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INTEGRAL results on the electromagnetic counterparts of gravitational waves
Mereghetti, S.; Savchenko, V.; Ferrigno, C.; Kuulkers, E.; Ubertini, P.; Bazzano, A.; Bozzo, E.; Brandt, S.; Chenevez, J.; Courvoisier, T. J.-L. Total number of authors: 21
Published in: Proceedings of the THESEUS Workshop 2017
Publication date: 2018
Document Version Early version, also known as pre-print
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Citation (APA): Mereghetti, S., Savchenko, V., Ferrigno, C., Kuulkers, E., Ubertini, P., Bazzano, A., Bozzo, E., Brandt, S., Chenevez, J., Courvoisier, T. J-L., Diehl, R., Hanlon, L., von Kienlin, A., Laurent, P., Lebrun, F., Lutovinov, A., Martin-Carrillo, A., Natalucci, L., Roques, J. P., ... Sunyaev, R. (2018). INTEGRAL results on the electromagnetic counterparts of gravitational waves. In L. Amati, E. Bozzo, M. Della Valle, D. Gotz, & P. O'Brien (Eds.), Proceedings of the THESEUS Workshop 2017 http://www.isdc.unige.ch/theseus/proceedings.html
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arXiv:1801.05359v1 [astro-ph.HE] 16 Jan 2018 e.SAI.Vl 0 0 00, Vol. S.A.It. Mem. ntehr -a soft / X-ray observations hard to the devoted European in the Agency of mission Space main the since is operating 2002, satellite, INTEGRAL The Introduction 1. c At2008 SAIt .Bazzano A. .Mereghetti S. NERLrslso h electromagnetic the on results INTEGRAL .Lutovinov A. 10 11 12 13 Abstract. e words. Key tw first the during interferometers. obtained LIGO/Virgo results advanced the the review multi briefly for we generally, more Here electromagneti and, the of signals waves study gravitational and identification satellit the INTEGRAL for the sky, ities whole the of coverage continuous 6 5 4 8 7 9 3 2 1 .Diehl R. T,Bidn 2,D-80Knes ygy Denmark Gar Lyngby, Physik, Italy f¨ur Kongens, Extraterrestrische Max-Planck-Institut 00133-Rome, DK-2800 100, 327, Cavaliere Building del DTU, Fosso Via IAPS, INAF, u lc oote ´oi uut 50 ai ee 3 Fra 13, Cedex Paris 75205 L´eonie Duquet, et Domont Sorbonne Alice Paris rue de Ireland Observatoire CEA/Irfu, 4, CNRS/IN2P3, Dublin APC, Belfield, Dublin, College University S/RUSp E aly 19 i-u-vteCdx Fra Cedex, Gif-sur-Yvette 91191 Saclay, CEA DSM/IRFU/SAp, pc eerhIsiue rfouny 43,179 Mos 117997 84/32, Profsoyuznaya Institute, Research Space ocwIsiueo hsc n ehooy ogpun,1 Dolgoprudny, Technology, and Physics of Institute Moscow nvri´ olue RP v oh,B 44,F308T F-31028 44346, BP Roche, Av. 9 Universit´e IRAP; Toulouse; P o srpyis alShazcidSr ,Garchin 1, Karl-Schwarzschild-Str. Astrophysics, for MPI S/SE,Kpela ,20 ZNodik h Netherland The Noordwijk, AZ 2201 1, Keplerlaan ESA/ESTEC, NF AFMln,vaEBsii1,I213Mln,Italy d’ chemin Milano, Geneva, I-20133 of University 15, ISDC, E.Bassini via IASF-Milano, INAF, oneprso rvttoa waves gravitational of counterparts hnst t ihobtadasto opeetr detectors complementary of set a and orbit high its to Thanks 4 6 .Bozzo E. , rvttoa ae am-a bursts Gamma-ray – waves Gravitational .Hanlon L. , 1 .Savchenko V. , 10 , 11 .Martin-Carrillo A. , .Siegert T. 2 .Brandt S. , γ 7 .vnKienlin von A. , ryrange -ray 2 6 .Ferrigno C. , n .Sunyaev R. and , 5 .Chenevez J. , cga 6C-20Vrox Switzerland Versoix, CH-1290 16 Ecogia, ´ hsc,ta a eetyetrda ex- thanks 2017), an Heuvel den entered astro- (van recently phase multi-messenger citing has of that tool context physics, powerful the prop- particularly unique a in few it make A resolution erties 2003). angular al. et and (Winkler spectral high with 7 .Natalucci L. , 6 .Laurent P. , 2 .Kuulkers E. , 10 5 Memorie msegrastrophysics. -messenger .J-.Courvoisier J.-L. T. , , aito soitdto associated radiation c 13 hn,Germany ching, -54,Germany D-85741, g a nqecapabil- unique has e 4 8 bevn usof runs observing o .P Roques P. J. , , 9 o,Russia cow, 10,Russia 41700, .Lebrun F. , della 3 uos,France oulouse, .Ubertini P. , nce. ai Cit´e, 10 Paris nce s providing 9 12 , , 4 2 , , S.Mereghetti et al.: Electromagnetic counterparts of GW with INTEGRAL 1 to the high sensitivity reached by the LIGO/Virgo interferometers for gravita- tional waves (GW) and by the new gen- eration of neutrino detetectors. The instruments on board INTEGRAL, besides providing high sensitivity with good imaging and spectroscopic capabili- ties over a wide field of view (∼900 deg2), are able to detect