A broadband radio study of the average profile and giant pulses from PSR B1821-24A

We present the results of wide-band (720-2400 MHz) study of PSR B1821-24A (J1824-2452A, M28A), an energetic millisecond pulsar visible in radio, X-rays and gamma-rays. In radio, the pulsar has a complex average profile which spans >85% of the spin period and exhibits strong evolution with observing frequency. For the first time we measure phase-resolved polarization properties and spectral indices of radio emission throughout almost all of the on-pulse window. We combine this knowledge with the high-energy information to compare M28A to other known gamma-ray millisecond pulsars and to speculate that M28A's radio emission originates in multiple regions within its magnetosphere (i.e. both in the slot or outer gaps near the light cylinder and at lower altitudes above the polar cap). M28A is one of the handful of pulsars which are known to emit Giant Pulses (GPs) -- short, bright radio pulses of unknown nature. We report a drop in the linear polarization of the average profile in both windows of GP generation and also a `W'-shaped absorption feature (resembling a double notch), partly overlapping with one of the GP windows. The GPs themselves have broadband spectra consisting of multiple patches with fractional spectral width ($\Delta\nu/\nu$) of about 0.07. Although our time resolution was not sufficient to resolve the GP structure on the microsecond scale, we argue that GPs from this pulsar most closely resemble the GPs from the main pulse of the Crab pulsar, which consist of a series of narrowband nanoshots.Comment: 16 pages, 8 figures, accepted to Ap

Green Bank Telescope Observations of the Eclipse of Pulsar "A" in the Double Pulsar Binary PSR J0737-3039

We report on the first Green Bank Telescope observations at 427, 820 and 1400 MHz of the newly discovered, highly inclined and relativistic double pulsar binary. We focus on the brief eclipse of PSR J0737-3039A, the faster pulsar, when it passes behind PSR J0737-3039B. We measure a frequency-averaged eclipse duration of 26.6 +/- 0.6 s, or 0.00301 +/- 0.00008 in orbital phase. The eclipse duration is found to be significantly dependent on radio frequency, with eclipses longer at lower frequencies. Specifically, eclipse duration is well fit by a linear function having slope (-4.52 +/- 0.03) x 10^{-7} orbits/MHz. We also detect significant asymmetry in the eclipse. Eclipse ingress takes 3.51 +/- 0.99 times longer than egress, independent of radio frequency. Additionally, the eclipse lasts (40 +/- 7) x 10^{-5} in orbital phase longer after conjunction, also independent of frequency. We detect significant emission from the pulsar on short time scales during eclipse in some orbits. We discuss these results in the context of a model in which the eclipsing material is a shock-heated plasma layer within the slower PSR J0737-3039B's light cylinder, where the relativistic pressure of the faster pulsar's wind confines the magnetosphere of the slower pulsar.Comment: 12 pages, 3 figure

Evaluation of direct current motors in vacuum

Simulated space environment evaluation of direct current motor material

Practical Methods for Continuous Gravitational Wave Detection using Pulsar Timing Data

Gravitational Waves (GWs) are tiny ripples in the fabric of space-time predicted by Einstein's General Relativity. Pulsar timing arrays (PTAs) are well poised to detect low frequency ($10^{-9}$ -- $10^{-7}$ Hz) GWs in the near future. There has been a significant amount of research into the detection of a stochastic background of GWs from supermassive black hole binaries (SMBHBs). Recent work has shown that single continuous sources standing out above the background may be detectable by PTAs operating at a sensitivity sufficient to detect the stochastic background. The most likely sources of continuous GWs in the pulsar timing frequency band are extremely massive and/or nearby SMBHBs. In this paper we present detection strategies including various forms of matched filtering and power spectral summing. We determine the efficacy and computational cost of such strategies. It is shown that it is computationally infeasible to use an optimal matched filter including the poorly constrained pulsar distances with a grid based method. We show that an Earth-term-matched filter constructed using only the correlated signal terms is both computationally viable and highly sensitive to GW signals. This technique is only a factor of two less sensitive than the computationally unrealizable optimal matched filter and a factor of two more sensitive than a power spectral summing technique. We further show that a pairwise matched filter, taking the pulsar distances into account is comparable to the optimal matched filter for the single template case and comparable to the Earth-term-matched filter for many search templates. Finally, using simulated data optimal quality, we place a theoretical minimum detectable strain amplitude of $h>2\times 10^{-15}$ from continuous GWs at frequencies on the order $\sim1/T_{\rm obs}$.Comment: submitted to Ap

