arXiv subjects

Renxin Xu

Publications and source records attributed to Renxin Xu.

At least 55 records · Page 3Linked to original sources

arXiv2025

Discovery of 15 new pulsars at high Galactic Latitudes with FAST

We present the discovery and timing results of 15 pulsars discovered in a high Galactic latitude survey conducted with the Five-hundred-meter Aperture Spherical Telescope (FAST). The survey targeted a region as close as possible to the Galactic Center, encompassing an area near the Galactic Bulge. The newly discovered pulsars consist of eleven normal pulsars and four millisecond pulsars (MSPs). Among the MSPs, three are identified in binary systems with orbital periods of ~3.1, 4.6 and 12.5 days, respectively. We have successfully obtained coherent timing solutions for three of the normal pulsars (PSRs J1745-0059, J1746-0156 and J1800-0059). Furthermore, within our data set we found that four pulsars (three new and one known) show mode-changing and/or subpulse drifting phenomena. Comparing our discoveries with simulations of the Galactic disk and Bulge MSP populations indicates that these new pulsars are most likely located in the disk. Nonetheless, our discoveries demonstrate that deep surveys at high Galactic latitudes have significant potential to enhance our understanding of the MSP population in the direction of the Bulge.

astro-ph.HE↗

arXiv2025

Systematic study of the composition of Type I X-ray burst ashes: Neutron star structure v.s. Reaction rate uncertainties

In this study, we calculate for the first time the impacts of neutron star(NS) structure on the type I X-ray burst ashes using the \texttt{MESA} code. We find an increased mass fraction of the heavier elements with increasing surface gravity (increase mass or decrease radius), resulting in a higher average mass number ($A_{\rm ash}$) of burst ashes (except for higher mass NS due to the competition between the envelope temperature and the recurrence time). The burst strength ($α$) increases as surface gravity increases, which indicates the positive correlation between $A_{\rm ash}$ and $α$ with changes in surface gravity. If the $α$ value is higher, heavier $p$-nuclei should be produced by the type I X-ray burst nucleosynthesis. Besides, the effects of various burst input parameters, e.g. base heating ($Q_{\rm b}$), metallicity ($Z$) and some new reaction rates are calculated for comparison. We find that the heavier nuclei synthesis is inversely correlated to the base heating/metallicity, the smaller the base heating/metallicity, the greater the mass fraction of the heavier elements. The $α$ value decreases as $Q_{\rm b}$ or $Z$ decreases, which also indicates the positive correlation between $A_{\rm ash}$ and $α$ with variation in $Q_{\rm b}$ or $Z$. The new reaction rates from the $(p,γ)$ reactions on $^{17}\rm{F}$, $^{19}\rm{F}$, $^{26}\rm{P}$, $^{56}\rm{Cu}$, $^{65}\rm{As}$, and $(α,p)$ reaction on $^{22}\rm{Mg}$ have only minimal effects on burst ashes. In hydrogen-rich X-ray binary systems, nuclei heavier than $^{64}\rm{Ge}$ are fertile produced with larger NS mass, smaller NS radius, smaller base heating and smaller metallicity.

astro-ph.HE↗

arXiv2025

Science objectives of the Einstein Probe mission

The Einstein Probe (EP) is an interdisciplinary mission of time-domain and X-ray astronomy. Equipped with a wide-field lobster-eye X-ray focusing imager, EP will discover cosmic X-ray transients and monitor the X-ray variability of known sources in 0.5-4 keV, at a combination of detecting sensitivity and cadence that is not accessible to the previous and current wide-field monitoring missions. EP can perform quick characterisation of transients or outbursts with a Wolter-I X-ray telescope onboard. In this paper, the science objectives of the Einstein Probe mission are presented. EP is expected to enlarge the sample of previously known or predicted but rare types of transients with a wide range of timescales. Among them, fast extragalactic transients will be surveyed systematically in soft X-rays, which include γ-ray bursts and their variants, supernova shock breakouts, and the predicted X-ray transients associated with binary neutron star mergers. EP will detect X-ray tidal disruption events and outbursts from active galactic nuclei, possibly at an early phase of the flares for some. EP will monitor the variability and outbursts of X-rays from white dwarfs, neutron stars and black holes in our and neighbouring galaxies at flux levels fainter than those detectable by the current instruments, and is expected to discover new objects. A large sample of stellar X-ray flares will also be detected and characterised. In the era of multi-messenger astronomy, EP has the potential of detecting the possible X-ray counterparts of gravitational wave events, neutrino sources, and ultra-high energy γ-ray and cosmic ray sources. EP is expected to help advance the studies of extreme objects/phenomena and their underlying physical processes revealed in the dynamic X-ray universe, as well as studies in other areas of X-ray astronomy.