transient γ-ray signals from every direction in the sky, as discussed in more detail in section 2. The other crucial property of INTEGRAL in this context is its highly eccentric orbit, with a period of 2.7 days. This allows uninterrupted observations of virtually the whole sky for 85% of the time (i.e. when the satellite is above the van Allen radiation belts). Note that, Fig. 1. Relative sensitivity of the different de- contrary to what happens for satellites in tectors on board INTEGRAL as a function of low earth orbits, the fraction of the sky the photon arrival direction (Savchenko et al. occulted by the Earth is negligible (from 2017c). The shaded regions indicate the vari- 0.05% at perigee to a maximum of ∼0.4% ation in sensitivity in the different azimuthal when INTEGRAL is close to the radiation directions. A burst with duration of 1 s and belts). In addition, all the data are contin- Comptonized spectrum with α = −0.5 and E uously transmitted to ground in real time p = 600 keV has been assumed. and can be processed at the INTEGRAL Science Data Center (Courvoisier et al. 2003), with a latency of only a few seconds from the time of their on-board acquisition (Mereghetti et al. 2003). In the next sections we describe the per- formance of the INTEGRAL instruments and review the results obtained during the O1 (September 2015 – January 2016) and O2 (December 2016 – August 2017) observ- ing runs of the LIGO/Virgo detectors.
2. INTEGRAL performances The INTEGRAL satellite carries two main instruments, SPI and IBIS, operating at hard X-ray / soft γ-ray energies, comple- mented by an X-ray and an optical tele- Fig. 2. Same as Fig. 1 but for a burst with scope (JEM-X and OMC). All these instru- duration of 8 s and a Band spectrum with ments observe simultaneously and point in α = −1, β = −2.4 and Ep = 300 keV. Note the same direction. IBIS uses two position- that in this case the IBIS/VETO provides a sensitive detectors (ISGRI and PICsIt) better sensitivity than the SPI/ACS for events coupled to a coded mask to provide images coming from the bottom direction. in the range from 20 keV to 10 MeV over a field of view of ∼30◦ × 30◦ with an angu- 2 S.Mereghetti et al.: Electromagnetic counterparts of GW with INTEGRAL lar resolution of 12 arcmin (Ubertini et al. other transient events can be found in 2003). A similar field of view and en- Savchenko et al. (2017c). ergy range are covered by SPI. Its angu- lar resolution is worse than that of IBIS, but thanks to its germanium detectors, 3. Results SPI provides an excellent spectral reso- 3.1. GW 150914 lution which makes it particularly useful to search for narrow lines from electron- The first gravitational wave signal sig- positron annihilation or nuclear deexcita- nificantly detected during the O1 run of tion (Vedrenne et al. 2003). the Advanced LIGO interferometer, GW Both the IBIS and SPI telescopes in- 150914, was located inside an uncertainty region (90% confidence) with area of 630 clude active anticoincidence systems, based 2 on BGO scintillators, that can be used very deg (Abbott et al. 2016b). At the time of effectively as omnidirectional detectors ca- the GW trigger, INTEGRAL was pointing pable to monitor the entire sky. Due to away from the GW error region, but its ori- their different geometry and to the pres- entation was optimal to cover the whole un- ence of surrounding absorbing material, the certainty region with the SPI/ACS. Indeed response of these detectors is a significant in 95% of the error region the achieved sen- (and energy-dependent) function of the ar- sitivity was within 20% of the best value, rival direction of the photons (see Fig. 1 providing constraining upper limits on the and 2). The highest sensitivity is given by fluence above 75 keV of possible counter- the SPI anticoincidence shield (SPI/ACS), parts (Savchenko et al. 2016). These limits which is sensitive to photons of energy depend on the assumed duration ∆t of the above ∼75 keV and provides light curves event, and to a lesser extent, on its sky po- with fixed binning of 50 ms of the to- sition and spectral shape. For typical GRB tal count rate of the whole detector. The spectra, the 3σ upper limits range from 2 × 10−8 erg cm−2 (∆t=50 ms) to ∼10−6 IBIS/Veto is sensitive in the 100 keV - 10 −2 MeV range and provides light curves with erg cm (∆t=10 s). a time resolution of 8 s. Due to the lack A possible hard X-ray transient lasting of spectral and directional information of ∼1 s was detected with the Fermi/GBM in- these detectors, whose main purpose is to strument about 0.4 s after the GW trigger shield the focal planes of the respective tele- time (Connaughton et al. 2016). The sig- scopes, it is necessary to assume a spec- nificance of this event and its association tral shape and sky position to convert their to the GW source are subject of discussion measured count rates to photon fluxes in (Greiner et al. 2016; Connaughton et al. physical units. 