Fermi-LAT Upper Limits on Gamma-ray Bursts Daniel Kocevski Kavli Institute for Particle Astrophysics and Cosmology SLAC - Stanford University On behalf of the Fermi collaboration
Fermi GRB Detections 291 GBM detections 10 LAT detections Rate ~ 0.7 events/day LAT is detecting roughly 3% Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
LAT Upper Limits on GRBs What are the upper limits to the 0.1-300 GeV flux for GBM only bursts? Can we rule out high energy emission for these events? XRT How do these upper limits compare GBM to the expected flux? Could point to interesting physics LAT Count Map of hypothetical GRB Position Intrinsic spectral breaks? EBL or γ-γ absorption? Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
LAT Field of View (FOV) Detector Fields of View GBM: Full unocculted sky LAT: ~2.4 Sr LAT does not see all events Bursts in the LAT FOV For this analysis: Expected counts vs. boresight angle < 65° from the LAT Boresight Sensitivity decreased rapidly Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
Burst Demographics GBM Detections 25 291 events 20 Number of events GBM Events in LAT FOV: 15 154 events (52%) 10 LAT Detections 5 10 events was ~3.4% 0 0 20 40 60 80 100 120 140 LAT Zenith Angle 10 events now ~ 6.4% Angle of GRB to the LAT boresight at GBM trigger What can we say about remaining 52%? Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
Flux Upper Limits Method Flux Upper Limit Calculations Method based on Helene 1983 Spatially: 10 degree ROI Temporally: T90 Also searched -200 to 200 s Spectral Slope 1•10 � 4 Assumed beta = -2.2 for all events 8•10 � 5 Flux Limit 95% (Photons cm � 2 s � 1 ) 6•10 � 5 Flux limit weakly depends on beta 4•10 � 5 Obtain 95% CL flux (or flux limit) and the 2•10 � 5 significance of detection (TS value) 0 � 3.0 � 2.8 � 2.6 � 2.4 � 2.2 � 2.0 � 1.8 � Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
LAT Flux Upper Limits GBM events in LAT FOV: 20 152 events (52%) 15 Events with flux upper limits Number of events 117 events (45%) 10 Assume beta = -2.2 5 Median flux limit (0.1 to 300 GeV) 0 � 5.0 � 4.5 � 4.0 � 3.5 Log ~ 2.6x10 -5 photons cm -2 s -1 LAT Flux Limit (photons cm � 2 s � 1) Over T90 duration Implications for extra components? Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
Extra Components SSC & pion decay components No obvious evidence Flux (photons cm -2 s -1 ) Unless significantly delayed SSC? Synchrotron Implications of flux limits Y < 1, ε B > ε e γ >> 100, Epk SSC >> 10 GeV 100 GeV 1 0 0 K e V 10 GeV EBL attenuation? No evidence for EBL in any LAT event Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
GBM Extrapolation Bright BGO Sample NaI7 GBM events with > 70 BGO cts But no LAT detections 79 events 33 in LAT FOV BGO1 Performed spec fits with rmfit 15 events in “Gold” sample Median beta ~ -2.3 GRB 090620.400 Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
GRB 090620.400 LAT Flux Limit = 3.6x10 -5 Expected LAT Flux = 9.2x10 -8 Epk = 149.5 +/-3.69 Alpha = -0.21 +/- 0.04 Beta = -2.65 +/- 0.11 Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
GRB 081207.680 LAT Flux Limit = 3.0x10 -4 Expected LAT Flux = 1.0x10 -3 Epk = 433.4 +/- 18.7 Alpha = -0.66 +/- 0.03 Beta = -2.19 +/- 0.13 Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
Possible Explanations EBL attenuation Test: Should vary with redshift Flux (photons cm -2 s -1 ) γ - γ abortion due to compactness Synchrotron Lower end of the Γ distribution? SSC? Test: Correlate with Epk, Eiso? 100 GeV 1 0 0 K e V 10 GeV Finely tuned SSC peak Mimic a spectral cutoff Test: Detailed spectral fits Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
Conclusions Only ~52% of Fermi GRBs are in LAT field of view Flux limit ~ few x 10 -5 photons cm -2 s -1 Bright BGO Sample ~80% of “Gold” sample do not predict LAT counts ~20% do show discrepancies that hint at spectral curvature Explanations? Extra-galactic background light attenuation gamma-gamma absorption due to compactness at the source Finely tuned SSC peak located near the break Look for a paper on this topic soon! Daniel Kocevski - 2009 Fermi Symposium, Nov 2nd-5th 2009
Recommend
More recommend
