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Register a new userDiscovery and modelling of broad-scale plasma lensing in black-widow pulsar J2051$-$0827
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We report on an unusually bright observation of PSR J2051$-$0827 recorded during a regular monitoring campaign of black-widow pulsar systems with the Effelsberg 100-m telescope. Through fortunate coincidence, a particularly bright scintillation maximum is simultaneous with the eclipse by the companion, enabling precise measurements of variations in the flux density, dispersion measure (DM), and scattering strength throughout the eclipse. The flux density is highly variable throughout the eclipse, with a peak 1.7 times the average away from the eclipse, and yet does not significantly decrease on average. We recover the flux density variations from the measured DM variations using geometric optics, with a relative velocity as the only free parameter. We measure an effective velocity of (470 $pm$ 10) km/s, consistent with the relative orbital motion of the companion, suggesting that the outflow velocity of the lensing material is low, or is directly along the line of sight. The 2 per cent uncertainty on the effective velocity is a formal error; systematics related to our current model are likely to dominate, and we detail several extensions to the model to be considered in a full treatment of lensing. This is a demonstration of the causal link between DM and lensing; the flux density variations can be predicted directly through the derivatives of DM. Going forward, this approach can be applied to investigate the dynamics of other eclipsing systems, and to investigate the physical nature of scintillation and lensing in the ionized interstellar medium.
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Timing results for the black-widow pulsar J2051-0827 are presented, using a 21-year dataset from four European Pulsar Timing Array telescopes and the Parkes radio telescope. This dataset, which is the longest published to date for a black-widow system, allows for an improved analysis that addresses previously unknown biases. While secular variations, as identified in previous analyses, are recovered, short-term variations are detected for the first time. Concurrently, a significant decrease of approx. 2.5x10-3 cm-3 pc in the dispersion measure associated with PSR J2051-0827 is measured for the first time and improvements are also made to estimates of the proper motion. Finally, PSR J2051-0827 is shown to have entered a relatively stable state suggesting the possibility of its eventual inclusion in pulsar timing arrays.
In regions with strongly varying electron density, radio emission can be magnified significantly by plasma lensing. In the presence of magnetic fields, magnification in time and frequency will be different for two circular polarizations. We show how these effects can be used to measure or constrain the magnetic field parallel to the line of sight, $B_parallel$, as well as its spatial structure, $sigma_{B_parallel}$, in the lensing region. In addition, we discuss how generalized Faraday rotation can constrain the strength of the perpendicular field, $B_perp$. We attempt to make such measurements for the Black Widow pulsar, PSR~B1957+20, in which plasma lensing was recently discovered. For this system, pressure equilibrium suggests $Bgtrsim 20,$G at the interface between the pulsar and companion winds, where the radio eclipse starts and ends, and where most lensing occurs. We find no evidence for large-scale magnetic fields, with, on average, $B_parallel=0.02pm0.09,$G over the egress lensing region. From individual lensing events, we strongly constrain small scale magnetic structure to $sigma_B<10,$mG, thus excluding scenarios with a strong but rapidly varying field. Finally, from the lack of reduction of average circular polarization in the same region, we rule out a strong, quasi-transverse field. We cannot identify any plausible scenario in which a large magnetic field in this system is concealed, leaving the nature of the interface between the pulsar and companion winds an enigma. Our method can be applied to other sources showing plasma lensing, including other eclipsing pulsars and fast radio bursts, to study the local properties of the magnetic field.
