Pulsations in short gamma ray bursts from black hole-neutron star mergers

Nicholas Stone, Abraham Loeb, and Edo Berger

Phys. Rev. D 87, 084053 – Published 23 April 2013

Abstract

The precise of short gamma ray bursts (SGRBs) remains an important open question in relativistic astrophysics. Increasingly, observational evidence suggests the merger of a binary compact object system as the source for most SGRBs, but it is currently unclear how to distinguish observationally between a binary neutron star progenitor and a black hole-neutron star progenitor. We suggest the quasiperiodic signal of jet precession as an observational signature of SGRBs originating in mixed binary systems, and quantify both the fraction of mixed binaries capable of producing SGRBs and the distributions of precession amplitudes and periods. The difficulty inherent in disrupting a neutron star outside the horizon of a stellar mass black hole biases the jet precession signal towards low amplitude and high frequency. Precession periods of $\sim 0.01 - 0.1 s$ and disk-black hole spin misalignments $\sim 10 °$ are generally expected, although sufficiently high viscosity may prevent the accumulation of multiple precession periods during the SGRB. The precessing jet will naturally cover a larger solid angle in the sky than would standard SGRB jets, enhancing observability for both prompt emission and optical afterglows.

Received 19 September 2012

DOI:https://doi.org/10.1103/PhysRevD.87.084053

Authors & Affiliations

Nicholas Stone, Abraham Loeb, and Edo Berger^*

Astronomy Department, Harvard University, 60 Garden Street, Cambridge, Massachusetts 02138, USA

^*nstone@cfa.harvard.edu

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Vol. 87, Iss. 8 — 15 April 2013

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Images

Figure 1
(a) Time evolution of $T_{prec}$ assuming a viscously spreading disk structure given by Eq. (4). Black dotted curves represent $α = 0.1$ , dashed magenta curves $α = 0.03$ , and solid blue curves $α = 0.01$ . Thick curves are for nearly equatorial disruptions with $a_{BH} = 0.9$ , while thin curves are for $a_{BH} = 0.9$ and initial spin-orbit misalignment of 70° or equivalently a nearly aligned disruption with $a \approx 0.5$ . The dash-dotted red line is $\propto t^{4 / 3}$ , the rough time evolution of $T_{prec}$ . (b) and (c) show $N_{cycles}$ , the accumulated number of cycles for $0.1 s < t < 1 s$ and $0.01 s < t < 1 s$ , respectively.Reuse & Permissions
Figure 2
The probability distribution of postmerger misalignment angles $ψ_{d}$ for scenarios A–H (labeled as panels (a)–(h), respectively). The vertical axis is a probability and the horizontal axis is $ψ_{d}$ . The dark blue curves show the $ψ_{d}$ distribution for all $2 \times 10^{5}$ BH-NS mergers in each scenario, while the smaller, light orange curves show only the subset of events where the NS is disrupted outside the ISSO (i.e., the subset where a disk and jet can form). Generally, $f_{GRB}$ , the ratio of the light orange area to the dark blue area, falls for bottom-heavy spin distributions and smaller NS radii. All distributions of $ψ_{d}$ cut off fairly sharply above 50°.Reuse & Permissions
Figure 3
The probability distribution of postmerger misalignment angles $ψ_{d}$ for scenarios I and J (in panels (a) and (b), respectively), illustrating the dependence of $f_{GRB}$ and $ψ_{d}$ on the strictness of our requirement for $M_{dis} / M_{NS}$ . Specifically, in the top panel we have imposed the strict requirement that $M_{dis} / M_{NS} > 0.2$ in order to produce a SGRB, while in the lower panel we have imposed the much laxer requirement of $M_{dis} / M_{NS} > 0.01$ . In all other respects both of these cases are identical to scenario a. Results in the bottom panel are quite similar to scenario a, but in the top panel, $f_{GRB}$ has been strongly suppressed, particularly at higher $ψ_{d}$ . The axes and curves are the same as in Fig. 2.Reuse & Permissions
Figure 4
Probability distributions of $N_{cycles}$ for scenarios A–H. As in Fig. 1, the dotted black lines represent $α = 0.1$ , the dashed magenta lines $α = 0.03$ , and the solid blue lines $α = 0.01$ . There is relatively little variation between progenitor scenarios, with the notable exception of scenario e (slow spins). The total number of precession cycles accumulated depends strongly on $α$ .Reuse & Permissions
Figure 5
Each contour plot in this figure shows the range of “halfway” precession periods (vertical axis) and postmerger misalignment angles $ψ_{d}$ (horizontal axis) for every BH-NS merger that produces a disk. $T_{prec}$ is plotted in seconds, and in all scenarios is generally $\leq 0.4 s$ , although we note again that it will grow with time as the disk viscously spreads outward. As in previous plots, we show scenarios A–H. The logarithmic contour scale is included in panel (e); the range in colors covers a probability range of 2 orders of magnitude. In this plot we use the fiducial value $α = 0.03$ .Reuse & Permissions

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