he Binary Neutron Star Merger
10.2 Olint Follow-up
10.2.2 bhowtr plus crak: Prompt Emission btarh
To further improve the sensitivity to a potential neutrino counterpart, a shower reconstruction was used to search for all lavour neutrino events, for the irst time in an A follow-up analysis, in parallel with the long time-scale emission analysis presented in the following section. hese two analyses have been announced together with the oicial announcement of the GW170817 detection on October 16thand published on November 30th, 2017, in collaboration with the I C , Pierre Auger, LIGO and Virgo collaborations.
he optimization strategy is identical for both the track and shower samples. Hereater, it will be developed for showers on which I worked. he implementation for the track sample will be summarized aterwards.
he goal of this analysis is to search for a prompt neutrino emission, therefore, as for previous analyses, a neutrino will be considered correlated in time if it occurs within the time window of k 500 s around the time of the merger.
bhowtr Optimization
Once again the optimization aims at maximizing the signal probability keeping the probability to observe one event from the background smaller than 3� = �.7 · ��−3 so that one event passing the cuts lead to a��detection.
he event selection used for previous point source searchesα[94] is used here as a basis.
However events are selected as downward-going and thanks to the space and time correlation, the Shcut can be totally relaxed. he selection cuts can be seen in Tableα10.1, the variables are detailed in sections 2.6andα5.3.
he optimization is done by relaxing the likelihood muon veto cutℒ� and optimizing the spatial cut, theregion of interest, in which events are considered spatially compatible ⁸given the resolution⁹ with the merger. Basically, it is designed as follows: For diferent radii of the region of interest RoI, the cut onℒ�is optimized to reduce thebackground probabilityto 3�. hen, the
RoImaximizing the signal probability using a −2spectrum is chosen. his will be developed in what follows.
As seen in sectionα5.3.2, ℒ� was introduced as a likelihood ratio between cosmic showers and atmospheric muons that is deined from an upward-going sample. To make a consistent
cablt 10.1–List ou tht stltrtion ruts applits uor showtrs.he usual cuts are deined in sectionα5.3.2.
RoIis the angular distance of the event to the source and|�evt− �src|the time between the merger and the neutrino candidate.
Criterion Condition
Triggered T3 or 3N
Track Veto Not selected as a track Containment �Sh� ���m� |�| � �5�m
Figurt 10.2–Probability to stttrt a bakgrouns tvtnt rorrtlatts in spart ans timt as a uunrtion ou tht rut on L� for a time slice of 1000 seconds and a radius of the region of interest of 24°.
use ofℒ�here, it has been redeined using the same probability density distributions but from a downward-going sample.
he optimization onℒ�is done from the curve represented on igureα10.2which shows the probability to have a background event correlated with GW170817 as a function of the cut on ℒ�. It comes from the anti-cumulative distribution ofℒ�for atmospheric muons and neutrinos from the Monte Carlo simulation. All downward-going events of the run are used, with the approximation that the background rate is constant on all the ield of view and all the run1. hen the curve is rescaled to match the data: the number of events before the cut on ℒ� is scaled to the corresponding number of events in the dataset ⁸25αevents⁹. hen, this is rescaled to the 1 000 s time window and to the region of interest. Finally, this is ited by a hyperbolic tangent to get the value of the cut leading to a probability to detect a background event of less than 3�.
For each pair of ⁸ RoI, ℒ�⁹, the acceptance is computed. he acceptance being deined as the constant of proportionality between the signal lux normalization and the expected number of signal event. he pair of ⁸ RoI, ℒ�⁹ maximizing the acceptance is chosen as shown in
Ta-1his would have a small impact on the signiicance of a potential detection but not on the upper limit set.
Figurt 10.3–Proportion ou rtmaining signal ans rtjtrtts bakgrouns attr tht rut onℒ�. Blue:
remaining astrophysical neutrinos, Red: rejected atmospheric muons, Green: rejected atmospheric neutrinos. he black lines show the values corre-sponding to the cut onℒ� of -13.
cablt 10.2 –Optimization ou tht sizt ou tht rtgion ou inttrtst aoI ans tht muon vtto ℒ� by maximizing the acceptance keeping a background probability of 3�. he bold line is the maximum.
RoI ℒ�cut Acceptance
bleα10.2. It results in rejecting 96⁵ of the background keeping 68⁵ of the signal as can be seen in igureα10.3. One can see in igureα10.4that more than 80 ⁵ of the signal events remaining ater cuts are contained in a region of interest with a radius of 24°, the median angular resolution being of 6°. he energy range deined as containing 90 ⁵ of the signal is [23 TeV, 16 PeV].
crak Optimization
For the track sample, the same optimization strategy is used, ΛTr being the quality cut which is optimized in parallel with the size of the region of interest. Figureα10.5 illustrates the opti-mization process, it results on a cut on ΛTr at -5.5 and a value of RoI of 1.5°. he inal cuts are shown in Tableα10.3. he region of interest is much smaller than for the showers thanks to the good angular resolution of the tracks, the median being at 0.5°. Even the energy range is slightly higher for the tracks ⁸[32 TeV, 22 PeV]⁹ contrary to the galactic plane analysis. his is due to the higher muon background to reject in the downward-going track sample, as rejecting background cut in the signal mostly lower energy events.
Ater unblinding, zero showers and ive tracks have been seen in the time window of k 500 s around the coalescence but none of them felt inside of the region of interest as can be seen in
Figurt 10.6 –Map ou tht ntutrino uollow-up ou tht GW170817 tvtnt. Localization of the 90 ⁵ credible region of GW170817 ⁸red contour⁹ and the NGCα4993 galaxy hosting the merger ⁸black plus symbol⁹ as well as the directions of A ’s ⁸blue diamond⁹ and I C ’s ⁸green crosses⁹ neutrino candidates within 500 s of the merger. A and I C ’s horizons separating downward-going and upward-going events ⁸dashed blue and green lines respectively⁹ are also shown with the Auger’s ields of view of Earth-skimming ⁸darker blue⁹ and downward-going ⁸lighter blue⁹ directions.α[147]
igureα10.6. A limit combining tracks and showers has been put on the neutrino lux expected from the merger, it is described in the chapterα11.