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The multimessenger detection of GW170817 showed that binary neutron star (BNS) mergers are progenitors of (at least some) short gamma-ray bursts (GRBs), and that short GRB jets (and their afterglows) can have structures (and observational properties) more complex than predicted by the standard top-hat jet scenario. Indeed, the emission from the structured jet launched in GW170817 peaked in the radio band (centimeter wavelengths) at ≈100 days since merger—a timescale much longer than the typical time span of radio follow-up observations of short GRBs. Moreover, radio searches for a potential late-time radio flare from the fast tail of the neutron-rich debris that powered the kilonova associated with GW170817 (AT 2017gfo) have extended to even longer timescales (years after the merger). In light of this, here we present the results of an observational campaign targeting a sample of seven, years-old GRBs in the Swift/BAT sample with no redshift measurements and no promptly identified X-ray counterpart. Our goal is to assess whether this sample of short GRBs could harbor nearby BNS mergers, searching for the late-time radio emission expected from their ejecta. We found one radio candidate counterpart for one of the GRBs in our sample, GRB 111126A, though an origin related to emission from star formation or from an active galactic nucleus in its host galaxy cannot be excluded without further observations.

 

We present optical, radio, and X-ray observations of a rapidly evolving transient SN2019wxt (PS19hgw), discovered during the search for an electromagnetic counterpart to the gravitational-wave (GW) trigger S191213g. Although S191213g was not confirmed as a significant GW event in the off-line analysis of LIGO-Virgo data, SN2019wxt remained an interesting transient due to its peculiar nature. The optical/near-infrared (NIR) light curve of SN2019wxt displayed a double-peaked structure evolving rapidly in a manner analogous to currently known ultrastripped supernovae (USSNe) candidates. This double-peaked structure suggests the presence of an extended envelope around the progenitor, best modeled with two components: (i) early-time shock-cooling emission and (ii) late-time radioactive⁵⁶Ni decay. We constrain the ejecta mass of SN2019wxt atM_ej≈ 0.20M_⊙, which indicates a significantly stripped progenitor that was possibly in a binary system. We also followed up SN2019wxt with long-term Chandra and Jansky Very Large Array observations spanning ∼260 days. We detected no definitive counterparts at the location of SN2019wxt in these long-term X-ray and radio observational campaigns. We establish the X-ray upper limit at 9.93 × 10⁻¹⁷erg cm⁻²s⁻¹and detect an excess radio emission from the region of SN2019wxt. However, there is little evidence for SN1993J- or GW170817-like variability of the radio flux over the course of our observations. A substantial host-galaxy contribution to the measured radio flux is likely. The discovery and early-time peak capture of SN2019wxt in optical/NIR observations during EMGW follow-up observations highlight the need for dedicated early, multiband photometric observations to identify USSNe.

 

Abstract GW170817 is the first binary neutron star (NS) merger detected in gravitational waves (GWs) and photons, and so far remains the only GW event of its class with a definitive electromagnetic counterpart. Radio emission from the structured jet associated with GW170817 has faded below the sensitivity achievable via deep radio observations with the most sensitive radio arrays currently in operation. Hence, we now have the opportunity to probe the radio re-brightening that some models predict, which should emerge at late times from the interaction of the dynamically stripped merger ejecta with the interstellar medium. Here we present the latest results from our deep radio observations of the GW170817 field with the Karl G. Jansky Very Large Array (VLA), 4.5 yr after the merger. Our new data at 3 GHz do not show any compelling evidence for emission in excess to the tail of the jet afterglow (<3.3 μ Jy), confirming our previous results. We thus set new constraints on the dynamical ejecta afterglow models. These constraints favor single-speed ejecta with energies ≲10 50 erg (for an ejecta speed of β 0 = 0.5), or steeper energy–speed distributions of the kilonova ejecta. Our results also suggest larger values of the cold, nonrotating maximum NS mass in equal-mass scenarios. However, without a detection of the dynamical ejecta afterglow, obtaining precise constraints on the NS equation of state remains challenging. 

One of the open questions following the discovery of GW170817 is whether neutron star (NS) mergers are the only astrophysical sites capable of producingr-process elements. Simulations have shown that 0.01–0.1M_⊙ofr-process material could be generated in the outflows originating from the accretion disk surrounding the rapidly rotating black hole that forms as a remnant to both NS mergers and collapsing massive stars associated with long-duration gamma-ray bursts (collapsars). The hallmark signature ofr-process nucleosynthesis in the binary NS merger GW170817 was its long-lasting near-infrared (NIR) emission, thus motivating a systematic photometric study of the light curves of broad-lined stripped-envelope (Ic-BL) supernovae (SNe) associated with collapsars. We present the first systematic study of 25 SNe Ic-BL—including 18 observed with the Zwicky Transient Facility and 7 from the literature—in the optical/NIR bands to determine what quantity ofr-process material, if any, is synthesized in these explosions. Using semi-analytic models designed to account forr-process production in SNe Ic-BL, we perform light curve fitting to derive constraints on ther-process mass for these SNe. We also perform independent light curve fits to models without ther-process. We find that ther-process-free models are a better fit to the light curves of the objects in our sample. Thus, we find no compelling evidence ofr-process enrichment in any of our objects. Further high-cadence infrared photometric studies and nebular spectroscopic analysis would be sensitive to smaller quantities ofr-process ejecta mass or indicate whether all collapsars are completely devoid ofr-process nucleosynthesis.

 

ABSTRACT The origin of fast radio bursts (FRBs) still remains a mystery, even with the increased number of discoveries in the last 3 yr. Growing evidence suggests that some FRBs may originate from magnetars. Large, single-dish telescopes such as Arecibo Observatory (AO) and Green Bank Telescope (GBT) have the sensitivity to detect FRB 121102-like bursts at gigaparsec distances. Here, we present searches using AO and GBT that aimed to find potential radio bursts at 11 sites of past gamma-ray bursts that show evidence for the birth of a magnetar. We also performed a search towards GW170817, which has a merger remnant whose nature remains uncertain. We place $10\sigma$ fluence upper limits of ≈0.036 Jy ms at 1.4 GHz and ≈0.063 Jy ms at 4.5 GHz for the AO data and fluence upper limits of ≈0.085 Jy ms at 1.4 GHz and ≈0.098 Jy ms at 1.9 GHz for the GBT data, for a maximum pulse width of ≈42 ms. The AO observations had sufficient sensitivity to detect any FRB of similar luminosity to the one recently detected from the Galactic magnetar SGR 1935+2154. Assuming a Schechter function for the luminosity function of FRBs, we find that our non-detections favour a steep power-law index (α ≲ −1.1) and a large cut-off luminosity (L0 ≳ 1041 erg s−1).