The Center for Particle and Gravitational Astrophysics includes, since 2012, also the activities of the predecessor Center for Gravitational Wave Physics, established in 2000 incorporating members of the Physics and Astronomy & Astrophysics departments at Penn State, as well as a number of visitors, postdocs and graduate students. The activities include research in numerical relativity, astrophysics of gravitational wave sources, gravitational wave detection techniques (e.g. LIGO) and other techniques of detecting gravitational waves (e.g. Pulsar Timing Arrays). My own emphasis here is gravitational astrophysics.
The idea that the merger of binary neutron stars (BNS) driven together by their emission of gravitational waves (GWs) would lead to a gamma-ray bursts (GRB) dates back to Paczynski (1986). Previosuly, it had already been hypothesized that such BNS mergers would occur frequently enough to provide an attractive target for a GW detector (such as the present LIGO, Virgo, etc.). With Martin Rees, starting in 1991, we discussed how these BNS mergers, right after the emissio of the burst of GWs, would produce a burst of gamma-rays, based on the tidal disruption, heating and subsequent ejection along the rotational axis of a jet of high entropy matter at relativistic velocities (Meszaros and Rees, 1992a, 1992b). Shocks in these relatvistic jets (Rees and Meszaros, 1992; 1994) would accelerate particles producing the non-thermal prompt GRB gamma-ray emission. This is the standard model of the so-called short GRBs (sGRB), whose hard promtp gamma-ray ligt curve lasts roughly under 2 seconds. This picture fitted the GRB phenomenology very well, as far as the electromagetic (i.e. photon) emission; but the proof that it was coincident with a burst of GWs remained unavailable, until August 17, 2017 (see below).
With Shiho Kobayashi, we studied the gravitational radiation production and detectability from various proposed progenitor models, gamma-ray burst, in particular compact mergers of binary neutron stars (DNS, or BNS) and neutron star-black hole binaries, which would give rise to short GRBs, and massive stellar collapses (collapsars) which lead to long GRBs. The first two models have in common a high angular rotation rate, and the final stage involves a rotating black hole and accretion disk system, while in the collapsar one can hypothesize that a high core roation rate leads to relativistic instabilites leading to a bar, resulting in a similar final outcome. We considered the in-spiral, merger and ringing phases, and calculated the strain and frequency of the GWs expected, at distances based on occurrence rate estimates. We estimated the probability of detection of gravitational waves by the advanced LIGO system from the different GRB scenarios, assuming for the NS-NS and NS-BH scenarios the use of numerical wave templates, while for collapsar long GRBs a cross-correlation technique with two co-aligned detectors was assumed. For the BNS, the gravitational wave chirp signal of the in-spiral phase should be detectable by the advanced LIGO, associated with the GRB electromagnetic signal (see Figure on left). On the other hand, collapsar GRB models would be expected to be at best only marginally detectable by the advanced LIGO. Another investigation with S. Kobayashi (2003) concerned the relation between polarized gravitational waves from GRB and their electromagnetic radiation, both of which should depend on angle in a predictable way. While measurements will require more advanced detectors, such relationships provide valuable constraints on the production mechanisms.
LIGO proved its mettle by discovering GWs from merging stellar mass black hole binaries (BBHs), starting in September 14, 2016, with GW150914 , a BBH systaem of 36 and 29 solar masses. This was followed quickly with the discovery of several other binary black holes. Disapointingly, no electromagnetic radiation of any type was detected from these binaries. These BBHs all had masses roughly an order of magnitude larger than the typical neutron star mass of about 1.4 solar masses, and hence the BBHs higher GW luminosity made them easier to detect than neutron star binaries. These discoveries were recognized with the Physics Nobel prize in 2017.
Then, on August 17, 2017, LIGO finally discovered its first binary neutron star (BNS) merger, GW170817, which was followed, 1.7 seconds later, by a short gamma-ray burst seen by the Fermi GBM satellite detector, GRB170817A, as well as by the INTEGRAL satellite (see figure on the right; top two panels GBM, third panel INTEGRAL, bottom panel LIGO, showing the increasing chirp frequency). This long-awaited proof-positive that binary neutron star mergers produce both gravitaional waves and gamma-tay bursts can be taken (apart from the neutrinos and the light from the supernova SN1987a) as marking the beginning of the multi-messenger astrophysics era, where at least two completely different but complementary types of messengers (in this case gravitons and photons) are used to study the same object.
