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1. Multimessenger Constraints on Magnetic Fields in Merging Black Hole–Neutron Star Binaries

   https://doi.org/10.3847/1538-4357/ac4bdb  

   D’Orazio, Daniel J. ; Haiman, Zoltán ; Levin, Janna ; Samsing, Johan ; Vigna-Gómez, Alejandro ( March 2022 , The Astrophysical Journal)

   The LIGO–Virgo–KAGRA Collaboration recently detected gravitational waves (GWs) from the merger of black hole–neutron star (BHNS) binary systems GW200105 and GW200115. No coincident electromagnetic (EM) counterparts were detected. While the mass ratio and BH spin in both systems were not sufficient to tidally disrupt the NS outside the BH event horizon, other, magnetospheric mechanisms for EM emission exist in this regime and depend sensitively on the NS magnetic field strength. Combining GW measurements with EM flux upper limits, we place upper limits on the NS surface magnetic field strength above which magnetospheric emission models would have generated an observable EM counterpart. We consider fireball models powered by the black hole battery mechanism, where energy is output in gamma rays over ≲1 s. Consistency with no detection by Fermi-GBM or INTEGRAL SPI-ACS constrains the NS surface magnetic field to ≲10¹⁵G. Hence, joint GW detection and EM upper limits rule out the theoretical possibility that the NSs in GW200105 and GW200115, and the putative NS in GW190814, retain dipolar magnetic fields ≳10¹⁵G until merger. They also rule out formation scenarios where strongly magnetized magnetars quickly merge with BHs. We alternatively rule out operation of the BH-battery-powered fireball mechanism in these systems.more »This is the first multimessenger constraint on NS magnetic fields in BHNS systems and a novel approach to probe fields at this point in NS evolution. This demonstrates the constraining power that multimessenger analyses of BHNS mergers have on BHNS formation scenarios, NS magnetic field evolution, and the physics of BHNS magnetospheric interactions.

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2. Evidence for X-Ray Emission in Excess to the Jet-afterglow Decay 3.5 yr after the Binary Neutron Star Merger GW 170817: A New Emission Component

   https://doi.org/10.3847/2041-8213/ac504a  

   Hajela, A. ; Margutti, R. ; Bright, J. S. ; Alexander, K. D. ; Metzger, B. D. ; Nedora, V. ; Kathirgamaraju, A. ; Margalit, B. ; Radice, D. ; Guidorzi, C. ; et al ( March 2022 , The Astrophysical Journal Letters)

   For the first ∼3 yrs after the binary neutron star merger event GW 170817, the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium withn< 0.01 cm⁻³. We report on observational evidence for an excess of X-ray emission atδt> 900 days after the merger. WithL_x≈ 5 × 10³⁸erg s⁻¹at 1234 days, the recently detected X-ray emission represents a ≥3.2σ(Gaussian equivalent) deviation from the universal post-jet-break model that best fits the multiwavelength afterglow at earlier times. In the context ofJetFitafterglow models, current data represent a departure with statistical significance ≥3.1σ, depending on the fireball collimation, with the most realistic models showing excesses at the level of ≥3.7σ. A lack of detectable 3 GHz radio emission suggests a harder broadband spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e., a kilonova afterglow. In this context, we present a set of ab initio numerical relativity binary neutron star (BNS) merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the mergermore »ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented.

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3. Searching for Gravitational-wave Counterparts Using the Transiting Exoplanet Survey Satellite

   https://doi.org/10.3847/2041-8213/acca70  

   Mo, Geoffrey ; Jayaraman, Rahul ; Fausnaugh, Michael ; Katsavounidis, Erik ; Ricker, George R. ; Vanderspek, Roland ( April 2023 , The Astrophysical Journal Letters)

   In 2017, the LIGO and Virgo gravitational-wave (GW) detectors, in conjunction with electromagnetic (EM) astronomers, observed the first GW multimessenger astrophysical event, the binary neutron star (BNS) merger GW170817. This marked the beginning of a new era in multimessenger astrophysics. To discover further GW multimessenger events, we explore the synergies between the Transiting Exoplanet Survey Satellite (TESS) and GW observations triggered by the LIGO–Virgo–KAGRA Collaboration (LVK) detector network. TESS's extremely wide field of view (∼2300 deg²) means that it could overlap with large swaths of GW localizations, which often span hundreds of square degrees or more. In this work, we use a recently developed transient detection pipeline to search TESS data collected during the LVK’s third observing run, O3, for any EM counterparts. We find no obvious counterparts brighter than about 17th magnitude in the TESS bandpass. Additionally, we present end-to-end simulations of BNS mergers, including their detection in GWs and simulations of light curves, to identify TESS's kilonova discovery potential for the LVK's next observing run (O4). In the most optimistic case, TESS will observe up to one GW-found BNS merger counterpart per year. However, TESS may also find up to five kilonovae that did not trigger themore »LVK network, emphasizing that EM-triggered GW searches may play a key role in future kilonova detections. We also discuss how TESS can help place limits on EM emission from binary black hole mergers and rapidly exclude large sky areas for poorly localized GW events.

