Abstract
We describe an image-based method that uses two radio criteria, compactness, and spectral index, to identify promising pulsar candidates among Fermi Large Area Telescope (LAT) unassociated ...sources. These criteria are applied to those radio sources from the Giant Metrewave Radio Telescope all-sky survey at 150 MHz (TGSS ADR1) found within the error ellipses of unassociated sources from the 3FGL catalogue and a preliminary source list based on 7 yr of LAT data. After follow-up interferometric observations to identify extended or variable sources, a list of 16 compact, steep-spectrum candidates is generated. An ongoing search for pulsations in these candidates, in gamma rays and radio, has found 6 ms pulsars and one normal pulsar. A comparison of this method with existing selection criteria based on gamma-ray spectral and variability properties suggests that the pulsar discovery space using Fermi may be larger than previously thought. Radio imaging is a hitherto underutilized source selection method that can be used, as with other multiwavelength techniques, in the search for Fermi pulsars.
The pulsar PSR J1756−2251 resides in a relativistic double neutron star binary system with a 7.67-h orbit. We have conducted long-term precision timing on more than 9 yr of data acquired from five ...telescopes, measuring five post-Keplerian parameters. This has led to several independent tests of general relativity (GR), the most constraining of which shows agreement with the prediction of GR at the 4 per cent level. Our measurement of the orbital decay rate disagrees with that predicted by GR, likely due to systematic observational biases. We have derived the pulsar distance from parallax and orbital decay measurements to be
$0.73_{-0.24}^{+0.60}\,$
kpc (68 per cent) and <1.2 kpc (95 per cent upper limit), respectively; these are significantly discrepant from the distance estimated using Galactic electron density models. We have found the pulsar mass to be 1.341 ± 0.007 M⊙, and a low neutron star (NS) companion mass of 1.230 ± 0.007 M⊙. We also determined an upper limit to the spin–orbit misalignment angle of 34° (95 per cent) based on a system geometry fit to long-term profile width measurements. These and other observed properties have led us to hypothesize an evolution involving a low mass–loss, symmetric supernova progenitor to the second-formed NS companion, as is thought to be the case for the double pulsar system PSR J0737−3039A/B. This would make PSR J1756−2251 the second compact binary system providing concrete evidence for this type of NS formation channel.
We search for an isotropic stochastic gravitational-wave background (GWB) in the newly released 11 year data set from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). ...While we find no evidence for a GWB, we place constraints on a population of inspiraling supermassive black hole (SMBH) binaries, a network of decaying cosmic strings, and a primordial GWB. For the first time, we find that the GWB constraints are sensitive to the solar system ephemeris (SSE) model used and that SSE errors can mimic a GWB signal. We developed an approach that bridges systematic SSE differences, producing the first pulsar-timing array (PTA) constraints that are robust against SSE errors. We thus place a 95% upper limit on the GW-strain amplitude of AGWB < 1.45 × 10−15 at a frequency of f = 1 yr−1 for a fiducial f−2/3 power-law spectrum and with interpulsar correlations modeled. This is a factor of ∼2 improvement over the NANOGrav nine-year limit calculated using the same procedure. Previous PTA upper limits on the GWB (as well as their astrophysical and cosmological interpretations) will need revision in light of SSE systematic errors. We use our constraints to characterize the combined influence on the GWB of the stellar mass density in galactic cores, the eccentricity of SMBH binaries, and SMBH-galactic-bulge scaling relationships. We constrain the cosmic-string tension using recent simulations, yielding an SSE-marginalized 95% upper limit of G < 5.3 × 10−11-a factor of ∼2 better than the published NANOGrav nine-year constraints. Our SSE-marginalized 95% upper limit on the energy density of a primordial GWB (for a radiation-dominated post-inflation universe) is GWB(f) h2 < 3.4 × 10−10.
