Search for very-high-energy emission with HAWC from GW170817 event
SSearch for very-high-energy emission with HAWCfrom GW170817 event
Antonio Galván-Gámez ∗ ; Nissim Fraija and M. Magdalena González The HAWC Collaboration † [email protected] , [email protected] , [email protected] The detection of the gravitational wave GW170817 defined a breakthrough in multi-messengerastronomy. For the first time, a gravitational wave transient detected by the Laser Interferom-eter Gravitational-Wave Observatory (LIGO) and Virgo interferometer was associated with afaint electromagnetic gamma-ray counterpart reported by the Gamma-ray Burst Monitor (GBM)aboard on the Fermi satellite. GRB 170817A was followed up by an enormous observationalcampaign covering a large fraction of the electromagnetic spectrum. In this work, we use the datafrom High Altitude Water Cherenkov (HAWC) gamma-ray observatory to search for very-high-energy (VHE) TeV photons in coincidence with the X-ray emission from GRB 170817A. Sinceno counts were observed up to ∼
120 days after the trigger time, we derive and report the corre-sponding upper limits in the energy range from 1 to 100 TeV. In addition, we extend the analysisto GRBs with similar features proposed by A. von Kienlin. ∗ Speaker. † For collaboration list, see PoS(ICRC2019) 1177. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ a s t r o - ph . H E ] A ug ollow up to GRB 170817A Antonio Galván-Gámez
1. Introduction
One of the events defining a breakthrough in the multi-messenger astronomy is the detectionof the gravitational wave (therefore GW) GW170817 [1, 2] by the LIGO and Virgo observatories.This gravitational wave was identified as a binary system consisting of neutron stars. The mergerwas associated with a short Gamma Ray Burst (from now sGRB or GRB) produced ∼
2s [3] afterthe GW event. The GRB was detected by the Gamma-ray Burst Monitor (GBM) on board theFermi spacecraft [4]. Since the detection of the prompt emission, several of optical telescopes onground started one of the most exhaustive follow up campaigns of the modern astronomy. The firstdetection of the optical counterparts came from the Large Binocular Telescope Observatory (LBT)[5] ∼
10 hours after the merger. The most interesting detection to the purposes of TeV counterpartsearches came from the observation of the X-ray [6, 7] and radio [8] emissions 9 and 16 days afterthe prompt emission, respectively. For the first time in GRBs astronomy, the maximum fluxes werereached ∼
120 days after the trigger time. Different models based on external shocks have beendeveloped to explain this burst [9, 10, 11, 12, 13, 14]. In general, TeV photons are likely generatedby synchrotron self-Compton (SSC) emission from X-ray photons [15, 16, 17, 18, 19, 20, 21]. Forthis reason, in this work we search for TeV emission up to the time of the maximum X-ray flux.Recently von Kienlin et. al. [22], using the data from the Fermi-GBM 10 yr catalog , identi-fied, applying a two-step selection supported by a statistical criteria using bayesian block analysis,13 sGRBs with similar features to GRB 170817A. Since HAWC scans the whole sky continuously,it is ideal and possible to search for TeV emission in coincidence with these bursts.The paper is arranged as follows. In Section 2 we present the bursts studied here. In Section3 we describe briefly the analysis performed. Finally, in Section 4, discussion and results arepresented.
2. Follow-up to GRB 170817A and sGRBs alike.
GRB 170817A triggered Fermi GBM at 2017 September 17 12:41:20 UTC. The Fermi GBMlocalization constrained this burst to a region at α = h m and δ = − ◦ (J2000.0). Immediately,GRB 170817A was followed up by a massive observational campaign covering a large fraction ofthe electromagnetic spectrum. In X-ray bands, this burst was detected by the Chandra and XMM-Newton satellites, in optical bands, non-thermal observations was revealed by the Hubble SpaceTelescope and in radio, bands at 3 and 6 GHz were identified by the Very Large Array (VLA).The GW170917/GRB 170817A event a few hours later entered the field of view of two ofthe TeV γ -ray observatories: the High Energy Stereoscopic System (H.E.S.S.) Telescope and theHAWC observatory. Observations with the HAWC experiment started on 2017 August 17 at 20:53UTC and finished 2.03 hr later. Although no significant excess of counts was detected by HAWC,an upper limit of 1 . × − erg cm − s − for an energy range of 4 - 100 TeV was derived.Recently, A. von Kienlin in [22] analyzed the GBM catalog looking for similarities with othersGRBs. These authors proposed a sample of sGRBs with similar characteristics to GRB 170817A.This is derived from the GBM data by applying cuts on the observable features (e.g. T < 5 s, the FERMIGBRST - Fermi GBM Burst Catalog ollow up to GRB 170817A Antonio Galván-Gámez main pulse was followed by a soft tail emission, etc). Those bursts are clearly interesting to searchfor TeV emission at long time scales as in GRB 170817A.
