arXiv subjects

K. Stovall

Publications and source records attributed to K. Stovall.

82 records · Page 5Linked to original sources

arXiv2013

VLBI astrometry of PSR J2222-0137: a pulsar distance measured to 0.4% accuracy

The binary pulsar J2222-0137 is an enigmatic system containing a partially recycled millisecond pulsar and a companion of unknown nature. Whilst the low eccentricity of the system favors a white dwarf companion, an unusual double neutron star system is also a possibility, and optical observations will be able to distinguish between these possibilities. In order to allow the absolute luminosity (or upper limit) of the companion object to be properly calibrated, we undertook astrometric observations with the Very Long Baseline Array to constrain the system distance via a measurement of annual geometric parallax. With these observations, we measure the parallax of the J2222-0137 system to be 3.742 +0.013 -0.016 milliarcseconds, yielding a distance of 267.3 +1.2 -0.9 pc, and measure the transverse velocity to be 57.1 +0.3 -0.2 km/s. Fixing these parameters in the pulsar timing model made it possible to obtain a measurement of Shapiro delay and hence the system inclination, which shows that the system is nearly edge-on (sin i = 0.9985 +/- 0.0005). Furthermore, we were able to detect the orbital motion of J2222-0137 in our VLBI observations and measure the longitude of ascending node. The VLBI astrometry yields the most accurate distance obtained for a radio pulsar to date, and is furthermore the most accurate parallax for any radio source obtained at "low" radio frequencies (below ~5 GHz, where the ionosphere dominates the error budget). Using the astrometric results, we show the companion to J2222-0137 will be easily detectable in deep optical observations if it is a white dwarf. Finally, we discuss the implications of this measurement for future ultra-high-precision astrometry, in particular in support of pulsar timing arrays.

astro-ph.SR↗

arXiv2013

PEACE: Pulsar Evaluation Algorithm for Candidate Extraction -- A software package for post-analysis processing of pulsar survey candidates

Modern radio pulsar surveys produce a large volume of prospective candidates, the majority of which are polluted by human-created radio frequency interference or other forms of noise. Typically, large numbers of candidates need to be visually inspected in order to determine if they are real pulsars. This process can be labor intensive. In this paper, we introduce an algorithm called PEACE (Pulsar Evaluation Algorithm for Candidate Extraction) which improves the efficiency of identifying pulsar signals. The algorithm ranks the candidates based on a score function. Unlike popular machine-learning based algorithms, no prior training data sets are required. This algorithm has been applied to data from several large-scale radio pulsar surveys. Using the human-based ranking results generated by students in the Arecibo Remote Command enter programme, the statistical performance of PEACE was evaluated. It was found that PEACE ranked 68% of the student-identified pulsars within the top 0.17% of sorted candidates, 95% within the top 0.34%, and 100% within the top 3.7%. This clearly demonstrates that PEACE significantly increases the pulsar identification rate by a factor of about 50 to 1000. To date, PEACE has been directly responsible for the discovery of 47 new pulsars, 5 of which are millisecond pulsars that may be useful for pulsar timing based gravitational-wave detection projects.

astro-ph.IM↗

arXiv2012

The PALFA Survey: Going to great depths to find radio pulsars

The on-going PALFA survey is searching the Galactic plane (|b| < 5 deg., 32 < l < 77 deg. and 168 < l < 214 deg.) for radio pulsars at 1.4 GHz using ALFA, the 7-beam receiver installed at the Arecibo Observatory. By the end of August 2012, the PALFA survey has discovered 100 pulsars, including 17 millisecond pulsars (P < 30 ms). Many of these discoveries are among the pulsars with the largest DM/P ratios, proving that the PALFA survey is capable of probing the Galactic plane for millisecond pulsars to a much greater depth than any previous survey. This is due to the survey's high sensitivity, relatively high observing frequency, and its high time and frequency resolution. Recently the rate of discoveries has increased, due to a new more sensitive spectrometer, two updated complementary search pipelines, the development of online collaborative tools, and access to new computing resources. Looking forward, focus has shifted to the application of artificial intelligence systems to identify pulsar-like candidates, and the development of an improved full-resolution pipeline incorporating more sophisticated radio interference rejection. The new pipeline will be used in a complete second analysis of data already taken, and will be applied to future survey observations. An overview of recent developments, and highlights of exciting discoveries will be presented.

