Toward an optimal search strategy of optical and gravitational wave emissions from binary neutron star coalescence

Observations of an optical source coincident with gravitational wave emission detected from a binary neutron star coalescence will improve the confidence of detection, provide host galaxy localisation, and test models for the progenitors of short gamma ray bursts. We employ optical observations of three short gamma ray bursts, 050724, 050709, 051221, to estimate the detection rate of a coordinated optical and gravitational wave search of neutron star mergers. Model R-band optical afterglow light curves of these bursts that include a jet-break are extrapolated for these sources at the sensitivity horizon of an Advanced LIGO/Virgo network. Using optical sensitivity limits of three telescopes, namely TAROT (m=18), Zadko (m=21) and an (8-10) meter class telescope (m=26), we approximate detection rates and cadence times for imaging. We find a median coincident detection rate of 4 yr^{-1} for the three bursts. GRB 050724 like bursts, with wide opening jet angles, offer the most optimistic rate of 13 coincident detections yr^{-1}, and would be detectable by Zadko up to five days after the trigger. Late time imaging to m=26 could detect off-axis afterglows for GRB 051221 like bursts several months after the trigger. For a broad distribution of beaming angles, the optimal strategy for identifying the optical emissions triggered by gravitational wave detectors is rapid response searches with robotic telescopes followed by deeper imaging at later times if an afterglow is not detected within several days of the trigger.

The Zadko Telescope: A Southern Hemisphere Telescope for Optical Transient Searches, Multi-Messenger Astronomy and Education

The new 1-m f/4 fast-slew Zadko Telescope was installed in June 2008 about 70 km north of Perth, Western Australia. It is the only metre-class optical facility at this southern latitude between the east coast of Australia and South Africa, and can rapidly image optical transients at a longitude not monitored by other similar facilities. We report on first imaging tests of a pilot program of minor planet searches, and Target of Opportunity observations triggered by the Swift satellite. In 12 months, 6 gamma-ray burst afterglows were detected, with estimated magnitudes; two of them, GRB 090205 (z = 4.65) and GRB 090516 (z = 4.11), are among the most distant optical transients imaged by an Australian telescope. Many asteroids were observed in a systematic 3-month search. In September 2009, an automatic telescope control system was installed, which will be used to link the facility to a global robotic telescope network; future targets will include fast optical transients triggered by highenergy satellites, radio transient detections, and LIGO gravitational wave candidate events. We also outline the importance of the facility as a potential tool for education, training, and public outreach.

Probing the low-luminosity GRB population with new generation satellite detectors

We compare the detection rates and redshift distributions of low-luminosity (LL) GRBs localized by Swift with those expected to be observed by the new generation satellite detectors on GLAST (now Fermi) and, in future, EXIST. Although the GLAST burst telescope will be less sensitive than Swifts in the 15--150 keV band, its large field-of-view implies that it will double Swifts detection rate of LL bursts. We show that Swift, GLAST and EXIST should detect about 1, 2 & 30 LL GRBs, respectively, over a 5-year operational period. The burst telescope on EXIST should detect LL GRBs at a rate of more than an order of magnitude greater than that of Swifts BAT. We show that the detection horizon for LL GRBs will be extended from $z simeq 0.4$ for Swift to $z simeq 1.1$ in the EXIST era. Also, the contribution of LL bursts to the observed GRB redshift distribution will contribute to an identifiable feature in the distribution at $z simeq 1$.

Where are the missing gamma ray burst redshifts?

In the redshift range z = 0-1, the gamma ray burst (GRB) redshift distribution should increase rapidly because of increasing differential volume sizes and strong evolution in the star formation rate. This feature is not observed in the Swift redshift distribution and to account for this discrepancy, a dominant bias, independent of the Swift sensitivity, is required. Furthermore, despite rapid localization, about 40-50% of Swift and pre-Swift GRBs do not have a measured redshift. We employ a heuristic technique to extract this redshift bias using 66 GRBs localized by Swift with redshifts determined from absorption or emission spectroscopy. For the Swift and HETE+BeppoSAX redshift distributions, the best model fit to the bias in z < 1 implies that if GRB rate evolution follows the SFR, the bias cancels this rate increase. We find that the same bias is affecting both Swift and HETE+BeppoSAX measurements similarly in z < 1. Using a bias model constrained at a 98% KS probability, we find that 72% of GRBs in z < 2 will not have measurable redshifts and about 55% in z > 2. To achieve this high KS probability requires increasing the GRB rate density in small z compared to the high-z rate. This provides further evidence for a low-luminosity population of GRBs that are observed in only a small volume because of their faintness.