arXiv subjects

Mallory S. E. Roberts

Publications and source records attributed to Mallory S. E. Roberts.

At least 19 recordsLinked to original sources

arXiv2026

Understanding Pulsar Wind Nebulae with the SKA

Produced by the interaction between the ``pulsar wind'' powered by the rotational energy of a neutron star and its surroundings, the study of pulsar wind nebulae (PWNe) provides vital insight into the physics of neutron star magnetospheres and ultra-relativistic outflows. Spatially-resolved studies of the continuum and polarized radio emission of these sources are vital for understanding the production of $e^\pm$ in the magnetospheres of neutron stars, the acceleration of these particles (and potentially baryons) to $\gtrsim10^{15}~{\rm eV}$ energies, and their propagation within the PWN and in the surrounding interstellar medium. The significant improvements in sensitivity, dynamic range, timing capabilities offered by the Square Kilometer Array have the potential to greatly improve our understanding of the origin of some of the highest energy particles produced in the Milky Way.

astro-ph.HE↗

arXiv2025

Understanding Pulsar Wind Nebulae with the SKA

Produced by the interaction between the ``pulsar wind'' powered by the rotational energy of a neutron star and its surroundings, the study of pulsar wind nebulae (PWNe) provides vital insight into the physics of neutron star magnetospheres and ultra-relativistic outflows. Spatially-resolved studies of the continuum and polarized radio emission of these sources are vital for understanding the production of $e^\pm$ in the magnetospheres of neutron stars, the acceleration of these particles to $\gtrsim10^{15}~{\rm eV}$ energies, and the propagation of these particles within the PWN as well as the surrounding interstellar medium. The significant improvements in sensitivity, dynamic range, timing capabilities offered by the Square Kilometer Array have the potential to significantly improve our understanding of the origin of some of the highest energy particles produced in the Milky Way.

astro-ph.HE↗

arXiv2025

Radio Observation of the Pulsar Wind Nebula in SNR G11.2-0.3

Pulsar wind nebulae (PWNe) are important sources for understanding galactic high-energy processes, but it is controversial until now about how high-energy particles in PWNe are accelerated and transported. Lacking radio counterparts of X-ray PWNe (the proposed acceleration sites) introduce difficulties to better understandings in multi wavelengths. Our recent 3, 6, and 16\,cm high-resolution observations of G11.2$-$0.3 PWN with the Australia Telescope Compact Array (ATCA) uniquely show morphological similarity with its X-ray PWN (a torus/jet feature). Spectral indices of the radio torus and jet are around -0.09 and -0.10, respectively. Meanwhile for the jet region, the spectral break between radio and X-ray spectra implies particle acceleration mechanisms other than a diffusive shock acceleration. Polarization results suggest a helical B-field inside the jet, the equipartition B-field strength of which is below 100\,$μ$G.

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arXiv2022

High-Resolution Radio Study of the Dragonfly Nebula

The Dragonfly Nebula (G75.2$+$0.1) powered by the young pulsar J2021$+$3651 is a rare pulsar wind nebula (PWN) that shows double tori and polar jets enclosed by a bow-shock structure in X-rays. We present new radio observations of this source taken with the Very Large Array (VLA) at 6 GHz. The radio PWN has an overall size about two times as large as the X-ray counterpart, consisting of a bright main body region in the southwest, a narrow and fainter bridge region in the northeast, and a dark gap in between. The nebula shows a radio spectrum much softer than that of a typical PWN. This could be resulting from compression by the ram pressure as the system travels mildly supersonically in the interstellar medium (ISM). Our polarization maps reveal a highly ordered and complex $B$-field structure. This can be explained by a toroidal field distorted by the pulsar motion.

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arXiv2020

The GBT 350-MHz Drift Scan Pulsar Survey. III. Detection of a magnetic field in the eclipsing material of PSR J2256-1024

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 per meter squared and a corresponding line-of-sight magnetic field of 3.5(17) mG.

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arXiv2018

The Green Bank North Celestial Cap Pulsar Survey III: 45 New Pulsar Timing Solutions

We provide timing solutions for 45 radio pulsars discovered by the Robert C. Byrd Green Bank Telescope. These pulsars were found in the Green Bank North Celestial Cap pulsar survey, an all-GBT-sky survey being carried out at a frequency of 350 MHz. We include pulsar timing data from the Green Bank Telescope and Low Frequency Array. Our sample includes five fully recycled millisecond pulsars (MSPs, three of which are in a binary system), a new relativistic double neutron star system, an intermediate mass binary pulsar, a mode-changing pulsar, a 138-ms pulsar with a very low magnetic field, and several nulling pulsars. We have measured two post-Keplerian parameters and thus the masses of both objects in the double neutron star system. We also report a tentative companion mass measurement via Shapiro delay in a binary MSP. Two of the MSPs can be timed with high precision and have been included in pulsar timing arrays being used to search for low-frequency gravitational waves, while a third MSP is a member of the black widow class of binaries. Proper motion is measurable in five pulsars and we provide an estimate of their space velocity. We report on an optical counterpart to a new black widow system and provide constraints on the optical counterparts to other binary MSPs. We also present a preliminary analysis of nulling pulsars in our sample. These results demonstrate the scientific return of long timing campaigns on pulsars of all types.

