Elements of the potential spectrum of SGRB170817A=LVT1708?? "after-glow"

Jonah Kanner and Alan Weinstein


Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share.

Gravitational wave modes

The recent measurement from the advanced interferometric LIGO detectors [1] of the first direct gravitational-wave (GW) signal from what has been interpreted as the inspiral, merger and ringdown of a binary system of black holes [2] marks, in many respects, the beginning of GW astronomy. As additional advanced detectors such as Virgo [3], and KAGRA [4] are going to become operational in the next few years, we are likely to soon witness also signals from the inspiral and post-merger of neutron-star binaries or neutron-star–black-hole binaries... 
What we now know rather reliably is that the merger of a binary neutronstar (BNS) system inevitably leads to the formation of a massive metastable object, which can either collapse promptly to a black hole or survive up to thousands of seconds [13], emitting gravitational and electromagnetic radiation [14–17]. Furthermore, if the neutron stars have large magnetic fields and extended magnetospheres, the inspiral can be accompanied by a precursor electromagnetic signal [18], while the merger can lead to instabilities [19, 20] and to the formation of magnetically confined jet structures once a torus is formed around the black hole [21, 22]. Hence, the prospects of a multimessenger gravitational and electromagnetic signal are particularly good in the case of merger of binary neutron stars... 
In addition to a strong electromagnetic signal, the merger of a neutron-star binary also promises a GW signal that will contain important signatures of the equation of state (EOS) of matter at nuclear densities. These signatures are contained both in the inspiral and in the post-merger signals. The former is reasonably well understood analytically [23–27] and can be tracked accurately with advanced high-order numerical codes [28–30] and over many orbits now [31]. More importantly, the instantaneous GW frequency at amplitude maximum fmax has been shown to correlate closely with the tidal deformability of the two stars [32–34]. The latter part of the signal has already been studied in the past [9, 35, 36], but it has become the focus of attention particularly over the last few years [34, 37–48]. This large bulk of work has reached some generally agreed upon conclusions, but also has raised points that are a matter of debate that we hope to clarify here.  
Let us therefore start by summarising the aspects of the post-merger GW signal that seem to be robust and confirmed by several groups employing a variety of numerical methods and mathematical approximations. Given a realistic BNS system, namely, with a total mass between ∼ 2.4 and ∼ 2.8 M  , and a mass difference between the two components that is ∼ 20% or less, then the spectrum of the post-merger GW signal will present at least three strong peaks [34, 37–42]. These peaks were dubbed f1, f2 and f3 in Refs. [34, 42] and were found to satisfy the following approximate relation: f2 ≃ (f1 + f3)/2. A simple mechanical toy model was also presented in [34], that provided an intuitive explanation on the origin of these peaks and why they should be almost equally spaced. In addition to these three peaks, another peak can be identified in the power spectral density (PSD) of the GW signal, although not always. This is given by the coupling between the 𝓁 = 2 = m fundamental mode (which yields the f2 peak) and a quasi-radial (fundamental) axisymmetric mode, i.e., with 𝓁 = 2, m = 0; this mode was dubbed f2-0 in Ref. [38]. Finally, Ref. [45] introduced the concept of the fspiral peak frequency and associated it to a “rotating pattern of a deformation of spiral shape”.
Example of the GW emission around the merger, i.e., 1 ms before the (gray-shaded area) and 25 ms after the merger. It refers to a fiducial mass of M = 1.300 M with a representative stiff equation of state (EOS) (i.e., GNH3) of matter. The top part of reports the gravitational strain h+ for a source at 50 Mpc, while the bottom part the corresponding spectrogram. Also marked with horizontal lines of different type and colour are the various frequencies discussed so far in the literature (we indicate with f2,i the values in the transient phase to distinguish them from the values f2 attained in the subsequent quasi-stationary evolution of the GW signal)Note that all of the peaks present in the short transient stage (i.e., t . 3 ms) essentially disappear in the quasi-stationary evolution. The only exception is the f2 peak, which slightly evolves from the f2,i frequency. In the case of a stiff EOS a trace of the f2-0 mode is still present, although at very low amplitudes
... A  systematic analysis of {the gravitational-wave signal emitted during the inspiral, merger and post-merger of {a} 56 neutron-star binaries sample {spaning across six different nuclear-physics equations of state and ten masses} has allowed us to obtain a rather robust picture of the spectral properties of the GW signal and hopefully clarify a number of aspects that have been debated recently in the literature. In essence, our most important findings can be summarised as follows: 
• the instantaneous GW frequency when the amplitude reaches its first maximum is related quasi-universally with the tidal deformability of the two stars. 
• this correlation is observed for binaries with masses that do not differ of more than 20%. 
• the spectral properties vary during the post-merger phase with a marked difference between a transient phase lasting a few millisecond after the merger and a following quasi-stationary phase. 
• the most robust features of the post-merger signal pertain four frequencies: f1, f2, f3, and f2-0, where f2 ≃  (f1 + f3)/2 and f2-0 is the result of a mode coupling. 
• when distinguishing the spectral peaks between these two phases, a number of ambiguities in the identification of the peaks disappear, leaving a rather simple and robust picture. 
• “universal” relations are found between the spectral features and the physical properties of the neutron stars. 
• for all of the correlations between the spectral features and the stellar properties, simple analytic expressionscan be found either in terms of the dimensionless tidal deformability or of the stellar compactness. 
When considered as a whole and in the light of recent direct detection of GWs [2], these results open the exciting and realistic prospects of constraining the EOS of nuclear matter via GW observations of merging BNSs.
(Submitted on 1 Apr 2016 (v1), last revised 1 Jun 2016 (this version, v2))



