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

By B. Metzger, A. Piro. 2013

230 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/97245c4c461c3c95feb640c34cd64a5c9c4c6399

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In Astronomy and Astrophysics

1. Bing Zhang
2. B. Metzger
3. E. Berger
4. Pawan Kumar
5. E. Quataert
6. B. Margalit
7. D. Kasen
8. T. Thompson

Citations in the corpus

• INTEGRAL Detection of the First Prompt Gamma-Ray Signal Coincident with the Gravitational-wave Event GW170817 (2017, background): “The possibility of forming a (short-lived) magnetar in a BNS merger has been extensively discussed in the past (e.g., Price & Rosswog 2006; Metzger & Piro 2014; Giacomazzo et al. 2015; Fernández & Metzger 2016).”
• Constraining the Maximum Mass of Neutron Stars from Multi-messenger Observations of GW170817 (2017, background): “Several works have explored the potentially exotic EM signals of BNS mergers in cases when a long-lived SMNS or stable neutron star remnant is formed (Metzger et al. 2008b; Bucciantini et al. 2012; Yu et al. 2013; Metzger & Bower 2014; Metzger & Piro 2014; Gao et al. 2016; Siegel & Ciolfi 2016a,b).”
• RAPIDLY EVOLVING AND LUMINOUS TRANSIENTS FROM PAN-STARRS1 (2014, background): “Both Yu et al. (2013) and Metzger & Piro (2014) investigate the electromagnetic counterparts that would be produced from the formation of such a system if the magnetar spin-down energy is channeled into the ejecta from the merger, as opposed to a collimated jet.”
• The Cow: Discovery of a Luminous, Hot, and Rapidly Evolving Transient (2018, methodology): “In particular, theMej = 0.1 M model with a magnetic field of 1015 G of Metzger & Piro (2014) predicts a UV/optical transient with a similar peak luminosity, decline rate, and effective temperature to that of AT2018cow.”
• Red or blue? A potential kilonova imprint of the delay until black hole formation following a neutron star merger (2014, background): “If the long-lived HMNS is magnetized, its rotational spin-down could also power the excess X-rays observed following this event (Fan et al. 2013; Fong et al. 2014; Metzger & Piro 2014).”
• The THESEUS space mission concept: science case, design and expected performances (2017, background): “wn emission reprocessed by the matter surrounding the merger site (isotropically ejected during and after merger), with luminosities in the range 1043-1048 erg s 1 and time scales of minutes to days (Metzger and Piro 2014; Siegel and Ciolfi 2016a,b). Alternatively, X-ray emission may come from direct”
• Science case for the Einstein telescope (2019, background): “Even if never observed so far, this emission is expected to be powerful and nearly isotropic [74, 75].”
• The physics of gamma-ray bursts & relativistic jets (2014, background): “If a supra-massivemagnetar is born during the merger, such a merger-nova could be brighter due to the additional energy injection into the ejecta from the magnetar [744,745].”
• THE MILLISECOND MAGNETAR CENTRAL ENGINE IN SHORT GRBs (2015, background): “…radiation even if the short GRB jet does not beam towards earth, so that some interesting observational signatures are expected to be associated with gravitational wave signals in the Advanced LIGO/Virgo era (Zhang 2013; Gao et al. 2013; Yu et al. 2013; Fan et al. 2013b; Metzger & Piro 2014).”
• Millisecond Magnetar Birth Connects FRB 121102 to Superluminous Supernovae and Long-duration Gamma-Ray Bursts (2017, background): “…in a very young magnetar remnant due to the much higher pair creation rate due to γ − γ annihilation, as is expected less than months after the explosion while the compactness parameter of the nebula is still 1, leading to a pair formation cascade (Metzger et al. 2014; Metzger & Piro 2014).”
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. V. Rising X-Ray Emission from an Off-axis Jet (2017, background): “L 10 10X 43 44 erg s−1 to photoionize the ejecta, as is clearly not satisfied by the observed source L 10X 40 erg s−1 (Metzger & Piro 2014).”
• Kilonova light curves from the disc wind outflows of compact object mergers (2014, background): “In this case, the kilonova emission could be significantly enhanced by rotational energy injected by the magnetized remnant (Yu, Zhang & Gao 2013; Kisaka, Ioka & Takami 2015; Metzger & Piro 2014).”
• A living theory catalogue for fast radio bursts (2018, background): “The variable radio source associated with FRB 121102 is consistent with the giant flare theory, too: it may be emission directly from the MWN, the shock interaction between the flare and the MWN, or afterglow from an off-axis long gamma ray burst (LGRB) (such that only the afterglow is observed) (Me”
• The Binary Neutron Star Event LIGO/Virgo GW170817 160 Days after Merger: Synchrotron Emission across the Electromagnetic Spectrum (2018, background): “7, the same magnetar engines would produce luminous optical emission at early times (Metzger & Piro 2014) in excess to the observed bolometric luminosity from GW170817 and for this reason are ruled out.”