We report the discovery of rising X-ray emission from the binary neutron star merger event GW170817. This is the first detection of X-ray emission from a gravitational-wave (GW) source. Observations ...acquired with the Chandra X-ray Observatory (CXO) at t 2.3 days post-merger reveal no significant emission, with L x 3.2 × 10 38 erg s − 1 (isotropic-equivalent). Continued monitoring revealed the presence of an X-ray source that brightened with time, reaching L x 9 × 10 38 erg s − 1 at 15.1 days post-merger. We interpret these findings in the context of isotropic and collimated relativistic outflows (both on- and off-axis). We find that the broadband X-ray to radio observations are consistent with emission from a relativistic jet with kinetic energy E k ∼ 10 49 − 50 erg , viewed off-axis with θ obs ∼ 20 ° - 40 ° . Our models favor a circumbinary density n ∼ 10 − 4 - 10 − 2 cm − 3 , depending on the value of the microphysical parameter ϵ B = 10 − 4 - 10 − 2 . A central-engine origin of the X-ray emission is unlikely. Future X-ray observations at t 100 days, when the target will be observable again with the CXO, will provide additional constraints to solve the model degeneracies and test our predictions. Our inferences on θ obs are testable with GW information on GW170817 from advanced LIGO/Virgo on the binary inclination.
We present observations of PS16dtm (also known as SN 2016ezh), a luminous transient that occurred at the nucleus of a narrow-line Seyfert 1 galaxy hosting a 106 M black hole. The light curve shows ...that PS16dtm exhibited a plateau phase for ∼100 days, during which it showed no color evolution, maintained a blackbody temperature of K, and radiated at approximately the Eddington luminosity of the supermassive black hole (SMBH). The spectra exhibit multicomponent hydrogen emission lines and strong Fe ii emission, show little time evolution, and closely resemble the spectra of NLS1s while being distinct from those of Type IIn supernovae (SNe IIn). Moreover, PS16dtm is undetected in the X-rays to a limit an order of magnitude below an archival X-ray detection of its host galaxy. These observations strongly link PS16dtm to activity associated with the SMBH and are difficult to reconcile with an SN origin or known forms of active galactic nucleus (AGN) variability. Therefore, we argue that PS16dtm is a tidal disruption event (TDE) in which the accretion of the stellar debris powers the rise in the continuum and excitation of the preexisting broad-line region, while obscuring the X-ray-emitting region of the preexisting AGN disk. We predict that PS16dtm will remain bright for years and that the X-ray emission will reappear on a similar timescale as the accretion rate declines. Placing PS16dtm in the context of other TDEs, we find that TDEs in AGN galaxies are more efficient and reach Eddington luminosities, likely due to interaction of the stellar debris with the preexisting accretion disk.
We present continued radio and X-ray observations of the relativistic tidal disruption event Swift J164449.3+573451 extending to δt 2000 days after discovery. The radio data were obtained with the ...Very Large Array (VLA) as part of a long-term program to monitor the energy and dynamical evolution of the jet and to characterize the parsec-scale environment around a previously dormant supermassive black hole. We combine these data with Chandra observations and demonstrate that the X-ray emission following the sharp decline at δt 500 days is likely due to the forward shock. We constrain the synchrotron cooling frequency and the microphysical properties of the outflow for the first time. We find that the cooling frequency evolves through the optical/NIR band at δt 10-200 days, corresponding to ϵB 10−3, well below equipartition; the X-ray data demonstrate that this deviation from equipartition holds to at least δt 2000 days. We thus recalculate the physical properties of the jet over the lifetime of the event, no longer assuming equipartition. We find a total kinetic energy of EK 4 × 1051 erg and a transition to non-relativistic expansion on the timescale of our latest observations (700 days). The density profile is approximately R−3/2 at 0.3 pc and 0.7 pc, with a plateau at intermediate scales, characteristic of Bondi accretion. Based on its evolution thus far, we predict that Sw 1644+57 will be detectable at centimeter wavelengths for decades to centuries with existing and upcoming radio facilities. Similar off-axis events should be detectable to z ∼ 2, but with a slow evolution that may inhibit their recognition as transient events.
