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Register a new userCandidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational Wave Event S190521g
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We report the first plausible optical electromagnetic (EM) counterpart to a (candidate) binary black hole (BBH) merger. Detected by the Zwicky Transient Facility (ZTF), the EM flare is consistent with expectations for a kicked BBH merger in the accretion disk of an active galactic nucleus (AGN), and is unlikely ($<O(0.01%$)) due to intrinsic variability of this source. The lack of color evolution implies that it is not a supernovae and instead is strongly suggestive of a constant temperature shock. Other false-positive events, such as microlensing or a tidal disruption event, are ruled out or constrained to be $<O(0.1%$). If the flare is associated with S190521g, we find plausible values of: total mass $ M_{rm BBH} sim 100 M_{odot}$, kick velocity $v_k sim 200, {rm km}, {rm s}^{-1}$ at $theta sim 60^{circ}$ in a disk with aspect ratio $H/a sim 0.01$ (i.e., disk height $H$ at radius $a$) and gas density $rho sim 10^{-10}, {rm g}, {rm cm}^{-3}$. The merger could have occurred at a disk migration trap ($a sim 700, r_{g}$; $r_g equiv G M_{rm SMBH} / c^2$, where $M_{rm SMBH}$ is the mass of the AGN supermassive black hole). The combination of parameters implies a significant spin for at least one of the black holes in S190521g. The timing of our spectroscopy prevents useful constraints on broad-line asymmetry due to an off-center flare. We predict a repeat flare in this source due to a re-encountering with the disk in $sim 1.6, {rm yr}, (M_{rm SMBH}/10^{8}M_{odot}), (a/10^{3}r_{g})^{3/2}$.
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We present the results from a search for the electromagnetic counterpart of the LIGO/Virgo event S190510g using the Dark Energy Camera (DECam). S190510g is a binary neutron star (BNS) merger candidate of moderate significance detected at a distance of 227$pm$92 Mpc and localized within an area of 31 (1166) square degrees at 50% (90%) confidence. While this event was later classified as likely non-astrophysical in nature within 30 hours of the event, our short latency search and discovery pipeline identified 11 counterpart candidates, all of which appear consistent with supernovae following offline analysis and spectroscopy by other instruments. Later reprocessing of the images enabled the recovery of 6 more candidates. Additionally, we implement our candidate selection procedure on simulated kilonovae and supernovae under DECam observing conditions (e.g., seeing, exposure time) with the intent of quantifying our search efficiency and making informed decisions on observing strategy for future similar events. This is the first BNS counterpart search to employ a comprehensive simulation-based efficiency study. We find that using the current follow-up strategy, there would need to be 19 events similar to S190510g for us to have a 99% chance of detecting an optical counterpart, assuming a GW170817-like kilonova. We further conclude that optimization of observing plans, which should include preference for deeper images over multiple color information, could result in up to a factor of 1.5 reduction in the total number of followups needed for discovery.
We present the first fully relativistic prediction of the electromagnetic emission from the surrounding gas of a supermassive binary black hole system approaching merger. Using a ray-tracing code to post-process data from a general relativistic 3-d MHD simulation, we generate images and spectra, and analyze the viewing angle dependence of the light emitted. When the accretion rate is relatively high, the circumbinary disk, accretion streams, and mini-disks combine to emit light in the UV/EUV bands. We posit a thermal Compton hard X-ray spectrum for coronal emission; at high accretion rates, it is almost entirely produced in the mini-disks, but at lower accretion rates it is the primary radiation mechanism in the mini-disks and accretion streams as well. Due to relativistic beaming and gravitational lensing, the angular distribution of the power radiated is strongly anisotropic, especially near the equatorial plane.
