Constraining the high-energy emission from gamma-ray bursts with Fermi

M. Ackermann, M. Ajello, L. Baldini, G. Barbiellini, M. G. Baring, K. Bechtol, R. Bellazzini, R. D. Blandford, E. D. Bloom, E. Bonamente, A. W. Borgland, E. Bottacini, A. Bouvier, M. Brigida, R. Buehler, S. Buson, G. A. Caliandro, R. A. Cameron, C. Cecchi, E. Charles & 119 others A. Chekhtman, J. Chiang, S. Ciprini, R. Claus, J. Cohen-Tanugi, S. Cutini, F. D'Ammando, F. De Palma, C. D. Dermer, E. Do Couto E Silva, P. S. Drell, A. Drlica-Wagner, C. Favuzzi, Y. Fukazawa, P. Fusco, F. Gargano, D. Gasparrini, N. Gehrels, S. Germani, N. Giglietto, F. Giordano, M. Giroletti, T. Glanzman, J. Granot, I. A. Grenier, J. E. Grove, D. Hadasch, Y. Hanabata, A. K. Harding, E. Hays, D. Horan, G. Jóhannesson, Jun Kataoka, J. Knödlseder, D. Kocevski, M. Kuss, J. Lande, F. Longo, F. Loparco, M. N. Lovellette, P. Lubrano, M. N. Mazziotta, J. McEnery, S. McGlynn, P. F. Michelson, W. Mitthumsiri, M. E. Monzani, E. Moretti, A. Morselli, I. V. Moskalenko, S. Murgia, M. Naumann-Godo, J. P. Norris, E. Nuss, T. Nymark, T. Ohsugi, A. Okumura, N. Omodei, E. Orlando, J. H. Panetta, D. Parent, V. Pelassa, M. Pesce-Rollins, F. Piron, G. Pivato, J. L. Racusin, S. Rainò, R. Rando, S. Razzaque, A. Reimer, O. Reimer, S. Ritz, F. Ryde, C. Sgrò, E. J. Siskind, E. Sonbas, G. Spandre, P. Spinelli, M. Stamatikos, Łukasz Stawarz, D. J. Suson, H. Takahashi, T. Tanaka, J. G. Thayer, J. B. Thayer, L. Tibaldo, M. Tinivella, G. Tosti, T. Uehara, J. Vandenbroucke, V. Vasileiou, G. Vianello, V. Vitale, A. P. Waite, V. Connaughton, M. S. Briggs, S. Guirec, A. Goldstein, J. M. Burgess, P. N. Bhat, E. Bissaldi, A. Camero-Arranz, J. Fishman, G. Fitzpatrick, S. Foley, D. Gruber, P. Jenke, R. M. Kippen, C. Kouveliotou, S. McBreen, C. Meegan, W. S. Paciesas, R. Preece, A. Rau, D. Tierney, A. J. Van Der Horst, A. Von Kienlin, C. Wilson-Hodge, S. Xiong

    Research output: Contribution to journal › Article

    60 Citations (Scopus)

    Abstract

    We examine 288 gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Space Telescope's Gamma-ray Burst Monitor (GBM) that fell within the field of view of Fermi's Large Area Telescope (LAT) during the first 2.5years of observations, which showed no evidence for emission above 100MeV. We report the photon flux upper limits in the 0.1-10GeV range during the prompt emission phase as well as for fixed 30s and 100s integrations starting from the trigger time for each burst. We compare these limits with the fluxes that would be expected from extrapolations of spectral fits presented in the first GBM spectral catalog and infer that roughly half of the GBM-detected bursts either require spectral breaks between the GBM and LAT energy bands or have intrinsically steeper spectra above the peak of the νF ν spectra (E pk). In order to distinguish between these two scenarios, we perform joint GBM and LAT spectral fits to the 30 brightest GBM-detected bursts and find that a majority of these bursts are indeed softer above E pk than would be inferred from fitting the GBM data alone. Approximately 20% of this spectroscopic subsample show statistically significant evidence for a cutoff in their high-energy spectra, which if assumed to be due to γγ attenuation, places limits on the maximum Lorentz factor associated with the relativistic outflow producing this emission. All of these latter bursts have maximum Lorentz factor estimates that are well below the minimum Lorentz factors calculated for LAT-detected GRBs, revealing a wide distribution in the bulk Lorentz factor of GRB outflows and indicating that LAT-detected bursts may represent the high end of this distribution.

