The emergence of a new source of X-rays from the binary neutron star merger GW170817

The binary neutron-star (BNS) merger GW170817 is the first celestial object from which both gravitational waves (GWs) and light have been detected enabling critical insight on the pre-merger (GWs) and post-merger (light) physical properties of these phenomena. For the first $sim 3$ years after the merger the detected radio and X-ray radiation has been dominated by emission from a structured relativistic jet initially pointing $sim 15-25$ degrees away from our line of sight and propagating into a low-density medium. Here we report on observational evidence for the emergence of a new X-ray emission component at $delta t>900$ days after the merger. The new component has luminosity $L_x approx 5times 10^{38}rm{erg s^{-1}}$ at 1234 days, and represents a $sim 3.5sigma$ - $4.3sigma$ excess compared to the expectations from the off-axis jet model that best fits the multi-wavelength afterglow of GW170817 at earlier times. A lack of detectable radio emission at 3 GHz around the same time suggests a harder broadband spectrum than the jet afterglow. These properties are consistent with synchrotron emission from a mildly relativistic shock generated by the expanding merger ejecta, i.e. a kilonova afterglow. In this context our simulations show that the X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. However, radiation from accretion processes on the compact-object remnant represents a viable alternative to the kilonova afterglow. Neither a kilonova afterglow nor accretion-powered emission have been observed before.

A GMRT 150 MHz Search for Variables and Transients in Stripe 82

We have carried out a dedicated transient survey of 300 deg$^2$ of the SDSS Stripe 82 region using the Giant Meterwavelength Radio Telescope (GMRT) at 150 MHz. Our multi-epoch observations, together with the TGSS survey, allow us to probe variability and transient activity on four different timescales, beginning with 4 hours, and up to 4 years. Data calibration, RFI flagging, source finding and transient search were carried out in a semi-automated pipeline incorporating the SPAM recipe. This has enabled us to produce superior-quality images and carry out reliable transient search over the entire survey region in under 48 hours post-observation. Among the few thousand unique point sources found in our 5$sigma$ single-epoch catalogs (flux density thresholds of about 24 mJy, 20 mJy, 16 mJy and 18 mJy on the respective timescales), we find $<$0.08%, 0.01%, $<$0.06% and 0.05% to be variable (beyond a significance of 4$sigma$ and fractional variability of 30%) on timescales of 4 hours, 1 day, 1 month and 4 years respectively. This is substantially lower than that in the GHz sky, where $sim$1% of the persistent point sources are found to be variable. Although our survey was designed to probe a superior part of the transient phase space, our transient sources did not yield any significant candidates. The transient (preferentially extragalactic) rate at 150 MHz is therefore $<$0.005 on timescales of 1 month and 4 years, and $<$0.002 on timescales of 1 day and 4 hours, beyond 7$sigma$ detection threshold. We put these results in the perspective with the previous studies and give recommendations for future low-frequency transient surveys.

Two years of non-thermal emission from the binary neutron star merger GW170817: rapid fading of the jet afterglow and first constraints on the kilonova fastest ejecta

We present Chandra and VLA observations of GW170817 at ~521-743 days post merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time non-thermal emission follows the expected evolution from an off-axis relativistic jet, with a steep temporal decay $F_{ u}propto t^{-1.95pm0.15}$ and a simple power-law spectrum $F_{ u}propto u^{-0.575pm0.007}$. We present a new method to constrain the merger environment density based on diffuse X-ray emission from hot plasma in the host galaxy and we find $nle 9.6 times 10^{-3},rm{cm^{-3}}$. This measurement is independent from inferences based on the jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy $E_0 = 1.5_{-1.1}^{+3.6}times 10^{49},rm{erg}$ ($E_{iso}= 2.1_{-1.5}^{+6.4}times10^{52}, rm{erg}$), jet opening angle $theta_{0}= 5.9^{+1.0}_{-0.7},rm{deg}$ with characteristic Lorentz factor $Gamma_j = 163_{-43}^{+23}$, expanding in a low-density medium with $n_0 = 2.5_{-1.9}^{+4.1} times 10^{-3}, rm{cm^{-3}}$ and viewed $theta_{obs} = 30.4^{+4.0}_{-3.4}, rm{deg}$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p=2.15^{+0.01}_{-0.02}$. The shock microphysics parameters are constrained to $epsilon_{rm{e}} = 0.18_{-0.13}^{+0.30}$ and $epsilon_{rm{B}}=2.3_{-2.2}^{+16.0} times 10^{-3}$. We investigate the presence of X-ray flares and find no statistically significant evidence of $ge2.5sigma$ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy $E_k^{KN}propto (Gammabeta)^{-alpha}$ into the environment, finding that shallow stratification indexes $alphale6$ are disfavored.