The possible electromagnetic counterparts of the first high-probability NSBH merger LIGO/Virgo S190814bv

LIGO/Virgo S190814 bv is the first high-probability neutron star–black hole(NSBH)merger candidate,whose gravitational waves(GWs)triggered LIGO/Virgo detectors at21:10:39.012957 UT,14 August 2019.It has a probability>99%of being an NSBH merger,with a low false alarm rate(FAR)of one per 1.559 e+25 years.For an NSBH merger,electromagnetic counterparts(especially short gamma-ray bursts(GRBs))are generally expected.However,no electromagnetic counterpart has been found in the extensive follow-up observing campaign.In the present work,we propose a novel explanation for this null result.In our scenario,LIGO/Virgo S190814 bv is just a GW mirror image of the real NSBH merger which should have been detected before 14 September 2015,but at that time we had no ability to detect its GW signals.The electromagnetic counterparts associated with the real NSBH merger should be found in the archive data before 14 September 2015.In this work,we indeed find nine short GRBs that are possibly electromagnetic counterparts.

Progress of GECAM Observation Research

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor(GECAM)is a constellation with four instruments(launch date):GECAM-A/B(10 December 2020),GECAM-C(27 July 2022)and GECAM-D(13 March 2024),which are dedicated to monitoring gamma-ray transients in all-sky.The primary science objectives of GECAM include Gamma-Ray Bursts(GRBs),Soft Gamma-ray Repeaters(SGRs),high energy counterparts of Gravitation Wave(GW)and Fast Radio Burst(FRB),Solar Flares(SFLs),as well as Terrestrial Gamma-ray Flashes(TGFs)and Terrestrial Electron Beams(TEBs).A series of observations and research have been made since the launch of GECAM-A/B.GECAM observations provide new insights into these highenergy transients,demonstrating the unique role of GECAM in the“multi-wavelength,multi-messenger”era.

Insight-HXMT observations of the first binary neutron star merger GW170817

Finding the electromagnetic （EM） counterpart of binary compact star merger, especially the binary neutron star （BNS） merger, is critically important for gravitational wave （GW） astronomy, cosmology and fundamental physics. On Aug. 17, 2017, Advanced LIGO and Fermi/GBM independently triggered the first BNS merger, GW170817, and its high energy EM counterpart, GRB 170817A, respectively, resulting in a global observation campaign covering gamma-ray, X-ray, UV, optical, IR, radio as well as neutrinos. The High Energy X-ray telescope （HE） onboard Insight-HXMT （Hard X-ray Modulation Telescope） is the unique high-energy gamma-ray telescope that monitored the entire GW localization area and especially the optical counterpart （SSS17a/AT2017gfo） with very large collection area （M000 cm2） and microsecond time resolution in 0.2-5 MeV. In addition, Insight-HXMT quickly implemented a Target of Opportunity （TOO） observation to scan the GW localization area for potential X-ray emission from the GW source. Although Insight-HXMT did not detect any significant high energy （0.2-5 MeV） radiation from GW170817, its observation helped to confirm the unexpected weak and soft nature of GRB 170817A. Meanwhile, Insight-HXMT/HE provides one of the most stringent constraints （-10-7 to 104 erg/cm2/s） for both GRB170817A and any other possible precursor or extended emissions in 0.2-5 MeV, which help us to better understand the properties of EM radiation from this BNS merger. Therefore the observation of Insight-HXMT constitutes an important chapter in the full context of multi-wavelength and multi-messenger observation of this historical GW event.

Forecast for cosmological parameter estimation with gravitational-wave standard sirens from the LISA-Taiji network

LISA and Taiji are expected to form a space-based gravitational-wave(GW)detection network in the future.In this work,we make a forecast for the cosmological parameter estimation with the standard siren observation from the LISA-Taiji network.We simulate the standard siren data based on a scenario with configuration angle of 40°between LISA and Taiji.Three models for the population of massive black hole binary(MBHB),i.e.,popⅢ,Q3d,and Q3nod,are considered to predict the events of MBHB mergers.We find that,based on the LISA-Taiji network,the number of electromagnetic(EM)counterparts detected is almost doubled compared with the case of single Taiji mission.Therefore,the LISA-Taiji network’s standard siren observation could provide much tighter constraints on cosmological parameters.For example,solely using the standard sirens from the LISA-Taiji network,the constraint precision of H;could reach 1.3%.Moreover,combined with the CMB data,the GW-EM observation based on the LISA-Taiji network could also tightly constrain the equation of state of dark energy,e.g.,the constraint precision of w reaches about 4%,which is comparable with the result of CMB+BAO+SN.It is concluded that the GW standard sirens from the LISA-Taiji network will become a useful cosmological probe in understanding the nature of dark energy in the future.