arXiv subjects

P. A. Gentile

Publications and source records attributed to P. A. Gentile.

10 recordsLinked to original sources

arXiv2023

Comparing recent PTA results on the nanohertz stochastic gravitational wave background

The Australian, Chinese, European, Indian, and North American pulsar timing array (PTA) collaborations recently reported, at varying levels, evidence for the presence of a nanohertz gravitational wave background (GWB). Given that each PTA made different choices in modeling their data, we perform a comparison of the GWB and individual pulsar noise parameters across the results reported from the PTAs that constitute the International Pulsar Timing Array (IPTA). We show that despite making different modeling choices, there is no significant difference in the GWB parameters that are measured by the different PTAs, agreeing within $1σ$. The pulsar noise parameters are also consistent between different PTAs for the majority of the pulsars included in these analyses. We bridge the differences in modeling choices by adopting a standardized noise model for all pulsars and PTAs, finding that under this model there is a reduction in the tension in the pulsar noise parameters. As part of this reanalysis, we "extended" each PTA's data set by adding extra pulsars that were not timed by that PTA. Under these extensions, we find better constraints on the GWB amplitude and a higher signal-to-noise ratio for the Hellings and Downs correlations. These extensions serve as a prelude to the benefits offered by a full combination of data across all pulsars in the IPTA, i.e., the IPTA's Data Release 3, which will involve not just adding in additional pulsars, but also including data from all three PTAs where any given pulsar is timed by more than as single PTA.

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arXiv2020

The NANOGrav 11-year Data Set: Constraints on Planetary Masses Around 45 Millisecond Pulsars

We search for extrasolar planets around millisecond pulsars using pulsar timing data and seek to determine the minimum detectable planetary masses as a function of orbital period. Using the 11-year data set from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), we look for variations from our models of pulse arrival times due to the presence of exoplanets. No planets are detected around the millisecond pulsars in the NANOGrav 11-year data set, but taking into consideration the noise levels of each pulsar and the sampling rate of our observations, we develop limits that show we are sensitive to planetary masses as low as that of the moon. We analyzed potential planet periods, P, in the range 7 days < P < 2000 days, with somewhat smaller ranges for some binary pulsars. The planetary mass limit for our median-sensitivity pulsar within this period range is 1 M_moon (P / 100 days)^(-2/3).

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arXiv2019

High-Precision X-ray Timing of Three Millisecond Pulsars with NICER: Stability Estimates and Comparison with Radio

The Neutron Star Interior Composition Explorer (NICER) is an X-ray astrophysics payload on the International Space Station. It enables unprecedented high-precision timing of millisecond pulsars without the pulse broadening and delays due to dispersion and scattering within the interstellar medium that plague radio timing. We present initial timing results from a year of data on the millisecond pulsars PSR B1937+21 and PSR J0218+4232, and nine months of data on PSR B1821-24. NICER time-of-arrival uncertainties for the three pulsars are consistent with theoretical lower bounds and simulations based on their pulse shape templates and average source and background photon count rates. To estimate timing stability, we use the $σ_z$ measure, which is based on the average of the cubic coefficients of polynomial fits to subsets of timing residuals. So far we are achieving timing stabilities $σ_z \approx 3 \times 10^{-14}$ for PSR B1937+21 and on the order of $10^{-12}$ for PSRs B1821$-$24 and J0218+4232. Within the span of our \textit{NICER} data we do not yet see the characteristic break point in the slope of $σ_z$; detection of such a break would indicate that further improvement in the cumulative root-mean-square (RMS) timing residual is limited by timing noise. We see this break point in our comparison radio data sets for PSR B1821-24 and PSR B1937+21 on time scales of $> 2$ years.

