arXiv subjects

S. J. Dang

Publications and source records attributed to S. J. Dang.

16 recordsLinked to original sources

arXiv2026

Single-Pulse Study of the Pseudo-nulling Pulsar PSR J1820-0509 Based on FAST Observations

Using two observations obtained with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we present a detailed single-pulse analysis of the high-nulling pulsar PSR J1820-0509. We measure an exceptionally high nulling fraction of approximately 81.78%, significantly exceeding previous estimates from Parkes observations. The single-pulse energy distribution exhibits a clear bimodal structure, consistent with classical nulling behavior. However, stacking the identified null pulses reveals a statistically significant residual profile above the noise level, indicating that the nulls correspond to a very weak emission state rather than a complete cessation of radio emission. The pulsar shows clustered burst activities spanning several hundred rotation periods, with prominent quasi-periodicities at 1191 +/- 81 and 590 +/- 15 pulse periods in the two observations. Based on temporal clustering and integrated profile morphology, we identify three distinct emission modes (A, B, and C) and a pseudo-null state (D). These modes exhibit systematic differences in pulse morphology, polarization, and energy statistics. The pulse width-energy relations reveal clear transitions between low- and high-energy regimes. The energy distributions of Modes A and C are well described by lognormal functions, while Mode B follows a composite Gaussian-lognormal distribution. These results suggest that the radio emission of PSR J1820-0509 is governed by multiple quasi-stable magnetospheric states. The presence of weak emission during pseudo-nulls, together with systematic mode-dependent variations, supports the interpretation that pulsar nulling reflects transitions between different magnetospheric activity levels rather than a complete shutdown of emission.

astro-ph.HE↗

arXiv2025

Single-Pulse Morphology of PSR J1935+1616 (B1933+16) Based on archival data from FAST

We utilized archived data from the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) to analyze the single-pulse profile morphology of PSR J1935$+$1616 (B1933$+$16). The results show that PSR J1935$+$1616 exhibits significant micropulses as well as various changes in single-pulse profile morphology. In the FAST archived data, a total of 969 single pulses with microstructure were identified, accounting for 9.69$\%$ of the total pulse sample, with characteristic widths of $127.63^{+70.74}_{-46.25}$ $μ$s. About half of these pulses display quasiperiodic micropulses, with a periodicity of 231.77 $\pm$ 9.90 $μ$s. Among the 520 single pulses with quasiperiodic microstructure, 208 also exhibit quasiperiodicity in circular polarization, with a characteristic period of $244.70^{+45.66}_{-21.05}$ $μ$s. The micropulse characteristic width in circular polarization is 106.52 $\pm$ 46.14 $μ$s. Compared to normal pulses, the relative energy (E/ ) of single pulse with microstructure follows a double Gaussian distribution, while that of normal pulses follows a single Gaussian distribution. Based on the intensity of the leading and trailing components in the single-pulse profile morphology of PSR J1935+1616, we classified the pulses into four morphological modes (A, B, C, and D). The relative energy distribution of pulses in mode A is significantly different from the others, following a double Gaussian distribution, while the relative energy distributions in modes B, C, and D follow a single Gaussian distribution. Our study also suggests a possible correlation between micropulses and single-pulse profile morphology. Single pulse with micropulses are most likely to occur in mode A, while their occurrence is least likely in mode D.

astro-ph.HE↗

arXiv2025

Unveiling the emission properties of three long-period pulsars using FAST

We detail the emission behaviors of three long-period pulsars detected using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) during the CRAFTS survey. Their rotational periods range from 1.83 s to 4.75 s, and the null fractions measure between 28% and 53%. PSR J1945+1211 and PSR J2323+1214 exhibited quasi-periodic nulls, with duration of around 57 seconds. The longest null was observed in PSR J1945+1211, lasting 76 seconds. PSR J2323+1214 displayed varying null fractions between its leading and trailing components. For the first time in PSR J2323+1214, we detected five dwarf pulses, which are much weaker and narrower pulses than typical burst pulses. In addition, we investigate the microstructure of PSR J1900-0134 for the first time, revealing intricate pulses of up to 2.05 milliseconds and noting its complex emission characteristics. Bright pulses occur in all of these sources at different rates. These observations suggest complex magnetospheric processes, potentially related to magnetic reconnections, and provide insights into the origins of bright and microstructure pulses, as well as their distinctions from ordinary pulses.

astro-ph.HE↗

arXiv2024

Follow-up timing of 12 pulsars discovered in Commensal Radio Astronomy FAST Survey