astro-ph.HE↗

arXiv2024

Thirty Meter Telescope International Observatory Detailed Science Case 2024

The Thirty Meter Telescope (TMT) International Observatory (TIO) will be a revolutionary leap forward in astronomical observing capabilities, enabling us to address some of the most profound questions about the universe. From unraveling the mysteries of dark matter and dark energy to exploring the origins of stars and planets, TMT will transform our understanding of the cosmos. The TIO Detailed Science Case (DSC) presents science goals that inform the top-level requirements for the observatory's design and operations, including the telescope, enclosure, instruments, and adaptive optics system.

astro-ph.IM↗

arXiv2024

An Extreme Radio Fluctuation of Pulsar B1929$+$10

We report the detection of an extreme flux decrease accompanied by clear dispersion measure (DM) and rotation measure (RM) variations for pulsar B1929+10 during the 110-minute radio observation with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The radio flux decreases by 2 to 3 orders of magnitude within a rapid time scale of about 20 minutes. Meanwhile, the variations of DM and RM are approximately 0.05 pc cm$^{-3}$ and 0.7 rad m$^{-2}$, respectively. Frequency-dependent analysis of DM indicates an extremely weak chromatic DM feature, which does not notably affect the radiative behavior detected. Moreover, the pulsar timing analysis shows an additional time delay from 100 $μ$s to 400 $μ$s in the event. These results are speculated to be due to the eclipse and bend for the radio emission of pulsar B1929+10 by a highly dense outflow from the pulsar. This not only impacts the intrinsic radio emission feature but also affects the pulsar timing behavior. Nevertheless, a plasma lens effect lasting around 20 minutes could also be responsible for the event.

astro-ph.HE↗

arXiv2024

Radio Pulsar B0950$+$08: Radiation in Magnetosphere and Sparks above Surface

We observed the nearby 100$\%$-duty-cycle radio pulsar B0950+08 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We obtained the polarization profile for its entire rotation, which enabled us to investigate its magnetospheric radiation geometry and the polar cap sparking pattern. After we excluded part of the profile in which the linear polarization factor is low ($\lesssim 30 \%$) and potentially contaminated by position angle jumps, the rest of the polarization position angle swing fits a classical rotating vector model (RVM) well. The bestfit RVM indicates that the inclination angle, $α$, and the impact angle, $β$, of this pulsar, are 100.5$^{\circ}$ and $-$33.2$^{\circ}$, respectively, suggesting that the radio emission comes from two poles.We find that, in such RVM geometry, either the annular vacuum gap or the core vacuum gap model would require that the radio emissions come from a high-altitude magnetosphere with heights from $\sim 0.25~R_{\rm LC}$ to $\sim 0.56~R_{\rm LC}$, with $R_{\rm LC}$ being the light cylinder radius. Both the main and inter-pulses' sparking points are located away from the magnetic pole, which could be relevant to the physical conditions on the pulsar surface.

astro-ph.HE↗

arXiv2024

PSR B0943+10: Mode Switch, Polar Cap Geometry, and Orthogonally Polarized Radiation

