NASA Technical Reports Server (NTRS) 20170001311: Searching for EM Signals Coincident with aLIGO GW Candidates Using the Fermi GBM

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by  NASA Technical Reports Server (NTRS)

Searching for EM Signals Coincident with
aLIGO GW Candidates using the Fermi GBM
Tyson B. Littenberg (NASA/MSFC)
on behalf of the 

Fermi GBM Team, LIGO Scientific Collaboration, and Virgo Scientific Collaboration
LIGO/Virgo & Fermi GBM Partnership
• GBM & LVC have a unique data-sharing
agreement
• GBM receives GW candidates in low-
latency for GRB follow-up
• GBM provides sub-threshold GRBs to
LVC for GW follow-up
LIGO/Virgo & Fermi GBM Partnership
This report focuses on searching for GBM counterparts to CBC triggers.
Onboard All Sky
triggered “Burst”
search search
Sub-
All Sky
threshold
GBM “CBC ” LVC
targeted
search
search
Sub-
Targeted
threshold
GRB
un-targeted
Follow-up
search
The GBM+LVC follow-up searches
• During O1
• GBM searched for offline CBC triggers in archival data and received “EM Alerts” for low-
latency analysis
• GW Offline Analysis:
• Combine GstLAL & PyCBC coincidences
• Use best calibration at the time (not final, but little difference in triggers)
• GBM Analysis
• Search for contemporaneous triggered and un-targeted GRBs
• Use trigger time to seed sub-threshold targeted GBM search
O1 analysis
All results are still under review by LVC & GBM
Selecting GW triggers
PRELIMINARY
Background sample Search sample
• PyCBC & GstLAL offline using best
-2
10
calibration available at the time
-3
10
• Search sample: FAR < ~1/day -4
10
-5
10
• Background sample: FAR >
-6
~1/15min W 10
G
-7
R 10
• Triggers combined into superset. A
F -8 LVT151012
10
• For overlapping triggers higher 10-9
FAR discarded -10
10
GW150914
 GW151226

-11
10
-12
10
0 20 40 60 80 100 120 140 160
t - t (days)
start
Search for coincident GBM triggers
PRELIMINARY
1
• Find time offset between each GRB
0.9 background
search
found by GBM and it’s nearest CBC
0.8
trigger
0.7
• No discernible difference between
0.6
background and search .
f
.
d 0.5
.
c
• 90% of samples (bkgd and search) 0.4
had offsets > +/- 104 s with nearest
0.3
triggered GRB
0.2
• No candidates from search sample 0.1
occurred within O(100) seconds of
0
2 3 4 5
a triggered GRB 10 10 10 10
|∆T| (s)
Targeted search (GBM sub-threshold events)
PRELIMINARY
• Search algorithm developed by L. • GW150914 seed produces most
Blackburn et al
 significant search result w/ FAR~2x10-4 Hz.

[Astrophys.J.Suppl. 217 (2015) no.1, 8] [Astrophys.J. 826 (2016) no.1, L6]
• Search performed over 3 template • 2nd lowest FAR is chance coincidence
spectra: “soft,” “normal,” and “hard” with solar flare
Combining targeted-search results
PRELIMINARY
• Computing False Alarm Probability [Astrophys.J. 826 (2016) no.1, L6]:
• GBM transient found in follow-up
of GW150914 has lowest FAP of
combined search (~2x10-3 )
• GW150914-GBM significance is too
low to declare an unambiguous EM
counterpart. More observations are
needed to reach a sound
conclusion.
• No other candidates had FAR<
1/100
LIGO/Virgo & Fermi GBM Partnership
[Goldstein et al, arXiv:1612.02395]
• New in O2
• Low latency, autonomous, sub-threshold
targeted GBM search for CBC online triggers
• Manual follow-up of significant “burst”
candidates.
• LVC receives sub threshold GBM candidates
from “un-targeted” search.