What if GW170817... and what's up GW170817/GRB170817A/AT2017gfo

.... proved the existence of unexpectedly heavy neutron star(s)?

The news that will be reported tomorrow by LIGO-Virgo detectors and several tenths of  telescopes will mark the real beginning of multimessenger astronomy with gravitational waves (SN 1987A provided the first coïncident detection of neutrino and electromagnetic signals) and will probably confirm the broad lines of reasoning relating short gamma ray bursts and binary neutron star or black-hole neutron star mergers. It will be the first detection of such binary objects and a fantastic opportunity to test numerical relativity with magnetohydrodynamical models but I can't help dreaming tonight about the possibility to find some hints of new physics. After all, the masses of  the black hole mergers detected up to now was unexpectedly high too ...

The discovery of the pulsar PSR J1614-2230 [1] led to several interpretative problems on the physics of neutron stars and to the possibility that the standard theory of such objects could be revised including also the other anomalous objects that observations are revealing [2, 3]. In particular, for a realistic description of nuclear matter, one needs to account for the appearance of exotic particle at densities ∼5−8×10¹⁴ g/cm³ [4]. Despite of this requirement, the equation of state (EoS) considerably softens (Mmax∼1.5−1.6M_⊙) (see [5–9]) and therefore the maximal neutron star mass results reduced. Some approaches have been recently proposed to solve the problem [10–20], however there is no final agreement on the solution of the puzzle. In particular, the required maximal mass (∼2M_⊙) can be obtained for hyperon EoS from more complex model of strong interaction than simple σωω-model with realistic hyperon-meson couplings. However, it is worth noticing that one cannot derive a reliable M-R relation for neutron stars from observations because there are no precise radius measurements for any stars [21]. On the other hand, the maximal limit of neutron star mass can increases considerably due to strong magnetic field inside the star. The observations of gamma-ray repeaters and anomalous X-ray pulsars may indicate the existence of magnetic fields of the order 10¹⁵ G on the stellar surface. In the center of the star, the magnetic field can exceed 10¹⁸ G. Realistic EoS with hyperons and quarks in presence of strong magnetic field are considered in [22–25]. The maximal mass of neutron star can exceed, in these cases, 3M_⊙  for Bc ∼3.3×10¹⁸ G. Therefore it seems that the existence of neutron stars with masses exceeding considerably two solar mass (without strong magnetic field) is impossible in the framework of General Relativity (GR). Despite of this theoretical constraint, it is interesting to note that there are several observational indications in favor of maximal masses that exceed this limit. In fact, masses of B1957+20 and 4U 1700-377 are estimated as ∼ 2.4M_⊙ [26, 27] while for PSR J1748-2021B , the mass limit reaches M ∼ 2.7M_⊙ [28]. In principle, observational data on neutron stars (mainly the mass-radius M −R relation) can be used to investigate possible deviations from GR as probe for alternative gravity theories...

Our considerations show that considerable increasing of mass can be achieved adopting cubic f(R) gravity corrections. Thus, the possibility of supermassive (M > 4M_⊙) neutron stars with R∼12−15 km in modified gravity seems, in principle, realistic. If such stars will be explicitly observed, this could be considered as a clear signature that some self-gravitating systems can violate General Relativity constraints in favor of modified gravity. On the other hand, quadratic f(G) gravity corrections indicate that another interesting effect is possible: namely stable stars with central densities close to ρc=1.5−2.0 GeV/fm³ (and therefore with high strangeness fraction) can exist. The field strength in the center can exceed 8 × 10¹⁸ G. This limit cannot be achieved in General Relativity by using standard observed matter. As a general remark, we can say that the puzzles related to the existence of extreme neutron stars could be realistically addressed by supposing the emergence of corrections and extensions to the General Relativity in the strong field regimes. In some sense, the mechanism could be similar to that supposed in the early-time inflation where higher-order curvature terms naturally emerge into dynamics. Beside the explanation of anomalous compact star, this approach could be considered as an independent probe for modified gravity with respect to the analogue descriptions invoked for dark matter and dark energy.

