Precision pulsar timing requires optimization against measurement errors and astrophysical variance from the neutron stars themselves and the interstellar medium. We investigate optimization of ...arrival-time precision as a function of radio frequency and bandwidth. We find that increases in bandwidth that reduce the contribution from receiver noise are countered by the strong chromatic dependence of interstellar effects and intrinsic pulse-profile evolution. The resulting optimal frequency range is therefore telescope and pulsar dependent. We demonstrate the results for five pulsars included in current pulsar timing arrays and determine that they are not optimally observed at current center frequencies. For those objects, we find that better choices of total bandwidth as well as center frequency can improve the arrival-time precision. Wideband receivers centered at somewhat higher frequencies with respect to the currently adopted receivers can reduce required overall integration times and provide significant improvements in arrival-time uncertainty by a factor of in most cases, assuming a fixed integration time. We also discuss how timing programs can be extended to pulsars with larger dispersion measures through the use of higher-frequency observations.
The highest velocity neutron stars establish stringent constraints on natal kicks, asymmetries in supernova core collapse, and the evolution of close binary systems. Here we present the first results ...of a long-term pulsar astrometry program using the VLBA. We measure a proper motion and parallax for the pulsar B1508+55, leading to model-independent estimates of its distance (2.37 super(+) sub(-) super(0) sub(0) super(.) sub(.) super(2) sub(2) super(3) sub(0) kpc) and transverse velocity (1083 super(+) sub(-) super(1) sub(9) super(0) sub(0) super(3) km s super(-1)), the highest velocity directly measured for a neutron star. We trace the pulsar back from its present Galactic latitude of 52.3 to a birth site in the Galactic plane near the Cyg OB associations, and find that it will inevitably escape the Galaxy. Binary disruption alone is insufficient to impart the required birth velocity, and a natal kick is indicated. A composite scenario including a large kick along with binary disruption can plausibly account for the high velocity.
ABSTRACT We present initial pulsar results from the first station of the Long Wavelength Array (LWA1) obtained during the commissioning period of LWA1 and in early science results. We present ...detections of periodic emission from 44 previously known pulsars, including 3 millisecond pulsars. The effects of the interstellar medium (ISM) on pulsar emission are significantly enhanced at the low frequencies of the LWA1 band (10-88 MHz), making LWA1 a very sensitive instrument for characterizing changes in the dispersion measure (DM) and other effects from the ISM. Pulsars also often have significant evolution in their pulse profile at low frequency and a break in their spectral index. We report DM measurements for 44 pulsars, mean flux density measurements for 36 pulsars, and multi-frequency component spacing and widths for 15 pulsars with more than one profile component. For 27 pulsars, we report spectral index measurements within our frequency range. We also introduce the LWA1 Pulsar Data Archive, which stores reduced data products from LWA1 pulsar observations. Reduced data products for the observations presented here can be found in the archive. Reduced data products from future LWA1 pulsar observations will also be made available through the archive.
The detection of radio pulsars within the central few parsecs of the Galaxy would provide a unique probe of the gravitational and magneto-ionic environments in the Galactic center (GC) and, if close ...enough to Sgr A*, precise tests of general relativity in the strong-field regime. While it is difficult to find pulsars at radio wavelengths because of interstellar scattering, the payoff from detailed timing of pulsars in the GC warrants a concerted effort. To motivate pulsar surveys and help define search parameters for them, we constrain the pulsar number and spatial distribution using a wide range of multiwavelength measurements. These include the five known radio pulsars within 15' of Sgr A*, non-detections in high-frequency pulsar surveys of the central parsec, radio and gamma-ray measurements of diffuse emission, a catalog of radio point sources from an imaging survey, infrared observations of massive star populations in the central few parsecs, candidate pulsar wind nebulae in the inner 20 pc, and estimates of the core-collapse supernova rate based on X-ray measurements. We find that under current observational constraints, the inner parsec of the Galaxy could harbor as many as ~10 super(3) active radio pulsars that are beamed toward Earth. Such a large population would distort the low-frequency measurements of both the intrinsic spectrum of Sgr A* and the free-free absorption along the line of sight of Sgr A*.
