Duncan Neill
Gather.town id
TA04
Poster Title
Resonant Shattering Flares as Multimessenger Probes of the Nuclear Symmetry Energy
Institution
University of Bath
Abstract (short summary)
The behaviour of the nuclear symmetry energy near saturation density is important for our understanding of dense nuclear matter. This density dependence can be parameterised by the nuclear symmetry energy and its derivatives evaluated at nuclear saturation density. In our work we have found that the quadrupole (ℓ=2) core-crust interface mode of a neutron star is sensitive to these parameters, through the (density-weighted) shear-speed within the crust, which is in turn dependent on the symmetry energy profile of dense matter. We have calculated the frequency at which the neutron star crust-core interface mode must be driven by the tidal field of its binary partner to trigger a Resonant Shattering Flare (RSF). With this, we demonstrate that coincident multimessenger timing of an RSF and gravitational wave chirp from a neutron star merger would enable us to place constraints on the symmetry energy parameters that are competitive with those from current nuclear experiments.
Plain text (extended) Summary
Resonant Shattering Flares (RSFs) are gamma-ray flares produced by resonant excitation of one of the modes of oscillation within a neutron star (NS) by the tidal field of its binary partner. This can occur during the late stages of the binary in-spiral of BHNS and NSNS systems. The crust-core interface mode (i-mode) has properties that make it the one most likely to trigger an RSF. This mode is dependent on the composition of the NS crust, which is in turn dependent on the nuclear symmetry energy parameters. Therefore, the properties of RSFs allow us to constrain these important parameters.
Possible EM counterparts to BHNS mergers include SGRBs and Kilonovae. However, both of these require that the NS be tidally disrupted by the black hole (BH). A massive and/or low spin BH may swallow its NS before tidal disruption occurs, resulting in no SGRB or Kilonova. RSFs on the other hand do not require tidal disruption, but instead require that resonant mode excitation occur before the inner-most stable circular orbit (ISCO). Similarly to tidal disruption, whether this occurs is dependent on the BH mass and spin, but the requirements are much weaker. We calculate the percentage of BHNS systems that satisfy each of these requirements (for both the upper and lower limit on BH spin), finding that, for the more realistic lower limit on BH spin, SGRBs and Kilonovae can only occur in a small fraction of mergers, while RSFs could be produced in almost all of them!
While RSFs may be produced in many BHNS mergers, whether they are luminous enough to be observed depends on the NS magnetic field distribution. The magnetic field determines the rate at which energy is transferred from seismic waves to pair-photon fireball shells. A stronger field will result in more shells being emitted during the resonance window, which in turn means more shell collisions and hence a brighter flare. We calculate the luminosities of 10 years of RSFs from BHNS mergers, and compare them to the source luminosity requirement for detection by Fermi/GBM. Using our upper and lower limits on the NS magnetic field strength distribution results in 4.1 and 0.5 detectable RSFs per year, respectively. Even in the lower limit, this rate is high enough that RSFs may be more commonly observed products of BHNS mergers than SGRBs or Kilonovae!
When resonant i-mode excitation triggers an RSF, the frequency of the GWs from the system will be (approximately) equal to the mode’s frequency. Therefore, a multi-messenger detection of GWs and a RSF can be used to measure the i-mode frequency. The i-mode frequency is strongly dependent of the properties of the NS crust, which are in turn dependent on the nuclear symmetry energy parameters. Using a set of equations of state (EOSs) parametrised by the first three symmetry energy parameters (J, L and Ksym), we are able to find surfaces of constant i-mode frequency in the J-L-Ksym parameter space. A GW detection at the frequency corresponding to one of these surfaces would constrain the parameters to lie on that surface (within a small degree of uncertainty). We compare these RSF constraints on J and L to the combined constraints from terrestrial nuclear experiments, and find that they are of comparable strength!
Possible EM counterparts to BHNS mergers include SGRBs and Kilonovae. However, both of these require that the NS be tidally disrupted by the black hole (BH). A massive and/or low spin BH may swallow its NS before tidal disruption occurs, resulting in no SGRB or Kilonova. RSFs on the other hand do not require tidal disruption, but instead require that resonant mode excitation occur before the inner-most stable circular orbit (ISCO). Similarly to tidal disruption, whether this occurs is dependent on the BH mass and spin, but the requirements are much weaker. We calculate the percentage of BHNS systems that satisfy each of these requirements (for both the upper and lower limit on BH spin), finding that, for the more realistic lower limit on BH spin, SGRBs and Kilonovae can only occur in a small fraction of mergers, while RSFs could be produced in almost all of them!
While RSFs may be produced in many BHNS mergers, whether they are luminous enough to be observed depends on the NS magnetic field distribution. The magnetic field determines the rate at which energy is transferred from seismic waves to pair-photon fireball shells. A stronger field will result in more shells being emitted during the resonance window, which in turn means more shell collisions and hence a brighter flare. We calculate the luminosities of 10 years of RSFs from BHNS mergers, and compare them to the source luminosity requirement for detection by Fermi/GBM. Using our upper and lower limits on the NS magnetic field strength distribution results in 4.1 and 0.5 detectable RSFs per year, respectively. Even in the lower limit, this rate is high enough that RSFs may be more commonly observed products of BHNS mergers than SGRBs or Kilonovae!
When resonant i-mode excitation triggers an RSF, the frequency of the GWs from the system will be (approximately) equal to the mode’s frequency. Therefore, a multi-messenger detection of GWs and a RSF can be used to measure the i-mode frequency. The i-mode frequency is strongly dependent of the properties of the NS crust, which are in turn dependent on the nuclear symmetry energy parameters. Using a set of equations of state (EOSs) parametrised by the first three symmetry energy parameters (J, L and Ksym), we are able to find surfaces of constant i-mode frequency in the J-L-Ksym parameter space. A GW detection at the frequency corresponding to one of these surfaces would constrain the parameters to lie on that surface (within a small degree of uncertainty). We compare these RSF constraints on J and L to the combined constraints from terrestrial nuclear experiments, and find that they are of comparable strength!
URL
dn431@bath.ac.uk
Poster file