Extreme neutron stars from Extended Theories of Gravity

Artyom V. Astashenok, Salvatore Capozziello, Sergei D. Odintsov

(Submitted on 17 Aug 2014 (v1), last revised 24 Nov 2014 (this version, v2))

Constraints on tabulated equations-of-state (EoS) from the posterior distribution of  the maximum neutron star (NS) mass (mmax) (derived from inferring the distribution of NS masses, assuming the mass distribution can be modeled as the sum of two Gaussians with a hard cut at mmax). From this figure it’s clear that this analysis of the NS mass distribution strongly favors some EoS models relative to others, at odds ratios of up to 15:1. This is a vast improvement over previous maximum mass considerations, where any EoS that supported NSs with >2M was considered equally acceptable based on maximum mass considerations alone. These results fall nicely in line with independent constraints on the maximum NS mass derived from the assumption that short GRBs are produced primarily by coalescing NSs with rapidly collapsing remnants (Lawrence et al. 2015; Fryer et al. 2015). This begins to paint a coherent picture of upper limits on the maximum NS mass from astrophysical observations and considerations. (from Evidence for a maximum mass cut-off in the neutron star mass distribution and constraints on the equation of state Justin Alsing, Hector O. Silva, Emanuele Berti (Submitted on 22 Sep 2017 (v1), last revised 27 Sep 2017 (this version, v2)).

//Update Oct 16th, 2017

A posteriori: no suprise from GW170817 on neutron star mass...

Two-dimensional posterior distribution for the component masses m1 and m2 in the rest frame of the {GW170817} source for the low spin scenario (|χ| < 0.05, blue) and the high-spin scenario (|χ| < 0.89, red). The colored contours enclose 90% of the probability from the joint posterior probability density function for m1 and m2. The shape of the two dimensional posterior is determined by a line of constant M and its width is determined by the uncertainty in M. The widths of the marginal distributions (shown on axes, dashed lines enclose 90% probability away from equal mass of 1.36M⊙) is strongly affected by the choice of spin priors. The result using the low-spin prior (blue) is consistent with the masses of all known binary neutron star systems.

//update October 18th

A new less model dependent upper limit for neutron star masses derived from GW-GRB-KN170817

We combine electromagnetic (EM) and gravitational wave (GW) information on the binary neutron star (NS) merger GW170817 in order to constrain the radii R ns   and maximum mass M max   of NSs. GW170817 was followed by a range of EM counterparts, including a weak gamma-ray burst (GRB), kilonova (KN) emission from the radioactive decay of the merger ejecta, and X-ray/radio emission consistent with being the synchrotron afterglow of a more powerful off-axis jet. The type of compact remnant produced in the immediate merger aftermath, and its predicted EM signal, depend sensitively on the high-density NS equation of state (EOS). For a soft EOS which supports a low M max   , the merger undergoes a prompt collapse accompanied by a small quantity of shock-heated or disk wind ejecta, inconsistent with the large quantity ≳10 −2 M ⊙   of lanthanide-free ejecta inferred from the KN. On the other hand, if M max   is sufficiently large, then the merger product is a rapidly-rotating supramassive NS (SMNS), which must spin-down before collapsing into a black hole. A fraction of the enormous rotational energy necessarily released by the SMNS during this process is transferred to the ejecta, either into the GRB jet (energy E GRB   ) or the KN ejecta (energy E ej   ), also inconsistent with observations. By combining the total binary mass of GW170817 inferred from the GW signal with conservative upper limits on E GRB   and E ej   from EM observations, we constrain the likelihood probability of a wide-range of previously-allowed EOS. These two constraints delineate an allowed region of the M max −R ns   parameter space, which once marginalized over NS radius places an upper limit of M max ≲2.17M ⊙   (90\%), which is tighter or arguably less model-dependent than other current constraints.

Constraining the Maximum Mass of Neutron Stars From Multi-Messenger Observations of GW170817
Authors: Ben Margalit, Brian Metzger
(Submitted on 16 Oct 2017)