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Register a new userExtinct radio pulsars as a source of subrelativistic positrons
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Extinct radio pulsars, in which stationary, self-sustaining generation of a relativistic electron-positron plasma becomes impossible when rotation brakes down, can be sources of a subrelativistic flux of positrons and electrons. We assume that the observed excess of positrons in the bulge and the disc of the Galaxy is associated with these old neutron stars. The production of pairs in their magnetospheres occurs due to one-photon absorption of gamma quanta of the Galactic and extragalactic backgrounds. The cascade process of plasma production leads to the flux of positrons escaping from the open magnetosphere $simeq 3 times 10^{34} text{ s}^{-1}$. The total flux of positrons from all old Galactic neutron stars with rotational periods $1.5 < P < 35$ s is $simeq 3 times 10^{43} text{ s}^{-1}$. The energy of positrons is less than $simeq 10$ MeV. The estimated characteristics satisfy the requirements for the positron source responsible for the 511-keV Galactic annihilation line.
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L. Guillemot (for the Fermi LAT Collaboration
, for the Fermi Pulsarn Search Consortium
,
2012
Observations of pulsars with the Large Area Telescope (LAT) on the Fermi satellite have revolutionized our view of the gamma-ray pulsar population. For the first time, a large number of young gamma-ray pulsars have been discovered in blind searches of the LAT data. More generally, the LAT has discovered many new gamma-ray sources whose properties suggest that they are powered by unknown pulsars. Radio observations of gamma-ray sources have been key to the success of pulsar studies with the LAT. For example, radio observations of LAT-discovered pulsars provide constraints on the relative beaming fractions, which are crucial for pulsar population studies. Also, radio searches of LAT sources with no known counterparts have been very efficient, with the discovery of over forty millisecond pulsars. I review radio follow-up studies of LAT-discovered pulsars and unidentified sources, and discuss some of the implications of the results.
We propose a new method to detect off-pulse (unpulsed and/or continuous) emission from pulsars, using the intensity modulations associated with interstellar scintillation. Our technique involves obtaining the dynamic spectra, separately for on-pulse window and off-pulse region, with time and frequency resolutions to properly sample the intensity variations due to diffractive scintillation, and then estimating their mutual correlation as a measure of off-pulse emission, if any. We describe and illustrate the essential details of this technique with the help of simulations, as well as real data. We also discuss advantages of this method over earlier approaches to detect off-pulse emission. In particular, we point out how certain non-idealities inherent to measurement set-ups could potentially affect estimations in earlier approaches, and argue that the present technique is immune to such non-idealities. We verify both of the above situations with relevant simulations. We apply this method to observation of PSR B0329+54 at frequencies 730 and 810 MHz, made with the Green Bank Telescope and present upper limits for the off-pulse intensity at the two frequencies. We expect this technique to pave way for extensive investigations of off-pulse emission with the help of even existing dynamic spectral data on pulsars and of course with more sensitive long-duration data from new observations.
LOFAR offers the unique capability of observing pulsars across the 10-240 MHz frequency range with a fractional bandwidth of roughly 50%. This spectral range is well-suited for studying the frequency evolution of pulse profile morphology caused by both intrinsic and extrinsic effects: such as changing emission altitude in the pulsar magnetosphere or scatter broadening by the interstellar medium, respectively. The magnitude of most of these effects increases rapidly towards low frequencies. LOFAR can thus address a number of open questions about the nature of radio pulsar emission and its propagation through the interstellar medium. We present the average pulse profiles of 100 pulsars observed in the two LOFAR frequency bands: High Band (120-167 MHz, 100 profiles) and Low Band (15-62 MHz, 26 profiles). We compare them with Westerbork Synthesis Radio Telescope (WSRT) and Lovell Telescope observations at higher frequencies (350 and1400 MHz) in order to study the profile evolution. The profiles are aligned in absolute phase by folding with a new set of timing solutions from the Lovell Telescope, which we present along with precise dispersion measures obtained with LOFAR. We find that the profile evolution with decreasing radio frequency does not follow a specific trend but, depending on the geometry of the pulsar, new components can enter into, or be hidden from, view. Nonetheless, in general our observations confirm the widening of pulsar profiles at low frequencies, as expected from radius-to-frequency mapping or birefringence theories. We offer this catalog of low-frequency pulsar profiles in a user friendly way via the EPN Database of Pulsar Profiles (http://www.epta.eu.org/epndb/).
We present high signal-to-noise, full polarization pulse profiles for 40 bright, slowly-rotating (non-recycled) pulsars using the new Ultra-Wideband Low-frequency (UWL; 704-4032 MHz) receiver on the Parkes radio telescope. We obtain updated and accurate interstellar medium parameters towards these pulsars (dispersion measures and Faraday rotation measures), and reveal Faraday dispersion towards PSR J1721-3532 caused by interstellar scattering. We find general trends in the pulse profiles including decreasing fractional linear polarization and increasing degree of circular polarization with increasing frequency, consistent with previous studies, while also revealing new features and frequency evolution. This demonstrates results that can be obtained using UWL monitoring observations of slow pulsars, which are valuable for improving our understanding of pulsar emission and the intervening interstellar medium. The calibrated data products are publicly available.
Pulsars play a crucial astrophysical role as the highly energetic compact radio, X-ray, and gamma-ray sources. Our previous works show that the radio pulsars found as the pulsing gamma sources by the Large Area Telescope (LAT) on the board of the Fermi Gamma-Ray Space Telescope have high values of magnetic field near the light cylinder, two-three orders of magnitude stronger comparing with the magnetic fields of radio pulsars: $log B_{lc}$ (G) are 3.60-3.95 and 1.75. Moreover, their losses of the rotation energy are also three orders higher than the corresponding values for the main group of radio pulsars on average:$log dot E$ (erg/s) = 35.37-35.53 and 32.64. The correlation between gamma-ray luminosities and radio luminosities is found. It allows us to select those objects from all set of the known radio pulsars that can be detected as gamma pulsars with the high probability. We give the list of such radio pulsars and propose to search for gamma emisson from these objects. On the other hand, the known catalog of gamma pulsars contains some sources which are not known as radio pulsars at this moment. Some of them have the large values of gamma luminosities and according to the obtained correlation, we can expect marked radio emission from these objects. We give the list of such pulsars and expected flux densities to search for radiation at frequencies 1400 and 111 MHz.
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