We report deep Chandra X-ray Observatory (CXO), Hubble Space Telescope (HST), and Karl J. Jansky Very Large Array (VLA) observations of the binary neutron star event GW170817 at t < 160 days after ...merger. These observations show that GW170817 has been steadily brightening with time and might have now reached its peak, and constrain the emission process as non-thermal synchrotron emission where the cooling frequency c is above the X-ray band and the synchrotron frequency m is below the radio band. The very simple power-law spectrum extending for eight orders of magnitude in frequency enables the most precise measurement of the index p of the distribution of non-thermal relativistic electrons accelerated by a shock launched by a neutron star (NS)-NS merger to date. We find p = 2.17 0.01, which indicates that radiation from ejecta with Γ ∼ 3-10 dominates the observed emission. While constraining the nature of the emission process, these observations do not constrain the nature of the relativistic ejecta. We employ simulations of explosive outflows launched in NS ejecta clouds to show that the spectral and temporal evolution of the non-thermal emission from GW170817 is consistent with both emission from radially stratified quasi-spherical ejecta traveling at mildly relativistic speeds, and emission from off-axis collimated ejecta characterized by a narrow cone of ultra-relativistic material with slower wings extending to larger angles. In the latter scenario, GW170817 harbored a normal short gamma-ray burst (SGRB) directed away from our line of sight. Observations at t ≤ 200 days are unlikely to settle the debate, as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material.
We present Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA) radio observations of GW170817, the first Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo ...gravitational wave (GW) event from a binary neutron star merger and the first GW event with an electromagnetic (EM) counterpart. Our data include the first observations following the discovery of the optical transient at both the centimeter (13.7 hr post-merger) and millimeter (2.41 days post-merger) bands. We detect faint emission at 6 GHz at 19.47 and 39.23 days after the merger, but not in an earlier observation at 2.46 days. We do not detect cm/mm emission at the position of the optical counterpart at frequencies of 10-97.5 GHz at times ranging from 0.6 to 30 days post-merger, ruling out an on-axis short gamma-ray burst (SGRB) for energies 10 48 erg. For fiducial SGRB parameters, our limits require an observer viewer angle of 20°. The radio and X-ray data can be jointly explained as the afterglow emission from an SGRB with a jet energy of ∼ 10 49 - 10 50 erg that exploded in a uniform density environment with n ∼ 10 − 4 - 10 − 2 cm−3, viewed at an angle of ∼20°-40° from the jet axis. Using the results of our light curve and spectral modeling, in conjunction with the inference of the circumbinary density, we predict the emergence of late-time radio emission from the deceleration of the kilonova (KN) ejecta on a timescale of ∼5-10 years that will remain detectable for decades with next-generation radio facilities, making GW170817 a compelling target for long-term radio monitoring.
We present new observations of the binary neutron star merger GW170817 at Δt 220-290 days post-merger, at radio (Karl G. Jansky Very Large Array; VLA), X-ray (Chandra X-ray Observatory), and optical ...(Hubble Space Telescope; HST) wavelengths. These observations provide the first evidence for a turnover in the X-ray light curve, mirroring a decline in the radio emission at 5 significance. The radio-to-X-ray spectral energy distribution exhibits no evolution into the declining phase. Our full multi-wavelength data set is consistent with the predicted behavior of our previously published models of a successful structured jet expanding into a low-density circumbinary medium, but pure cocoon models with a choked jet cannot be ruled out. If future observations continue to track our predictions, we expect that the radio and X-ray emission will remain detectable until ∼1000 days post-merger.
Since the discovery of superluminous supernovae (SLSNe) in the last decade, it has been known that these events exhibit bluer spectral energy distributions than other supernova subtypes, with ...significant output in the ultraviolet. However, the event Gaia16apd seems to outshine even the other SLSNe at rest-frame wavelengths below ∼3000 . Yan et al. have recently presented HST UV spectra and attributed the UV flux to low iron-group abundance in the outer ejecta, and hence reduced line blanketing. Here, we present UV and optical light curves over a longer baseline in time, revealing a rapid decline at UV wavelengths despite a typical optical evolution. Combining the published UV spectra with our own optical data, we demonstrate that Gaia16apd has a much hotter continuum than virtually any SLSN at maximum light, but it cools rapidly thereafter and is indistinguishable from the others by ∼10-15 days after peak. Comparing the equivalent widths of UV absorption lines with those of other events, we show that the excess UV continuum is a result of a more powerful central power source, rather than a lack of UV absorption relative to other SLSNe or an additional component from interaction with the surrounding medium. These findings strongly support the central-engine hypothesis for hydrogen-poor SLSNe. An explosion ejecting Mej = 4.8(0.2/κ) M , where κ is the opacity in cm2 g−1, and forming a magnetar with spin period P = 2 ms, and B = 2 × 1014 G (lower than other SLSNe with comparable rise times) can consistently explain the light curve evolution and high temperature at peak. The host metallicity, Z = 0.18 Z , is comparable to other SLSNe.
