Exploring Fermi-LAT Extended GeV Emission with Stacking Analysis

Exploring Fermi-LAT Extended GeV Emission with Stacking Analysis Judy Racusin (NASA/GSFC) Jim Chiang (Stanford) on behalf of the Fermi-LAT Collaboration

Motivation LAT extended emission has been seen in several bright LAT bursts (080916C, 090510, 090902B, 090926A) Physical origin of emission unclear/controversial Relationship to broadband observations can provide hints to origin of extended emission GRB 080916C: Abdo et al. (2009) GRB 090902B: Abdo et al. (2009) de Pasquale et al., ApJL, 709, 146

Methods Swift GRB sample provides accurate localizations and broadband afterglow observations for comparison LAT survey mode scans through positions of Swift GRBs sometime within 2 orbits (~10 ks), as well as anywhere else on the sky Stack LAT unbinned likelihood profiles of each Swift burst and compare to broadband observations in overlapping time windows Split sample by various characteristics to see how stacked likelihood profiles compare Construct broadband stacked SEDs to compare emission components 3

Sample 15 155 BAT GRBs from Aug 2008 (start of Fermi Science Operations) to December 2010 Exclude individual detections 090510 - Ackerman et al. 2010, de Pasquale et al. 2010 100728A - LAT team, submitted to ApJL Require XRT detections UVOT detection of 58, non-detection 97 142 long bursts, 13 short bursts :" 10 long!& N ;!" 5 short & 0 0.1 1 10 100 BAT T 90 (s) "!" #!" $!" %!" &!" '!" ( )*+,-./0120,3!&!&",4056,3078,29 : 6

Integration Window LAT Extended emission has been observed to last up to a few thousand seconds in bright bursts Integrate over any observation windows in first 10 ks Number of BAT GRBs 140 120 100 80 60 40 in LAT FoV in XRT FoV 20 0 1 10 100 1000 10000 Time since BAT Trigger (s) 5

X-ray Overlap All X-ray afterglow light curves and spectral fits from Leicester XRT light curve repository (Evans et al. 2007, 2009) Flares removed Light curves fit with (broken) power laws (methods of Racusin et al. 2009) Using light curve fits, fluxes from LAT intervals were averaged to compare to LAT fluxes Only afterglow components included for now (post-steep decay) 9:;<(4$=7 &$(>,?5(4,23(/+ ' (- & 5 &$ 8 &$ % &$ &$ &$ && &$ &' &$ &7 &$ &6!"#$%&$'$( Average XRT Flux in LAT Window LAT Windows & &$ &$$ &$$$ &$$$$ )*+,(-*./,(#0)(12*33,2(4-5 6

LAT Stacking Standard unbinned likelihood analysis for each GRB: Assume point source at XRT location + Galactic diffuse + Extragalactic diffuse contributions Events in 15 degree radius of interest during first 10 ks after BAT trigger 0. bandpass, split into 0.1-1, to estimate average photon index (2.1) Derive likelihood profile (TS) and 95% confidence upper limit for each burst Stack TS: The peak of the TS profile provides stacked maximum likelihood estimate of the population average flux 7

Stacked LAT Counts Map 8

Full Sample Stacking Full energy range 0. averaged over first 10 ks 155 GRBs, Flux = 4.7x10-8 ph cm -2 s -1, TS=20 Split energy averaged over first 10 ks: : Flux = 4.0x10-8 ph cm -2 s -1, TS=8 : Flux = 4.0x10-9 ph cm -2 s -1, TS=14 Photon index = 2.07 (consistent with assumed index) Peak of TS curve indicates most probable flux for sample in energy range 0. 9

Stacked SED!"!"!"!! (0 ()+12(34 # (5! -!"!#!"!/!"!$ + 1" Mean X-ray! F! - 1" LAT Stacked Flux #X$~1 Evidence for additional component (SSC)?!"!. 677(89:5!"!%!" "!" #!" $!" %!" & 10 '()*+,-

Splitting the Sample - Prompt Emission Properties Long 0. Long (142) TS(0.) = 21.6 TS() = 8.6 TS() = 14.6 Short (13) TS(0.) = 0 TS() = 0 TS() = 0 Short High Fluence 0. High Fluence (78) TS(0.) = 30.9 TS() = 14.2 TS() = 19.4 Low Fluence (77) TS(0.) = 0 TS() = 0 TS() = 0 BAT Fluence 15-150 kev Low Fluence