astro-ph.IM↗

arXiv2012

Two Millisecond Pulsars Discovered by the PALFA Survey and a Shapiro Delay Measurement

We present two millisecond pulsar discoveries from the PALFA survey of the Galactic plane with the Arecibo telescope. PSR J1955+2527 is an isolated pulsar with a period of 4.87 ms, and PSR J1949+3106 has a period of 13.14 ms and is in a 1.9-day binary system with a massive companion. Their timing solutions, based on 4 years of timing measurements with the Arecibo, Green Bank, Nançay and Jodrell Bank telescopes, allow precise determination of spin and astrometric parameters, including precise determinations of their proper motions. For PSR J1949+3106, we can clearly detect the Shapiro delay. From this we measure the pulsar mass to be 1.47(+0.43/-0.31) solar masses, the companion mass to be 0.85(+0.14/-0.11) solar masses and the orbital inclination to be i = 79.9(+1.6/-1.9) degrees, where uncertainties correspond to +/- 1-σ confidence levels. With continued timing, we expect to also be able to detect the advance of periastron for the J1949+3106 system. This effect, combined with the Shapiro delay, will eventually provide very precise mass measurements for this system and a test of general relativity.

astro-ph.SR↗

arXiv2012

Four Highly Dispersed Millisecond Pulsars Discovered in the Arecibo PALFA Galactic Plane Survey

We present the discovery and phase-coherent timing of four highly dispersed millisecond pulsars (MSPs) from the Arecibo PALFA Galactic plane survey: PSRs J1844+0115, J1850+0124, J1900+0308, and J1944+2236. Three of the four pulsars are in binary systems with low-mass companions, which are most likely white dwarfs, and which have orbital periods on the order of days. The fourth pulsar is isolated. All four pulsars have large dispersion measures (DM > 100 pc cm-3), are distant (> 3.4 kpc), faint at 1.4 GHz (< 0.2 mJy), and are fully recycled (with spin periods P between 3.5 and 4.9 ms). The three binaries also have very small orbital eccentricities, as expected for tidally circularized, fully recycled systems with low-mass companions. These four pulsars have DM/P ratios that are among the highest values for field MSPs in the Galaxy. These discoveries bring the total number of confirmed MSPs from the PALFA survey to fifteen. The discovery of these MSPs illustrates the power of PALFA for finding weak, distant MSPs at low-Galactic latitudes. This is important for accurate estimates of the Galactic MSP population and for the number of MSPs that the Square Kilometer Array can be expected to detect.

astro-ph.SR↗

arXiv2012

First Light for the First Station of the Long Wavelength Array

The first station of the Long Wavelength Array (LWA1) was completed in April 2011 and is currently performing observations resulting from its first call for proposals in addition to a continuing program of commissioning and characterization observations. The instrument consists of 258 dual-polarization dipoles, which are digitized and combined into beams. Four independently-steerable dual-polarization beams are available, each with two "tunings" of 16 MHz bandwidth that can be independently tuned to any frequency between 10 MHz and 88 MHz. The system equivalent flux density for zenith pointing is ~3 kJy and is approximately independent of frequency; this corresponds to a sensitivity of ~5 Jy/beam (5sigma, 1 s); making it one of the most sensitive meter-wavelength radio telescopes. LWA1 also has two "transient buffer" modes which allow coherent recording from all dipoles simultaneously, providing instantaneous all-sky field of view. LWA1 provides versatile and unique new capabilities for Galactic science, pulsar science, solar and planetary science, space weather, cosmology, and searches for astrophysical transients. Results from LWA1 will detect or tightly constrain the presence of hot Jupiters within 50 parsecs of Earth. LWA1 will provide excellent resolution in frequency and in time to examine phenomena such as solar bursts, and pulsars over a 4:1 frequency range that includes the poorly understood turnover and steep-spectrum regimes. Observations to date have proven LWA1's potential for pulsar observing, and just a few seconds with the completed 256-dipole LWA1 provide the most sensitive images of the sky at 23 MHz obtained yet. We are operating LWA1 as an open skies radio observatory, offering ~2000 beam-hours per year to the general community.

astro-ph.IM↗

arXiv2012

Discovery of the Optical/Ultraviolet/Gamma-ray Counterpart to the Eclipsing Millisecond Pulsar J1816+4510