Gamma  

Binary neutron star mergers are considered to be the most favorable sources that produce electromagnetic (EM) signals associated with gravitational waves (GWs)These mergers are the likely progenitors of short duration gamma-ray bursts (GRBs). The brief gamma-ray emission (the “prompt” GRB emission) is produced by ultra-relativistic jets, as a result, this emission is strongly beamed over a small solid angle along the jet. It is estimated to be a decade or more before a short GRB jet within the LIGO volume points along our line of sight. For this reason, the study of the prompt signal as an EM counterpart to GW events has been largely ignored. We argue that for a realistic jet model, one whose luminosity and Lorentz factor vary smoothly with angle, the prompt signal can be detected for a significantly broader range of viewing angles. This can lead to a new type of EM counterpart, an “off-axis” short GRB. Our estimates and simulations show that it is feasible to detect these signals with the aid of the temporal coincidence from a LIGO trigger, even if the observer is substantially misaligned with respect to the jet.
A schematic of a short GRB jet. Mergers produce GWs detectable by LIGO and are the likely progenitors of short GRBs. The prompt emission from the jet’s luminous core (routinely observed as a short GRB) is strongly beamed and can only be detected by observers located within θj from the jet axis. However, the jet is expected to have a lateral structure that moves slower and is fainter than the luminous core. Given the proximity of a LIGO-triggered short GRB, Fermi and Swift can potentially detect the prompt emission from this lateral structure even if the jet is misaligned with respect to our line of sight by an angle θobs  < 5θj

(Submitted on 24 Aug 2017)


Gamma ray bursts seem to be divided into two distinct classes, long duration soft gamma ray bursts (GRBs) that usually last more than 2 seconds and short hard bursts (SHBs) that usually last less than 2 seconds [1]. While there is clear observational evidence that most of the long duration GRBs are produced in broad line supernova explosions of type Ic [2], the origin of SHBs is still unknown. Supernova origin is ruled out [3], and although there is no solid evidence, it is widely believed [4] that SHBs are produced mainly in merger of neutron stars in close binaries [5]. SHBs are often followed by extended emission (EE) with a much lower luminosity that lasts a couple of minutes and is taken over during its fast decay by a long duration afterglow [6]. The origin of the EE and the afterglow of SHBs are also not known [4], although it has been suggested long ago that rotationally powered MSPs or magnetars powered by magnetic energy, which are born in neutron star mergers, dominate or contribute to the afterglow of SHBs [7]. Recently this possibility has attracted increasing attention [8].  
In this letter, we show that the X-ray light curves of the afterglow of SHBs, which were measured with the Swift X-ray telescope (XRT) are indeed those expected from the launch of highly relativistic narrowly collimated jets which produce the SHBs in the birth of rotationally powered millisecond pulsars (MSPs). Such MSPs are the smoking guns from the production of SHBs. They seem to be present in all SHBs, which rules out the birth of stellar black holes or magnetars powered by the decay of their ultra-strong magnetic field as the origin of most SHBs. The highly relativistic jets that produce SHBs in neutron star mergers [5,9] or/and in phase transition of neutron stars to more compact stars (quark stars ?) in compact binaries, within or without globular clusters [10], produce the SHBs with or without an extended emission (EE), respectively... 

