Some compact objects observed in gravitational wave events have masses in the gap between known neutron stars (NSs) and black holes (BHs). The nature of these mass gap objects is unknown, as is the ...formation of their host binary systems. We report pulsar timing observations made with the Karoo Array Telescope (MeerKAT) of PSR J0514-4002E, an eccentric binary millisecond pulsar in the globular cluster NGC 1851. We found a total binary mass of 3.887 ± 0.004 solar masses (
), and multiwavelength observations show that the pulsar's binary companion is also a compact object. The companion's mass (2.09 to 2.71
, 95% confidence interval) is in the mass gap, indicating either a very massive NS or a low-mass BH. We propose that the companion formed in a merger between two earlier NSs.
We report the discovery of an eclipsing binary millisecond pulsar in the globular cluster M92 (NGC 6341) with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). PSR J1717+4308A, or ...M92A, has a pulse frequency of 316.5 Hz (3.16 ms) and a dispersion measure of 35.45 pc cm−3. The pulsar is a member of a binary system with an orbital period of 0.20 days around a low-mass companion that has a median mass of ∼0.18 M . From observations so far, at least two eclipsing events have been observed in each orbit. The longer one lasted for ∼5000 s in the orbital phase range 0.1-0.5. The other lasted for ∼500 s and occurred between 1000 and 2000 s before or after the longer eclipsing event. The lengths of these two eclipsing events also change. These properties suggest that J1717+4308A is a "red-back" system with a low-mass main-sequence or sub-giant companion. Timing observations of the pulsar and further searches of the data for additional pulsars are ongoing.
Abstract
Close encounters between neutron stars and main-sequence stars occur in globular clusters and may lead to various outcomes. Here we study encounters resulting in the tidal disruption of the ...star. Using
N
-body models, we predict the typical stellar masses in these disruptions and the dependence of the event rate on the host cluster properties. We find that tidal disruption events occur most frequently in core-collapsed globular clusters and that roughly 25% of the disrupted stars are merger products (i.e., blue straggler stars). Using hydrodynamic simulations, we model the tidal disruptions themselves (over timescales of days) to determine the mass bound to the neutron star and the properties of the accretion disks formed. In general, we find roughly 80%–90% of the initial stellar mass becomes bound to the neutron star following disruption. Additionally, we find that neutron stars receive impulsive kicks of up to about 20 km s
−1
as a result of the asymmetry of unbound ejecta; these kicks place these neutron stars on elongated orbits within their host cluster, with apocenter distances well outside the cluster core. Finally, we model the evolution of the (hypercritical) accretion disks on longer timescales (days to years after disruption) to estimate the accretion rate onto the neutron stars and accompanying spin-up. As long as ≳1% of the bound mass accretes onto the neutron star, millisecond spin periods can be attained. We argue the growing numbers of isolated millisecond pulsars observed in globular clusters may have formed, at least in part, through this mechanism. In the case of significant mass growth, some of these neutron stars may collapse to form low-mass (≲3
M
⊙
) black holes.
ABSTRACT We report on an effort to extract and monitor interstellar scintillation parameters in regular timing observations collected for the North American Nanohertz Observatory for Gravitational ...Waves pulsar timing array. Scattering delays are measured by creating dynamic spectra for each pulsar and observing epoch of wide-band observations centered near 1500 MHz and carried out at the Green Bank Telescope and the Arecibo Observatory. The ∼800 MHz wide frequency bands imply dramatic changes in scintillation bandwidth across the bandpass, and a stretching routine has been included to account for this scaling. For most of the 10 pulsars for which the scaling has been measured, the bandwidths scale with frequency less steeply than expected for a Kolmogorov medium. We find estimated scattering delay values that vary with time by up to an order of magnitude. The mean measured scattering delays are similar to previously published values and are slightly higher than predicted by interstellar medium models. We investigate the possibility of increasing the timing precision by mitigating timing errors introduced by the scattering delays. For most of the pulsars, the uncertainty in the time of arrival of a single timing point is much larger than the maximum variation of the scattering delay, suggesting that diffractive scintillation remains as only a negligible part of their noise budget.
ABSTRACT
We present the first measurement of a non-zero magnetic field in the eclipsing material of a black widow pulsar. Black widows are millisecond pulsars which are ablating their companions; ...therefore they are often proposed as one potential source of isolated millisecond pulsars. PSR J2256–1024 is an eclipsing black widow discovered at radio wavelengths and later also observed in the X-ray and gamma parts of the spectrum. Here we present the radio timing solution for PSR J2256–1024; polarization profiles at 350, 820, and 1500 MHz; and an investigation of changes in the polarization profile due to eclipsing material in the system. In the latter we find evidence of Faraday rotation in the linear polarization shortly after eclipse, measuring a rotation measure of 0.44(6) rad m−2 and a corresponding line-of-sight magnetic field of ∼1.11(16) mG.
ABSTRACT Using the nine-year radio-pulsar timing data set from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), collected at Arecibo Observatory and the Green Bank ...Telescope, we have measured the positions, proper motions, and parallaxes for 37 millisecond pulsars. We report twelve significant parallax measurements and distance measurements, and eighteen lower limits on distance. We compare these measurements to distances predicted by the NE2001 interstellar electron density model and find them to be in general agreement. We use measured orbital-decay rates and spin-down rates to confirm two of the parallax distances and to place distance upper limits on other sources; these distance limits agree with the parallax distances with one exception, PSR J1024-0719, which we discuss at length. Using the proper motions of the 37 NANOGrav pulsars in combination with other published measurements, we calculate the velocity dispersion of the millisecond pulsar population in Galactocentric coordinates. We find the radial, azimuthal, and perpendicular dispersions to be 46, 40, and 24 , respectively, in a model that allows for high-velocity outliers; or 81, 58, and 62 for the full population. These velocity dispersions are far smaller than those of the canonical pulsar population, and are similar to older Galactic disk populations. This suggests that millisecond pulsar velocities are largely attributable to their being an old population rather than being artifacts of their birth and evolution as neutron star binary systems. The components of these velocity dispersions follow similar proportions to other Galactic populations, suggesting that our results are not biased by selection effects.
