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Optical and X-ray emission from stable millisecond magnetars formed from the merger of binary neutron stars

Metzger, Brian D. and Piro, Anthony L. (2014) Optical and X-ray emission from stable millisecond magnetars formed from the merger of binary neutron stars. Monthly Notices of the Royal Astronomical Society, 439 (4). pp. 3916-3930. ISSN 0035-8711.

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The coalescence of binary neutron stars (NSs) may in some cases produce a stable massive NS remnant rather than a black hole. Due to the substantial angular momentum from the binary, such a remnant is born rapidly rotating and likely acquires a strong magnetic field (a ‘millisecond magnetar’). Magnetic spin-down deposits a large fraction of the rotational energy from the magnetar behind the small quantity of mass ejected during the merger. If the magnetar outflow is indeed trapped behind the ejecta (instead of placing most of its energy into a collimated jet), this has the potential for creating a bright transient that could be useful for determining whether an NS or black hole was formed in the merger. We investigate the expected signature of such an event, including for the first time the important impact of e^± pairs injected by the millisecond magnetar into the surrounding nebula. These pairs cool via synchrotron and inverse Compton emission, producing a pair cascade and hard X-ray spectrum. A fraction of these X-rays are absorbed by the ejecta walls and re-emitted as thermal radiation, leading to an optical/UV transient peaking at a luminosity of ∼10^(43)–10^(44) erg s^(−1) on a time-scale of several hours to days. This is dimmer than predicted by simpler analytic models because the large optical depth of e^± pairs across the nebula suppresses the efficiency with which the magnetar spin-down luminosity is thermalized. Nevertheless, the optical/UV emission is more than two orders of magnitude brighter than a radioactively powered ‘kilonova’. In some cases, nebular X-rays are sufficiently luminous to re-ionize the ejecta, in which case non-thermal X-rays escape the ejecta unattenuated with a similar peak luminosity and time-scale as the optical radiation. We discuss the implications of our results for the temporally extended X-ray emission that is observed to follow some short gamma-ray bursts (GRBs), including the kilonova candidates GRB 080503 and GRB 130603B.

Item Type:Article
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URLURL TypeDescription Paper
Metzger, Brian D.0000-0002-4670-7509
Piro, Anthony L.0000-0001-6806-0673
Additional Information:© 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. First published online: March 3, 2014. Accepted 2014 February 4. Received 2014 January 9; in original form 2013 November 6. We are grateful to Todd Thompson for many helpful discussions and for detailed comments on the manuscript. We thank Andrei Beloborodov, Indrek Vurm and Romain Hascoet for their help in developing the model for millisecond PWN adapted for use in this paper. We also thank John Beacom, He Gao, Eliot Quataert and Bing Zhang for helpful discussions. ALP is supported through NSF grants AST-1205732, PHY-1068881, PHY-1151197 and the Sherman Fairchild Foundation.
Funding AgencyGrant Number
Sherman Fairchild FoundationUNSPECIFIED
Subject Keywords: radiation mechanisms: non-thermal gamma-ray burst: general stars: magnetars stars: magnetic field stars: neutron
Issue or Number:4
Record Number:CaltechAUTHORS:20140530-141831941
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Official Citation:Brian D. Metzger and Anthony L. Piro Optical and X-ray emission from stable millisecond magnetars formed from the merger of binary neutron stars MNRAS (April 21, 2014) Vol. 439 3916-3930 first published online March 3, 2014 doi:10.1093/mnras/stu247
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:46015
Deposited By: Ruth Sustaita
Deposited On:30 May 2014 21:44
Last Modified:09 Mar 2020 13:19

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