PSR B1257+12: a quark star with planets?

A recent observation has shown that PSR B1257+12 could have quite small X-ray emitting area, only about 2000 m$^2$, which is more than three orders smaller than the canonical polar cap size. We suggest here that PSR B1257+12 could be a low-mass quark star with radius of $R \simeq 0.6$ km and mass of M \simeq 3\times10^{-4}\msun. Such a low-mass quark star system may form in an accretion induced collapse process or a collision process of two quark stars.Comment: 3 pages, 1figure, poster at the international conference "Astrophysics of Compact Objects" (July 1-7, 2007, Huangshan, China

Gravitational Microlensing by Neutron Stars and Radio Pulsars: Event Rates, Timescale Distributions, and Mass Measurements

We investigate properties of Galactic microlensing events in which a stellar object is lensed by a neutron star. For an all-sky photometric microlensing survey, we determine the number of lensing events caused by $\sim10^{5}$ potentially-observable radio pulsars to be $\sim0.2\ \rm{yr^{-1}}$ for $10^{10}$ background stellar sources. We expect a few detectable events per year for the same number of background sources from an astrometric microlensing survey. We show that such a study could lead to precise measurements of radio pulsar masses. For instance, if a pulsar distance could be constrained through radio observations, then its mass would be determined with a precision of $\sim10\%$. We also investigate the time-scale distributions for neutron star events, finding that they are much shorter than had been previously thought. For photometric events towards the Galactic centre that last $\sim15$ days, around $7\%$ will have a neutron star lens. This fraction drops rapidly for longer time-scales. Away from the bulge region we find that neutron stars will contribute $\sim40\%$ of the events that last less than $\sim10$ days. These results are in contrast to earlier work which found that the maximum fraction of neutron star events would occur on time-scales of hundreds of days.Comment: 10 pages, accepted for publication in ApJ. v2 updated to reflect change of title in proof stag

The Radiation Structure of PSR B2016++28 Observed with FAST

With the largest dish Five-hundred-meter Aperture Spherical radio Telescope (FAST), both the mean and single pulses of PSR B2016$+$28, especially including the single-pulse structure, are investigated in detail in this study. The mean pulse profiles at different frequencies can be well fitted in a conal model, and the peak separation of intensity-dependent pulse profiles increases with intensity. The integrated pulses are obviously frequency dependent (pulse width decreases by $\sim\,20\%$ as frequency increases from 300 MHz to 750 MHz), but the structure of single pulses changes slightly (the corresponding correlation scale decreases by only $\sim\,1\%$). This disparity between mean and single pulses provides independent evidence for the existence of the RS-type vacuum inner gap, indicating a strong bond between particles on the pulsar surface. Diffused drifting sub-pulses are analyzed. The results show that the modulation period along pulse series ($P_3$) is positively correlated to the separation between two adjacent sub-pulses ($P_2$). This correlation may hint a rough surface on the pulsar, eventually resulting in the irregular drift of sparks. All the observational results may have significant implications in the dynamics of pulsar magnetosphere and are discussed extensively in this paper.Comment: Sci. China-Phys. Mech. Astron. 62, 959505 (2019

Study of Three Rotating Radio Transients with FAST

Rotating radio transients (RRATs) are peculiar astronomical objects whose emission mechanism remains under investigation. In this paper, we present observations of three RRATs, J1538+2345, J1854+0306 and J1913+1330, observed with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Specifically, we analyze the mean pulse profiles and temporal flux density evolutions of the RRATs. Owing to the high sensitivity of FAST, the derived burst rates of the three RRATs are higher than those in previous reports. RRAT J1854+0306 exhibited a time-dynamic mean pulse profile, whereas RRAT J1913+1330 showed distinct radiation and nulling segments on its pulse intensity trains. The mean pulse profile variation with frequency is also studied for RRAT J1538+2345 and RRAT J1913+1330, and the profiles at different frequencies could be well fitted with a cone-core model and a conal-beam model, respectively.Comment: Sci. China-Phys. Mech. Astron. 62, 959503 (2019