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arXiv2018

X-Ray and Optical Properties of Black Widows and Redbacks

Black widows and redbacks are binary systems consisting of a millisecond pulsar in a close binary with a companion having matter driven off of its surface by the pulsar wind. X-rays due to an intra-binary shock have been observed from many of these systems, as well as orbital variations in the optical emission from the companion due to heating and tidal distortion. We have been systematically studying these systems in radio, optical and X-rays. Here we will present an overview of X-ray and optical studies of these systems, including new XMM-Newton and NuStar data obtained from several of them, along with new optical photometry.

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arXiv2017

The High Frequency Radio Emission of the Galactic Center Magnetar SGR J1745-29 During a Transitional Period

The origin of the high-frequency radio emission detected from several magnetars is poorly understood. In this paper, we report the ~40 GHz properties of SGR J1745-29 as measured using Jansky Very Large Array (JVLA) and Robert C. Byrd Green Bank Telescope (GBT) observations between 2013 October 26 and 2014 May 31. Our analysis of a Q-band (45 GHz) GBT observation on 2014 April 10 resulted in the earliest detection of pulsed radio emission at high frequencies (>20 GHz); we found that the average pulse has a singly peaked profile with width ~75 ms (~2% of the 3.764 s pulse period) and an average pulsed flux density of ~100 mJy. We also detected very bright, short (<10 ms) single pulses during ~70% of this neutron star's rotations, and the peak flux densities of these bright pulses follow the same log-normal distribution as measured at 8.5 GHz. Additionally, our analysis of contemporaneous JVLA observations suggest that its 41/44 GHz flux density varied between ~1-4 mJy during this period, with a ~2x change observed on ~20 minute timescales during a JVLA observation on 2014 May 10. Such a drastic change over short time-scales is inconsistent with the radio emission resulting from a shock powered by the magnetar's supersonic motion through the surrounding medium, and instead is dominated by pulsed emission generated in its magnetosphere.

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arXiv2017

A search for extragalactic pulsars in the Local Group galaxies IC 10 and Barnard's Galaxy

As of today, more than 2500 pulsars have been found, nearly all in the Milky Way, with the exception of ~28 pulsars in the Small and Large Magellanic Clouds. However, there have been few published attempts to search for pulsars deeper in our Galactic neighborhood. Two of the more promising Local Group galaxies are IC 10 and NGC 6822 (also known as Barnard's Galaxy) due to their relatively high star formation rate and their proximity to our galaxy. IC 10 in particular, holds promise as it is the closest starburst galaxy to us and harbors an unusually high number of Wolf-Rayet stars, implying the presence of many neutron stars. We observed IC 10 and NGC 6822 at 820 MHz with the Green Bank Telescope for ~15 and 5 hours respectively, and put a strong upper limit of 0.1 mJy on pulsars in either of the two galaxies. We also performed single pulse searches of both galaxies with no firm detections.

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arXiv2016

G11.2-0.3: The Young Remnant of a Stripped-Envelope Supernova

We present results of a 400-ks Chandra observation of the young shell supernova remnant (SNR) G11.2-0.3, containing a pulsar and pulsar-wind nebula (PWN). We measure a mean expansion rate for the shell since 2000 of 0.0277+/-0.0018% per yr, implying an age between 1400 and 2400 yr, and making G11.2-0.3 one of the youngest core-collapse SNRs in the Galaxy. However, we find very high absorption ($A_V \sim 16^m \pm 2^m$), confirming near-IR determinations and ruling out a claimed association with the possible historical SN of 386 CE. The PWN shows strong jets and a faint torus within a larger, more diffuse region of radio emission and nonthermal X-rays. Central soft thermal X-ray emission is anticorrelated with the PWN; that, and more detailed morphological evidence, indicates that the reverse shock has already reheated all ejecta and compressed the PWN. The pulsar characteristic energy-loss timescale is well in excess of the remnant age, and we suggest that the bright jets have been produced since the recompression. The relatively pronounced shell and diffuse hard X-ray emission in the interior, enhanced at the inner edge of the shell, indicate that the immediate circumstellar medium into which G11.2-0.3 is expanding was quite anisotropic. We propose a possible origin for G11.2-0.3 in a stripped-envelope progenitor that had lost almost all its envelope mass, in an anisotropic wind or due to binary interaction, leaving a compact core whose fast winds swept previously lost mass into a dense irregular shell, and which exploded as a Type cIIb or Ibc supernova.

