Continued Radio Observations of GW170817 3.5 yr Post-merger

Abstract

We present new radio observations of the binary neutron star merger GW170817 carried out with the Karl G. Jansky Very large Array (VLA) more than 3 yr after the merger. Our combined data set is derived by coadding more than ≈32 hr of VLA time on-source, and as such provides the deepest combined observation (rms sensitivity ≈0.99 μJy) of the GW170817 field obtained to date at 3 GHz. We find no evidence for a late-time radio rebrightening at a mean epoch of t ≈ 1200 days since merger, in contrast to a ≈2.1σ excess observed at X-ray wavelengths at the same mean epoch. Our measurements agree with expectations from the post-peak decay of the radio afterglow of the GW170817 structured jet. Using these results, we constrain the parameter space of models that predict a late-time radio rebrightening possibly arising from the high-velocity tail of the GW170817 kilonova ejecta, which would dominate the radio and X-ray emission years after the merger (once the structured jet afterglow fades below detection level). Our results point to a steep energy-speed distribution of the kilonova ejecta (with energy-velocity power-law index α  5). We suggest possible implications of our radio analysis, when combined with the recent tentative evidence for a late-time rebrightening in the X-rays, and highlight the need for continued radio-to-X-ray monitoring to test different scenarios. Unified Astronomy Thesaurus concepts: Radio continuum emission (1340); Radio transient sources (2008); X-ray transient sources (1852)

Estimating outflow masses and velocities in merger simulations: Impact of r -process heating and neutrino cooling

“…Multimessenger observations of neutron star-neutron star (NSNS) and black hole-neutron star (BHNS) binaries provide us with information about the properties of dense matter [1][2][3], the origin of heavy elements [4][5][6], the population of compact objects [7], their formation mechanism [8], and even the expansion rate of the Universe [9,10]. So far, one NSNS merger (GW170817) has been observed through both gravitational and electromagnetic waves [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. A few additional mergers that likely involved at least one neutron star have been observed in gravitational waves only, most notably the likely NSNS merger GW190425 [26] and the likely BHNS mergers GW200105 and GW200115 [27].…”

Electromagnetic counterparts of binary neutron star mergers leading to a strongly magnetized long-lived remnant neutron star

“…Interestingly, the X-ray flux observed around 3.5 years after GW170817 exceeds the expected flux of the jet afterglow, suggesting that the ejecta afterglow starts to dominate over the jet component (Hajela et al 2021;Troja et al 2022). However, the origin of the X-ray excess is sill under debate because the radio flux is still consistent with the prediction of the jet afterglow (Balasubramanian et al 2021).…”

Inclination Estimates from Off-Axis GRB Afterglow Modelling

“…Similar to the non-spreading Gaussian model, both of the two-component models give good agreement with the observed latetime radio and X-ray decline, and an inclination of ι = 0.23 ± 0.01 rad (13.2 ± 0.6 degrees) without spreading and ι = 0.32 +0. 22 −0.02 rad (18.3 +12.6 −1.3 degrees) with lateral spreading. The future detection of GW-EM afterglows from core dominated relativistic jets could be used to help constrain the degree of lateral spreading in GRB afterglows; however, the effects of jet structure choice would need to be better understood or the physically expected jet structure for short GRB jets found (e.g., [6,38,39] and Nativi et al in prep).…”

“…A real test of the effects of lateral spreading on the parameter estimation for GW-EM, GRB afterglows is demonstrated by fitting, via Markov Chain Monte Carlo (MCMC), the four jet structure models with/without spreading to the data from GRB 170817A-we use the latest radio through X-ray frequency data as listed in [21][22][23][24]. For the fits to GRB 170817A, we fix p = 2.15 (e.g., [17,25]) and allow all other parameters to vary using a flat prior in each case as defined in Table 1.…”