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1. SN2019wxt: An Ultrastripped Supernova Candidate Discovered in the Electromagnetic Follow-up of a Gravitational Wave Trigger

   https://doi.org/10.3847/1538-4357/acd5d5  

   Shivkumar, Hinna ; Jaodand, Amruta D. ; Balasubramanian, Arvind ; Fremling, Christoffer ; Corsi, Alessandra ; Tzanidakis, Anastasios ; Nissanke, Samaya ; Kasliwal, Mansi ; Brightman, Murray ; Raaijmakers, Geert ; et al ( July 2023 , The Astrophysical Journal)

   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.

    

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2. 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 magnetar 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. 

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3. 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 merger 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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4. SN 2017gci: a nearby Type I Superluminous Supernova with a bumpy tail

   https://doi.org/10.1093/mnras/staa4035  

   Fiore, A ; Chen, T-W ; Jerkstrand, A ; Benetti, S ; Ciolfi, R ; Inserra, C ; Cappellaro, E ; Pastorello, A ; Leloudas, G ; Schulze, S ; et al ( February 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We present and discuss the optical spectrophotometric observations of the nearby (z = 0.087) Type I superluminous supernova (SLSN I) SN 2017gci, whose peak K-corrected absolute magnitude reaches Mg = −21.5 mag. Its photometric and spectroscopic evolution includes features of both slow- and of fast-evolving SLSN I, thus favoring a continuum distribution between the two SLSN-I subclasses. In particular, similarly to other SLSNe I, the multiband light curves (LCs) of SN 2017gci show two re-brightenings at about 103 and 142 d after the maximum light. Interestingly, this broadly agrees with a broad emission feature emerging around 6520 Å after ∼51 d from the maximum light, which is followed by a sharp knee in the LC. If we interpret this feature as Hα, this could support the fact that the bumps are the signature of late interactions of the ejecta with a (hydrogen-rich) circumstellar material. Then we fitted magnetar- and CSM-interaction-powered synthetic LCs on to the bolometric one of SN 2017gci. In the magnetar case, the fit suggests a polar magnetic field Bp ≃ 6 × 1014 G, an initial period of the magnetar Pinitial ≃ 2.8 ms, an ejecta mass $M_{\rm ejecta}\simeq 9\, \mathrm{M}_\odot $ and an ejecta opacity $\kappa \simeq 0.08\, \mathrm{cm}^{2}\, \rm{g}^{-1}$. A CSM-interaction scenario would imply a CSM mass $\simeq 5\, \mathrm{M}_\odot $ and an ejecta mass $\simeq 12\, \mathrm{M}_\odot $. Finally, the nebular spectrum of phase  + 187 d was modeled, deriving a mass of $\sim 10\, {\rm M}_\odot$ for the ejecta. Our models suggest that either a magnetar or CSM interaction might be the power sources for SN 2017gci and that its progenitor was a massive ($40\, {\rm M}_\odot$) star. 

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5. On the nature of fast blue optical transients

   https://doi.org/10.1093/mnras/stac1717  

   Lyutikov, Maxim ( June 2022 , Monthly Notices of the Royal Astronomical Society)

   Short rise times of fast blue optical transients (FBOTs) require very light ejected envelopes, $M_{\rm ej} \le 10^{-1} \, \mathrm{M}_\odot$, much smaller than of a typical supernova. The detection by Chandra of X-ray emission in AT2020mrf of LX ∼ 1042 erg s−1 after 328 d implies total, overall dominant, X-ray energetics at the gamma-ray burst level of ∼6 × 1049 erg. We further develop a model of Lyutikov and Toonen, whereby FBOTs are the results of a late accretion-induced collapse of the product of double white dwarf (WD) merger between ONeMg WD and another WD. Small ejecta mass, and the rarity of FBOTs, results from the competition between mass-loss from the merger product to the wind, and ashes added to the core, on a time-scale of ∼103–104 yr. FBOTs proper come from central engine-powered radiation-dominated forward shock as it propagates through ejecta. All the photons produced by the central source deep inside the ejecta escape almost simultaneously, producing a short bright event. The high-energy emission is generated at the highly relativistic and highly magnetized termination shock, qualitatively similar to pulsar wind nebulae. The X-ray bump observed in AT2020mrf by SRG/eROSITA, predicted by Lyutikov and Toonen, is coming from the breakout of the engine-powered shock from the ejecta into the preceding wind. The model requires total energetics of just few × 1050 erg, slightly above the observed X-rays. We predict that the system is hydrogen poor.

    

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