George Hobbs

Gravitational waves from binary supermassive black holes missing in pulsar observations

Placing bounds on gravitational wave detection Gravitational waves are expected to be generated by the interaction of the massive bodies in black-hole binary systems. As gravitational waves distort spacetime, it should be possible to verify their existence as they interfere with the pulses emitted by millisecond pulsars. However, after monitoring 24 pulsars with the Parkes radio telescope for 12 years, Shannon et al. found no detectable variation in pulsar records. This nondetection result indicates that a new detection strategy for gravitational waves is needed. Science, this issue p. 1522 A lack of observed variations in the timing of pulsars places constraints on the detection of gravitational waves. Gravitational waves are expected to be radiated by supermassive black hole binaries formed during galaxy mergers. A stochastic superposition of gravitational waves from all such binary systems would modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrained the characteristic amplitude of this background, Ac,yr, to be <1.0 × 10−15 with 95% confidence. This limit excludes predicted ranges for Ac,yr from current models with 91 to 99.7% probability. We conclude that binary evolution is either stalled or dramatically accelerated by galactic-center environments and that higher-cadence and shorter-wavelength observations would be more sensitive to gravitational waves.

The International Pulsar Timing Array: first data release

The highly stable spin of neutron stars can be exploited for a variety of (astro)physical investigations. In particular, arrays of pulsars with rotational periods of the order of milliseconds can be used to detect correlated signals such as those caused by gravitational waves. Three such pulsar timing arrays (PTAs) have been set up around the world over the past decades and collectively form the International PTA (IPTA). In this paper, we describe the first joint analysis of the data from the three regional PTAs, i.e. of the first IPTA data set. We describe the available PTA data, the approach presently followed for its combination and suggest improvements for future PTA research. Particular attention is paid to subtle details (such as underestimation of measurement uncertainty and long-period noise) that have often been ignored but which become important in this unprecedentedly large and inhomogeneous data set. We identify and describe in detail several factors that complicate IPTA research and provide recommendations for future pulsar timing efforts. The first IPTA data release presented here (and available on-line) is used to demonstrate the IPTAs potential of improving upon gravitational-wave limits

Gravitational Wave Astronomy with the SKA

On a time scale of years to decades, gravitational wave (GW) astronomy will become a reality. Low frequency (∼10 −9 Hz) GWs are detectable through long-term timing observations of the most stable pulsars. Radio observatories worldwide are currently carrying out observing programmes to detect GWs, with data sets being shared through the International Pulsar Timing Array project. One of the most likely sources of low frequency GWs are supermassive black hole binaries (SMBHBs), detectable as a background due to a large number of binaries, or as continuous or burst emission from individual sources. No GW signal has yet been detected, but stringent constraints are already being placed on galaxy evolution models. The SKA will bring this research to fruition. In this chapter, we describe how timing observations using SKA1 will contribute to detecting GWs, or can confirm a detection if a first signal already has bee n identified when SKA1 commences observations. We describe how SKA observations will identify the source(s) of a GW signal, search for anisotropies in the background, improve models of galaxy evolution, test theories of gravity, and characterise the early inspiral phase o f a SMBHB system. We describe the impact of the large number of millisecond pulsars to be discovered by the SKA; and the observing cadence, observation durations, and instrumentation required to reach the necessary sensitivity. We describe the noise processes that wi ll influence the achievable precision with the SKA. We assume a long-term timing programme using the SKA1-MID array and consider the implications of modifications to the current desig n. We describe the possible benefits from observations using SKA1-LOW. Finally, we describe GW detection prospects with SKA1 and SKA2, and end with a description of the expectations of GW astronomy.