    Original languageEnglish
    Article number121
    JournalAstrophysical Journal
    Volume754
    Issue number2
    DOIs
    Publication statusPublished - 2012 Aug 1

    Fingerprint

    gamma ray bursts
    monitors
    outflow
    bursts
    energy
    telescopes
    field of view
    Fermi Gamma-ray Space Telescope
    distribution
    catalogs
    energy bands
    extrapolation
    energy spectra
    cut-off
    actuators
    attenuation
    photons
    estimates

    Keywords

    • gamma rays: general
    • gamma-ray burst: general

    ASJC Scopus subject areas

    • Space and Planetary Science
    • Astronomy and Astrophysics

    Cite this

    Ackermann, M., Ajello, M., Baldini, L., Barbiellini, G., Baring, M. G., Bechtol, K., ... Xiong, S. (2012). Constraining the high-energy emission from gamma-ray bursts with Fermi. Astrophysical Journal, 754(2), [121]. https://doi.org/10.1088/0004-637X/754/2/121

    Constraining the high-energy emission from gamma-ray bursts with Fermi. / Ackermann, M.; Ajello, M.; Baldini, L.; Barbiellini, G.; Baring, M. G.; Bechtol, K.; Bellazzini, R.; Blandford, R. D.; Bloom, E. D.; Bonamente, E.; Borgland, A. W.; Bottacini, E.; Bouvier, A.; Brigida, M.; Buehler, R.; Buson, S.; Caliandro, G. A.; Cameron, R. A.; Cecchi, C.; Charles, E.; Chekhtman, A.; Chiang, J.; Ciprini, S.; Claus, R.; Cohen-Tanugi, J.; Cutini, S.; D'Ammando, F.; De Palma, F.; Dermer, C. D.; Do Couto E Silva, E.; Drell, P. S.; Drlica-Wagner, A.; Favuzzi, C.; Fukazawa, Y.; Fusco, P.; Gargano, F.; Gasparrini, D.; Gehrels, N.; Germani, S.; Giglietto, N.; Giordano, F.; Giroletti, M.; Glanzman, T.; Granot, J.; Grenier, I. A.; Grove, J. E.; Hadasch, D.; Hanabata, Y.; Harding, A. K.; Hays, E.; Horan, D.; Jóhannesson, G.; Kataoka, Jun; Knödlseder, J.; Kocevski, D.; Kuss, M.; Lande, J.; Longo, F.; Loparco, F.; Lovellette, M. N.; Lubrano, P.; Mazziotta, M. N.; McEnery, J.; McGlynn, S.; Michelson, P. F.; Mitthumsiri, W.; Monzani, M. E.; Moretti, E.; Morselli, A.; Moskalenko, I. V.; Murgia, S.; Naumann-Godo, M.; Norris, J. P.; Nuss, E.; Nymark, T.; Ohsugi, T.; Okumura, A.; Omodei, N.; Orlando, E.; Panetta, J. H.; Parent, D.; Pelassa, V.; Pesce-Rollins, M.; Piron, F.; Pivato, G.; Racusin, J. L.; Rainò, S.; Rando, R.; Razzaque, S.; Reimer, A.; Reimer, O.; Ritz, S.; Ryde, F.; Sgrò, C.; Siskind, E. J.; Sonbas, E.; Spandre, G.; Spinelli, P.; Stamatikos, M.; Stawarz, Łukasz; Suson, D. J.; Takahashi, H.; Tanaka, T.; Thayer, J. G.; Thayer, J. B.; Tibaldo, L.; Tinivella, M.; Tosti, G.; Uehara, T.; Vandenbroucke, J.; Vasileiou, V.; Vianello, G.; Vitale, V.; Waite, A. P.; Connaughton, V.; Briggs, M. S.; Guirec, S.; Goldstein, A.; Burgess, J. M.; Bhat, P. N.; Bissaldi, E.; Camero-Arranz, A.; Fishman, J.; Fitzpatrick, G.; Foley, S.; Gruber, D.; Jenke, P.; Kippen, R. M.; Kouveliotou, C.; McBreen, S.; Meegan, C.; Paciesas, W. S.; Preece, R.; Rau, A.; Tierney, D.; Van Der Horst, A. J.; Von Kienlin, A.; Wilson-Hodge, C.; Xiong, S.

