2018MNRAS.481.2581L -
Mon. Not. R. Astron. Soc., 481, 2581-2589 (2018/December-1)
Late-time evolution of afterglows from off-axis neutron star mergers.
LAMB G.P., MANDEL I. and RESMI L.
Abstract (from CDS):
Gravitational-wave-detected neutron star mergers provide an opportunity to investigate short gamma-ray burst (GRB) jet afterglows without the GRB trigger. Here we show that the post-peak afterglow decline can distinguish between an initially ultrarelativistic jet viewed off-axis and a mildly relativistic wide-angle outflow. Post-peak the afterglow flux will decline as Fν ∝ t–α. The steepest decline for a jet afterglow is α > 3p/4 or > (3p + 1)/4, for an observation frequency below and above the cooling frequency, respectively, where p is the power-law index of the electron energy distribution. The steepest decline for a mildly relativistic outflow, with initial Lorentz factor Γ0 <= 2, is α <= (15p - 19)/10 or α <= (15p - 18)/10, in the respective spectral regimes. If the afterglow from GW170817 fades with a maximum index α > 1.5, then we are observing the core of an initially ultrarelativistic jet viewed off the central axis, while a decline with α <= 1.4 after ∼5-10 peak times indicates that a wide-angled and initially Γ0 <= 2 outflow is responsible. At twice the peak time, the two outflow models fall on opposite sides of α ≃ 1. So far, two post-peak X-ray data points at 160 and 260 d suggest a decline consistent with an off-axis jet afterglow. Follow-up observations over the next 1-2 yr will test this model.
Abstract Copyright:
© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
Journal keyword(s):
gravitational waves - gamma-ray burst: general
Simbad objects:
1
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