The frequency dependence of radio pulse arrival times provides a probe of structures in the intervening media. Demorest et al. was the first to show a short-term (∼100-200 days) reduction in the ...electron content along the line of sight to PSR J1713+0747 in data from 2008 (approximately MJD 54750) based on an apparent dip in the dispersion measure of the pulsar. We report on a similar event in 2016 (approximately MJD 57510), with average residual pulse-arrival times −3.0, −1.3, and −0.7 s at 820, 1400, and 2300 MHz, respectively. Timing analyses indicate possible departures from the standard −2 dispersive-delay dependence. We discuss and rule out a wide variety of potential interpretations. We find the likeliest scenario to be lensing of the radio emission by some structure in the interstellar medium, which causes multiple frequency-dependent pulse arrival-time delays.
We report the timing results for PSR J2234+0611, a 3.6 ms pulsar in a 32 day, eccentric (e = 0.13) orbit with a helium white dwarf. The precise timing and eccentric nature of the orbit allow ...measurements of an unusual number of parameters: (a) a precise proper motion of 27.10(3) mas yr−1 and a parallax of 1.05(4) mas resulting in a pulsar distance of 0.95(4) kpc; enabling an estimate of the transverse velocity, 123(5) km s−1. Together with previously published spectroscopic measurements of the systemic radial velocity, this allows a 3D determination of the system's velocity; (b) precise measurements of the rate of advance of periastron yields a total system mass of M ; (c) a Shapiro delay measurement, h3 = 82 14 ns, despite the orbital inclination not being near 90°; combined with the measurement of the total mass yields a pulsar mass of and a companion mass of (d) we measure precisely the secular variation of the projected semimajor axis and detect a significant annual orbital parallax; together these allow a determination of the 3D orbital geometry of the system, including an unambiguous orbital inclination ( ) and a position angle for the line of nodes ( ). We discuss the component masses to investigate the hypotheses previously advanced to explain the origin of eccentric MSPs. The unprecedented determination of the 3D position, motion, and orbital orientation of the system, plus the precise pulsar and WD masses and the latter's optical detection make this system a unique test of our understanding of white dwarfs and their atmospheres.
Abstract
In this work, we present polarization profiles for 23 millisecond pulsars observed at 820 and 1500 MHz with the Green Bank Telescope as part of the NANOGrav pulsar timing array. We calibrate ...the data using Mueller matrix solutions calculated from observations of PSRs B1929+10 and J1022+1001. We discuss the polarization profiles, which can be used to constrain pulsar emission geometry, and present both the first published radio polarization profiles for nine pulsars and the discovery of very low-intensity average profile components (“microcomponents”) in four pulsars. We obtain the Faraday rotation measures for each pulsar and use them to calculate the Galactic magnetic field parallel to the line of sight for different lines of sight through the interstellar medium. We fit for linear and sinusoidal trends in time in the dispersion measure and Galactic magnetic field and detect magnetic field variations with a period of 1 yr in some pulsars, but overall find that the variations in these parameters are more consistent with a stochastic origin.
ABSTRACT We compute upper limits on the nanohertz-frequency isotropic stochastic gravitational wave background (GWB) using the 9 year data set from the North American Nanohertz Observatory for ...Gravitational Waves (NANOGrav) collaboration. Well-tested Bayesian techniques are used to set upper limits on the dimensionless strain amplitude (at a frequency of 1 yr−1) for a GWB from supermassive black hole binaries of A gw < 1.5 × 10 − 15 . We also parameterize the GWB spectrum with a broken power-law model by placing priors on the strain amplitude derived from simulations of Sesana and McWilliams et al. Using Bayesian model selection we find that the data favor a broken power law to a pure power law with odds ratios of 2.2 and 22 to one for the Sesana and McWilliams prior models, respectively. Using the broken power-law analysis we construct posterior distributions on environmental factors that drive the binary to the GW-driven regime including the stellar mass density for stellar-scattering, mass accretion rate for circumbinary disk interaction, and orbital eccentricity for eccentric binaries, marking the first time that the shape of the GWB spectrum has been used to make astrophysical inferences. Returning to a power-law model, we place stringent limits on the energy density of relic GWs, gw ( f ) h 2 < 4.2 × 10 − 10 . Our limit on the cosmic string GWB, gw ( f ) h 2 < 2.2 × 10 − 10 , translates to a conservative limit on the cosmic string tension with G < 3.3 × 10 − 8 , a factor of four better than the joint Planck and high-l cosmic microwave background data from other experiments.
