TY - JOUR
T1 - Constraints on the bulk Lorentz factor of gamma-ray burst jets from Fermi/LAT upper limits
AU - Nava, L.
AU - Desiante, R.
AU - Longo, F.
AU - Celotti, A.
AU - Omodei, N.
AU - Vianello, G.
AU - Bissaldi, E.
AU - Piran, T.
N1 - Publisher Copyright:
© 2016 The Authors.
PY - 2017/2/11
Y1 - 2017/2/11
N2 - It is largely recognized that gamma-ray burst (GRB) jets involve ultrarelativistic motion. However, the value of the Lorentz factor Γ0 is still not clear and only lower limits are known for most bursts. We suggest here a new method to obtain upper limits on Γ0. The early highenergy synchrotron afterglow flux depends strongly on Γ0. Upper limits on GeV emission therefore provide upper limits on Γ0. Applying this method to 190 Fermi GRBs which have not been detected by the Fermi-LAT, we place upper limits on the high-energy afterglow flux, and in turn on Γ0. For bursts at a typical redshift z = 2, we find values of the order of 200 (and above) for a homogeneous density medium, and in the range 100-400 for a wind-like medium. These upper limits are consistent with (and are very close to) lower limits and direct estimates inferred using other methods, suggesting that the typical Lorentz factors of GRB jets are of the order of a few hundred.
AB - It is largely recognized that gamma-ray burst (GRB) jets involve ultrarelativistic motion. However, the value of the Lorentz factor Γ0 is still not clear and only lower limits are known for most bursts. We suggest here a new method to obtain upper limits on Γ0. The early highenergy synchrotron afterglow flux depends strongly on Γ0. Upper limits on GeV emission therefore provide upper limits on Γ0. Applying this method to 190 Fermi GRBs which have not been detected by the Fermi-LAT, we place upper limits on the high-energy afterglow flux, and in turn on Γ0. For bursts at a typical redshift z = 2, we find values of the order of 200 (and above) for a homogeneous density medium, and in the range 100-400 for a wind-like medium. These upper limits are consistent with (and are very close to) lower limits and direct estimates inferred using other methods, suggesting that the typical Lorentz factors of GRB jets are of the order of a few hundred.
KW - Gamma-rays: general
KW - Radiation mechanisms: non-thermal
UR - http://www.scopus.com/inward/record.url?scp=85014785197&partnerID=8YFLogxK
U2 - 10.1093/mnras/stw2771
DO - 10.1093/mnras/stw2771
M3 - ???researchoutput.researchoutputtypes.contributiontojournal.article???
AN - SCOPUS:85014785197
SN - 0035-8711
VL - 465
SP - 811
EP - 819
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 1
ER -