Motivated by the recent discovery of 30 new millisecond pulsars in Terzan 5, made using the Green Bank Telescope's S-band receiver and the Pulsar Spigot spectrometer, we have set out to use the same ...observing system in a systematic search for pulsars in other globular clusters. Here we report on the discovery of five new pulsars in NGC 6440 and three in NGC 6441; each cluster previously had one known pulsar. Using the most recent distance estimates to these clusters, we conclude that there are as many potentially observable pulsars in NGC 6440 and NGC 6441 as in Terzan 5. We present timing solutions for all of the pulsars in these globular clusters. Four of the new discoveries are in binary systems; one of them, PSR J1748-2021B (NGC 6440B), has a wide (image days) and eccentric orbit. This allowed a measurement of its rate of advance of periastron, image. If this is a purely relativistic effect, the total mass of this binary system is image (1 image) implying a median pulsar mass of image. There is a 1% probability that the inclination is low enough that pulsar mass is below 2 image, and 0.10% probability that it is between 1.20 and 1.44 image. If confirmed, this anomalously large mass would strongly constrain the equation of state for dense matter. The other highly eccentric binary, PSR J1750-37A, has image, and image, implying a total system mass of image and, along with the mass function, maximum and median pulsar masses of 1.65 and 1.26 image, respectively.
Abstract
We report on the first pulsar and transient survey of the Galactic Center (GC) with the Atacama Large Millimeter/submillimeter Array (ALMA). The observations were conducted during the Global ...Millimeter VLBI Array campaign in 2017 and 2018. We carry out searches using time series of both total intensity and other polarization components in the form of Stokes parameters. We incorporate acceleration and its derivative in the pulsar search, and also search in segments of the entire observation to compensate for potential orbital motion of the pulsar. While no new pulsar is found, our observations yield the polarization profile of the GC magnetar PSR J1745−2900 at millimeter wavelength for the first time, which turns out to be nearly 100% linearly polarized. Additionally, we estimate the survey sensitivity placed by both system and red noise, and evaluate its capability of finding pulsars in orbital motion with either Sgr A* or a binary companion. We show that the survey is sensitive to only the most luminous pulsars in the known population and future observations with ALMA in Band-1 will deliver significantly deeper survey sensitivity on the GC pulsar population.
Abstract
Terzan 5 is a rich globular cluster within the galactic bulge containing 39 known millisecond pulsars, the largest known population of any globular cluster. These faint pulsars do not have ...sufficient signal-to-noise ratio (S/N) to measure reliable flux density or polarization information from individual observations in general. We combined over 5.2 days of archival data, at 1500 and 2000 MHz, taken with the Green Bank Telescope over the past 12 years. We created high-S/N profiles for 32 of the pulsars and determined precise rotation measures (RMs) for 28. We used the RMs, pulsar positions, and dispersion measures to map the projected parallel component of the Galactic magnetic field toward the cluster. The 〈
B
∣∣
〉 shows a rough gradient of ∼6 nG arcsec
−1
(∼160 nG pc
−1
) or, fractionally, a change of ∼20% in the R.A. direction across the cluster, implying Galactic magnetic field variability at sub-parsec scales. We also measured average flux densities
S
ν
for the pulsars, ranging from ∼10
μ
Jy to ∼2 mJy, and an average spectral index
α
= −1.35, where
S
ν
∝
ν
α
. This spectral index is flatter than most known pulsars, likely a selection effect due to the high frequencies used in pulsar searches to mitigate dispersion and scattering. We used flux densities from each observation to constrain the scintillation properties toward the cluster, finding strong refractive modulation on timescales of months. The inferred pulsar luminosity function is roughly power law, with slope
(
d
log
N
)
/
(
d
log
L
)
=
−
1
at the high-luminosity end. At the low-luminosity end, there are incompleteness effects, implying that Terzan 5 contains many more pulsars.
We used ultra-deep observations obtained with the NIRCam aboard the
James Webb
Space Telescope to explore the stellar population of NGC 6440: a typical massive, obscured, and contaminated globular ...cluster formed and orbiting within the Galactic bulge. Leveraging the exceptional capabilities of this camera, we sampled the cluster down to about five magnitudes below the main sequence turn-off in the (
m
F
115
W
,
m
F
115
W
−
m
F
200
W
) colour–magnitude diagram. After carefully accounting for differential extinction and contamination by field interlopers, we find that the main sequence splits into two branches, each above and below the characteristic knee. By comparing the morphology of the colour–magnitude diagram with a suitable set of isochrones, we argue that the upper main sequence bi-modality is likely due to the presence of a He-enriched stellar population with a helium spread of Δ
Y
= 0.04. The lower main sequence bi-modality can be attributed to variations in the abundance of water (i.e., oxygen) with ΔO/Fe ∼ −0.4. This is the first evidence of both helium and oxygen abundance variations in a globular cluster purely based on JWST observations. These results open the window for future in-depth investigations of the multiple population phenomenon in clusters located in the Galactic bulge, which were previously unfeasible with near-UV observations, due to prohibitive reddening and crowding conditions.
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