From astrobites: “Neutron Star Mergers May Help Short GRBs Go ‘Boom’ “
Title: Binary neutron star mergers: a jet engine for short gamma-ray bursts
Authors: Milton Ruiz, Ryan N. Lang, Vasileios Paschalidis, & Stuart L. Shapiro
First Author’s Institution: University of Illinois at Urbana-Champaign; Universidad Industrial de Santander, Colombia
Status: Published in ApJ Letters, Volume 824, Number 1 (2016 June 3) [open access]
The LIGO Scientific Collaboration‘s direct detection of gravitational waves (GWs) is the first whisper from an era of so-called “multimessenger astronomy,” of which astronomers have thus far only been able to dream.
The ability to probe an astrophysical event in both the electromagnetic spectrum and in gravitational waves would allow for groundbreaking scientific activities, such as unique tests of general relativity (GR) and understanding the interior properties of neutron stars (NSs). Among the transient events able to produce a strong signal simultaneously in GWs and the EM spectrum are black hole-black hole (BH-BH) mergers, NS-BH mergers, and NS-NS mergers. Intriguingly, NS-BH and NS-NS mergers are strongly favored progenitors of gamma-ray bursts (GRBs), making them ideal multimessenger candidates.
Do Neutron Star Mergers Actually Produce GRBs?
GRBs are the brightest events in our universe.
They’re known to fall into two (fairly) distinct categories: short-duration, “hard” GRBs and long-duration, “soft” GRBs. Short GRBs last ∼0.1 seconds and are spectrally hard, meaning they have a high ratio of high-frequency to low-frequency emission. These make up about 20-30% of the total GRB population. Long GRBs last ∼10-30 seconds and are associated with type Ic supernovae. The short variety may be powered by the coalescence of compact binaries (like double NSs), but this is still an active area of research.
Previous studies have simulated the merging of a BH-NS system, and found that they are able to power short GRB jets when the dipole magnetic field (think: bar magnet) of the neutron star extends all the way from its interior to exterior, as in a pulsar. However, the BH-NS merger did not produce jets when the dipole field was confined to the interior of the neutron star. Similar simulations of NS-NS mergers produced conflicting, unclear results. The authors therefore made a new attempt at trying to simulate a NS-NS merger in order to determine if they are also capable of producing short GRB jets, and if the magnetic field arrangement mattered (like in the BH-NS case). In other words, is the BH necessary for a short GRB to occur?
Figure 1: This series of images from the Ruiz et al. publication shows the stages of NS merging. Colors indicate rest mass density (for example, the NSs are shown in purple in the first panel). The process starts from two separate, orbiting NSs with dipole magnetic fields (white lines). Their orbit shrinks as energy is lost to gravitational radiation, and tidal forces cause them to become oblate as they draw nearer. Eventually a high-mass NS is formed, which collapses into a black hole. Helical jets with twisted magnetic fields are finally formed with energies comparable to observed short GRBs. Figure 1 in the paper.
For their simulations, it was necessary to include a combination of general relativistic and magnetohydrodynamic (relating to the electrical conductivity of fluids) parameters. The NSs were given astrophysically plausible but intentionally strong magnetic fields of ∼1015 G. The simulation of two nonrotating NSs is initialized at a separation distance prior to merging (see Figure 1). Both the “pulsar” case, in which the magnetic field extends outside the NS, and the interior-only case were studied. The authors found that both of these scenarios prompted the formation of short GRB jets, unlike the BH-NS case. The collimated jets that form are consistent with an effect called the “Blandford-Znajek Process” in which jets are formed from the twisting of magnetic field lines above and below a BH (see bottom-middle panel of Figure 1). In addition to the simulations producing mildly relativistic jets, the observed accretion timescales and energy outputs match expected values for short GRBs. In short, NS-NS mergers can probably power short GRBs, no matter if their magnetic fields are pulsar-like or not.
Is the Era of Multimessenger Astronomy Here?
Studies such as this are a boon to the new era of GW astronomy. Though electromagnetic followup and localization of GW sources was difficult for the first LIGO detections, we should feel encouraged that such a powerful phenomenon — that of the GRB — might coincide with a detectable GW event. These capabilities will almost definitely yield answers to questions about fundamental physics that can’t be found in the lab.
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