    In: Astrophysical Journal, Vol. 754, No. 2, 121, 01.08.2012.

    Research output: Contribution to journal › Article

    Ackermann, M, Ajello, M, Baldini, L, Barbiellini, G, Baring, MG, Bechtol, K, Bellazzini, R, Blandford, RD, Bloom, ED, Bonamente, E, Borgland, AW, Bottacini, E, Bouvier, A, Brigida, M, Buehler, R, Buson, S, Caliandro, GA, Cameron, RA, Cecchi, C, Charles, E, Chekhtman, A, Chiang, J, Ciprini, S, Claus, R, Cohen-Tanugi, J, Cutini, S, D'Ammando, F, De Palma, F, Dermer, CD, Do Couto E Silva, E, Drell, PS, Drlica-Wagner, A, Favuzzi, C, Fukazawa, Y, Fusco, P, Gargano, F, Gasparrini, D, Gehrels, N, Germani, S, Giglietto, N, Giordano, F, Giroletti, M, Glanzman, T, Granot, J, Grenier, IA, Grove, JE, Hadasch, D, Hanabata, Y, Harding, AK, Hays, E, Horan, D, Jóhannesson, G, Kataoka, J, Knödlseder, J, Kocevski, D, Kuss, M, Lande, J, Longo, F, Loparco, F, Lovellette, MN, Lubrano, P, Mazziotta, MN, McEnery, J, McGlynn, S, Michelson, PF, Mitthumsiri, W, Monzani, ME, Moretti, E, Morselli, A, Moskalenko, IV, Murgia, S, Naumann-Godo, M, Norris, JP, Nuss, E, Nymark, T, Ohsugi, T, Okumura, A, Omodei, N, Orlando, E, Panetta, JH, Parent, D, Pelassa, V, Pesce-Rollins, M, Piron, F, Pivato, G, Racusin, JL, Rainò, S, Rando, R, Razzaque, S, Reimer, A, Reimer, O, Ritz, S, Ryde, F, Sgrò, C, Siskind, EJ, Sonbas, E, Spandre, G, Spinelli, P, Stamatikos, M, Stawarz, Ł, Suson, DJ, Takahashi, H, Tanaka, T, Thayer, JG, Thayer, JB, Tibaldo, L, Tinivella, M, Tosti, G, Uehara, T, Vandenbroucke, J, Vasileiou, V, Vianello, G, Vitale, V, Waite, AP, Connaughton, V, Briggs, MS, Guirec, S, Goldstein, A, Burgess, JM, Bhat, PN, Bissaldi, E, Camero-Arranz, A, Fishman, J, Fitzpatrick, G, Foley, S, Gruber, D, Jenke, P, Kippen, RM, Kouveliotou, C, McBreen, S, Meegan, C, Paciesas, WS, Preece, R, Rau, A, Tierney, D, Van Der Horst, AJ, Von Kienlin, A, Wilson-Hodge, C & Xiong, S 2012, 'Constraining the high-energy emission from gamma-ray bursts with Fermi', Astrophysical Journal, vol. 754, no. 2, 121. https://doi.org/10.1088/0004-637X/754/2/121
    Ackermann M, Ajello M, Baldini L, Barbiellini G, Baring MG, Bechtol K et al. Constraining the high-energy emission from gamma-ray bursts with Fermi. Astrophysical Journal. 2012 Aug 1;754(2). 121. https://doi.org/10.1088/0004-637X/754/2/121
    Ackermann, M. ; Ajello, M. ; Baldini, L. ; Barbiellini, G. ; Baring, M. G. ; Bechtol, K. ; Bellazzini, R. ; Blandford, R. D. ; Bloom, E. D. ; Bonamente, E. ; Borgland, A. W. ; Bottacini, E. ; Bouvier, A. ; Brigida, M. ; Buehler, R. ; Buson, S. ; Caliandro, G. A. ; Cameron, R. A. ; Cecchi, C. ; Charles, E. ; Chekhtman, A. ; Chiang, J. ; Ciprini, S. ; Claus, R. ; Cohen-Tanugi, J. ; Cutini, S. ; D'Ammando, F. ; De Palma, F. ; Dermer, C. D. ; Do Couto E Silva, E. ; Drell, P. S. ; Drlica-Wagner, A. ; Favuzzi, C. ; Fukazawa, Y. ; Fusco, P. ; Gargano, F. ; Gasparrini, D. ; Gehrels, N. ; Germani, S. ; Giglietto, N. ; Giordano, F. ; Giroletti, M. ; Glanzman, T. ; Granot, J. ; Grenier, I. A. ; Grove, J. E. ; Hadasch, D. ; Hanabata, Y. ; Harding, A. K. ; Hays, E. ; Horan, D. ; Jóhannesson, G. ; Kataoka, Jun ; Knödlseder, J. ; Kocevski, D. ; Kuss, M. ; Lande, J. ; Longo, F. ; Loparco, F. ; Lovellette, M. N. ; Lubrano, P. ; Mazziotta, M. N. ; McEnery, J. ; McGlynn, S. ; Michelson, P. F. ; Mitthumsiri, W. ; Monzani, M. E. ; Moretti, E. ; Morselli, A. ; Moskalenko, I. V. ; Murgia, S. ; Naumann-Godo, M. ; Norris, J. P. ; Nuss, E. ; Nymark, T. ; Ohsugi, T. ; Okumura, A. ; Omodei, N. ; Orlando, E. ; Panetta, J. H. ; Parent, D. ; Pelassa, V. ; Pesce-Rollins, M. ; Piron, F. ; Pivato, G. ; Racusin, J. L. ; Rainò, S. ; Rando, R. ; Razzaque, S. ; Reimer, A. ; Reimer, O. ; Ritz, S. ; Ryde, F. ; Sgrò, C. ; Siskind, E. J. ; Sonbas, E. ; Spandre, G. ; Spinelli, P. ; Stamatikos, M. ; Stawarz, Łukasz ; Suson, D. J. ; Takahashi, H. ; Tanaka, T. ; Thayer, J. G. ; Thayer, J. B. ; Tibaldo, L. ; Tinivella, M. ; Tosti, G. ; Uehara, T. ; Vandenbroucke, J. ; Vasileiou, V. ; Vianello, G. ; Vitale, V. ; Waite, A. P. ; Connaughton, V. ; Briggs, M. S. ; Guirec, S. ; Goldstein, A. ; Burgess, J. M. ; Bhat, P. N. ; Bissaldi, E. ; Camero-Arranz, A. ; Fishman, J. ; Fitzpatrick, G. ; Foley, S. ; Gruber, D. ; Jenke, P. ; Kippen, R. M. ; Kouveliotou, C. ; McBreen, S. ; Meegan, C. ; Paciesas, W. S. ; Preece, R. ; Rau, A. ; Tierney, D. ; Van Der Horst, A. J. ; Von Kienlin, A. ; Wilson-Hodge, C. ; Xiong, S. / Constraining the high-energy emission from gamma-ray bursts with Fermi. In: Astrophysical Journal. 2012 ; Vol. 754, No. 2.
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    title = "Constraining the high-energy emission from gamma-ray bursts with Fermi",
    abstract = "We examine 288 gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Space Telescope's Gamma-ray Burst Monitor (GBM) that fell within the field of view of Fermi's Large Area Telescope (LAT) during the first 2.5years of observations, which showed no evidence for emission above 100MeV. We report the photon flux upper limits in the 0.1-10GeV range during the prompt emission phase as well as for fixed 30s and 100s integrations starting from the trigger time for each burst. We compare these limits with the fluxes that would be expected from extrapolations of spectral fits presented in the first GBM spectral catalog and infer that roughly half of the GBM-detected bursts either require spectral breaks between the GBM and LAT energy bands or have intrinsically steeper spectra above the peak of the νF ν spectra (E pk). In order to distinguish between these two scenarios, we perform joint GBM and LAT spectral fits to the 30 brightest GBM-detected bursts and find that a majority of these bursts are indeed softer above E pk than would be inferred from fitting the GBM data alone. Approximately 20{\%} of this spectroscopic subsample show statistically significant evidence for a cutoff in their high-energy spectra, which if