Even though SN 2012cg is one of the best-studied Type Ia supernovae to date, the nature of its progenitor system has been debated in numerous studies. Specifically, it is difficult to reconcile ...recent claims of the detection of a ∼6 MS companion with recent deep, late-time flux limits. In this study we add three new constraints. (1) We analyze a new high-signal-to-noise, nebular-phase, Large Binocular Telescope/MODS spectrum of SN 2012cg and place an upper limit on the amount of low-velocity, solar-abundance material removed from a possible companion of . (2) We use Swift X-ray observations to constrain the pre-explosion mass-loss rate to be for . (3) We carefully reanalyze a prediscovery MASTER image, and with published light curves of SN 2012cg we estimate the time of first light and conservatively constrain the radius of a Roche-lobe overflowing companion to be . These observations disagree with a large nearby companion, and when considered with other studies of SN 2012cg's progenitor system, essentially rule out a non-degenerate companion.
We present the results from a sensitive X-ray survey of 26 nearby hydrogen-poor superluminous supernovae (SLSNe-I) with Swift, Chandra, and XMM. This data set constrains the SLSN evolution from a few ...days until ∼2000 days after explosion, reaching a luminosity limit Lx ∼ 1040 erg s−1 and revealing the presence of significant X-ray emission possibly associated with PTF 12dam. No SLSN-I is detected above , suggesting that the luminous X-ray emission Lx ∼ 1045 erg s−1 associated with SCP 60F6 is not common among SLSNe-I. We constrain the presence of off-axis gamma-ray burst (GRB) jets, ionization breakouts from magnetar engines and the density in the sub-parsec environments of SLSNe-I through inverse Compton emission. The deepest limits rule out the weakest uncollimated GRB outflows, suggesting that if the similarity of SLSNe-I with GRB/SNe extends to their fastest ejecta, then SLSNe-I are either powered by energetic jets pointed far away from our line of sight (θ > 30°), or harbor failed jets that do not successfully break through the stellar envelope. Furthermore, if a magnetar central engine is responsible for the exceptional luminosity of SLSNe-I, our X-ray analysis favors large magnetic fields G and ejecta masses , in agreement with optical/UV studies. Finally, we constrain the pre-explosion mass-loss rate of stellar progenitors of SLSNe-I. For PTF 12dam we infer , suggesting that the SN shock interaction with an extended circumstellar medium is unlikely to supply the main source of energy powering the optical transient and that some SLSN-I progenitors end their lives as compact stars surrounded by a low-density medium similar to long GRBs and type Ib/c SNe.
We present Chandra and Very Large Array 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 nonthermal ...emission follows the expected evolution of an off-axis relativistic jet, with a steep temporal decay F ∝ t − 1.95 0.15 and power-law spectrum F ∝ − 0.575 0.007 . 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 find n ≤ 9.6 × 10 − 3 cm − 3 . This measurement is independent from inferences based on 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 × 10 49 erg ( E iso = 2.1 − 1.5 + 6.4 × 10 52 erg ) and jet opening angle θ 0 = 5.9 − 0.7 + 1.0 deg with characteristic Lorentz factor Γ j = 163 − 43 + 23 , expanding in a low-density medium with n 0 = 2.5 − 1.9 + 4.1 × 10 − 3 cm − 3 and viewed θ obs = 30.4 − 3.4 + 4.0 deg off-axis. The synchrotron emission originates from a power-law distribution of electrons with index p = 2.15 − 0.02 + 0.01 . The shock microphysics parameters are constrained to ϵ e = 0.18 − 0.13 + 0.30 and ϵ B = 2.3 − 2.2 + 16.0 × 10 − 3 . Furthermore, we investigate the presence of X-ray flares and find no statistically significant evidence of ≥2.5 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 ∝ ( Γ β ) − into the environment, finding that shallow stratification indexes ≤ 6 are disfavored. Future radio and X-ray observations will refine our inferences on the fastest kilonova ejecta properties.
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