The Fermi Gamma-ray Burst Monitor reported the possible detection of the gamma-ray counterpart of a binary black hole merger event, GW150914. We show that the gamma-ray emission is caused by a relativistic outflow with Lorentz factor larger than 10. Subsequently, debris outflow pushes the ambient gas to form a shock, which is responsible for the afterglow synchrotron emission. We find that the 1.4 GHz radio flux peaks at $sim10^5$ sec after the burst trigger. If the ambient matter is dense enough with density larger than $sim10^{-2}$ cm$^{-3}$, then the peak radio flux is $sim0.1$ mJy, which is detectable with radio telescopes such as the Very Large Array. The optical afterglow peaks earlier than the radio, and if the ambient matter density is larger than $sim0.1$ cm$^{-3}$, the optical flux is detectable with large telescopes such as the Subaru Hyper Suprime-Cam. To reveal the currently unknown mechanisms of the outflow and its gamma-ray emission associated with the binary black hole merger event, follow-up electromagnetic observations of afterglows are important. Detection of the afterglow will localize the sky position of the gravitational wave and the gamma-ray emissions, and it will support the physical association between them.
We study the impact of gas accretion on the orbital evolution of black-hole binaries initially at large separation in the band of the planned Laser Interferometer Space Antenna (LISA). We focus on two sources: (i)~stellar-origin black-hole binaries~(SOBHBs) that can migrate from the LISA band to the band of ground-based gravitational-wave observatories within weeks/months; and (ii) intermediate-mass black-hole binaries~(IMBHBs) in the LISA band only. Because of the large number of observable gravitational-wave cycles, the phase evolution of these systems needs to be modeled to great accuracy to avoid biasing the estimation of the source parameters. Accretion affects the gravitational-wave phase at negative ($-4$) post-Newtonian order, and is therefore dominant for binaries at large separations. If accretion takes place at the Eddington or at super-Eddington rate, it will leave a detectable imprint on the dynamics of SOBHBs. In optimistic astrophysical scenarios, a multiwavelength strategy with LISA and a ground-based interferometer can detect about $10$ (a few) SOBHB events for which the accretion rate can be measured at $50%$ ($10%$) level. In all cases the sky position can be identified within much less than $0.4,{rm deg}^2$ uncertainty. Likewise, accretion at $gtrsim 10%$ ($gtrsim 100%$) of the Eddington rate can be measured in IMBHBs up to redshift $zapprox 0.1$ ($zapprox 0.5$), and the position of these sources can be identified within less than $0.01,{rm deg}^2$ uncertainty. Altogether, a detection of SOBHBs or IMBHBs would allow for targeted searches of electromagnetic counterparts to black-hole mergers in gas-rich environments with future X-ray detectors (such as Athena) and radio observatories (such as SKA).
We present optical follow-up imaging obtained with the Katzman Automatic Imaging Telescope, Las Cumbres Observatory Global Telescope Network, Nickel Telescope, Swope Telescope, and Thacher Telescope of the LIGO/Virgo gravitational wave (GW) signal from the neutron star-black hole (NSBH) merger GW190814. We searched the GW190814 localization region (19 deg$^{2}$ for the 90th percentile best localization), covering a total of 51 deg$^{2}$ and 94.6% of the two-dimensional localization region. Analyzing the properties of 189 transients that we consider as candidate counterparts to the NSBH merger, including their localizations, discovery times from merger, optical spectra, likely host-galaxy redshifts, and photometric evolution, we conclude that none of these objects are likely to be associated with GW190814. Based on this finding, we consider the likely optical properties of an electromagnetic counterpart to GW190814, including possible kilonovae and short gamma-ray burst afterglows. Using the joint limits from our follow-up imaging, we conclude that a counterpart with an $r$-band decline rate of 0.68 mag day$^{-1}$, similar to the kilonova AT 2017gfo, could peak at an absolute magnitude of at most $-17.8$ mag (50% confidence). Our data are not constraining for red kilonovae and rule out blue kilonovae with $M>0.5 M_{odot}$ (30% confidence). We strongly rule out all known types of short gamma-ray burst afterglows with viewing angles $<$17$^{circ}$ assuming an initial jet opening angle of $sim$$5.2^{circ}$ and explosion energies and circumburst densities similar to afterglows explored in the literature. Finally, we explore the possibility that GW190814 merged in the disk of an active galactic nucleus, of which we find four in the localization region, but we do not find any candidate counterparts among these sources.
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