A first-order MeV-scale cosmological phase transition (PT) can generate a peak in the power spectrum of stochastic gravitational wave background around nanohertz frequencies. With the recent International Pulsar Timing Array data release two covering nanohertz frequencies, we search for such a phase transition signal. For the standard 4-parameter PT model, we obtain the PT temperature $T_\star\in$ [66 MeV, 30 GeV], which indicates that dark or QCD phase transitions occurring below 66 MeV have been ruled out at $2\,σ$ confidence level. This constraint is much tighter than $T_\star\sim$ [1 MeV, 100 GeV] from NANOGrav. We also give much tighter $2\,σ$ bounds on the PT duration $H_\star/β>0.1$, strength $α_\star>0.39$ and friction $η<2.74$ than NANOGrav. For the first time, we find a positive correlation between $\mathrm{log}_{10}T_\star$ and $\mathrm{log}_{10}H_\star/β$ implying that PT temperature increases with increasing bubble nucleation rate. To avoid large theoretical uncertainties in calculating PT spectrum, we make bubble spectral shape parameters $a$, $b$, $c$ and four PT parameters free together, and confront this model with data. We find that pulsar timing is very sensitive to the parameter $a$, and give the first clear constraint $a=1.27_{-0.54}^{+0.71}$ at $1\,σ$ confidence level.
After NANOGrav, the IPTA collaboration also reports a strong evidence of a stochastic gravitation wave background. This hint has very important implications for fundamental physics. With the recent IPTA data release two, we attempt to search signals of light new physics. and give new constraints on the audible axion, domain walls and cosmic strings models. We find that the best fit point corresponding to a decay constant $F\approx5\times10^{17}$ GeV and an axion mass $m_a\approx2\times10^{-13}$ eV from NANOGrav data is ruled out by IPTA at beyond $2σ$ confidence level. Fixing the coupling stre…
Nancay radiotelescope is involved in high precision timing since 20 years. Since 2004, a coherent dedispersion instrumentation enables numerous routine observations on more than 200 pulsars using half of the time if this 100-meters class radiotelescope. Two main programs are currently conducted. A large set of young and old pulsars is timed for a multi-wavelength approach, complementary to the very successful high energy observations of pulsars done by FERMI. A set of highly stable millisecond pulsars is monitored as our contribution to the European Pulsar Timing Array in order to probe the co…
We describe an analysis of the First International Pulsar Timing Array Data Challenge. We employ a robust, unbiased Bayesian framework developed by van Haasteren to study the three Open and Closed datasets, testing various models for each dataset and using MultiNest to recover the evidence for the purposes of Bayesian model-selection. The parameter constraints of the favoured model are confirmed using an adaptive MCMC technique. Our results for Closed1 favoured a gravitational-wave background with strain amplitude at f=1 yr-1, A, of (1.1 +/- 0.1) x 10^{-14}, power spectral-index gamma=4.30 +/-…
We provide brief notes on a particle swarm-optimisation approach to constraining the properties of a stochastic gravitational-wave background in the first International Pulsar Timing Array data-challenge. The technique employs many computational-agents which explore parameter space, remembering their most optimal positions and also sharing this information with all other agents. It is this sharing of information which accelerates the convergence of all agents to the global best-fit location in a very short number of iterations. Error estimates can also be provided by fitting a multivariate Gau…
We present a model-independent search for the gravitational wave background from cosmic domain walls (DWs) in the NANOGrav 12.5 years dataset and International PTA Data Release 2. DWs that annihilate at temperatures $\sim 20-50~\text{MeV}$ with tensions $\sim (40-100~\text{TeV})^3$ provide as good a fit to both datasets as the astrophysical background from supermassive black hole mergers. DWs may decay into the Standard Model (SM) or a dark sector. In the latter case we predict an abundance $ΔN_{\text{eff}}$ of dark radiation well within the reach of upcoming CMB surveys. Complementary signatu…
Pulsar timing arrays (PTAs) can be used to detect and study gravitational waves in the nanohertz band (i.e., wavelengths of order light-years). This requires high-precision, decades-long data sets from sensitive, instrumentally stable telescopes. NANOGrav and its collaborators in the International Pulsar Timing Array consortium are on the verge of the first detection of the stochastic background produced by supermassive binary black holes, which form via the mergers of massive galaxies. By providing Northern hemisphere sky coverage with exquisite sensitivity and higher frequency coverage compa…
Pulsar timing arrays are sensitive to gravitational waves from supermassive black hole (SMBH) binaries at orbital separations of << 1pc. There is currently an observational paucity of such systems, although they are central figures in studies of galaxy evolution, merger dynamics, and active nucleus formation. We review the prospects of detecting SMBH binaries through electromagnetic radiative processes thought to be associated with galaxy mergers and late-stage binary evolution. We then discuss the scientific goals of joint pulsar timing and electromagnetic studies of these systems, including …
Observing and timing a group of millisecond pulsars (MSPs) with high rotational stability enables the direct detection of gravitational waves (GWs). The GW signals can be identified from the spatial correlations encoded in the times-of-arrival of widely spaced pulsar-pairs. The Chinese Pulsar Timing Array (CPTA) is a collaboration aiming at the direct GW detection with observations carried out using Chinese radio telescopes. This short article serves as a `table of contents' for a forthcoming series of papers related to the CPTA Data Release 1 (CPTA DR1) which uses observations from the Five-h…
The 2012 International Pulsar Timing Array (IPTA) Mock Data Challenge (MDC) is designed to test current Gravitational Wave (GW) detection algorithms. Here we will briefly outline two detection algorithms for a stochastic background of gravitational waves, namely, a first-order likelihood method and an optimal statistic method and present our results from the closed MDC data sets.
The NANOGrav, Parkes, European, and International Pulsar Timing Array (PTA) Collaborations have reported evidence for a common-spectrum process that can potentially correspond to a stochastic gravitational wave background (SGWB) in the 1--100 nHz frequency range. We consider the scenario in which this signal is produced by magnetohydrodynamic (MHD) turbulence in the early Universe, induced by a nonhelical primordial magnetic field at the energy scale corresponding to the quark confinement phase transition. We perform MHD simulations to study the dynamical evolution of the magnetic field and co…
