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تسجيل مستخدم جديدConstraining the coalescence rate of supermassive black-hole binaries using pulsar timing
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Pulsar timing observations are used to place constraints on the rate of coalescence of supermassive black-hole (SMBH) binaries as a function of mass and redshift. In contrast to the indirect constraints obtained from other techniques, pulsar timing observations provide a direct constraint on the black-hole merger rate. This is possible since pulsar timing is sensitive to the gravitational waves (GWs) emitted by these sources in the final stages of their evolution. We find that upper bounds calculated from the recently published Parkes Pulsar Timing Array data are just above theoretical predictions for redshifts below 10. In the future, with improved timing precision and longer data spans, we show that a non-detection of GWs will rule out some of the available parameter space in a particular class of SMBH binary merger models. We also show that if we can time a set of pulsars to 10ns timing accuracy, for example, using the proposed Square Kilometre Array, it should be possible to detect one or more individual SMBH binary systems.
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Pulsar timing arrays (PTAs) are expected to detect gravitational waves (GWs) from individual low-redshift (z<1.5) compact supermassive (M>10^9 Msun) black hole (SMBH) binaries with orbital periods of approx. 0.1 - 10 yrs. Identifying the electromagne
tic (EM) counterparts of these sources would provide confirmation of putative direct detections of GWs, present a rare opportunity to study the environments of compact SMBH binaries, and could enable the use of these sources as standard sirens for cosmology. Here we consider the feasibility of such an EM identification. We show that because the host galaxies of resolved PTA sources are expected to be exceptionally massive and rare, it should be possible to find unique hosts of resolved sources out to redshift z=0.2. At higher redshifts, the PTA error boxes are larger, and may contain as many as 100 massive-galaxy interlopers. The number of candidates, however, remains tractable for follow-up searches in upcoming wide-field EM surveys. We develop a toy model to characterize the dynamics and the thermal emission from a geometrically thin, gaseous disc accreting onto a PTA-source SMBH binary. Our model predicts that at optical and infrared frequencies, the source should appear similar to a typical luminous active galactic nucleus (AGN). However, owing to the evacuation of the accretion flow by the binarys tidal torques, the source might have an unusually low soft X-ray luminosity and weak UV and broad optical emission lines, as compared to an AGN powered by a single SMBH with the same total mass. For sources near z=1, the decrement in the rest-frame UV should be observable as an extremely red optical color. These properties would make the PTA sources stand out among optically luminous AGN, and could allow their unique identification.
We have searched for continuous gravitational wave (CGW) signals produced by individually resolvable, circular supermassive black hole binaries (SMBHBs) in the latest EPTA dataset, which consists of ultra-precise timing data on 41 millisecond pulsars
. We develop frequentist and Bayesian detection algorithms to search both for monochromatic and frequency-evolving systems. None of the adopted algorithms show evidence for the presence of such a CGW signal, indicating that the data are best described by pulsar and radiometer noise only. Depending on the adopted detection algorithm, the 95% upper limit on the sky-averaged strain amplitude lies in the range $6times 10^{-15}<A<1.5times10^{-14}$ at $5{rm nHz}<f<7{rm nHz}$. This limit varies by a factor of five, depending on the assumed source position, and the most constraining limit is achieved towards the positions of the most sensitive pulsars in the timing array. The most robust upper limit -- obtained via a full Bayesian analysis searching simultaneously over the signal and pulsar noise on the subset of ours six best pulsars -- is $Aapprox10^{-14}$. These limits, the most stringent to date at $f<10{rm nHz}$, exclude the presence of sub-centiparsec binaries with chirp mass $cal{M}_c>10^9$M$_odot$ out to a distance of about 25Mpc, and with $cal{M}_c>10^{10}$M$_odot$ out to a distance of about 1Gpc ($zapprox0.2$). We show that state-of-the-art SMBHB population models predict $<1%$ probability of detecting a CGW with the current EPTA dataset, consistent with the reported non-detection. We stress, however, that PTA limits on individual CGW have improved by almost an order of magnitude in the last five years. The continuing advances in pulsar timing data acquisition and analysis techniques will allow for strong astrophysical constraints on the population of nearby SMBHBs in the coming years.
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