The light curve of the X-ray afterglow of short gamma ray burst (GRB) 130603B reported
in the Swift-XRT GRB light curve repository [14] and its best fit light curves of millisecond pulsar
powered by rotation and a magnetar powered by magnetic field energy...

Gravitational wave detection of relatively nearby neutron star mergers by Ligo-Virgo, followed by the detection of newly born orphan afterglows (of beamed away SHBs) from the same directions with an MSP-like light curve will prove beyond doubt the neutron star merger origin of SHBs
(Submitted on 15 Aug 2017 (v1), last revised 21 Aug 2017 (this version, v2))


Optical, IR and radio


“A priori detectability”, i. e. a priori probability that the EM counterpart of a compact binary inspiral is detected if the observation is performed at a time t after the merger, for observations in Radio at 1.4 GHz, in Infrared (IR) in the J band, and in Optical in the r band, with limiting fluxes of 0.5 mJy in Radio and 22.4 AB magnitude in IR and Optical. The Radio and Optical probabilities account only for the jet afterglow, while the IR probability accounts only for the dynamical ejecta macronova. Based on a series of simplifying assumptions
In order to construct the a priori detectability P(F(t) > Flim), one must assume some prior probability density of the model parameters. Let us consider a simple, illustrative example. First, we construct a synthetic population of NS-NS inspirals whose properties roughly reproduce those expected for the population detected by Advanced LIGO; then we associate to each of them a jet afterglow and a macronova, under some assumptions. The detectable fraction of lightcurves in a given band, at a given time, will then constitute our estimate of the a priori detectability for this particular case. For the jet afterglow, we assume that all SGRB jets have an isotropic kinetic energy EK = 10^50 erg and a halfopening angle θjet = 0.2 radians (11.5 deg), and that they are surrounded by a relatively tenuous interstellar medium with constant number density nISM = 0.01 cm−3 . We fix the microphysical parameters so that the only remaining parameters needed to predict the afterglow lightcurve of the SGRB are the distance dL and the viewing angle θv. We will link the viewing angle to the binary orbit inclination, and the distance will be obviously set equal to that of the binary... 
(Submitted on 19 Apr 2017 (v1), last revised 1 Aug 2017 (this version, v3))


