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Register a new userFermi-LAT observations of the LIGO/Virgo event GW170817
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Fermi-LAT Collaboration
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We present the Fermi Large Area Telescope (LAT) observations of the binary neutron star merger event GW170817 and the associated short gamma-ray burst (SGRB) GRB,170817A detected by the Fermi Gamma-ray Burst Monitor. The LAT was entering the South Atlantic Anomaly at the time of the LIGO/Virgo trigger ($t_{rm GW}$) and therefore cannot place constraints on the existence of high-energy (E $>$ 100 MeV) emission associated with the moment of binary coalescence. We focus instead on constraining high-energy emission on longer timescales. No candidate electromagnetic counterpart was detected by the LAT on timescales of minutes, hours, or days after the LIGO/Virgo detection. The resulting flux upper bound (at 95% C.L./) from the LAT is $4.5times$10$^{-10}$ erg cm$^{-2}$ s$^{-1}$ in the 0.1--1 GeV range covering a period from T0 + 1153 s to T0 + 2027 s. At the distance of GRB,170817A, this flux upper bound corresponds to a luminosity upper bound of 9.7$times10^{43}$ erg s$^{-1}$, which is 5 orders of magnitude less luminous than the only other LAT SGRB with known redshift, GRB,090510. We also discuss the prospects for LAT detection of electromagnetic counterparts to future gravitational wave events from Advanced LIGO/Virgo in the context of GW170817/GRB,170817A.
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The Fermi Large Area Telescope (LAT) has an instantaneous field of view covering $sim 1/5$ of the sky and completes a survey of the full sky every ~3 hours. It provides a continuous, all-sky survey of high-energy gamma-rays, enabling searches for transient phenomena over timescales from milliseconds to years. Among these phenomena could be electromagnetic counterparts to gravitational wave sources. In this paper, we present a detailed study of the LAT observations relevant to Laser Interferometer Gravitational-wave Observatory (LIGO) event GW150904 (Abbott et al. 2016), which is the first direct detection of gravitational waves and has been interpreted as due to coalescence of two stellar-mass black holes. The localization region for GW150904 was outside the LAT field of view at the time of the gravitational wave signal. However, as part of routine survey observations, the LAT observed the entire LIGO localization region within ~70 minutes of the trigger, and thus enabled a comprehensive search for a gamma-ray counterpart to GW150904. The study of the LAT data presented here did not find any potential counterparts to GW150904, but it did provide limits on the presence of a transient counterpart above 100 MeV on timescales of hours to days over the entire GW150904 localization region.
We present the emph{Fermi} Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) observations of the LIGO binary black hole merger (BBH) event GW170104. No candidate electromagnetic counterparts was detected by either GBM or LAT. A detailed analysis of the GBM and LAT data over timescales from seconds to days covering the LIGO localization region is presented. The resulting flux upper bound from the GBM is (5.2--9.4)$times$10$^{-7}$ erg cm$^{-2}$ s$^{-1}$ in the 10-1000 keV range and from the LAT is (0.2--13)$times$10$^{-9}$ erg cm$^{-2}$ s$^{-1}$ in the 0.1--1 GeV range. We also describe the improvements to our automated pipelines and analysis techniques for searching for and characterizing the potential electromagnetic counterparts for future gravitational wave events from Advanced LIGO/VIRGO.
With an instantaneous view of 70% of the sky, the Fermi Gamma-ray Burst Monitor (GBM) is an excellent partner in the search for electromagnetic counterparts to gravitational wave (GW) events. GBM observations at the time of the Laser Interferometer Gravitational-wave Observatory (LIGO) event GW150914 reveal the presence of a weak transient above 50 keV, 0.4~s after the GW event, with a false alarm probability of 0.0022 (2.9$sigma$). This weak transient lasting 1 s was not detected by any other instrument and does not appear connected with other previously known astrophysical, solar, terrestrial, or magnetospheric activity. Its localization is ill-constrained but consistent with the direction of GW150914. The duration and spectrum of the transient event are consistent with a weak short Gamma-Ray Burst arriving at a large angle to the direction in which Fermi was pointing, where the GBM detector response is not optimal. If the GBM transient is associated with GW150914, this electromagnetic signal from a stellar mass black hole binary merger is unexpected. We calculate a luminosity in hard X-ray emission between 1~keV and 10~MeV of $1.8^{+1.5}_{-1.0} times 10^{49}$~erg~s$^{-1}$. Future joint observations of GW events by LIGO/Virgo and Fermi GBM could reveal whether the weak transient reported here is a plausible counterpart to GW150914 or a chance coincidence, and will further probe the connection between compact binary mergers and short Gamma-Ray Bursts.
Following the reported discovery of the gravitational-wave pulse GW170817/ G298048 by three LIGO/Virgo antennae (Abbott et al., 2017a), the MASTER Global Robotic Net telescopes obtained the first image of the NGC 4993 galaxy after the NS+NS merging. The optical transient MASTER OTJ130948.10-232253.3/SSS17a was later found, which appears to be a kilonova resulting from a merger of two neutron stars. In this paper we report the independent detection and photometry of the kilonova made in white light and in B, V, and R filters. We note that luminosity of the discovered kilonova NGC 4993 is very close to another possible kilonova proposed early GRB 130603 and GRB 080503.
We report deep Chandra, HST and VLA observations of the binary neutron star event GW170817 at $t<160$ d 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 $ u_c$ is above the X-ray band and the synchrotron frequency $ u_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 $N(gamma)propto gamma^{-p}$ accelerated by a shock launched by a NS-NS merger to date. We find $p=2.17pm0.01$, which indicates that radiation from ejecta with $Gammasim3-10$ dominates the observed emission. While constraining the nature of the emission process, these observations do emph{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, emph{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 SGRB directed away from our line of sight. Observations at $tle 200$ days are unlikely to settle the debate as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material.
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