In this paper we report on $sim10$ years of observations of PSR J2051$-$0827, at radio frequencies in the range 110--4032 MHz. We investigate the eclipse phenomena of this black widow pulsar using model fits of increased dispersion and scattering of the pulsed radio emission as it traverses the eclipse medium. These model fits reveal variability in dispersion features on timescales as short as the orbital period, and previously unknown trends on timescales of months--years. No clear patterns are found between the low-frequency eclipse widths, orbital period variations and trends in the intra-binary material density. Using polarisation calibrated observations we present the first available limits on the strength of magnetic fields within the eclipse region of this system; the average line of sight field is constrained to be $10^{-4}$ G $lesssim B_{||} lesssim 10^2$ G, while for the case of a field directed near-perpendicular to the line of sight we find $B_{perp} lesssim 0.3$ G. Depolarisation of the linearly polarised pulses during the eclipse is detected and attributed to rapid rotation measure fluctuations of $sigma_{text{RM}} gtrsim 100$ rad m$^{-2}$ along, or across, the line of sights averaged over during a sub-integration. The results are considered in the context of eclipse mechanisms, and we find scattering and/or cyclotron absorption provide the most promising explanation, while dispersion smearing is conclusively ruled out. Finally, we estimate the mass loss rate from the companion to be $dot{M}_{text{C}} sim 10^{-12} M_odot$ yr$^{-1}$, suggesting that the companion will not be fully evaporated on any reasonable timescale.
We report the discovery of 1.97 ms period gamma-ray pulsations from the 75 minute orbital-period binary pulsar now named PSR J1653-0158. The associated Fermi Large Area Telescope gamma-ray source 4FGL J1653.6-0158 has long been expected to harbor a binary millisecond pulsar. Despite the pulsar-like gamma-ray spectrum and candidate optical/X-ray associations -- whose periodic brightness modulations suggested an orbit -- no radio pulsations had been found in many searches. The pulsar was discovered by directly searching the gamma-ray data using the GPU-accelerated Einstein@Home distributed volunteer computing system. The multi-dimensional parameter space was bounded by positional and orbital constraints obtained from the optical counterpart. More sensitive analyses of archival and new radio data using knowledge of the pulsar timing solution yield very stringent upper limits on radio emission. Any radio emission is thus either exceptionally weak, or eclipsed for a large fraction of the time. The pulsar has one of the three lowest inferred surface magnetic-field strengths of any known pulsar with $B_{rm surf} approx 4 times 10^{7},$G. The resulting mass function, combined with models of the companion stars optical light curve and spectra, suggests a pulsar mass $gtrsim 2,M_{odot}$. The companion is light-weight with mass $sim 0.01,M_{odot}$, and the orbital period is the shortest known for any rotation-powered binary pulsar. This discovery demonstrates the Fermi Large Area Telescopes potential to discover extreme pulsars that would otherwise remain undetected.
We report the discovery of the first radio pulsar associated with NGC 6712, an eclipsing black widow (BW) pulsar, J1853$-$0842A, found by high-sensitivity searches using the Five-hundred-meter Aperture Spherical radio Telescope. This 2.15 ms pulsar is in a 3.56 hr compact circular orbit with a very low mass companion likely of mass 0.018 to 0.036 $M_{rm odot}$ and exhibits eclipsing of the pulsar signal. Though the distance to PSR J1853$-$0842A predicted from its dispersion measure ($155.125 pm 0.004$ cm$^{-3}$ pc) and Galactic free electron density models are about 30% smaller than that of NGC 6712 obtained from interstellar reddening measurements, this is likely due to limited knowledge about the spiral arms and Scutum stellar cloud in this direction. Follow-up timing observations spanning 445 days allow us to localize the pulsars position to be 0.14 core radii from the center of NGC 6712 and measure a negative spin-down rate for this pulsar of $-2.39(2)times10^{-21}rm s s^{-1}$. The latter cannot be explained without the acceleration of the GC and decisively supports the association between PSR J1853--0842A and NGC 6712. Considering the maximum GC acceleration, Galactic acceleration, and Shklovskii effect, we place an upper limit on the intrinsic spin-down rate to be $1.11times10^{-20}rm~s~s^{-1}$. From an analysis of the eclipsing observations, we estimate the electron density of the eclipse region to be about $1.88times10^6rm cm^{-3}$. We also place an upper limit of the accretion rate from the companion is about $3.05times10^{-13}~M_{rm odot}rm~yr^{-1}$ which is comparable with some other BWs.
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