The energetic, eclipsing millisecond pulsar J1816+4510 was recently discovered in a low-frequency radio survey with the Green Bank Telescope. With an orbital period of 8.7 hr and minimum companion mass of 0.16 Msun it appears to belong to an increasingly important class of pulsars that are ablating their low-mass companions. We report the discovery of the gamma-ray counterpart to this pulsar, and present a likely optical/ultraviolet counterpart as well. Using the radio ephemeris we detect pulsations in the unclassified gamma-ray source 2FGL J1816.5+4511, implying an efficiency of ~25% in converting the pulsar's spin-down luminosity into gamma-rays and adding PSR J1816+4510 to the large number of millisecond pulsars detected by Fermi. The likely optical/UV counterpart was identified through position coincidence (<0.1") and unusual colors. Assuming that it is the companion, with R=18.27+/-0.03 mag and effective temperature >15,000 K it would be among the brightest and hottest of low-mass pulsar companions, and appears qualitatively different from other eclipsing pulsar systems. In particular, current data suggest that it is a factor of two larger than most white dwarfs of its mass, but a factor of four smaller than its Roche lobe. We discuss possible reasons for its high temperature and odd size, and suggest that it recently underwent a violent episode of mass loss. Regardless of origin, its brightness and the relative unimportance of irradiation make it an ideal target for a mass, and hence a neutron star mass, determination.

astro-ph.HE↗

arXiv2011

Arecibo PALFA Survey and Einstein@Home: Binary Pulsar Discovery by Volunteer Computing

We report the discovery of the 20.7 ms binary pulsar J1952+2630, made using the distributed computing project Einstein@Home in Pulsar ALFA survey observations with the Arecibo telescope. Follow-up observations with the Arecibo telescope confirm the binary nature of the system. We obtain a circular orbital solution with an orbital period of 9.4 hr, a projected orbital radius of 2.8 lt-s, and a mass function of f = 0.15 solar masses by analysis of spin period measurements. No evidence of orbital eccentricity is apparent; we set a 2-sigma upper limit e < 1.7e-3. The orbital parameters suggest a massive white dwarf companion with a minimum mass of 0.95 solar masses, assuming a pulsar mass of 1.4 solar masses. Most likely, this pulsar belongs to the rare class of intermediate mass binary pulsars. Future timing observations will aim to determine the parameters of this system further, measure relativistic effects, and elucidate the nature of the companion star.

astro-ph.GA↗

arXiv2010

Pulsar Discovery by Global Volunteer Computing

Einstein@Home aggregates the computer power of hundreds of thousands of volunteers from 192 countries to "mine" large data sets. It has now found a 40.8 Hz isolated pulsar in radio survey data from the Arecibo Observatory taken in February 2007. Additional timing observations indicate that this pulsar is likely a disrupted recycled pulsar. PSR J2007+2722's pulse profile is remarkably wide with emission over almost the entire spin period; the pulsar likely has closely aligned magnetic and spin axes. The massive computing power provided by volunteers should enable many more such discoveries.

astro-ph.GA↗

arXiv2010

Apparent Faster-Than-Light Pulse Propagation in Interstellar Space: A new probe of the Interstellar Medium

Radio pulsars emit regular bursts of radio radiation that propagate through the interstellar medium (ISM), the tenuous gas and plasma between the stars. Previously known dispersive properties of the ISM cause low frequency pulses to be delayed in time with respect to high frequency ones. This effect can be explained by the presence of free electrons in the medium. The ISM also contains neutral hydrogen which has a well known resonance at 1420.4 MHz. Electro-magnetic theory predicts that at such a resonance, the induced dispersive effects will be drastically different from those of the free electrons. Pulses traveling through a cloud of neutral hydrogen should undergo "anomalous dispersion", which causes the group velocity of the medium to be larger than the speed of light in vacuum. This superluminal group velocity causes pulses containing frequencies near the resonance to arrive earlier in time with respect to other pulses. Hence, these pulses appear to travel faster than light. This phenomenon is caused by an interplay between the time scales present in the pulse and the time scales present in the medium. Although counter-intuitive, it does not violate the laws of special relativity. Here, we present Arecibo observations of the radio pulsar PSR B1937+21 that show clear evidence of anomalous dispersion. Though this effect is known in laboratory physics, this is the first time it has been directly observed in an astrophysical context, and it has the potential to be a useful tool for studying the properties of neutral hydrogen in the Galaxy.

astro-ph.GA↗