The Center for
Gravitational Wave Physics
was established in 2000, incorporating members of the Physics and
Astronomy & Astrophysics departments at Penn State, as well as a number
of long-term senior visitors, postdoctoral fellows and graduate students.
It runs an active conference and workshop program, dealing with numercial
relativity, astrophysics of gravitational wave sources and gravitational
wave detection techniques. 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.
Research sponsor: NSF
``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)