assumed to be due to γγ attenuation, places limits on the maximum Lorentz factor associated with the relativistic outflow producing this emission. All of these latter bursts have maximum Lorentz factor estimates that are well below the minimum Lorentz factors calculated for LAT-detected GRBs, revealing a wide distribution in the bulk Lorentz factor of GRB outflows and indicating that LAT-detected bursts may represent the high end of this distribution.",
    keywords = "gamma rays: general, gamma-ray burst: general",
    author = "M. Ackermann and M. Ajello and L. Baldini and G. Barbiellini and Baring, {M. G.} and K. Bechtol and R. Bellazzini and Blandford, {R. D.} and Bloom, {E. D.} and E. Bonamente and Borgland, {A. W.} and E. Bottacini and A. Bouvier and M. Brigida and R. Buehler and S. Buson and Caliandro, {G. A.} and Cameron, {R. A.} and C. Cecchi and E. Charles and A. Chekhtman and J. Chiang and S. Ciprini and R. Claus and J. Cohen-Tanugi and S. Cutini and F. D'Ammando and {De Palma}, F. and Dermer, {C. D.} and {Do Couto E Silva}, E. and Drell, {P. S.} and A. Drlica-Wagner and C. Favuzzi and Y. Fukazawa and P. Fusco and F. Gargano and D. Gasparrini and N. Gehrels and S. Germani and N. Giglietto and F. Giordano and M. Giroletti and T. Glanzman and J. Granot and Grenier, {I. A.} and Grove, {J. E.} and D. Hadasch and Y. Hanabata and Harding, {A. K.} and E. Hays and D. Horan and G. J{\'o}hannesson and Jun Kataoka and J. Kn{\"o}dlseder and D. Kocevski and M. Kuss and J. Lande and F. Longo and F. Loparco and Lovellette, {M. N.} and P. Lubrano and Mazziotta, {M. N.} and J. McEnery and S. McGlynn and Michelson, {P. F.} and W. Mitthumsiri and Monzani, {M. E.} and E. Moretti and A. Morselli and Moskalenko, {I. V.} and S. Murgia and M. Naumann-Godo and Norris, {J. P.} and E. Nuss and T. Nymark and T. Ohsugi and A. Okumura and N. Omodei and E. Orlando and Panetta, {J. H.} and D. Parent and V. Pelassa and M. Pesce-Rollins and F. Piron and G. Pivato and Racusin, {J. L.} and S. Rain{\`o} and R. Rando and S. Razzaque and A. Reimer and O. Reimer and S. Ritz and F. Ryde and C. Sgr{\`o} and Siskind, {E. J.} and E. Sonbas and G. Spandre and P. Spinelli and M. Stamatikos and Łukasz Stawarz and Suson, {D. J.} and H. Takahashi and T. Tanaka and Thayer, {J. G.} and Thayer, {J. B.} and L. Tibaldo and M. Tinivella and G. Tosti and T. Uehara and J. Vandenbroucke and V. Vasileiou and G. Vianello and V. Vitale and Waite, {A. P.} and V. Connaughton and Briggs, {M. S.} and S. Guirec and A. Goldstein and Burgess, {J. M.} and Bhat, {P. N.} and E. Bissaldi and A. Camero-Arranz and J. Fishman and G. Fitzpatrick and S. Foley and D. Gruber and P. Jenke and Kippen, {R. M.} and C. Kouveliotou and S. McBreen and C. Meegan and Paciesas, {W. S.} and R. Preece and A. Rau and D. Tierney and {Van Der Horst}, {A. J.} and {Von Kienlin}, A. and C. Wilson-Hodge and S. Xiong",
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    TY - JOUR

