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Register a new userA tale of two mergers: constraints on kilonova detection in two short GRBs at z$sim$0.5
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We present a detailed multi-wavelength analysis of two short Gamma-Ray Bursts (sGRBs) detected by the Neil Gehrels Swift Observatory: GRB 160624A at $z=0.483$ and GRB 200522A at $z=0.554$. These sGRBs demonstrate very different properties in their observed emission and environment. GRB 160624A is associated to a late-type galaxy with an old stellar population ($approx$3 Gyr) and moderate on-going star formation ($approx$1 $M_{odot}$ yr$^{-1}$). Hubble and Gemini limits on optical/nIR emission from GRB 160624A are among the most stringent for sGRBs, leading to tight constraints on the allowed kilonova properties. In particular, we rule out any kilonova brighter than AT2017gfo, disfavoring large masses of wind ejecta ($lesssim$0.03 $M_odot$). In contrast, observations of GRB 200522A uncovered a luminous ($L_textrm{F125W}approx 10^{42}$ erg s$^{-1}$ at 2.3~d) and red ($r-Happrox 1.3$ mag) counterpart. The red color can be explained either by bright kilonova emission powered by the radioactive decay of a large amount of wind ejecta (0.03 $M_odot$ $lesssim$ $M$ $lesssim$ 0.1 $M_odot$) or moderate extinction, $E(B-V)approx0.1-0.2$ mag, along the line of sight. The location of this sGRB in the inner regions of a young ($approx$0.1 Gyr) star-forming ($approx$2-6 $M_{odot}$ yr$^{-1}$) galaxy and the limited sampling of its counterpart do not allow us to rule out dust effects as contributing, at least in part, to the red color.
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Massive, rapidly-spinning magnetar remnants produced as a result of binary neutron star (BNS) mergers may deposit a fraction of their energy into the surrounding kilonova ejecta, powering a synchrotron radio signal from the interaction of the ejecta with the circumburst medium. We present 6.0 GHz Very Large Array (VLA) observations of nine, low-redshift short gamma-ray bursts (SGRBs; $z<0.5$) on rest-frame timescales of $approx2.4-13.9$ yr following the bursts. We place $3sigma$ limits on radio continuum emission of $F_{ u}lesssim6-20,mu$Jy at the burst positions, or $L_{ u}lesssim(0.6-8.3)times10^{28}$erg s$^{-1}$Hz$^{-1}$. Comparing these limits with new light curve modeling which properly incorporates relativistic effects, we obtain limits on the energy deposited into the ejecta of $E_{ej}lesssim(0.6-6.7)times 10^{52}$erg ($E_{ej}lesssim(1.8-17.6)times10^{52}$erg) for an ejecta mass of $0.03,M_{odot}$ ($0.1,M_{odot}$). We present a uniform re-analysis of 27 SGRBs with $5.5-6.0$ GHz observations, and find that $gtrsim50%$ of SGRBs did not form stable magnetar remnants in their mergers. Assuming SGRBs are produced by BNS mergers drawn from the Galactic BNS population plus an additional component of high-mass GW194025-like mergers in a fraction $f_{GW190425}$ of cases, we place constraints on the maximum mass of a non-rotating neutron star (NS) ($M_{TOV}$), finding $M_{TOV}lesssim2.23,M_{odot}$ for $f_{GW190425}=0.4$; this limit increases for larger values of $f_{GW190425}$. The detection (or lack thereof) of radio remnants in untargeted surveys such as the VLA Sky Survey (VLASS) could provide more stringent constraints on the fraction of mergers that produce stable remnants. If $gtrsim30-300$ radio remnants are discovered in VLASS, this suggests that SGRBs are a biased population of BNS mergers in terms of the stability of the remnants they produce.
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We study the mass of quasar-hosting dark matter halos at z $sim$ 6 and further constrain the fraction of dark matter halos hosting an active quasar $f_{on}$ and the quasar beaming angle $i_{rm max}$ using observations of CII lines in the literature. We make assumptions that (1) more massive halos host brighter quasars, (2) a fraction of the halos host active quasars with a certain beaming angle, (3) cold gas in galaxies has rotational velocity $V_{rm circ}=alpha V_{rm max}$, and that (4) quasars point randomly on the sky. We find that for a choice of specific $alpha gtrsim 1$, the most likely solution has $f_{rm on} < 0.01$, corresponding to a small duty cycle of quasar activity. However, if we marginalize over $alpha$, for some choices of a prior a second solution with $f_{rm on}=1$ appears. Overall, our the constraints are highly sensitive to $alpha$ and hence inconclusive. Stronger constraints on $f_{rm on}$ can be made if we better understand the dynamics of cold gas in these galaxies.
We present multi-wavelength follow-up campaigns by the AstroSat-CZTI and GROWTH collaborations to search for an electromagnetic counterpart to the gravitational wave event GW170104. At the time of the GW170104 trigger, the AstroSat CZTI field-of-view covered 50.3% of the sky localization. We do not detect any hard X-ray (>100 keV) signal at this time, and place an upper limit of $approx 4.5 times 10^{-7}~{rm erg~cm}^{-2}{rm~s}^{-1}$ for a 1,s timescale. Separately, the ATLAS survey reported a rapidly fading optical source dubbed ATLAS17aeu in the error circle of GW170104. Our panchromatic investigation of ATLAS17aeu shows that it is the afterglow of an unrelated long, soft GRB~170105A, with only a fortuitous spatial coincidence with GW170104. We then discuss the properties of this transient in the context of standard long GRB afterglow models.
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