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, postdoctoral fellows and graduate students. The activities include research dealing with 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.
With Shiho Kobayashi, we have studied gravitational radiation from various proposed gamma-ray burst (GRB) progenitor models, in particular compact mergers and massive stellar collapses. These models have in common a high angular rotation rate, and the final stage involves a rotating black hole and accretion disk system. We consider the in-spiral, merger and ringing phases, and for massive collapses we consider the possible effects of asymmetric collapse and break-up, as well bar-mode instabilities in the disks. We calculate the strain and frequency of the gravitational waves expected from various progenitors, at distances based on occurrence rate estimates. Based on simplifying assumptions, we give estimates of the probability of detection of gravitational waves by the advanced LIGO system from the different GRB scenarios. For the NS-NS and NS-BH scenarios this is done assuming that wave templates will be available. If some fraction of GRBs are produced such coompact binary mergers, the gravitational wave chirp signal of the in-spiral phase should be detectable by the advanced LIGO within one year, associated with the GRB electromagnetic signal. For the BH-WD, BH-He and collapsar cases, where templates are likely to be uncertain at best, the signal/noise ratios are estimated using a cross-correlation technique with two co-aligned detectors. Under these assumptions, collapsar GRB models would be expected to be marginally detectable as gravitational wave sources by the advanced LIGO within one year of observations.
Another investigation with Shiho Kobayashi (2003) concerned the possible 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.
Gravitational waves and X-ray flares are 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 below 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 gravitational wave signal. The hardness of the X-ray flare may serve as a diagnostic of the mass of the black hole involved in the tidal disruption process.
Ioka and Meszaros (2005) considered the spatial clustering of massive black hole (MBH) mergers, and discuss possible ways to use gravitational wave observations in the space-born LISA 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 gamma-ray bursts is a common feature, which may be connected to the formation of a highly magnetized millisecond pulsar pumping energy into the fireball on timescales longer than the prompt emission. In this scenario, the nascent neutron star could undergo a secular bar-mode instability, leading to gravitational wave 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 gravitational wave signal emitted in association with an observed X-ray light-curve plateau, over relatively long 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
``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)
``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)
``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)