GW170817/GRB170817A was also detected, soon after the initial GRB and GW trigger, with a large number of other electromagnetic detectors, including X-ray, optical, IR and radio signals. The gamma rays appear to have been observed some 20 to 30 degrees off the jet axis (see Figure on the left), e.g. Ioka and Nakamura, 2017; Kasliwal, et al, 2017, while a broader outflow at large angles shows, after a day, a chracteristic macronova type of optical emission, believed to be powered by the decay of radioactive elements made by the r-process of rapid neutron capture, which also makes all the other stable A>56 elements, including gold, platinum, uranium, etc.
Gravitational waves and X-ray flares are also expected from tidal disruption of stars by a massive black hole. Using a relativistic smoothed particle hydrodynamics code, Kobayashi, Meszaros et al (2004) investigated the fate of main sequence and Helium stars in plunge orbits passing near Schwarzschild or a Kerr black holes of mass ~10^5-10^6 solar masses (see figure right). The quadrupole gravitational waves emitted during the tidal disruption process are described reasonable well by a point particle approximation even in the strong encounter case. X-ray flares (~ 1 keV for the disruption of solar-type stars by ~10^6 solar mass black holes) will be associated with the GW signal. The hardness of the X-ray flare may serve as a diagnostic of the mass of the black hole producing the tidal disruption.
Ioka and Meszaros (2005) considered the spatial clustering of massive black hole (MBH) mergers, and discussed possible ways to use GW observations in the space-born eLISA and DECIGO/BBO range for obtaining cosmological and cosmogonical information. Constraints on large scale structure (LSS) and merger histories would be possible through the detection of an alignment of the GW polarization direction with principal axes of the LSS. Constraints on the merger physics and the reionization epoch may be obtained by GW measurements of MBH correlation lengths, in the case when the MBH angular momentum loss occurs through gas drag. Such measurements would provide information about the LSS and the reionization epoch, as well as about the astrophysics of MBH mergers, additional to and independent of that obtained from electromagnetic signals.
Alessandra Corsi and Meszaros (2009) calculated the gravitational wave light curves expected under the hypothesis that GRBs are initially powered by a temporary magnetar phase. A shallow decay phase in the early X-ray afterglows of GRBs is a common feature, which may be connected to the formation of a highly magnetized millisecond pulsar pumping energy into the fireball. In this scenario, the nascent neutron star could undergo a secular bar-mode instability, leading to GW losses which affect the star's spin-down. In this case, nearby GRBs with isotropic energies of the order of 1E50 ergs would produce a detectable GW signal associated with an observed X-ray light-curve plateau, over timescales of minutes to about an hour. The peak amplitude of the GW signal would be delayed with respect to the gamma-ray burst trigger, thus offering gravitational wave interferometers such as the advanced Virgo and LIGO the challenging possibility of catching its signature on the fly. Research sponsor: NSF
``Tidal heating and mass loss in neutron star binaries - Implications for gamma-ray burst models", Meszaros, P. and Rees, M.J., 1992a, ApJ, 397:570
"High-entropy fireballs and jets in gamma-ray burst sources", Meszaros, P. and Rees, M.J., 1992b, MNRAS, 257:29P
"Relativistic fireballs - Energy conversion and time-scales", Rees, M.J. and Meszaros, P., 1992, MNRAS, 258:41P
"Unsteady outflow models for cosmological gamma-ray bursts", Rees, M.J. and Meszaros, P. 1994, ApJL, 430:L93
``Observation of Gravitational Waves from a Binary Black Hole Merger", LIGO-Virgo consortium, 2016, Phys.ReV.Lett, 116:061102
``Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A", LIGO, Virgo, INTEGRAL and Ferni/GBM collaborations, 2017, arXiv:1710.05834
``Polarized Gravitational Waves from Gamma-Ray Bursts", Shiho Kobayashi and Peter Meszaros, 2003, ApJ(Letters) 585, L89 (astro-ph/0212539)
``Gravitational Radiation from Gamma-Ray Burst Progenitors", Shiho Kobayashi and Peter Meszaros, 2002, ApJ, 589, 861 (astro-ph/0210211)
``Gravitational Wave and X-ray Signals from Stellar Disruption by a Massive Black Hole", Kobayashi, S, Laguna, P, Phinney, ES & Meszaros, P, 2004, ApJ, 615, 855 (astro-ph/0404173)
``Spatial Correlation of Massive Black Hole Mergers: Probing the Formation Mechanism and the Reionization", Ioka, K and Meszaros, P, 2005, ApJ, 635:143 (astro-ph/0502437)
``GRB Afterglow Plateaus and Gravitational Waves: Probing the Millisecond Magnetar Scenario", Corsi, A. and Meszaros, P., 2009, ApJ, 702:1711 (arXiv:0907.2290)