Possible sources of gravitational wave bursts detectable today

Abstract

We discuss the possibility that galactic gravitational wave sources might give burst signals at a rate of several events per year, detectable by state-of-the-art detectors. We are stimulated by the results of the data collected by the EXPLORER and NAUTILUS bar detectors in the 2001 run, which suggest an excess of coincidences between the two detectors, when the resonant bars are orthogonal to the galactic plane. Signals due to the coalescence of galactic compact binaries fulfill the energy requirements but are problematic for lack of known candidates with the necessary merging rate. We examine the limits imposed by galactic dynamics on the mass loss of the Galaxy due to GW emission, and we use them to put constraints also on the GW radiation from exotic objects, like binaries made of primordial black holes. We discuss the possibility that the events are due to GW bursts coming repeatedly from a single or a few compact sources. We examine different possible realizations of this idea, such as accreting neutron stars, strange quark stars, and the highly magnetized neutron stars ("magnetars") introduced to explain Soft Gamma Repeaters. Various possibilities are excluded or appear very unlikely, while others at present cannot be excluded.

Upper limits on gravitational wave bursts in LIGO’s second science run

“…[48]). The rate of such events is of order 200 events/year (or 0.55 events/day) [48,49]. Given the amplitude of the observed events by the resonant mass detectors, the corresponding h rss of the hypothetical events in our LIGO instruments will generally depend on the signal morphology.…”

Search for gravitational-wave bursts in LIGO data from the fourth science run

“…For such Gaussians our S4 search all-sky 90% sensitivity is 2.5 × 10 −20 Hz −1/2 (see table 3) and when accounting for optimal orientation, this improves by roughly a factor of 3, to 9.3 × 10 −21 Hz −1/2 . The rate of the EXPLORER and NAUTILUS events was of order 200 events year −1 (or 0.55 events day −1 ) [37,38]. A steady flux of gravitational-wave bursts at this rate is excluded by our present measurement at the 99.9% confidence level.…”

The search for gravitational waves

“…They have been looking closely at the energetics of the signals. Eugenio Coccia of the Rome group concludes that if the signals are from gravitational waves, standard sources such as coalescing compact binaries in the galaxy would not happen often enough; rather some more exotic source such as one or more repeating bursters in the plane of the galaxy would be needed [31]. Analysis of a further year of data is currently underway and may shed further light on the situation although preliminary indications from the Rome group suggest that the outcome is less than clear.…”