As one of the paradigm examples to probe into pulsar magnetospheric dynamics, PSR B0943+10 (J0946+0951) manifests representatively, showing mode switch, orthogonal polarization and subpulse drifting, frequently studied below 600 MHz. Here both integrated and single pulses are studied at a high frequency (1.25 GHz) with FAST. The mode switch is studied using a profile decomposition method. A phase space evolution for the pulsar's mode switch shows a strange-attractor-like pattern. The radiative geometry is proposed by fitting polarization position angles with the rotating vector model. The pulsar pulse profile is then mapped to the sparking locations on pulsar surface, and the differences between the main pulse's and the precursor component's radiative processes may explain the X-ray's synchronization with radio mode switch. Detailed single pulse studies on B0943+10's orthogonally polarized radiation are presented, which may support for certain models of radiative transfer of polarized emission. Especially, the difference in OPMs' circular polarization might reflect the cyclotron absorption in pulsar magnetospheres. B0943+10's B and Q modes evolve differently with frequency and have different proportions of orthogonal modes, which indicates possible magnetospheric changes during mode switch. For Q mode pulse profile, the precursor and the main pulse components are orthogonally polarized, and are probably originated from different depths in the magnetosphere. The findings could impact significantly on pulsar electrodynamics and the radiative mechanism related.

astro-ph.HE↗

arXiv2024

High priority targets for transient gravitational waves from glitching pulsars

Glitching pulsars are expected to be important sources of gravitational waves. In this paper, we explore six different models that propose the emission of transient continuous waves, lasting days to months, coincident with glitches. The maximal gravitational wave energy is calculated for each model, which is then used to determine whether associated gravitational waves could be detectable with LIGO-Virgo-KAGRA's O4 detectors. We provide an analytical approximation to calculate the signal-to-noise ratio which includes information about the source's sky position, improving on previous estimates that assume isotropic or sky and orientation averaged sensitivities. By analysing the entire glitching population, we find that certain models predict detectable signals in O4, whereas others do not. We also rank glitching pulsars by signal-to-noise ratio, based on archival data, and we find that for all models, the Vela pulsar (PSR J0835$-$4510) would provide the strongest signal. Moreover, PSR J0537$-$6910 is not expected to yield a detectable signal in O4, but will start becoming relevant for next generation detectors. Our analysis also extends to the entire pulsar population, regardless of whether they have glitched, and we provide a list of pulsars that would present a significant signal, if they were to glitch. Finally, we apply our analysis to the April 2024 Vela glitch and find that a signal should be detectable under certain models. The non-detection of a supposedly detectable signal would provide an efficiency factor that quantifies a model's contribution to gravitational wave emission, eventually leading to a differentiation of models and independent constraints on physical parameters.

astro-ph.HE↗

arXiv2024

AXPs/SGRs: strange stars with crusts?

The emission of Anomalous X-ray Pulsars (AXPs) and Soft Gamma-Ray Repeaters (SGRs) is believed to be powered by the dissipation of their strong magnetic fields, which coined the name `magnetar'. By combining timing and energy observational results, the magnetar model can be easily appreciated. From a timing perspective, the magnetic field strengths of AXPs and SGRs, calculated assuming dipole radiation, are extremely strong. From an energy perspective, the X-ray/soft gamma-ray luminosities of AXPs and SGRs are larger than their rotational energy loss rates (i.e., $ L_{\rm X}>\dot E_{\rm rot}$). It is thus reasonable to assume that the high-energy radiation comes from magnetic energy decay, and the magnetar model has been extensively discussed (or accepted). However, we argue that: (1) calculating magnetic fields by assuming that rotational energy loss is dominated by dipole radiation (i.e., $\dot{E}_{\rm rot}\simeq\dot{E}_μ$)) may be controversial, and we suggest that the energies carried by outflowing particles should also be considered; and (2) the fact that X-ray luminosity is greater than the rotational energy loss rate does not necessarily mean that the emission energy comes from the magnetic field decaying, which requires further observational testing. Furthermore, some observational facts conflict with the `magnetar' model, such as observations of anti-magnetars, high magnetic field pulsars, and radio and X-ray observations of AXPs/SGRs. Therefore, we propose a crusted strange star model as an alternative, which can explain many more observational facts of AXPs/SGRs.

astro-ph.HE↗

arXiv2024

On the energy budget of starquake-induced repeating fast radio bursts

With a growing sample of fast radio bursts (FRBs), we investigate the energy budget of different power sources within the framework of magnetar starquake triggering mechanism. During a starquake, the energy can be released in any form through strain, magnetic, rotational, and gravitational energies. The strain energy can be converted from other three kinds of energy during starquakes. The following findings are revealed: 1. The crust can store free magnetic energy of $\sim10^{46}$ erg by existing toroidal fields, sustaining $10^6$ bursts with frequent starquakes occurring due to crustal instability. 2. The strain energy develops as a rigid object spins down, which can be released during a global starquake accompanied by a glitch. However, it takes a long time to accumulate enough strain energy via spindown. 3. The rotational energy of a magnetar with $P\lesssim0.1\rm\,s$ can match the energy and luminosity budget of FRBs. 4. The budget of the total gravitational energy is high, but the mechanism and efficiency of converting this energy to radiation deserve further exploration.