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arXiv2018

PSR J2234+0611: A new laboratory for stellar evolution

We report 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 companion discovered as part of the Arecibo Observatory 327 MHz drift scan survey. The precise timing and the eccentric nature of the orbit allow precise measurements of an unusual number of parameters: a) a precise proper motion of 27.10(3) mas/yr and a parallax of 1.05(4) mas resulting in a pulsar distance of 0.95(4) kpc; this allows a precise estimate of the transverse velocity, 123(5) km/s. Together with previously published spectroscopic measurements of the systemic radial velocity, this allows a full 3-D determination of the system's velocity; b) precise measurements of the rate of advance of periastron, which after subtraction of the contribution of the proper motion yields a total system mass of $1.6518^{+0.0033}_{-0.0035}$ solar masses; c) a Shapiro delay measurement, h_3 = $82 \pm 14$ ns despite the orbital inclination not being near 90 deg; combined with the measurement of the total mass, this yields a pulsar mass of $1.353^{+0.014}_{-0.017}$ solar masses and a companion mass of $0.298^{+0.015}_{-0.012}$ solar masses; d) we measure precisely the secular variation of the projected semi-major axis and detect significant annual orbital parallax; together these allow a determination of the full 3-D orbital geometry, including an unambiguous orbital inclination (i = $138.7^{+2.5}_{-2.2}$ deg) and a position angle for the line of nodes (Omega = $44^{+5}_{-4}$ deg). We discuss the component masses to investigate hypotheses previously advanced to explain the origin of eccentric MSPs. The unprecedented determination of the full 3-D position, motion and orbital orientation of the system, plus the precisely measured pulsar and WD mass and the latter's optical detection make this system an unique test of our understanding of white dwarfs and their atmospheres.

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arXiv2018

The NANOGrav 11-year Data Set: Pulse Profile Variability

Access to 50 years of data has led to the discovery of pulsar emission and rotation variability on timescales of months and years. Most of this long-term variability has been seen in long-period pulsars, with relatively little focus on recycled millisecond pulsars. We have analyzed a 38-pulsar sub-set of the 45 millisecond pulsars in the NANOGrav 11-year data set, in order to review their pulse profile stability. The most variability, on any timescale, is seen in PSRs J1713+0747, B1937+21 and J2145-0750. The strongest evidence for long-timescale pulse profile changes is seen in PSRs B1937+21 and J1643-1224. We have focused our analyses on these four pulsars in an attempt to elucidate the causes of their profile variability. Effects of scintillation seem to be responsible for the profile modifications of PSR J2145-0750. We see evidence that imperfect polarization calibration contributes to the profile variability of PSRs J1713+0747 and B1937+21, along with radio frequency interference around 2 GHz, but find that propagation effects also have an influence. The changes seen in PSR J1643-1224 have been reported previously, yet elude explanation beyond their astrophysical nature. Regardless of cause, unmodeled pulse profile changes are detrimental to the accuracy of pulsar timing and must be incorporated into the timing models where possible.

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arXiv2018

The NANOGrav 12.5-Year Data Set: The Frequency Dependence of Pulse Jitter in Precision Millisecond Pulsars

Low-frequency gravitational-wave experiments require the highest timing precision from an array of the most stable millisecond pulsars. Several known sources of noise on short timescales in single radio-pulsar observations are well described by a simple model of three components: template-fitting from a finite signal-to-noise ratio, pulse phase/amplitude jitter from single-pulse stochasticity, and scintillation errors from short-timescale interstellar scattering variations. Currently template-fitting errors dominate, but as radio telescopes push towards higher signal-to-noise ratios, jitter becomes the next dominant term for most millisecond pulsars. Understanding the statistics of jitter becomes crucial for properly characterizing arrival-time uncertainties. We characterize the radio-frequency dependence of jitter using data on 48 pulsars in the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) timing program. We detect significant jitter in 43 of the pulsars and test several functional forms for its frequency dependence; we find significant frequency dependence for 30 pulsars. We find moderate correlations of rms jitter with pulse width (R = 0.62) and number of profile components (R = 0.40); the single-pulse rms jitter is typically ~1% of pulse phase. The average frequency dependence for all pulsars using a power-law model has index -0.42. We investigate the jitter variations for the interpulse of PSR~B1937+21 and find no significant deviations from the main pulse rms jitter. We also test the time-variation of jitter in two pulsars and find that systematics likely bias the results for high-precision pulsars. Pulsar timing array analyses must properly model jitter as a significant component of the noise within the detector.

