TY - JOUR
T1 - The broadband counterpart of the short GRB 200522A at z = 0.5536
T2 - a luminous kilonova or a collimated outflow with a reverse shock?
AU - Fong, W.
AU - Laskar, T.
AU - Rastinejad, J.
AU - Escorial, A. Rouco
AU - Schroeder, G.
AU - Barnes, J.
AU - Kilpatrick, C. D.
AU - Paterson, K.
AU - Berger, E.
AU - Metzger, B. D.
AU - Dong, Y.
AU - Nugent, A. E.
AU - Strausbaugh, R.
AU - Blanchard, P. K.
AU - Goyal, A.
AU - Cucchiara, A.
AU - Terreran, G.
AU - Alexander, K. D.
AU - Eftekhari, T.
AU - Fryer, C.
AU - Margalit, B.
AU - Margutti, R.
AU - Nicholl, Matt
PY - 2021/1/15
Y1 - 2021/1/15
N2 - We present the discovery of the radio afterglow and near-infrared (NIR) counterpart of the Swift short gamma-ray burst (GRB) GRB 200522A, located at a small projected offset of ≈1 kpc from the center of a young, star-forming host galaxy at z = 0.5536. The radio and X-ray luminosities of the afterglow are consistent with those of on-axis cosmological short GRBs. The NIR counterpart, revealed by our Hubble Space Telescope observations at a rest-frame time of ≈2.3 days, has a luminosity of ≈(1.3–1.7) × 1042 erg s−1. This is substantially lower than on-axis short GRB afterglow detections but is a factor of ≈8–17 more luminous than the kilonova of GW170817 and significantly more luminous than any kilonova candidate for which comparable observations exist. The combination of the counterpart's color (i − y = −0.08 ± 0.21; rest frame) and luminosity cannot be explained by standard radioactive heating alone. We present two scenarios to interpret the broadband behavior of GRB 200522A: a synchrotron forward shock with a luminous kilonova (potentially boosted by magnetar energy deposition), or forward and reverse shocks from a ≈14°, relativistic (Γ0 ≳ 10) jet. Models that include a combination of enhanced radioactive heating rates, low-lanthanide mass fractions, or additional sources of heating from late-time central engine activity may provide viable alternate explanations. If a stable magnetar was indeed produced in GRB 200522A, we predict that late-time radio emission will be detectable starting ≈0.3–6 yr after the burst for a deposited energy of ≈1053 erg. Counterparts of similar luminosity to GRB 200522A associated with gravitational wave events will be detectable with current optical searches to ≈250 Mpc.
AB - We present the discovery of the radio afterglow and near-infrared (NIR) counterpart of the Swift short gamma-ray burst (GRB) GRB 200522A, located at a small projected offset of ≈1 kpc from the center of a young, star-forming host galaxy at z = 0.5536. The radio and X-ray luminosities of the afterglow are consistent with those of on-axis cosmological short GRBs. The NIR counterpart, revealed by our Hubble Space Telescope observations at a rest-frame time of ≈2.3 days, has a luminosity of ≈(1.3–1.7) × 1042 erg s−1. This is substantially lower than on-axis short GRB afterglow detections but is a factor of ≈8–17 more luminous than the kilonova of GW170817 and significantly more luminous than any kilonova candidate for which comparable observations exist. The combination of the counterpart's color (i − y = −0.08 ± 0.21; rest frame) and luminosity cannot be explained by standard radioactive heating alone. We present two scenarios to interpret the broadband behavior of GRB 200522A: a synchrotron forward shock with a luminous kilonova (potentially boosted by magnetar energy deposition), or forward and reverse shocks from a ≈14°, relativistic (Γ0 ≳ 10) jet. Models that include a combination of enhanced radioactive heating rates, low-lanthanide mass fractions, or additional sources of heating from late-time central engine activity may provide viable alternate explanations. If a stable magnetar was indeed produced in GRB 200522A, we predict that late-time radio emission will be detectable starting ≈0.3–6 yr after the burst for a deposited energy of ≈1053 erg. Counterparts of similar luminosity to GRB 200522A associated with gravitational wave events will be detectable with current optical searches to ≈250 Mpc.
UR - http://www.scopus.com/inward/record.url?scp=85100293469&partnerID=8YFLogxK
U2 - 10.3847/1538-4357/abc74a
DO - 10.3847/1538-4357/abc74a
M3 - Article
SN - 0004-637X
VL - 906
JO - The Astrophysical Journal
JF - The Astrophysical Journal
IS - 2
M1 - 127
ER -