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Register a new userAGILE Observations of the Gravitational-wave Source GW170817: Constraining Gamma-Ray Emission from a NS-NS Coalescence
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Added by
Francesco Verrecchia
Publication date
2017
fields
Physics
and research's language is
English
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The LIGO-Virgo Collaboration (LVC) detected, on 2017 August 17, an exceptional gravitational-wave (GW) event temporally consistent within $sim,1.7 , rm s$ with the GRB 1708117A observed by Fermi-GBM and INTEGRAL. The event turns out to be compatible with a neutron star-neutron star (NS-NS) coalescence that subsequently produced a radio/optical/X-ray transient detected at later times. We report the main results of the observations by the AGILE satellite of the GW170817 localization region (LR) and its electromagnetic (e.m.) counterpart. At the LVC detection time $T_0$, the GW170817 LR was occulted by the Earth. The AGILE instrument collected useful data before and after the GW-GRB event because in its spinning observation mode it can scan a given source many times per hour. The earliest exposure of the GW170817 LR by the gamma-ray imaging detector (GRID) started about 935 s after $T_0$. No significant X-ray or gamma-ray emission was detected from the LR that was repeatedly exposed over timescales of minutes, hours, and days before and after GW170817, also considering Mini-calorimeter and Super-AGILE data. Our measurements are among the earliest ones obtained by space satellites on GW170817 and provide useful constraints on the precursor and delayed emission properties of the NS-NS coalescence event. We can exclude with high confidence the existence of an X-ray/gamma-ray emitting magnetar-like object with a large magnetic field of $10^{15} , rm G$. Our data are particularly significant during the early stage of evolution of the e.m. remnant.
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The LIGO/Virgo Collaboration (LVC) detected on 2017 January 4, a significant gravitational-wave (GW) event (now named GW170104). We report in this Letter the main results obtained from the analysis of hard X-ray and gamma-ray data of the AGILE mission that repeatedly observed the GW170104 localization region (LR). At the LVC detection time $T_0$ AGILE observed about 36% of the LR. The gamma-ray imaging detector did not reveal any significant emission in the energy range 50 MeV--30 GeV. Furthermore, no significant gamma-ray transients were detected in the LR that was repeatedly exposed over timescales of minutes, hours, and days. We also searched for transient emission using data near $T_0$ of the omnidirectional detector MCAL operating in the energy band 0.4--100 MeV. A refined analysis of MCAL data shows the existence of a weak event (that we call E2) with a signal-to-noise ratio of $4.4,sigma$ lasting about 32 ms and occurring $0.46,pm,0.05 ,rm s$ before $T_0$. A study of the MCAL background and of the false-alarm rate of E2 leads to the determinination of a post-trial significance between $2.4,sigma$ and $2.7,sigma$ for a temporal coincidence with GW170104. We note that E2 has characteristics similar to those detected from the weak precursor of the short GRB 090510. The candidate event E2 is worth consideration for simultaneous detection by other satellites. If associated with GW170104, it shows emission in the MeV band of a short burst preceding the final coalescence by 0.46 sec and involving $sim 10^{-7}$ of the total rest mass energy of the system.
Einstein Telescope (ET) is a planned third generation gravitational waves detector located in Europe. Its design will be different from currently build interferometers, because ET will consist of three interferometers rotated by a 60 deg with respect to each other in one plane. One of the biggest challenges for ET will be to determine sky position and distance to observed sources. If an object is observed in a few interferometers simultaneously one can estimate the position using traingulation from time delays, but so far there are no plans for a network of third generation detectors. Another possibility to deal with that problem is by using multimessenger approach, because redshift and sky position could be recovered from electromagnetic observations. In this paper we present a novel method of estimating distance and position in the sky of merging binaries. While our procedure is not as accurate as the multimessenger method, it can be applied to all observations, not just the ones with electromagnetic counterparts. We have shown that it is possible to significantly improve distance estimates using the measurements of the signal to noise ratio from all three interferometers .
The coalescence rate of two neutron stars (NS) is revisited. For estimation of the number of bound NS-NS and the probability of their coalescence in a timescale $tau$, the galactic star formation history, directly derived from observations, and the evolution of massive stars are considered. The newly established galactic merging rate is $(1.7pm 1.0) times 10^{-5} yr^{-1}$, while the local merging rate, including the contribution of elliptical galaxies, is about a factor of two higher, $3.4 times 10^{-5} yr^{-1}$. Using the present data basis on galaxy distribution in the local universe and the expected sensitivity of the first generation of laser beam interferometers, we estimate that one event should occur every 125 years for LIGO and one event each 148 years for VIRGO. The situation is considerably improved for advanced-LIGO since we predict that 6 events per year should be detected whereas for a recently proposed VIRGO new configuration, the event rate might increase up to 3 events every two years.
The exciting development of gravitational wave (GW) astronomy in the correlation of LIGO and VIRGO detection of GW signals makes possible to expect registration of effects of not only Binary Black Hole (BH) coalescence, but also Binary Neutron Star (NS) merging accompanied by electromagnetic (GRB) signal. Here we consider the possibility that a Neutron Star (NS), merging in a NS-NS or NS-BH system might be (soon) observed in correlation with any LIGO-VIRGO Gravitational Waves detection. We analyze as an example the recent case of the short GRB 170817A observed by Fermi and Integral. The associated Optical transient OT source in NGC4993 imply a rare near source, a consequent averaged large rate of such events (almost) compatible with expected NS NS merging rate. However the expected beamed GRB (or Short GRB) may be mostly aligned to a different direction than our. Therefore even soft GRB photons, more spread than hard ones, might be hardly able to shower to us. Nevertheless a prompt spiraling electron turbine jet in largest magnetic fields, at the base of the NS-NS collapse, might shine by its tangential synchrotron radiation in spread way with its skimming photons shining in large open disk. The consequent solid angle for such soft disk gamma radiation may be large enough to be nevertheless often observed.
We report the results of an extensive search in the AGILE data for a gamma-ray counterpart of the LIGO gravitational wave event GW150914. Currently in spinning mode, AGILE has the potential of covering with its gamma-ray instrument 80 % of the sky more than 100 times a day. It turns out that AGILE came within a minute from the event time of observing the accessible GW150914 localization region. Interestingly, the gamma-ray detector exposed about 65 % of this region during the 100 s time intervals centered at -100 s and +300 s from the event time. We determine a 2-sigma flux upper limit in the band 50 MeV - 10 GeV, $UL = 1.9 times 10^{-8} rm , erg , cm^{-2} , s^{-1}$ obtained about 300 s after the event. The timing of this measurement is the fastest ever obtained for GW150914, and significantly constrains the electromagnetic emission of a possible high-energy counterpart. We also carried out a search for a gamma-ray precursor and delayed emission over timescales ranging from minutes to days: in particular, we obtained an optimal exposure during the interval -150 / -30 s. In all these observations, we do not detect a significant signal associated with GW150914. We do not reveal the weak transient source reported by Fermi-GBM 0.4 s after the event time. However, even though a gamma-ray counterpart of the GW150914 event was not detected, the prospects for future AGILE observations of gravitational wave sources are decidedly promising.
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