2018). If confirmed, this would be a rather surprising result since GW 150914 was Finally, we note that, due to the high caused by the coalescence of two black holes penetrating nature of γ-rays, both ISGRI (Abbott et al. 2016a) and most models do and PICsIt are sensitive also to events com- not predict electromagnetic emission in this ing from sky regions outiside the imaging case. field of view. From a comparison of the A comparison of the Fermi/GBM re- relative number of counts revealed in all sults with the INTEGRAL upper limits is the different elements that constitute the not straightforward, owing to the poorly INTEGRAL payload it is possible to derive constrained spectrum and uncertain arrival some rough information on the sky location direction of this weak event. The GBM of transient events. response extends to lower energies than A more complete description of the per- that of the SPI/ACS and, in principle, formances of the INTEGRAL instruments the results of the two instruments could for the search of GW counterparts and be reconciled if the putative counterpart S.Mereghetti et al.: Electromagnetic counterparts of GW with INTEGRAL 3 of GW 150914 had a very soft spectral shape, different from that of the majority of GRBs. On the other hand, the GBM data favor a relatively hard spectrum (e.g. a Comptonized model with αcomp = −0.42 and Epeak > 1 MeV; Veres et al. (2016)) that would result in a significant signal in the SPI/ACS. Further work, also to investi- gate the relative intercalibration of the two instruments, is required to give a better as- sessment of the properties of this electro- Fig. 3. SPI/ACS light curve around the magnetic signal and its possible association time of GW 170817, binned at 100 ms (from to GW 150914. Savchenko et al. (2017a)). The vertical dashed line indicates the time of the GW trigger. 3.2. GW 151226 tent with the fluence estimated with the At the time of this event, produced AGILE/MCAL. by the coalescence of two black holes (Abbott et al. 2016c), INTEGRAL was not observing because it was close to the 3.4. GW 170814 perigee, below the Earth radiation belts. GW 170814 was the first event revealed by three gravitational waves interferometers. 3.3. GW 170104 Thanks to the inclusion of the Virgo data it was possible to derive a small localiza- GW 170104 was the first high-significance tion region of only 60 deg2 (Abbott et al. event revealed during the LIGO O2 observ- 2017b). Also in this case, no significant ing run. Also this signal was caused by the signals were found with the SPI/ACS merging of two black holes (Abbott et al. at or near the time of the GW trigger 2017a). It was localized within an uncer- (Savchenko et al. 2017b). tainty region (90% confidence) of ∼1200 An INTEGRAL follow-up observation deg2. This region was entirely visible started about two days after the GW trig- with good sensitivity by the SPI/ACS, ger and covered more than 90% of the local- but no significant signals were detected ization region in the imaging field of view (Savchenko et al. 2017a). The derived up- of IBIS and SPI with a maximum net ex- per limits are similar to those obtained for posure of ∼100 ks. No counterparts were GW 150914. For example, assuming a typ- found, with a 3σ upper limit on the av- ical spectrum for a short GRB (a cutoff erage flux of ∼3 mCrab (13 mCrab) in power-law with α = −0.5 and Ep = 600 the 20–80 keV (80–300 keV) energy range keV) the SPI/ACS 3σ upper limit for the (Savchenko et al. 2017e). 75-2000 keV fluence in a duration of 1 s × −7 −2 is below 2 10 erg cm in 95% of the 3.5. GW 170817 LIGO localization region. Verrecchia et al. (2017) reported the The first gravitational wave signal pro- possible detection of a weak and short (32 duced by the merger of two neutron stars ms) burst, occurring 0.46 s before the GW was revealed by the the LIGO/Virgo trigger time, in the data of the MCAL de- interferometers on August 17, 2017 tector on the AGILE satellite. For most of (Abbott et al. 2017c), while INTEGRAL the localization region of GW 170104 the was pointing toward the localization SPI/ACS provides an upper limit inconsis- region of the previous event, GW 4 S.Mereghetti et al.: Electromagnetic counterparts of GW with INTEGRAL