astro-ph.HE↗

arXiv2024

Free Energy of Anisotropic Strangeon Stars

Can pulsar-like compact objects release further huge free energy besides the kinematic energy of rotation? This is actually relevant to the equation of state of cold supra-nuclear matter, which is still under hot debate. Enormous energy is surely needed to understand various observations, such as $γ-$ray bursts, fast radio bursts and soft $γ-$ray repeaters. In this paper, the elastic/gravitational free energy of solid strangeon star is revisited for strangeon stars, with two anisotropic models to calculate in general relativity. It is found that huge free energy (> $10^{46}$ erg) could be released via starquakes, given an extremely small anisotropy ($(p_{\rm t}-p_{\rm r})/p_{\rm r} \sim 10^{-4}$, with $p_{\rm t}$/$p_{\rm r}$ the tangential/radial pressure), implying pulsar-like stars could have great potential of free energy release without extremely strong magnetic fields in solid strangeon star model.

astro-ph.HE↗

arXiv2024

Radial and Non-Radial Oscillations of Inverted Hybrid Stars

We study the radial and non-radial oscillations of Cross stars (CrSs), i.e., stars with a quark matter crust and a hadronic matter core in an inverted order compared to conventional hybrid stars. We draw comparisons of their oscillation modes with those of neutron stars, quark stars, and conventional hybrid stars. We find that the stellar stability analysis from the fundamental mode of radial oscillations, and the $g$, $f$ modes of non-radial oscillations are quite similar to those of conventional hybrid stars. However, due to the inverted stellar structure, the first non-radial $p$ mode of CrSs behaves in an inverted way and sits in a higher frequency domain compared to that of conventional hybrid stars. These results provide a direct way to discriminate CrSs from other types of compact stars via gravitational-wave probes. Specifically, compact stars emitting $g$-mode gravitational waves within the $0.5$-$1$ kHz range should be CrSs or conventional hybrid stars rather than neutron stars or pure quark stars, and a further GW detection of the first $p$ mode above 8 kHz or an identification of a decreasing trend of frequencies versus star masses associated with it will help identify the compact object to be a CrS rather than a conventional hybrid star.

astro-ph.HE↗

arXiv2024

The double-peaked type I X-ray bursts with different mass accretion rate and fuel composition

Using the MESA code, we have carried out a detailed survey of the available parameter space for the double-peaked type I X-ray bursts. We find that the double-peaked structure appears at mass accretion rate $\dot{M}$ in the range of $\sim(4-8)\times10^{-10}\,M_{\odot}/{\rm yr}$ when metallicity $Z=0.01$, while in the range of $\sim(4-8)\times10^{-9}\,M_{\odot}/\rm{yr}$ when $Z=0.05$. Calculations of the metallicity impact suggest that the double peaks will disappear when $Z\lesssim0.005$ for $\dot{M}=5\times10^{-10}\,M_{\odot}/\rm{yr}$ and $Z\lesssim0.04$ for $\dot{M}=5\times10^{-9}\,M_{\odot}/\rm{yr}$. Besides, the impacts of base heating $Q_{\rm b}$, as well as nuclear reaction waiting points: $^{22}\rm{Mg}$, $^{26}\rm{Si}$, $^{30}\rm{S}$, $^{34}\rm{Ar}$, $^{56}{\rm Ni}$, $^{60}\rm Zn$, $^{64}\rm{Ge}$, $^{68}\rm{Se}$, $^{72}\rm{Kr}$ have been explored. The luminosity of the two peaks decreases as $Q_{\rm b}$ increases. $^{68}{\rm Se}(p,γ){^{69}{\rm Br}}$ is the most sensitive reaction, the double peaks disappear assuming that $^{56}{\rm Ni}(p,γ)^{57}{\rm Cu}$ and $^{64}{\rm Ge}(p,γ)^{65}{\rm As}$ reaction rates have been underestimated by a factor of 100 and the $^{22}{\rm Mg}(α,p)^{25}{\rm Al}$ reaction rate has been overestimated by a factor of 100, which indicates that $^{22}{\rm Mg}$, $^{56}{\rm Ni}$, $^{64}{\rm Ge}$, $^{68}{\rm Se}$ are possibly the most important nuclear waiting points impedance. Comparisons to the double-peaked bursts from 4U 1636-53 and 4U 1730-22 suggest that the nuclear origins of double-peaked type I X-ray bursts are difficult to explain the observed larger peak times ($t_{\rm p,1}\gtrsim4\,{\rm s}$, $t_{\rm p,2}\gtrsim8\,{\rm s}$) and smaller peak ratio($r_{1,2}\lesssim0.5$). The composition of ashes from double-peaked bursts is very different from the single-peaked bursts especially for the heavier p-nuclei.

