Evidence for jet launching close to the black hole in GRB 101219B - a Fermi GRB dominated by thermal emission

TL;DR

This paper reports the detection of thermal blackbody emission in the prompt phase of GRB 101219B by Fermi GBM, providing insights into jet launching close to the black hole and the properties of the outflow.

Contribution

First observation of a blackbody spectrum in Fermi GBM data for a GRB, enabling calculation of jet launch radius and outflow parameters near the black hole.

Findings

Blackbody spectrum detected in prompt gamma-ray emission

Estimated Lorentz factor of the jet is 138±8

Jet launch radius close to the black hole, around 2.7×10^7 cm

Abstract

We present observations by the Fermi Gamma-Ray Space Telescope Gamma-Ray Burst Monitor (GBM) of the nearby (z=0.55) GRB 101219B. This burst is a long GRB, with an associated supernova and with a blackbody component detected in the early afterglow observed by the Swift X-ray Telescope (XRT). Here we show that the prompt gamma-ray emission has a blackbody spectrum, making this the second such burst observed by Fermi GBM. The properties of the blackbody, together with the redshift and our estimate of the radiative efficiency, makes it possible to calculate the absolute values of the properties of the outflow. We obtain an initial Lorentz factor Gamma=138\pm 8, a photospheric radius r_phot=4.4\pm 1.9 \times 10^{11} cm and a launch radius r_0=2.7\pm 1.6 \times 10^{7} cm. The latter value is close to the black hole and suggests that the jet has a relatively unobstructed path through the star. There is no smooth connection between the blackbody components seen by GBM and XRT, ruling out the scenario that the late emission is due to high-latitude effects. In the interpretation that the XRT blackbody is prompt emission due to late central engine activity, the jet either has to be very wide or have a clumpy structure where the emission originates from a small patch. Other explanations for this component, such as emission from a cocoon surrounding the jet, are also possible.