170814. The independent discovery by 4. Conclusions the Fermi/GBM (Goldstein et al. 2017) and by the SPI/ACS (Savchenko et al. About fifteen years after its launch, 2017d) of an electromagnetic signal clearly INTEGRAL has started a new exciting associated to GW 170817 is a milestone of phase of its scientific life by playing a ma- multi-messenger astrophysics. This event jor role in the era of multi-messenger as- has important physical and astrophysical trophysics. In the case of black hole binary implications on many phenomena, such as, mergers, it has provided unique upper lim- e.g., the speed of gravitational waves, the its that constrain the ratio of emitted elec- Lorentz invariance, the equivalence princi- tromagnetic to gravitational energy to val- < −7 − −5 ple, the equation of state of neutron stars ues Eγ/EGW ∼ 10 10 . In the case of and the physics of GRBs (Abbott et al. the first, and up to now single, GW event 2017d). produced by the coalescence of two neutron The SPI/ACS light curve around the stars, INTEGRAL has given a crucial in- time of GW 170817 is shown in Fig. 3. dependent confirmation of the short GRB The excess corresponding to GRB 170817 discovered by Fermi/GBM, as well as im- is detected with a signal to noise ratio portant upper limits on subsequent high- of 4.6, 1.9 s after the GW trigger time, energy emission on different timescales. As expected for such a faint γ-ray burst, The unique INTEGRAL performances no coincident signal was visible in all the discussed above are relevant also in the other INTEGRAL detectors, thus support- search for counterparts of astrophysical ing that the excess seen in the SPI/ACS neutrinos, as demonstrated in several re- was not due to particle background. We de- cent cases for which constraining upper lim- rived a 75-2000 keV fluence of (1.4 ± 0.4 its were provided (Savchenko et al. 2017f; ± 0.6) ×10−7 erg cm−2, where the latter Santander et al. 2017). value gives the systematic error due to the The THESEUS satellite (Amati et al. uncertainty on the assumed spectral model. 2017), proposed for the ESA M5 call for INTEGRAL carried out a follow-up ob- new missions, is planned to be operative servation, initially centered at the best in the years following 2030, when GW as- Fermi/GBM location of GRB 170817, and tronomy will be a mature field, well be- later repointed toward the optical counter- yond the current exploratory phase. The part, as soon as it was announced. This po- expected potentialities of THESEUS for sition was covered with a net exposure of multi-messenger astronomy are described more than 320 ks, starting about one day in Stratta et al. (2017), but it is difficult to after the GW event, but no X-ray or γ-ray anticipate the wealth and variety of phe- counterparts were detected. The 3σ upper nomena that THESEUS will address. limits on a long-lasting afterglow are of the The lesson that can be learned from order of ∼1–10 mCrab for energies below the INTEGRAL results described above, is ∼100 keV and of few hundreds of mCrab that unanticipated uses of a payload can in the MeV region. give important scientific contributions and Finally, thanks to the long INTEGRAL exciting results. By definition, it is diffi- follow-up observation, we could also search cult to optimize the mission for an unfore- for delayed bursting activity, as could be seen science exploitation, but some gen- expected if (at least temporarily) a mag- eral guidelines can be followed, as including netar is formed, and for the presence γ- the possibility of reconfiguration of the on- ray lines from r-process elements, such as board software (with the associated prob- I or Cs. In both cases the results were neg- lem of mantaining the required expertise ative, and no other mission could provide for an extended time period). 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