Abstract
We investigate the use of bright single pulses from the Crab pulsar to determine separately the dispersion measure (DM) for the Main Pulse and Interpulse components. We develop two ...approaches using cross-correlation functions (CCFs). The first method computes the CCF of the total intensity of each of the 64 frequency channels with a reference channel and converts the time lag of maximum correlation into a DM. The second method separately computes the CCF between every pair of channels for each individual bright pulse and extracts an average DM from the distribution of all channel-pair DMs. Both methods allow the determination of the DM with a relative uncertainty of better than 10
−5
and provide robust estimates for the uncertainty of the best-fit value. We find differences in DM between the Main Pulse, the Low Frequency Interpulse, and the High Frequency Interpulse using both methods in a frequency range from 4 to 6 GHz. Earlier observations of the High Frequency Interpulse carried out by Hankins et al. (2016) resulted in DM
HFIP
–DM
MP
of 0.010 ± 0.016 pc cm
−3
. Our results indicate a DM
HFIP
–DM
MP
of 0.0127 ± 0.0011 pc cm
−3
(with DM
comp
being the DM value of the respective emission component), confirming earlier results with an independent method. During our studies we also find a relation between the brightness of single pulses in the High Frequency Interpulse and their DM. We also discuss the application of the developed methods on the identification of substructures in the case of Fast Radio Bursts.
ABSTRACT
We searched for an isotropic stochastic gravitational wave background in the second data release of the International Pulsar Timing Array, a global collaboration synthesizing decadal-length ...pulsar-timing campaigns in North America, Europe, and Australia. In our reference search for a power-law strain spectrum of the form $h_c = A(f/1\, \mathrm{yr}^{-1})^{\alpha }$, we found strong evidence for a spectrally similar low-frequency stochastic process of amplitude $A = 3.8^{+6.3}_{-2.5}\times 10^{-15}$ and spectral index α = −0.5 ± 0.5, where the uncertainties represent 95 per cent credible regions, using information from the auto- and cross-correlation terms between the pulsars in the array. For a spectral index of α = −2/3, as expected from a population of inspiralling supermassive black hole binaries, the recovered amplitude is $A = 2.8^{+1.2}_{-0.8}\times 10^{-15}$. None the less, no significant evidence of the Hellings–Downs correlations that would indicate a gravitational-wave origin was found. We also analysed the constituent data from the individual pulsar timing arrays in a consistent way, and clearly demonstrate that the combined international data set is more sensitive. Furthermore, we demonstrate that this combined data set produces comparable constraints to recent single-array data sets which have more data than the constituent parts of the combination. Future international data releases will deliver increased sensitivity to gravitational wave radiation, and significantly increase the detection probability.
Abstract
The primary goal of the search for extraterrestrial intelligence is to gain an understanding of the prevalence of technologically advanced beings (organic or inorganic) in the Galaxy. One ...way to approach this is to look for technosignatures: remotely detectable indicators of technology, such as temporal or spectral electromagnetic emissions consistent with an artificial source. With the new Commensal Open-Source Multimode Interferometer Cluster (COSMIC) digital backend on the Karl G. Jansky Very Large Array (VLA), we aim to conduct a search for technosignatures that is significantly more comprehensive, sensitive, and efficient than previously attempted. The COSMIC system is currently operational on the VLA, recording data and designed with the flexibility to provide user-requested modes. This paper describes the hardware system design, the current software pipeline, and plans for future development.
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