We present phase-connected timing ephemerides, polarization pulse profiles and Faraday rotation measurements of 12 pulsars discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in the Commensal Radio Astronomy FAST Survey (CRAFTS). The observational data for each pulsar span at least one year. Among them, PSR J1840+2843 shows subpulse drifting, and five pulsars are detected to exhibit pulse nulling phenomena. PSR J0640$-$0139 and PSR J2031$-$1254 are isolated MSPs with stable spin-down rates ($\dot{P}$) of $4.8981(6) \times $10$^{-20}$\,s\,s$^{-1}$ and $6.01(2) \times $10$^{-21}$\,s\,s$^{-1}$, respectively. Additionally, one pulsar (PSR J1602$-$0611) is in a neutron star - white dwarf binary system with 18.23-d orbit and a companion of $\leq$ 0.65M$_{\odot}$. PSR J1602$-$0611 has a spin period, companion mass, and orbital eccentricity that are consistent with the theoretical expectations for MSP - Helium white dwarf (He - WD) systems. Therefore, we believe it might be an MSP-He WD binary system. The locations of PSRs J1751$-$0542 and J1840+2843 on the $P-\dot{P}$ diagram are beyond the traditional death line. This indicates that FAST has discovered some low $\dot{E}$ pulsars, contributing new samples for testing pulsar radiation theories. We estimated the distances of these 12 pulsars based on NE2001 and YMW16 electron density models, and our work enhances the dataset for investigating the electron density model of the Galaxy.

astro-ph.HE↗

arXiv2024

Statistical analysis of pulsar flux density distribution

This study presents a comprehensive analysis of the spectral properties of 886 pulsars across a wide frequency range from 20MHz to 343.5GHz, including a total of 86 millisecond pulsars. The majority of the pulsars exhibit power-law behavior in their spectra, although some exceptions are observed. Five different spectral models, namely simple power-law, broken power-law, low-frequency turn-over, high-frequency cut-off, and double turn-over, were employed to explore the spectral behaviors. The average spectral index for pulsars modeled with a simple power-law is found to be -1.64 +/-0.80, consistent with previous studies. Additionally, significant correlations between the spectral index and characteristic parameters are observed particularly in millisecond pulsars, while no strong correlation is observed in normal pulsars. Different models show variations in the most influential characteristic parameters associated with the spectral index, indicating diverse dominant radiation mechanisms in millisecond pulsars.Finally, this study identifies 22 pulsars of the Gigahertz-peaked Spectra (GPS) type for the first time based on the Akaike information criterion.

astro-ph.HE↗

arXiv2024

Investigation of profile shifting and subpulse movement in PSR J0344-0901 with FAST

We report two phenomena detected in PSR J0344$-$0901 from two observations conducted at frequency centered at 1.25 GHz using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The first phenomenon manifests as shifting in the pulse emission to later longitudinal phases and then gradually returns to its original location. The event lasts for about 216 pulse periods, with an average shift of about $0.7^\circ$ measured at the peak of the integrated profile. Changes in the polarization position angle (PPA) are detected around the trailing edge of the profile, together with an increase in the profile width. The second phenomenon is characterized by the apparent movement of subpulses, which results in different subpulse track patterns across the profile window. For the first time in this pulsar, we identify four emission modes, each with unique subpulse movement, and determine the pattern periods for three of the emission modes. Pulse nulling was not detected. Modeling of the changes in the PPA using the rotating vector model gives an inclination angle of $75.12^\circ \pm 3.80^\circ$ and an impact parameter of $-3.17^\circ \pm 5.32^\circ$ for this pulsar. We speculate that the subpulse movement may be related to the shifting of the pulse emission.

astro-ph.HE↗

arXiv2024

Pulse Jitter and Single-pulse Variability in Millisecond Pulsars

Understanding the jitter noise resulting from single-pulse phase and shape variations is important for the detection of gravitational waves using pulsar timing array. We presented measurements of jitter noise and single-pulse variability of 12 millisecond pulsars that are part of the International Pulsar Timing Array sample using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We found that the levels of jitter noise can vary dramatically among pulsars. A moderate correlation with a correlation coefficient of 0.57 between jitter noise and pulse width is detected. To mitigate jitter noise, we performed matrix template matching using all four Stokes parameters. Our results revealed a reduction in jitter noise ranging from 6.7\% to 39.6\%. By performing longitude-resolved fluctuation spectrum analysis, we identified periodic intensity modulations in 10 pulsars. In PSR J0030+0451, we detected single-pulses with energies more than 10 times the average pulse energy, suggesting the presence of giant pulses. We also observed a periodic mode-changing phenomenon in PSR J0030+0451. We examined the achievable timing precision by selecting a sub-set of pulses with a specific range of peak intensity, but no significant improvement in timing precision is achievable.