    T1 - Constraining the high-energy emission from gamma-ray bursts with Fermi

    AU - Ackermann, M.

    AU - Ajello, M.

    AU - Baldini, L.

    AU - Barbiellini, G.

    AU - Baring, M. G.

    AU - Bechtol, K.

    AU - Bellazzini, R.

    AU - Blandford, R. D.

    AU - Bloom, E. D.

    AU - Bonamente, E.

    AU - Borgland, A. W.

    AU - Bottacini, E.

    AU - Bouvier, A.

    AU - Brigida, M.

    AU - Buehler, R.

    AU - Buson, S.

    AU - Caliandro, G. A.

    AU - Cameron, R. A.

    AU - Cecchi, C.

    AU - Charles, E.

    AU - Chekhtman, A.

    AU - Chiang, J.

    AU - Ciprini, S.

    AU - Claus, R.

    AU - Cohen-Tanugi, J.

    AU - Cutini, S.

    AU - D'Ammando, F.

    AU - De Palma, F.

    AU - Dermer, C. D.

    AU - Do Couto E Silva, E.

    AU - Drell, P. S.

    AU - Drlica-Wagner, A.

    AU - Favuzzi, C.

    AU - Fukazawa, Y.

    AU - Fusco, P.

    AU - Gargano, F.

    AU - Gasparrini, D.

    AU - Gehrels, N.

    AU - Germani, S.

    AU - Giglietto, N.

    AU - Giordano, F.

    AU - Giroletti, M.

    AU - Glanzman, T.

    AU - Granot, J.

    AU - Grenier, I. A.

    AU - Grove, J. E.

    AU - Hadasch, D.

    AU - Hanabata, Y.

    AU - Harding, A. K.

    AU - Hays, E.

    AU - Horan, D.

    AU - Jóhannesson, G.

    AU - Kataoka, Jun

    AU - Knödlseder, J.

    AU - Kocevski, D.

    AU - Kuss, M.

    AU - Lande, J.

    AU - Longo, F.

    AU - Loparco, F.

    AU - Lovellette, M. N.

    AU - Lubrano, P.

    AU - Mazziotta, M. N.

    AU - McEnery, J.

    AU - McGlynn, S.

    AU - Michelson, P. F.

    AU - Mitthumsiri, W.

    AU - Monzani, M. E.

    AU - Moretti, E.

    AU - Morselli, A.

    AU - Moskalenko, I. V.

    AU - Murgia, S.

    AU - Naumann-Godo, M.

    AU - Norris, J. P.

    AU - Nuss, E.

    AU - Nymark, T.

    AU - Ohsugi, T.

    AU - Okumura, A.

    AU - Omodei, N.

    AU - Orlando, E.

    AU - Panetta, J. H.

    AU - Parent, D.

    AU - Pelassa, V.

    AU - Pesce-Rollins, M.

    AU - Piron, F.

    AU - Pivato, G.

    AU - Racusin, J. L.

    AU - Rainò, S.

    AU - Rando, R.

    AU - Razzaque, S.

    AU - Reimer, A.

    AU - Reimer, O.

    AU - Ritz, S.

    AU - Ryde, F.

    AU - Sgrò, C.

    AU - Siskind, E. J.

    AU - Sonbas, E.

    AU - Spandre, G.

    AU - Spinelli, P.

    AU - Stamatikos, M.

    AU - Stawarz, Łukasz

    AU - Suson, D. J.

    AU - Takahashi, H.

    AU - Tanaka, T.

    AU - Thayer, J. G.

    AU - Thayer, J. B.

    AU - Tibaldo, L.

    AU - Tinivella, M.

    AU - Tosti, G.

    AU - Uehara, T.

    AU - Vandenbroucke, J.

    AU - Vasileiou, V.

    AU - Vianello, G.

    AU - Vitale, V.

    AU - Waite, A. P.

    AU - Connaughton, V.

    AU - Briggs, M. S.

    AU - Guirec, S.

    AU - Goldstein, A.

    AU - Burgess, J. M.

    AU - Bhat, P. N.