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arXiv2015

Properties and Evolution of the Redback Millisecond Pulsar Binary PSR J2129-0429

PSR J2129-0429 is a "redback" eclipsing millisecond pulsar binary with an unusually long 15.2 hour orbit. It was discovered by the Green Bank Telescope in a targeted search of unidentified Fermi gamma-ray sources. The pulsar companion is optically bright (mean $m_R = 16.6$ mag), allowing us to construct the longest baseline photometric dataset available for such a system. We present ten years of archival and new photometry of the companion from LINEAR, CRTS, PTF, the Palomar 60-inch, and LCOGT. Radial velocity spectroscopy using the Double-Beam Spectrograph on the Palomar 200-inch indicates that the pulsar is massive: $1.74\pm0.18 M_\odot$. The G-type pulsar companion has mass $0.44\pm0.04 M_\odot$, one of the heaviest known redback companions. It is currently 95\% Roche-lobe filling and only mildly irradiated by the pulsar. We identify a clear 13.1 mmag yr$^{-1}$ secular decline in the mean magnitude of the companion as well as smaller-scale variations in the optical lightcurve shape. This behavior may indicate that the companion is cooling. Binary evolution calculations indicate that PSR J2129-0429 has an orbital period almost exactly at the bifurcation period between systems that converge into tighter orbits as black widows and redbacks and those that diverge into wider pulsar--white dwarf binaries. Its eventual fate may depend on whether it undergoes future episodes of mass transfer and increased irradiation.

astro-ph.SR↗

arXiv2015

X-Ray Studies of Redbacks

We consider the X-ray properties of the redback class of eclipsing millisecond pulsars. These are transitional systems between accreting low-mass X-ray binaries and binary millisecond pulsars orbiting white dwarfs, and hence their companions are non-degenerate and nearly Roche-lobe filling. The X-ray luminosity seems to scale with the fraction of the pulsar sky subtended by the companion, suggesting the shock region is not much larger than the companion, which is supported by modeling of the orbital light curves. The typical X-ray photon spectral index is $\sim 1$ and the typical 0.3-8 keV X-ray efficiency, assuming a shock size on the order of the companion's Roche lobe cross-section, is on the order of 10%. We present an overview of previous investigations, and present new observations of two redbacks, a Chandra observation of PSR J1628$-$3205 and a XMM-Newton observation of PSR J2129$-$0429. The latter shows a clearly double peaked orbital light curve with variation of the non-thermal flux by a factor of $\sim 11$, with peaks around orbital phases 0.6 and 0.9. We suggest the magnetic field of the companion plays a significant role in the X-ray emission from intrabinary shocks in redbacks.

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arXiv2014

Intrabinary Shock Emission from Black Widows and Redbacks

Eclipsing millisecond pulsars in close ($P_b < 1$~day) binary systems provide a different view of pulsar winds and shocks than do isolated pulsars. Since 2009, the numbers of these systems known in the Galactic field has increased enormously. We have been systematically studying many of these newly discovered systems at multiple wavelengths. Typically, the companion is nearly Roche-lobe filling and heated by the pulsar which drives mass loss from the companion. The pulsar wind shocks with this material just above the surface of the companion. We discuss various observational properties of this shock, including radio eclipses, orbitally modulated X-ray emission, and the potential for $γ$-ray emission. Redbacks, whose companions are likely non-degenerate and significantly more massive, generally have more luminous shocks than black widows which have very low mass companions. This is expected since the more massive redback companions intercept a greater fraction of the pulsar wind. We also compare these systems to accreting millisecond pulsars, which may be progenitors of black widows and in some cases can pass back and forth between redback and accretion phases.

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arXiv2013

The hunt for new pulsars with the Green Bank Telescope

The Green Bank Telescope (GBT) is the largest fully steerable radio telescope in the world and is one of our greatest tools for discovering and studying radio pulsars. Over the last decade, the GBT has successfully found over 100 new pulsars through large-area surveys. Here I discuss the two most recent---the GBT 350 MHz Drift-scan survey and the Green Bank North Celestial Cap survey. The primary science goal of both surveys is to find interesting individual pulsars, including young pulsars, rotating radio transients, exotic binary systems, and especially bright millisecond pulsars (MSPs) suitable for inclusion in Pulsar Timing Arrays, which are trying to directly detect gravitational waves. These two surveys have combined to discover 85 pulsars to date, among which are 14 MSPs and many unique and fascinating systems. I present highlights from these surveys and discuss future plans. I also discuss recent results from targeted GBT pulsar searches of globular clusters and Fermi sources.