astro-ph.HE↗

arXiv2023

Discovery of four pulsars in a pilot survey at intermediate Galactic latitudes with FAST

We present the discovery and timing results of four pulsars discovered in a pilot survey at intermediate Galactic latitudes with the Five-hundred Aperture Spherical Telescope (FAST). Among these pulsars, two belong to the category of millisecond pulsars (MSPs) with spin periods of less than 20 ms. The other two fall under the classification of "mildly recycled" pulsars, with massive white dwarfs as companions. Remarkably, this small survey, covering an area of 4.7 $deg^2$ , led to the discovery of four recycled pulsars. Such success underscores the immense potential of future surveys at intermediate Galactic latitudes. In order to assess the potential yield of MSPs, we conducted population simulations and found that both FAST and Parkes new phased array feed surveys, focusing on intermediate Galactic latitudes, have the capacity to uncover several hundred new MSPs.

astro-ph.HE↗

arXiv2022

Arecibo and FAST Timing Follow-up of twelve Millisecond Pulsars Discovered in Commensal Radio Astronomy FAST Survey

We report the phase-connected timing ephemeris, polarization pulse profiles, Faraday rotation measurements, and Rotating-Vector-Model (RVM) fitting results of twelve millisecond pulsars (MSPs) discovered with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in the Commensal radio Astronomy FAST survey (CRAFTS). The timing campaigns were carried out with FAST and Arecibo over three years. Eleven of the twelve pulsars are in neutron star - white dwarf binary systems, with orbital periods between 2.4 and 100 d. Ten of them have spin periods, companion masses, and orbital eccentricities that are consistent with the theoretical expectations for MSP - Helium white dwarf (He WD) systems. The last binary pulsar (PSR J1912$-$0952) has a significantly smaller spin frequency and a smaller companion mass, the latter could be caused by a low orbital inclination for the system. Its orbital period of 29 days is well within the range of orbital periods where some MSP - He WD systems have shown anomalous eccentricities, however, the eccentricity of PSR J1912$-$0952 is typical of what one finds for the remaining MSP - He WD systems.

astro-ph.HE↗

arXiv2022

Emission Variation of a Long-period Pulsar Discovered by the Five-hundred-meter Aperture Spherical Radio Telescope (FAST)

We report on the variation in the single-pulse emission from PSR J1900+4221 (CRAFTS 19C10) observed at frequency centered at 1.25 GHz using the Five-hundred-meter Aperture Spherical radio Telescope. The integrated pulse profile shows two distinct components, referred to here as the leading and trailing components, with the latter component also containing a third weak component. The single-pulse sequence reveals different emissions demonstrating as nulling, regular, and bright pulses, each with a particular abundance and duration distribution. There also exists pulses that follow a log-normal distribution suggesting the possibility of another emission, in which the pulsar is radiating weakly. Changes in the profile shape are seen across different emissions. We examine the emission variations in the leading and trailing components collectively and separately, and find moderate correlation between the two components. The inclination angle is estimated to be about 7° based on pulse-width, and we discuss that nulling in this pulsar does not seem to show correlation with age and rotation period.

hep-th↗

arXiv2022

Detection of strong scattering close to the eclipse region of PSR B1957+20

We present the first measurement of pulse scattering close to the eclipse region of PSR B1957+20, which is in a compact binary system with a low-mass star. We measured pulse scattering time-scales up to 0.2 ms close to the eclipse and showed that it scales with the dispersion measure (DM) excess roughly as $τ\proptoΔ{\rm DM}^{2}$. Our observations provide the first evidence of strong scattering due to multi-path propagation effects in the eclipsing material. We show that Kolmogorov turbulence in the eclipsing material with an inner scale of $\sim100$ m and an outer scale of the size of the eclipse region can naturally explain the observation. Our results show that the eclipsing material in such systems can be highly turbulent and suggest that scattering is one of the main eclipsing mechanisms at around 1.4 GHz.