Optimal strategies for continuous gravitational wave detection in pulsar timing arrays

Supermassive black hole binaries (SMBHBs) are expected to emit continuous gravitational waves in the pulsar timing array (PTA) frequency band ($10^{-9}$--$10^{-7}$ Hz). The development of data analysis techniques aimed at efficient detection and characterization of these signals is critical to the gravitational wave detection effort. In this paper we leverage methods developed for LIGO continuous wave gravitational searches, and explore the use of the $\mathcal{F}$-statistic for such searches in pulsar timing data. Babak & Sesana 2012 have already used this approach in the context of PTAs to show that one can resolve multiple SMBHB sources in the sky. Our work improves on several aspects of prior continuous wave search methods developed for PTA data analysis. The algorithm is implemented fully in the time domain, which naturally deals with the irregular sampling typical of PTA data and avoids spectral leakage problems associated with frequency domain methods. We take into account the fitting of the timing model, and have generalized our approach to deal with both correlated and uncorrelated colored noise sources. We also develop an incoherent detection statistic that maximizes over all pulsar dependent contributions to the likelihood. To test the effectiveness and sensitivity of our detection statistics, we perform a number of monte-carlo simulations. We produce sensitivity curves for PTAs of various configurations, and outline an implementation of a fully functional data analysis pipeline. Finally, we present a derivation of the likelihood maximized over the gravitational wave phases at the pulsar locations, which results in a vast reduction of the search parameter space.Comment: 11 pages, 5 figure

Locating the intense interstellar scattering towards the inner Galaxy

We use VLBA+VLA observations to measure the sizes of the scatter-broadened images of 6 of the most heavily scattered known pulsars: 3 within the Galactic Centre (GC) and 3 elsewhere in the inner Galactic plane. By combining the measured sizes with temporal pulse broadening data from the literature and using the thin-screen approximation, we locate the scattering medium along the line of sight to these 6 pulsars. At least two scattering screens are needed to explain the observations of the GC sample. We show that the screen inferred by previous observations of SGR J1745-2900 and Sgr A*, which must be located far from the GC, falls off in strength on scales < 0.2 degree. A second scattering component closer to (< 2 kpc) or even (tentatively) within (< 700 pc) the GC produces most or all of the temporal broadening observed in the other GC pulsars. Outside the GC, the scattering locations for all three pulsars are ~2 kpc from Earth, consistent with the distance of the Carina-Sagittarius or Scutum spiral arm. For each object the 3D scattering origin coincides with a known HII region (and in one case also a supernova remnant), suggesting that such objects preferentially cause the intense interstellar scattering seen towards the Galactic plane. We show that the HII regions should contribute > 25% of the total dispersion measure (DM) towards these pulsars, and calculate reduced DM distances. Those distances for other pulsars lying behind HII regions may be similarly overestimated.Comment: 16 pages, 10 figures, MNRAS, in pres

VLA Observations of Single Pulses from the Galactic Center Magnetar

We present the results of a 7-12 GHz phased-array study of the Galactic center magnetar J1745-2900 with the Karl G. Jansky Very Large Array (VLA). Using data from two 6.5 hour observations from September 2014, we find that the average profile is comprised of several distinct components at these epochs and is stable over $\sim$day timescales and $\sim$GHz frequencies. Comparison with additional phased VLA data at 8.7 GHz shows significant profile changes on longer timescales. The average profile at 7-12 GHz is dominated by the jitter of relatively narrow pulses. The pulses in each of the four main profile components seen in September 2014 are uncorrelated in phase and amplitude, though there is a small but significant correlation in the occurrence of pulses in two of the profile components. Using the brightest pulses, we measure the dispersion and scattering parameters of J1745-2900. A joint fit of 38 pulses gives a 10 GHz pulse broadening time of $\tau_{\rm sc, 10} = 0.09 \pm 0.03~\rm ms$ and a dispersion measure of ${\rm DM} = 1760^{+2.4}_{-1.3}~{\rm pc~cm}^{-3}$. Both of these results are consistent with previous measurements, which suggests that the scattering and dispersion measure of J1745-2900 may be stable on timescales of several years.Comment: 20 pages, 10 figures, published in Ap