Viewing angle constraint of LIGO/Virgo binary neutron star merger candidates with Fermi-GBM non-detection limits
The LIGO and Virgo scientific collaboration (LVC) alerted two binary neutron star (BNS) merger candidates, S190425z and S190426c. Fermi-GBM observed 55.6% (for S190425z) and 100% (for S190426c) of the probability regions of both events at the respective merger times, but no gamma-ray burst (GRB) was detected in either case. The derived luminosity upper limits suggest that a short GRB similar to GRB 170817A would not be detectable for both cases due to their larger distances than GW170817. Assuming that the jet profile obtained from the GW170817/GRB 170817A is quasi-universal for all BNS-GRB associations, we derive that the viewing angles of S190425z and S190426c should be and , respectively.
Subject headings:gamma-ray burst: general - gravitational waves
After the exciting discovery of the first binary neutron star (BNS) merger event GW170817 (Abbott et al., 2017a) and its associated gamma-ray burst (GRB) 170817A, kilonova, and multi-wavelength afterglows (Abbott et al., 2017b), the LIGO-Virgo scientific collaboration (LVC) lately reported two more candidates that may have the BNS merger origin, i.e., LIGO/Virgo S190425z and LIGO/Virgo S190426c (The LIGO and the Virgo Collaboration, 2019a, b).
S190425z was identified during the real-time processing of data from LIGO Livingston Observatory (L1) and the Virgo Observatory (V1) at 2019-04-25 08:18:05.017 UTC (The LIGO and the Virgo Collaboration, 2019a). The false alarm rate is estimated by the online analysis as Hz, or about one in 70 thousands of years. Since the source was not detected by LIGO Hanford (H1) and the signal-to-noise ratio (SNR) was below the threshold in V1, LVC provides a very poor localization constraint. Assuming the candidate is astrophysical in origin, the probability for classifying this GW event as BNS merger is greater than . Detailed data analysis is on going. Multi-band observations from radio to gamma-ray were immediately operated to search for its electromagnetic (EM) counterpart candidate, but no confident counterpart has been identified so far. The Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-Ray Observatory (Fermi-GBM) observed 55.6% of the probability region at the merger event time (Fermi/GBM Team, 2019a). There was no onboard trigger around the event time, and no counterpart candidate was identified with both automated, blind search and coherent search for a GRB signal (from 30 s around the merger time). The Fermi-GBM Team thus estimated that the intrinsic luminosity upper limit for a S190425z-associated-GRB, if any, is .
S190426c was identified during the real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1) and Virgo Observatory (V1) at 2019-04-26 15:21:55.337 UTC (The LIGO and the Virgo Collaboration, 2019b). The false alarm rate for this event is estimated by the online analysis as Hz, or about one in 1 year and 7 months. Assuming that the candidate is of an astrophysical in origin, the probability for classifying this GW event as a BNS merger is . Detailed data analysis is on going. Similar to S190425z, multi-band observations were carried out immediately to search for an EM counterpart of S190426c, but no confident counterpart has been identified yet. For S190426c, Fermi-GBM was observing 100% of the probability region at the merger event time (Fermi/GBM Team, 2019b). Again, there was no onboard trigger around the event time, and no counterpart candidate was identified with both automated, blind search and coherent search (from 30 s around merger time). The Fermi-GBM Team thus estimated the intrinsic luminosity upper limit for a S190426c-associated-GRB, if any, as .
The abnormally low prompt emission luminosity (Goldstein et al., 2017; Zhang et al., 2018) and the slow rising of the multi-wavelength lightcurves (Mooley et al., 2017; Lazzati et al., 2017; Troja et al., 2017, 2018; Lyman et al., 2018) of GRB 170817A suggested that the event is best interpreted as a structured jet (e.g. Zhang & Mészáros, 2002) with a large viewing angle from the jet axis. With a broad-band study and a multimessenger analysis including the GW constraints, Troja et al. (2018) proposed that the data of GW170817/GRB 170817A favor a Gaussian-shaped jet profile (Zhang & Mészáros, 2002)
for , where is the on-axis equivalent isotropic energy, is the charateristic angle of the core, and is the truncating angle of the jet. Such a Gaussian jet profile seems to be supported by numerical simulations of a short GRB jet propagates in a dynamical ejecta with a negligible waiting time of jet launching (Xie et al., 2018; Geng et al., 2019). In order to interpret the multi-wavelength EM observations and the viewing angle constraint from the GW analysis, Troja et al. (2018) proposed , and for the jet profile of GRB 170817A. The value of the Hubble constant reported by the Planck collaboration (Planck Collaboration XIII, 2016) was adopted.
In Figure 1, we plot the jet profile of GRB 170817A with the 1- region as proposed by Troja et al. (2018), the -ray emission luminosity of GRB 170817A, and the -ray emission luminosity upper limits for S190425z and S190426c. Note that in order to convert the energy profile in Troja et al. (2018) to luminosity profile, here we assume that the -ray radiation efficiency is 10%, the burst duration s and the spectrum is flat. Such a jet profile covers the regime of known short GRBs, so it is attractive to assume that all short GRBs has a quasi-universal structured jet. Assuming that both S190425z and S190426c are associated with a short GRB, whose jet profile is similar to that of GRB 170817A, we derive that the viewing angle of S190426c is , with the uncertainty mainly defined by the uncertainty of its luminosity distance. For S190425z, with an additional assumption that its location at the merger time was within the filed of view of Fermi-GBM, one can derive its viewing angle being , with the uncertainty again mainly defined by the uncertainty of its luminosity distance. It is worth noticing that Finstad et al. (2018) performed a joint analysis of the GW/EM observations and suggested a conservative lower limit on the viewing angle of for GRB 170817A. If one takes this limit, the lower boundary of GRB 170817A jet profile would become tighter (dashed line in Fig.1), so that the viewing angle constraint for both S190425z and S190426c would become tighter, i.e., for S190426c and for S190425z, respectively.
The constraints presented here are not relevant in the following situations: 1. S190425z and/or S190426c are not from BNS mergers; 2. S190425z was not in the field of view of GBM (e.g. blocked by Earth); 3. Not all BNS mergers are associated with short GRBs; and 4. BNS short GRBs do not share a quasi-universal jet structure. On the other hand, the hypothesis of a quasi-universal structured jet for all short GRBs seems to be consistent with the currently available data. A 170817A-like short GRB would be undetectable for S190425z and/or S190426c due to their larger distances than GW170817/GRB 170817A. Future joint observations of BNS GW sources by LVC and all sky gamma-ray detectors, such as GBM and the Chinese future mission Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) (Zhang et al., 2018), together with the event rate density studies (e.g. Sun et al., 2015; Zhang et al., 2018), will finally test the quasi-universal jet hypothesis. If our interpretation is correct, the constraints on viewing angle would be helpful for GW data analysis to reach better constraints on the binary properties.
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