Neutrino

Neutrinos serve as an important messenger to probe the physics of neutron star-neutron star (NS-NS) and neutron star-black hole (NS-BH) mergers. However, the detection of MeV neutrinos from compact binary mergers is difficult even with future detectors (e.g., the detection horizon for Hyper-Kamiokande is ∼ 5 Mpc; Sekiguchi et al. 2011). We here focus on the detectability of high-energy neutrinos from short gamma ray bursts (SGRBs). GRBs have been discussed as promising sources of high-energy neutrinos (e.g., Waxman & Bahcall 1997; Murase & Nagataki 2006a; Becker 2008). The IceCube Collaboration has put interesting limits on the parameter spaces of GRB neutrinos (e.g., Aartsen et al. 2017). Their conclusions mainly come from long GRBs, and limits on prompt neutrinos from SGRBs with the current statistics are weak since their fluence is much smaller. However, we may still have a chance to detect neutrinos from SGRBs. One possibility is occurrence of a very nearby SGRB. Another is, as suggested by Murase & Nagataki (2006b), high-energy neutrino emission by late prompt emission (such as flares and plateaus) with longer durations, where the neutrino production efficiency can be much higher than that for the prompt emission... 
Using the expected distance of GW detection from faceon binaries (∼ 300 Mpc), IceCube can detect neutrinos from less than 10 % of EEs in the moderate case and around half of EEs in the optimistic case...
High-energy neutrinos can serve as a powerful probe of cosmic-ray acceleration in SGRBs and physics of SGRB jets associated with NS-NS mergers. They can provide important clues to an outflow associated with latetime activities, whose mechanisms are highly uncertain. Several scenarios for late-time activities have been proposed to explain EEs, flares, and plateaus. For example, the fragmentation of the accretion disk (Perna et al. 2006) and its magnetic barrier (Liu et al. 2012) may lead to a considerable amount of baryons around the central engine. Low luminosities of the late-time activities (∼ < 1049 erg s−1 ) indicate a lower accretion rate of 10−4 M⊙ s −1 . The baryon loading is large if the outflow is dirty, whereas it can be small if the outflow is largely Poynting-dominated...
(Submitted on 23 Aug 2017)

Ultra-Heavy elements in cosmic rays

Coalescence of a close binary with two neutron stars or with a neutron star and a black hole are paid a great attention as promising sources of gravitational wave and neutrino emission, and plausible progenitors of short gamma-ray bursts (SGRBs) and other electromagnetic transients. Neutron star mergers (NSMs) are thought to be the most promising site for the rapid neutron capture process (r-process) nucleosynthesis, and a possible dominant source of r-process elements in the universe (Lattimer & Schramm 1974). In this paper, we show that NSMs are also a possible major source of the ultraheavy element component of cosmic rays. The r-process is one of the major nucleosynthetic processes to synthesize elements heavier than the iron group (Burbidge et al. 1957). The astronomical source of the r-process elements is a longstanding problem in nuclear astrophysics. Two scenarios have been proposed for the dominant astronomical sources of r-process elements. One is the core-collapse supernova (CCSN) scenario and the other is the NSM scenario (e.g., review by Cowan 1991). In this paper, the term “NSM” includes the coalescence of a neutron star - black hole binary. 

Though CCSNe were widely accepted as a major source for r-process elements more than a decade ago, there have been growing evidence supporting the NSM scenario in recent years. Theoretical studies of nucleosynthesis have revealed that elements at or above the second r-process peak are hard to be synthesized in the CCSNe (e.g. Wanajo et al. 2011; Wanajo 2013) excepting the model of the magneto-rotational driven explosion (e.g. Nishimura et al. 2015). On the other hand, all of the NSM ejecta should become r-process elements because of their very low electron fractions. One NSM event yields r-process elements with a mass of Mr,NSM ∼ 0.0001 − 0.1 M⊙ with a very high velocity of v ∼ 0.2c, where c is the speed of light (e.g. Hotokezaka et al. 2013). Resent numerical studies of nucleosynthesis in NSMs successfully reproduce the solar r-process abundance pattern (e.g., Bauswein et al. 2013; Wanajo et al. 2014). The radioactive decay of these rprocess nuclei powers a electromagnetic transient known as a kilonova or macronova, which emit photons mainly in the infra-red band. The observed infrared excess in the afterglow of GRB 130603B is consistent with kilonova models and supports NSMs as a major r-process source (Tanvir et al. 2013; Berger et al. 2013; Barnes & Kasen 2013; Tanaka & Hotokezaka 2013)... 
We still have poor observational knowledge about both UHCRs and NSMs. There are still significant uncertainties on the absolute flux and the abundance pattern of UHCRs. There are few observational data for the cosmic ray energy spectrum of elements heavier than iron. We still have no observational sample of confirmed NSMs, though many SGRBs and a few kilonova candidates are detected. Future observations of the abundance pattern and spectrum of UHCRs and their temporal evolutions will be useful probes to understand the nature of NSM ejecta and origin of r-process elements in combination of detailed studies of propagation of UHCRs. 
(Submitted on 18 Aug 2017)




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