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4. Early-time searches for coherent radio emission from short GRBs with the Murchison Widefield Array

   https://doi.org/10.1017/pasa.2021.58  

   Tian, J. ; Anderson, G. E. ; Hancock, P. J. ; Miller-Jones, J. C. ; Sokolowski, M. ; Rowlinson, A. ; Williams, A. ; Morgan, J. ; Hurley-Walker, N. ; Kaplan, D. L. ; et al ( January 2022 , Publications of the Astronomical Society of Australia)

   Abstract Many short gamma-ray bursts (GRBs) originate from binary neutron star mergers, and there are several theories that predict the production of coherent, prompt radio signals either prior, during, or shortly following the merger, as well as persistent pulsar-like emission from the spin-down of a magnetar remnant. Here we present a low frequency (170–200 MHz) search for coherent radio emission associated with nine short GRBs detected by the Swift and/or Fermi satellites using the Murchison Widefield Array (MWA) rapid-response observing mode. The MWA began observing these events within 30–60 s of their high-energy detection, enabling us to capture any dispersion delayed signals emitted by short GRBs for a typical range of redshifts. We conducted transient searches at the GRB positions on timescales of 5 s, 30 s, and 2 min, resulting in the most constraining flux density limits on any associated transient of 0.42, 0.29, and 0.084 Jy, respectively. We also searched for dispersed signals at a temporal and spectral resolution of 0.5 s and 1.28 MHz, but none were detected. However, the fluence limit of 80–100 Jy ms derived for GRB 190627A is the most stringent to date for a short GRB. Assuming the formation of a stable magnetarmore »for this GRB, we compared the fluence and persistent emission limits to short GRB coherent emission models, placing constraints on key parameters including the radio emission efficiency of the nearly merged neutron stars ( $\epsilon_r\lesssim10^{-4}$ ), the fraction of magnetic energy in the GRB jet ( $\epsilon_B\lesssim2\times10^{-4}$ ), and the radio emission efficiency of the magnetar remnant ( $\epsilon_r\lesssim10^{-3}$ ). Comparing the limits derived for our full GRB sample (along with those in the literature) to the same emission models, we demonstrate that our fluence limits only place weak constraints on the prompt emission predicted from the interaction between the relativistic GRB jet and the interstellar medium for a subset of magnetar parameters. However, the 30-min flux density limits were sensitive enough to theoretically detect the persistent radio emission from magnetar remnants up to a redshift of $z\sim0.6$ . Our non-detection of this emission could imply that some GRBs in the sample were not genuinely short or did not result from a binary neutron star merger, the GRBs were at high redshifts, these mergers formed atypical magnetars, the radiation beams of the magnetar remnants were pointing away from Earth, or the majority did not form magnetars but rather collapse directly into black holes.« less
5. Electromagnetic precursor flares from the late inspiral of neutron star binaries

   https://doi.org/10.1093/mnras/stac1909  

   Most, Elias R ; Philippov, Alexander A ( August 2022 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The coalescence of two neutron stars is accompanied by the emission of gravitational waves, and can also feature electromagnetic counterparts powered by mass ejecta and the formation of a relativistic jet after the merger. Since neutron stars can feature strong magnetic fields, the non-trivial interaction of the neutron star magnetospheres might fuel potentially powerful electromagnetic transients prior to merger. A key process powering those precursor transients is relativistic reconnection in strong current sheets formed between the two stars. In this work, we provide a detailed analysis of how the twisting of the common magnetosphere of the binary leads to an emission of electromagnetic flares, akin to those produced in the solar corona. By means of relativistic force-free electrodynamics simulations, we clarify the role of different magnetic field topologies in the process. We conclude that flaring will always occur for suitable magnetic field alignments, unless one of the neutron stars has a magnetic field significantly weaker than the other.