We present 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 hr observations from 2014 ...September, we find that the average profile is comprised of several distinct components at these epochs and is stable over ∼day timescales and ∼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 2014 September 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 and a dispersion measure of . 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.
The radio millisecond pulsar J1713+0747 is regarded as one of the highest-precision clocks in the sky and is regularly timed for the purpose of detecting gravitational waves. The International Pulsar ...Timing Array Collaboration undertook a 24 hr global observation of PSR J1713+0747 in an effort to better quantify sources of timing noise in this pulsar, particularly on intermediate (1-24 hr) timescales. We observed the pulsar continuously over 24 hr with the Arecibo, Effelsberg, GMRT, Green Bank, LOFAR, Lovell, Nançay, Parkes, and WSRT radio telescopes. The combined pulse times-of-arrival presented here provide an estimate of what sources of timing noise, excluding DM variations, would be present as compared to an idealized \sqrt{N} improvement in timing precision, where N is the number of pulses analyzed. In the case of this particular pulsar, we find that intrinsic pulse phase jitter dominates arrival time precision when the signal-to-noise ratio of single pulses exceeds unity, as measured using the eight telescopes that observed at L band/1.4 GHz. We present first results of specific phenomena probed on the unusually long timescale (for a single continuous observing session) of tens of hours, in particular interstellar scintillation, and discuss the degree to which scintillation and profile evolution affect precision timing. This paper presents the data set as a basis for future, deeper studies.
We report the discovery of up to 35 new supernova remnants (SNRs) from a 42" resolution, 90 cm multiconfiguration Very Large Array survey of the Galactic plane covering 4 5 < l < 22 0 and "b" < 1 25. ...Archival 20 cm, 11 cm, and 8 km data have also been used to identify the SNRs and constrain their properties. The 90 cm image is sensitive to SNRs with diameters 2.'5 to 50' and down to a surface brightness limit of 610 super(-21) W m super(-2) Hz super(-1)sr super(-1). This survey has nearly tripled the number of SNRs known in this part of the Galaxy and represents an overall 15% increase in the total number of Galactic SNRs. These results suggest that further deep, low-frequency surveys of the inner Galaxy will resolve the discrepancy between the expected number of Galactic SNRs and the significantly smaller number of currently known SNRs.
We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2°X 08 field around the Galactic center. This enlarges the number of known X-ray sources in the region by a factor ...of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Ms of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities of 4 X 1032 erg s-1 (0.5-8.0 keV; 90% confidence) over an area of 1 deg2, and up to an order of magnitude more sensitive in the deepest exposure (1.0 Ms) around Sgr A*. The positions of 60% of our sources are accurate to <1 '' (95% confidence), and 20% have positions accurate to <05. We search for variable sources, and find that 3% exhibit flux variations within an observation, and 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7 - 285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically accreting cataclysmic variable. We compare the spatial distribution of X-ray sources to a model for the stellar distribution, and find 2.8s evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l 01.
ABSTRACT
From an on-going survey of the Galactic bulge, we have discovered a number of compact, steep spectrum radio sources. In this study, we have carried out more detailed observations for two of ...these sources, located 43 arcmin and 12${_{.}^{\circ}}$7 from the Galactic Centre. Both sources have a very steep spectrum (α ≃ − 3) and are compact, with upper limits on the angular size of 1–2 arcsec. Their flux densities appear to be relatively steady on time-scales of years, months, and hours, with no indications of rapid variability or transient behaviour. We detect significant circularly polarized emission from both sources, but only weak or upper limits on linear polarization. Neither source has a counterpart at other wavelengths and deep, high-frequency searches fail to find pulsations. We compare their source properties with other known compact, non-thermal source populations in the bulge (e.g. X-ray binaries, magnetars, the Burper, and cataclysmic variables). Our existing data support the hypothesis that they are scatter broadened millisecond or recycled pulsars, either at the bulge or along the line of sight. We also consider the possibility that they may be a new population of Galactic radio sources which share similar properties as pulsars but lack pulsations; a hypothesis that can be tested by future large-scale synoptic surveys.
PDF