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arXiv2012

Surrounded by Spiders! New Black Widows and Redbacks in the Galactic Field

Over the last few years, the number of known eclipsing radio millisecond pulsar systems in the Galactic field has dramatically increased, with many being associated with Fermi gamma-ray sources. All are in tight binaries (orbital period < 24 hr) with many being classical "black widows" which have very low mass companions (companion mass Mc << 0.1 Msol) but some are "redbacks" with low mass (Mc ~ 0.2 - 0.4Msol) companions which are probably non-degenerate. These latter are systems where the mass transfer process may have only temporarily halted, and so are transitional systems between low mass X-ray binaries and ordinary binary millisecond pulsars. Here we review the new discoveries and their multi-wavelength properties, and briefly discuss models of shock emission, mass determinations, and evolutionary scenarios.

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arXiv2012

The Green Bank Telescope 350 MHz Drift-scan Survey I: Survey Observations and the Discovery of 13 Pulsars

Over the summer of 2007, we obtained 1191 hours of `drift-scan' pulsar search observations with the Green Bank Telescope at a radio frequency of 350 MHz. Here we describe the survey setup, search procedure, and the discovery and follow-up timing of thirteen pulsars. Among the new discoveries, one (PSR J1623-0841) was discovered only through its single pulses, two (PSRs J1327-0755 and J1737-0814) are millisecond pulsars, and another (PSR J2222-0137) is a mildly recycled pulsar. PSR J1327-0755 is a 2.7 ms pulsar at a DM of 27.9 pc cm^{-3} in a 8.7 day orbit with a minimum companion mass of 0.22 solar mass. PSR J1737-0814 is a 4.2 ms pulsar at a DM of 55.3 pc cm^{-3} in a 79.3 day orbit with a minimum companion mass of 0.06 solar mass. PSR J2222-0137 is a 32.8 ms pulsar at a very low DM of 3.27 pc cm^{-3} in a 2.4 day orbit with a minimum companion mass of 1.11 solar mass. It is most likely a white dwarf-neutron star system or an unusual low-eccentricity double neutron star system. Ten other pulsars discovered in this survey are reported in the companion paper Lynch et al. 2012.

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arXiv2012

The Green Bank Telescope 350 MHz Drift-scan Survey II: Data Analysis and the Timing of 10 New Pulsars, Including a Relativistic Binary

We have completed a 350 MHz drift scan survey using the Robert C. Byrd Green Bank Telescope with the goal of finding new radio pulsars, especially millisecond pulsars that can be timed to high precision. This survey covered ~10300 square degrees and all of the data have now been fully processed. We have discovered a total of 31 new pulsars, seven of which are recycled pulsars. A companion paper by Boyles et al. (2012) describes the survey strategy, sky coverage, and instrumental set-up, and presents timing solutions for the first 13 pulsars. Here we describe the data analysis pipeline, survey sensitivity, and follow-up observations of new pulsars, and present timing solutions for 10 other pulsars. We highlight several sources---two interesting nulling pulsars, an isolated millisecond pulsar with a measurement of proper motion, and a partially recycled pulsar, PSR J0348+0432, which has a white dwarf companion in a relativistic orbit. PSR J0348+0432 will enable unprecedented tests of theories of gravity.

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arXiv2011

A Radio Pulsar Search of the Gamma-ray Binaries LS I +61 303 and LS 5039

LS I +61 303 and LS 5039 are exceptionally rare examples of HMXBs with MeV-TeV emission, making them two of only five known or proposed "gamma-ray binaries". There has been disagreement within the literature over whether these systems are microquasars, with stellar winds accreting onto a compact object to produce high energy emission and relativistic jets, or whether their emission properties might be better explained by a relativistic pulsar wind colliding with the stellar wind. Here we present an attempt to detect radio pulsars in both systems with the Green Bank Telescope. The upper limits of flux density are between 4.1-14.5 uJy, and we discuss the null results of the search. Our spherically symmetric model of the wind of LS 5039 demonstrates that any pulsar emission will be strongly absorbed by the dense wind unless there is an evacuated region formed by a relativistic colliding wind shock. LS I +61 303 contains a rapidly rotating Be star whose wind is concentrated near the stellar equator. As long as the pulsar is not eclipsed by the circumstellar disk or viewed through the densest wind regions, detecting pulsed emission may be possible during part of the orbit.

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