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Register a new userEuropean Pulsar Timing Array Limits on Continuous Gravitational Waves from Individual Supermassive Black Hole Binaries
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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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The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) project currently observes 43 pulsars using the Green Bank and Arecibo radio telescopes. In this work we use a subset of 17 pulsars timed for a span of roughly five years (2005--2010). We analyze these data using standard pulsar timing models, with the addition of time-variable dispersion measure and frequency-variable pulse shape terms. Within the timing data, we perform a search for continuous gravitational waves from individual supermassive black hole binaries in circular orbits using robust frequentist and Bayesian techniques. We find that there is no evidence for the presence of a detectable continuous gravitational wave; however, we can use these data to place the most constraining upper limits to date on the strength of such gravitational waves. Using the full 17 pulsar dataset we place a 95% upper limit on the sky-averaged strain amplitude of $h_0lesssim 3.8times 10^{-14}$ at a frequency of 10 nHz. Furthermore, we place 95% emph{all sky} lower limits on the luminosity distance to such gravitational wave sources finding that the $d_L gtrsim 425$ Mpc for sources at a frequency of 10 nHz and chirp mass $10^{10}{rm M}_{odot}$. We find that for gravitational wave sources near our best timed pulsars in the sky, the sensitivity of the pulsar timing array is increased by a factor of $sim$4 over the sky-averaged sensitivity. Finally we place limits on the coalescence rate of the most massive supermassive black hole binaries.
The formation and growth processes of supermassive black holes (SMBHs) are not well constrained. SMBH population models, however, provide specific predictions for the properties of the gravitational-wave background (GWB) from binary SMBHs in merging galaxies throughout the Universe. Using observations from the Parkes Pulsar Timing Array, we constrain the fractional GWB energy density with 95% confidence to be ${Omega}_{GW}(H_0/73 {rm km} {rm s}^{-1} {rm Mpc}^{-1})^2 < 1.3 times 10^{-9}$ at a frequency of 2.8 nHz, which is approximately a factor of six more stringent than previous limits. We compare our limit to models of the SMBH population and find inconsistencies at confidence levels between 46% and 91%. For example, the standard galaxy formation model implemented in the Millennium simulations is inconsistent with our limit with 50% probability.
Observations indicate that nearly all galaxies contain supermassive black holes (SMBHs) at their centers. When galaxies merge, their component black holes form SMBH binaries (SMBHBs), which emit low-frequency gravitational waves (GWs) that can be detected by pulsar timing arrays (PTAs). We have searched the recently-released North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11-year data set for GWs from individual SMBHBs in circular orbits. As we did not find strong evidence for GWs in our data, we placed 95% upper limits on the strength of GWs from such sources as a function of GW frequency and sky location. We placed a sky-averaged upper limit on the GW strain of $h_0 < 7.3(3) times 10^{-15}$ at $f_mathrm{gw}= 8$ nHz. We also developed a technique to determine the significance of a particular signal in each pulsar using ``dropout parameters as a way of identifying spurious signals in measurements from individual pulsars. We used our upper limits on the GW strain to place lower limits on the distances to individual SMBHBs. At the most-sensitive sky location, we ruled out SMBHBs emitting GWs with $f_mathrm{gw}= 8$ nHz within 120 Mpc for $mathcal{M} = 10^9 , M_odot$, and within 5.5 Gpc for $mathcal{M} = 10^{10} , M_odot$. We also determined that there are no SMBHBs with $mathcal{M} > 1.6 times 10^9 , M_odot$ emitting GWs in the Virgo Cluster. Finally, we estimated the number of potentially detectable sources given our current strain upper limits based on galaxies in Two Micron All-Sky Survey (2MASS) and merger rates from the Illustris cosmological simulation project. Only 34 out of 75,000 realizations of the local Universe contained a detectable source, from which we concluded it was unsurprising that we did not detect any individual sources given our current sensitivity to GWs.
We present new limits on an isotropic stochastic gravitational-wave background (GWB) using a six pulsar dataset spanning 18 yr of observations from the 2015 European Pulsar Timing Array data release. Performing a Bayesian analysis, we fit simultaneously for the intrinsic noise parameters for each pulsar, along with common correlated signals including clock, and Solar System ephemeris errors, obtaining a robust 95$%$ upper limit on the dimensionless strain amplitude $A$ of the background of $A<3.0times 10^{-15}$ at a reference frequency of $1mathrm{yr^{-1}}$ and a spectral index of $13/3$, corresponding to a background from inspiralling super-massive black hole binaries, constraining the GW energy density to $Omega_mathrm{gw}(f)h^2 < 1.1times10^{-9}$ at 2.8 nHz. We also present limits on the correlated power spectrum at a series of discrete frequencies, and show that our sensitivity to a fiducial isotropic GWB is highest at a frequency of $sim 5times10^{-9}$~Hz. Finally we discuss the implications of our analysis for the astrophysics of supermassive black hole binaries, and present 95$%$ upper limits on the string tension, $Gmu/c^2$, characterising a background produced by a cosmic string network for a set of possible scenarios, and for a stochastic relic GWB. For a Nambu-Goto field theory cosmic string network, we set a limit $Gmu/c^2<1.3times10^{-7}$, identical to that set by the {it Planck} Collaboration, when combining {it Planck} and high-$ell$ Cosmic Microwave Background data from other experiments. For a stochastic relic background we set a limit of $Omega^mathrm{relic}_mathrm{gw}(f)h^2<1.2 times10^{-9}$, a factor of 9 improvement over the most stringent limits previously set by a pulsar timing array.
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 will modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrain the characteristic amplitude of this background, $A_{rm c,yr}$, to be < $1.0times10^{-15}$ with 95% confidence. This limit excludes predicted ranges for $A_{rm c,yr}$ from current models with 91-99.7% 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 result in an increased sensitivity to gravitational waves.
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