3. Analysis
The HAWC observatory is located next to the volcano Sierra Negra in the state of Puebla inMexico at an altitude of 4,100m a.s.l. HAWC is constituted by 300 Water Cherenkov Detectors(WCDs) as the main array and 345 smaller detectors to improve its sensitivity to showers of thehighest energies. HAWC has an instantaneous field of view of 2 sr, a duty cycle > 95% and anangular resolution (cid:38) ◦ . The optimal sensitivity of HAWC lies in the range of declination of δ ∈ [ − ◦ , ◦ ] and energies of 1 and 100 TeV.The details on the adopted analysis in this work, such as event reconstruction, systematicuncertainties, background subtraction and map making, are described in [23]. Here, specific detailsrelevant to our analysis are presented.Although HAWC continuously records extensive air showers in all directions over the horizon,its effective area is a function of the zenith angle of the primary gamma-ray. It is optimal for values< 45 ◦ and the contribution from those events outside a cone with open angle of ∼ ◦ is small. Asshown by [23] for a source like the crab nebula, it is expected that 90% of the signal arrives withinthe ∼ − . ∼
4. Results and Conclusions
Unfortunately, there was no TeV emission found for any of the bursts studied here. In particu-lar, the flux upper limits derived for GRB 170817A are presented in Figure 1. We have estimated atheoretical prediction of SSC emission from the observed X-ray emission. The upper limits are notconstraining as observed in Figure 1.From the GRBs identified by [22], only GRB 150101B, GRB 170111A and GRB 170817Awere within the HAWC’s field of view for at least one hour. In Table 1 the transit duration and thetotal exposure time for each of these bursts reported.2 ollow up to GRB 170817A
Antonio Galván-Gámez
As observed, even though GRB 150101B has the highest redshift observed, the derived fluxupper limit is comparable with the one obtained for GRB 170111A assuming a redshift of 0.009.The best flux upper limit is obtained for GRB 170817A because of their closeness and mediumlow exposure. The flux upper limit for GRB 170111A when assuming a redshift of 0.3 is also aconsequence of the very low exposure time. The decreasing behavior of the flux upper limits as afunction of time is a clear consequence of the increasing time window.HAWC continuously monitors the whole sky in order to search for VHE emission from sGRBsin timescales from seconds to days. In this work, we present the most interesting sGRBs in the fieldof view of HAWC and associated or possible associated to gravitational waves.GRB NAME RA(deg) Dec(deg) Error(deg) Transit duration(hr) Exposure Time(hr) RedshiftGRB 150101B 188.0 -11.0 0 4.51 464.58 0.134GRB 170111B 270.9 63.7 6.7 0.99 96.467 -GRB 170817A 197.5 -23.4 0 2.01 205.27 0.009
Table 1:
General characteristics of the short GRBs used in this study.
5. Acknowledgements
We acknowledge the support from: the US National Science Foundation (NSF) the US De-partment of Energy Office of High-Energy Physics; the Laboratory Directed Research and Devel-opment (LDRD) program of Los Alamos National Laboratory; Consejo Nacional de Ciencia y Tec-nología (CONACyT), México (grants 271051, 232656, 260378, 179588, 239762, 254964, 271737,258865, 243290, 132197, 281653)(Cátedras 873, 1563, 341), Laboratorio Nacional HAWC derayos gamma; L’OREAL Fellowship for Women in Science 2014; Red HAWC, México; DGAPA-UNAM (grants AG100317, IN111315, IN111716-3, IA102715, IN111419, IA102019, IN112218);VIEP-BUAP; PIFI 2012, 2013, PROFOCIE 2014, 2015; the University of Wisconsin Alumni Re-search Foundation; the Institute of Geophysics, Planetary Physics, and Signatures at Los AlamosNational Laboratory; Polish Science Centre grant DEC-2014/13/B/ST9/945, DEC-2017/27/B/ST9/02272;Coordinación de la Investigación Científica de la Universidad Michoacana; Royal Society - New-ton Advanced Fellowship 180385. Thanks to Scott Delay, Luciano Díaz and Eduardo Murrieta fortechnical support.
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