Splitting the Sample - Prompt Emission Properties Long 0. Long (142) TS(0.) = 21.6 TS() = 8.6 TS() = 14.6 Short (13) TS(0.) = 0 TS() = 0 TS() = 0 Short sample too small to tell Short High Fluence 0. High Fluence (78) TS(0.) = 30.9 TS() = 14.2 TS() = 19.4 Low Fluence (77) TS(0.) = 0 TS() = 0 TS() = 0 BAT Fluence 15-150 kev Low Fluence

Splitting the Sample - Prompt Emission Properties Long 0. Long (142) TS(0.) = 21.6 TS() = 8.6 TS() = 14.6 Short (13) TS(0.) = 0 TS() = 0 TS() = 0 Short burst sample too small to tell Short 15 10 long N 5 short 0 0.1 1 10 100 BAT T 90 (s)

Splitting the Sample - Prompt Emission Properties :"!& ;!" & "!" #!" $!" %!" &!" '!" ( )*+,-./0120,3!&!&",4056,3078,29 : 6 High Fluence 0. High Fluence (78) TS(0.) = 30.9 TS() = 14.2 TS() = 19.4 Low Fluence (77) TS(0.) = 0 TS() = 0 TS() = 0 BAT Fluence 15-150 kev Significant difference Low Fluence

Splitting the Sample - Afterglow Properties Flares (101) TS(0.) = 11.0 TS() = 3.4 Flares 0. TS() = 10.7 No Flares (54) TS(0.) = 8.8 TS() = 4.4 TS() = 3.0 Note: X-ray Flares not necessarily in LAT time windows No Flares 0. UVOT Detection UVOT Det (58) TS(0.) = 18.2 TS() = 12.1 TS() = 6.2 No UVOT Det (97) TS(0.) = 4.4 TS() = 0 TS() = 7.7 No UVOT Detection

Splitting the Sample - Afterglow Properties Flares (101) TS(0.) = 11.0 TS() = 3.4 Flares 0. TS() = 10.7 No Flares (54) TS(0.) = 8.8 TS() = 4.4 TS() = 3.0 Note: X-ray Flares not necessarily in LAT time windows Similar TS values No Flares 0. UVOT Detection UVOT Det (58) TS(0.) = 18.2 TS() = 12.1 TS() = 6.2 No UVOT Det (97) TS(0.) = 4.4 TS() = 0 TS() = 7.7 No UVOT Detection

Splitting the Sample - Afterglow Properties Flares (101) TS(0.) = 11.0 TS() = 3.4 Flares 0. TS() = 10.7 No Flares (54) TS(0.) = 8.8 TS() = 4.4 TS() = 3.0 Note: X-ray Flares not necessarily in LAT time windows Similar TS values No Flares UVOT Det (58) TS(0.) = 18.2 TS() = 12.1 TS() = 6.2 No UVOT Det (97) TS(0.) = 4.4 TS() = 0 TS() = 7.7 UVOT Detection significant UVOT Detection No UVOT Detection 0.

Splitting the Sample - Afterglow Properties #! (! *++,-./01,.2,& 3456,781891/:;,<''=> 3456,):;?81891/:;,<=!> 3456,3;:@A8BC8? Distribution of redshift for Swift Sample (2005-2009) Note: Not the same sample as LAT sample. Redshift measurement more likely with UVOT detection. ) "! Redshift? Strength of afterglow? GRB Environment? '!!! " # $ % & UVOT Detection 0. UVOT Det (58) TS(0.) = 18.2 TS() = 12.1 TS() = 6.2 No UVOT Det (97) TS(0.) = 4.4 TS() = 0 TS() = 7.7 No UVOT Detection

Conclusions and Future Work Despite individual non-detections, we can learn about LAT extended emission through stacked analysis Combined XRT+LAT SED hints at different emission components (SSC?) Extended emission is correlated with prompt emission fluence (already seen by GBM, Bissaldi et al. 2011, Nava et al. 2011) Extended emission is correlated with UVOT detection brightness of afterglow (strength of forward shock?) blueness of afterglow (redshift?) related to environment? Future Work Full characterization of BAT+XRT lightcurves Additional stacking splits (optical detection, NIR detection) Add UV/optical/NIR afterglow fluxes to stacked SED Temporal splits of stacking analysis 16