tokens: The GBM targeted search pipeline (Blackburn et al. 2015 ), which operates using a seed time and an optional seed sky map, was updated in preparation for the second observing run of Advanced LIGO (O2) to produce an overall lower false-alarm rate (FAR) and to be more sensitive to sGRB-like signals (Goldstein et al. 2016 ). The improvements implemented include replacing the “hard” spectral template with a more physically and observationally representative cutoff power-law spectrum, increasing the data temporal resolution, improving the background estimation, and utilizing localization information in the ranking statistic. 2.1.2. GBM Results The GBM was observing ∼82% of the LIGO sky map at the time of the LIGO trigger, and no onboard GBM trigger was recorded close in time to GW170104. The nearest triggers were more than 13 hr preceding and more than 10 hr following GW170104, and both of these triggers were due to steep non-astrophysical rate increases near the boundary of the SAA just before the detectors were turned off for passage. The untargeted search did not find any untriggered sGRB candidate on January 4. The nearest candidate preceding GW170104 was more than 15 hr earlier, and the nearest candidate following GW170104 was about 8 days later. The targeted search was run over a 1 minute window centered at t GW , searching for any signal on timescales from 0.256 s, increasing by factors of two up to 8.192 s (six timescales). As reported in Burns et al. ( 2017 ), no credible significant candidate was found. The most significant result from the search was on the longest timescale with the “soft” spectral template, which the search found to be marginally consistent with part of the LIGO sky map. Further inspection of the count-rate light curves for each detector reveal a slow-rising bump that begins ∼100 s before t GW and has a duration of ∼300 s. This bump, which is strongest in the GBM Na i channel 1 (12–25 keV) and does not appear above channel 2 (50 keV), is consistent with a low-level increase in magnetospheric activity as Fermi proceeds through its orbit. Using the GBM, we can set an upper bound on the impulsive flux expected from an sGRB-like signal. The current technique is to estimate the background in the same way that the targeted search does (unbinned Poisson maximum likelihood over a sliding window) and calculate the 3 σ upper bound in the count rate as three times the standard deviation of the background count rate. The count-rate upper bound can then be converted to a flux upper bound by assuming a photon model, folding it through the detector response and fitting the amplitude to determine the associated flux upper bound. We note that this technique produces a conservative upper limit because it utilizes a single GBM detector rather than all detectors over the entire LIGO map, which would be significantly more computationally challenging. We evaluate the limits in this way over the
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