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arXiv2018

The NANOGrav 11-year Data Set: Solar Wind Sounding Through Pulsar Timing

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has observed dozens of millisecond pulsars for over a decade. We have accrued a large collection of dispersion measure (DM) measurements sensitive to the total electron content between Earth and the pulsars at each observation. All lines of sight cross through the solar wind which produces correlated DM fluctuations in all pulsars. We develop and apply techniques for extracting the imprint of the solar wind from the full collection of DM measurements in the recently released NANOGrav 11-yr data set. We filter out long time scale DM fluctuations attributable to structure in the interstellar medium and carry out a simultaneous analysis of all pulsars in our sample that can differentiate the correlated signature of the wind from signals unique to individual lines of sight. When treating the solar wind as spherically symmetric and constant in time, we find the electron number density at 1 A.U. to be $7.9\pm0.2$ cm$^{-3}$. Our data shows little evidence of long-term variation in the density of the wind. We argue that our techniques paired with a high cadence, low radio frequency observing campaign of near-ecliptic pulsars would be capable of mapping out large-scale latitudinal structure in the wind.

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arXiv2018

A Second Chromatic Timing Event of Interstellar Origin toward PSR J1713+0747

The frequency dependence of radio pulse arrival times provides a probe of structures in the intervening media. Demorest et al. 2013 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 of approximately 3.0,-1.3, and -0.7 microseconds at 820, 1400, and 2300 MHz, respectively. Timing analyses indicate possible departures from the standard nu^-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.

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arXiv2018

The NANOGrav 11-year Data Set: Pulsar-timing Constraints On The Stochastic Gravitational-wave Background

We search for an isotropic stochastic gravitational-wave background (GWB) in the newly released $11$-year dataset from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). While we find no significant evidence for a GWB, we place constraints on a GWB from a population of supermassive black-hole binaries, cosmic strings, and a primordial GWB. For the first time, we find that the GWB upper limits and detection statistics 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 PTA constraints that are robust against SSE uncertainties. We thus place a $95\%$ upper limit on the GW strain amplitude of $A_\mathrm{GWB}<1.45\times 10^{-15}$ at a frequency of $f=1$ yr$^{-1}$ for a fiducial $f^{-2/3}$ power-law spectrum, and with inter-pulsar correlations modeled. This is a factor of $\sim 2$ improvement over the NANOGrav $9$-year limit, calculated using the same procedure. Previous PTA upper limits on the GWB will need revision in light of SSE systematic uncertainties. 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 cosmic-string tension using recent simulations, yielding an SSE-marginalized $95\%$ upper limit on the cosmic string tension of $Gμ< 5.3\times 10^{-11}$---a factor of $\sim 2$ better than the published NANOGrav $9$-year constraints. Our SSE-marginalized $95\%$ upper limit on the energy density of a primordial GWB (for a radiation-dominated post-inflation Universe) is $Ω_\mathrm{GWB}(f)h^2<3.4\times10^{-10}$.

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arXiv2016

PSR J1024-0719: A Millisecond Pulsar in an Unusual Long-Period Orbit

PSR J1024$-$0719 is a millisecond pulsar that was long thought to be isolated. However, puzzling results concerning its velocity, distance, and low rotational period derivative have led to reexamination of its properties. We present updated radio timing observations along with new and archival optical data that show PSR J1024$-$0719 is most likely in a long period (2$-$20 kyr) binary system with a low-mass ($\approx 0.4\,M_\odot$) low-metallicity ($Z \approx -0.9\,$ dex) main sequence star. Such a system can explain most of the anomalous properties of this pulsar. We suggest that this system formed through a dynamical exchange in a globular cluster that ejected it into a halo orbit, consistent with the low observed metallicity for the stellar companion. Further astrometric and radio timing observations such as measurement of the third period derivative could strongly constrain the range of orbital parameters.

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