Abstract

We discuss the different signals, in gravitational and electromagnetic waves, emitted during the merger of two compact stars. We will focus in particular on the possible contraints that those signals can provide on the equation of state of dense matter. Indeed, the stiffness of the equation of state and the particle composition of the merging compact stars strongly affect, e.g., the life time of the post-merger remnant and its gravitational wave signal, the emission of the short gamma-ray-burst, the amount of ejected mass and the related kilonova. The first detection of gravitational waves from the merger of two compact stars in August 2017, GW170817, and the subsequent detections of its electromagnetic counterparts, GRB170817A and AT2017gfo, is the first example of the era of “multi-messenger astronomy”: we discuss what we have learned from this detection on the equation of state of compact stars and we provide a tentative interpretation of this event, within the two families scenario, as being due to the merger of a hadronic star with a quark star.

Highlights

  • The observation, on 17 August 2017, of the coalescence of two compact objects characterized by masses in the typical neutron star (NS) range has marked the beginning of the so-called “multi-messenger astronomy” [1] (SN1987a, detected both by optical telescopes and by neutrino observatories, constitutes probably the first example of a multi-messenger astronomical event)

  • This implies that once gravitational wave (GW) will be detected from mergers, this will allow us to measure Mthreshold, and to obtain precious information on the structure of cold neutron stars and on the equation of state of dense nuclear matter

  • The prompt emission takes place when the process of quark deconfinement has reached the surface of the star reducing the baryonic pollution and the Extended Emission (EE) is due to the proto-magnetar that in this case is a Quark Star (QS) [17]

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Summary

Introduction

The observation, on 17 August 2017, of the coalescence of two compact objects characterized by masses in the typical neutron star (NS) range has marked the beginning of the so-called “multi-messenger astronomy” [1] (SN1987a, detected both by optical telescopes and by neutrino observatories, constitutes probably the first example of a multi-messenger astronomical event). The merger event has provided a signal in gravitational waves (GW170817) detected by Advanced. Gamma- Ray-Burst event (GRB170817A) delayed by 1.7 s with respect to the merger time [2] These two detections have been followed by multiple observations revealing the existence of electromagnetic (EM). The study of the optical counterpart of GW170817, called kilonova (AT2017gfo) because of its peak luminosity, has confirmed that NS mergers host r-processes responsible of the synthesis of the most heavy nuclei. It has provided information about the amount and features of the ejecta and these. Universe 2018, 4, 50 could give constraints about the importance of the different ejection mechanisms and of the features of the progenitors [3,4]

Expectations from the GW Signal
Ejection Mechanism and Features of the Outgoing Fluid
R-Processes
EM Counterpart
Analysis of the GW Signal
The Weak Gamma Emission of GRB170817A
Analysis of the Optical Transient
Role of Different Ejection Mechanisms
Findings
A Different Hypothesis: A Hadronic Star–Quark Star Merger
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