star XRBs are systematically higher for those of the black hole (BH) XRBs ($alphasim 0.05-0.15$ for BHs, and $alphasim 0.15-0.4$ for neutron stars). Our model predicts the transition luminosity may be higher than the peak luminosity provided $alpha$ is sufficiently high, which is able to explain a substantial fraction of outbursts in BHXRBs not reaching the thermally dominant accretion state.
We report the probable identification of the X-ray counterpart to the gamma-ray pulsar PSR J2021+4026 using imaging with the Chandra X-ray Observatory ACIS and timing analysis with the Fermi satellite. Given the statistical and systematic errors, the positions determined by both satellites are coincident. The X-ray source position is R.A. 20h21m30.733s, Decl. +40 deg 26 min 46.04sec (J2000) with an estimated uncertainty of 1.3 arsec combined statistical and systematic error. Moreover, both the X-ray to gamma-ray and the X-ray to optical flux ratios are sensible assuming a neutron star origin for the X-ray flux. The X-ray source has no cataloged infrared-to-visible counterpart and, through new observations, we set upper limits to its optical emission of i >23.0 mag and r > 25.2mag. The source exhibits an X-ray spectrum with most likely both a powerlaw and a thermal component. We also report on the X-ray and visible light properties of the 43 other sources detected in our Chandra observation.
A long-standing paradigm in astrophysics is that collisions- or mergers- of two neutron stars (NSs) form highly relativistic and collimated outflows (jets) powering gamma-ray bursts (GRBs) of short (< 2 s) duration. However, the observational support for this model is only indirect. A hitherto outstanding prediction is that gravitational wave (GW) events from such mergers should be associated with GRBs, and that a majority of these GRBs should be off-axis, that is, they should point away from the Earth. Here we report the discovery of the X-ray counterpart associated with the GW event GW170817. While the electromagnetic counterpart at optical and infrared frequencies is dominated by the radioactive glow from freshly synthesized r-process material in the merger ejecta, known as kilonova, observations at X-ray and, later, radio frequencies exhibit the behavior of a short GRB viewed off-axis. Our detection of X-ray emission at a location coincident with the kilonova transient provides the missing observational link between short GRBs and GWs from NS mergers, and gives independent confirmation of the collimated nature of the GRB emission.
Based on previously published multi-wavelength modelling of the GRB 170817A jet afterglow, that includes information from the VLBI centroid motion, we construct the posterior probability density distribution on the total energy in the bipolar jets launched by the GW170817 merger remnant. By applying a new numerical-relativity-informed fitting formula for the accretion disk mass, we construct the posterior probability density distribution of the GW170817 remnant disk mass. By combining the two, we estimate the accretion-to-jet energy conversion efficiency in this system, carefully accounting for uncertainties. The accretion-to-jet energy conversion efficiency in GW170817 is $etasim 10^{-3}$ with an uncertainty of slightly less than two orders of magnitude. This low efficiency is in good agreement with expectations from the $ ubar u$ mechanism, which therefore cannot be excluded by this measurement alone. Such an efficiency also agrees with that anticipated for the Blandford-Znajek mechanism, provided that the magnetic field in the disk right after the merger is predominantly toroidal (which is expected as a result of the merger dynamics).
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