astro-ph.HE↗

arXiv2021

Unusual Emission Variations Near the Eclipse of A Black Widow PSR J1720$-$0533

We report on an {unusually} bright observation of PSR J1720$-$0533 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The pulsar is in a black widow system that {was discovered by the Commensal Radio Astronomy FAST Survey (CRAFTS). By coincidence, a bright scintillation maximum was simultaneous with the eclipse in our observation which allowed for precise measurements of flux density variations, as well as dispersion measure (DM) and polarization.} We found that there are quasi-periodic pulse emission variations with a modulation period of $\sim$ {22\,s} during the ingress of the eclipse, which could be caused by plasma lensing. {No such periodic modulation was found during the egress of the eclipse. } {The linear polarization of the pulsar disappears before the eclipse, even before there is a visually obvious change in DM. We also found that the pulse scattering maybe play an important role in the eclipse of PSR J1720$-$0533.}

astro-ph.HE↗

arXiv2020

The Mode Switching in Pulsar J1326$-$6700

We report on a detailed study of the mode switching in pulsar J1326$-$6700 by analyzing the data acquired from the Parkes 64 m radio telescope at 1369 MHz. During the abnormal mode, the emission at the central and trailing components becomes extremely weak. Meanwhile, the leading emission shifts toward earlier longitude by almost 2°, and remains in this position for typically less than a minute. The mean flux density of the normal mode is almost five times that of the abnormal mode. Our data show that, for PSR J1326$-$6700, 85% of the time was spent in the normal mode and 15% was in the abnormal mode. The intrinsic distributions of mode timescales can be well described by Weibull distributions, which present a certain amount of memory in mode switching. Furthermore, a quasiperiodicity has been identified in the mode switching in pulsar J1326$-$6700. The estimated delay emission heights based on the kinematical effects indicate that the abnormal mode may have originated from higher altitude than the normal mode.

astro-ph.HE↗

arXiv2020

The two emission states of PSR B1534+12

We have observed PSR~B1534+12 (J1537+1155), a pulsar with a neutron star companion, using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We found that this pulsar shows two distinct emission states: a weak state with a wide pulse profile and a burst state with a narrow pulse profile. The weak state is always present. We cannot, with our current data, determine whether the pulse energy of the weak state follows a normal or a log-normal distribution. The burst state energy distribution follows a power-law. The amplitude of the single pulse emission in the burst state varies significantly; the peak flux intensity of the brightest pulse is 334 times stronger than that of the average pulse. We also examined the timing precision achievable using only bright pulses, which showed no demonstrable improvement because of pulse jitter and therefore quantified the jitter noise level for this pulsar.

astro-ph.HE↗

arXiv2020

Results of 12 Years of Pulsar Timing at Nanshan -- I

We have used the Nanshan 25-m Radio Telescope at Xinjiang Astronomical Observatory to obtain timing observations of 87 pulsars from 2002 July to 2014 March. Using the "Cholesky" timing analysis method we have determined positions and proper motions for 48 pulsars, 24 of which are improved positions compared to previously published values. We also present the first published proper motions for nine pulsars and improved proper motions for 21 pulsars using pulsar timing and position comparison method. The pulsar rotation parameters are derived and are more accurate than previously published values for 36 pulsars. Glitches are detected in three pulsars: PSRs J1722$-$3632, J1852$-$0635 and J1957+2831. For the first two, the glitches are large, with $Δν_g/ν> 10^{-6}$, and they are the first detected glitches in these pulsars. PSR J1722$-$3632 is the second oldest pulsar with large glitch. For the middle-age pulsars ($τ_c > 10^5$~yr), the calculated braking indices, $|n|$, are strongly correlated with $τ_c$ and the numbers of positive and negative values of $n$ are almost equal. For young pulsars ($τ_c < 10^5$~yr), there is no correlation between $|n|$ and $τ_c$ and most have $n>0$.

astro-ph.HE↗

arXiv2018

The Spin-down State Change and Mode Change Associated with Glitch Activity of PSR B2035+36

We presented timing results of PSR B2035+36 using ~9-yr observations with the Nanshan 25-m radio telescope. PSR B2035+36 was reported to exhibit significant changes in pulse profile correlated with spin-down state variations. We found that the pulsar underwent a glitch with a jump in the frequency of $Δν\sim12.4(5)\, \rm nHz$ around MJD 52950. Unusually, the spin-down rate increased persistently over $800$ days after the glitch, and the average spin-down rate of post-glitch was about $9.6\%$ larger than pre-glitch. Accompanied with the glitch activity, pulse profile became narrower. The pulsar began to switch between two emission modes after the glitch, with pulse width ($W_{\ 50 \rm mean}$) of $8.5(7)^{\circ}$ and $3.7(3)^{\circ}$, respectively. Besides that, the relatively narrow pulse profile gradually became dominant. All of the observations indicate that there should be connection between magnetospheric behavior and glitch activity. We discuss one possibility of magnetosphere fluctuation triggered by glitch event.

astro-ph.HE↗