    AU - Bissaldi, E.

    AU - Camero-Arranz, A.

    AU - Fishman, J.

    AU - Fitzpatrick, G.

    AU - Foley, S.

    AU - Gruber, D.

    AU - Jenke, P.

    AU - Kippen, R. M.

    AU - Kouveliotou, C.

    AU - McBreen, S.

    AU - Meegan, C.

    AU - Paciesas, W. S.

    AU - Preece, R.

    AU - Rau, A.

    AU - Tierney, D.

    AU - Van Der Horst, A. J.

    AU - Von Kienlin, A.

    AU - Wilson-Hodge, C.

    AU - Xiong, S.

    PY - 2012/8/1

    Y1 - 2012/8/1

    N2 - We examine 288 gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Space Telescope's Gamma-ray Burst Monitor (GBM) that fell within the field of view of Fermi's Large Area Telescope (LAT) during the first 2.5years of observations, which showed no evidence for emission above 100MeV. We report the photon flux upper limits in the 0.1-10GeV range during the prompt emission phase as well as for fixed 30s and 100s integrations starting from the trigger time for each burst. We compare these limits with the fluxes that would be expected from extrapolations of spectral fits presented in the first GBM spectral catalog and infer that roughly half of the GBM-detected bursts either require spectral breaks between the GBM and LAT energy bands or have intrinsically steeper spectra above the peak of the νF ν spectra (E pk). In order to distinguish between these two scenarios, we perform joint GBM and LAT spectral fits to the 30 brightest GBM-detected bursts and find that a majority of these bursts are indeed softer above E pk than would be inferred from fitting the GBM data alone. Approximately 20% of this spectroscopic subsample show statistically significant evidence for a cutoff in their high-energy spectra, which if assumed to be due to γγ attenuation, places limits on the maximum Lorentz factor associated with the relativistic outflow producing this emission. All of these latter bursts have maximum Lorentz factor estimates that are well below the minimum Lorentz factors calculated for LAT-detected GRBs, revealing a wide distribution in the bulk Lorentz factor of GRB outflows and indicating that LAT-detected bursts may represent the high end of this distribution.

    AB - We examine 288 gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Space Telescope's Gamma-ray Burst Monitor (GBM) that fell within the field of view of Fermi's Large Area Telescope (LAT) during the first 2.5years of observations, which showed no evidence for emission above 100MeV. We report the photon flux upper limits in the 0.1-10GeV range during the prompt emission phase as well as for fixed 30s and 100s integrations starting from the trigger time for each burst. We compare these limits with the fluxes that would be expected from extrapolations of spectral fits presented in the first GBM spectral catalog and infer that roughly half of the GBM-detected bursts either require spectral breaks between the GBM and LAT energy bands or have intrinsically steeper spectra above the peak of the νF ν spectra (E pk). In order to distinguish between these two scenarios, we perform joint GBM and LAT spectral fits to the 30 brightest GBM-detected bursts and find that a majority of these bursts are indeed softer above E pk than would be inferred from fitting the GBM data alone. Approximately 20% of this spectroscopic subsample show statistically significant evidence for a cutoff in their high-energy spectra, which if assumed to be due to γγ attenuation, places limits on the maximum Lorentz factor associated with the relativistic outflow producing this emission. All of these latter bursts have maximum Lorentz factor estimates that are well below the minimum Lorentz factors calculated for LAT-detected GRBs, revealing a wide distribution in the bulk Lorentz factor of GRB outflows and indicating that LAT-detected bursts may represent the high end of this distribution.

    KW - gamma rays: general

    KW - gamma-ray burst: general

    UR - http://www.scopus.com/inward/record.url?scp=84864225950&partnerID=8YFLogxK

    UR - http://www.scopus.com/inward/citedby.url?scp=84864225950&partnerID=8YFLogxK

    U2 - 10.1088/0004-637X/754/2/121

    DO - 10.1088/0004-637X/754/2/121

    M3 - Article

    VL - 754

    JO - Astrophysical Journal

    JF - Astrophysical Journal

    SN - 0004-637X

    IS - 2

    M1 - 121

    ER -

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