astro-ph.HE↗

arXiv2024

Narrow spectra of repeating fast radio bursts: A magnetospheric origin

Fast radio bursts (FRBs) can present a variety of polarization properties, and some of them have narrow spectra. We study spectral properties from perspectives of intrinsic radiation mechanisms and absorption during the waves propagating in the magnetosphere. The intrinsic radiation mechanisms are considered by invoking quasi-periodic bunch distribution and perturbations on charged bunches moving on curved trajectories. The narrow-band emission likely reflects some quasi-periodic structure on the bulk of bunches, which may be due to quasi-periodically sparking in a ``gap'' or quasi-monochromatic Langmuir waves. A sharp spike would appear at the spectrum if the perturbations can induce a monochromatic oscillation of bunches, however, it is hard to create a narrow spectrum because the Lorentz factor has large fluctuations so that the spike disappears. Both the bunching mechanism and perturbations scenarios share the same polarization properties with a uniformly distributed bulk of bunches. We investigate absorption effects including Landau damping and curvature self-absorption in the magnetosphere, which are significant at low frequencies. Subluminous O-mode photons can not escape from the magnetosphere due to the Landau damping, leading to a height-dependent lower frequency cut-off. Spectra can be narrow when the frequency cut-off is close to the characteristic frequency of curvature radiation, while such conditions can only be met sometimes. The spectral index is 5/3 at low-frequency bands due to the curvature self-absorption but not as steep as the observations. The intrinsic radiation mechanisms are more likely to generate the observed narrow spectra of FRBs rather than the absorption effects.

astro-ph.HE↗

arXiv2024

Non-symmetrical sparking may hint "zits'' on a pulsar surface

Pulsar electrodynamics could be relevant to the physics of stellar surface, which remains poorly understood for more than half a centenary and is difficult to probe due to the absence of direct and clear observational evidence. Nevertheless, highly-sensitive telescopes (e.g., China's Five-hundred-meter Aperture Spherical radio Telescope, FAST) may play an essential role in solving the problem since the predicted surface condition would have quite different characteristics in some models of pulsar structure, especially after the establishment of the standard model of particle physics. For instance, small hills (or ``zit'') may exist on solid strangeon star surface with rigidity, preferential discharge, i.e., gap sparking, may occur around the hills in the polar cap region. In this work, with the 110-min polarization observation of PSR B0950+08 targeted by FAST, we report that the gap sparking is significantly non-symmetrical to the meridian plane on which the rotational and magnetic axes lie. It is then speculated that this asymmetry could be the result of preferential sparking around zits which might rise randomly on pulsar surface. Some polarization features of both single pulses and the mean pulse, as well as the cross-correlation function of different emission regions, have also been presented.

astro-ph.HE↗

arXiv2024

Hybrid Strangeon Stars

It was conjectured that the basic units of the ground state of bulk strong matter may be strange-clusters called strangeons, and they can form self-bound strangeon stars that are highly compact. Strangeon stars can develop a strange quark matter (SQM) core at high densities, particularly in the color-flavor-locking phase, yielding a branch of hybrid strangeon stars. We explore the stellar structure and astrophysical implications of hybrid strangeon stars. We find that hybrid strangeon stars can meet various astrophysical constraints on pulsar masses, radii, and tidal deformabilities. Finally, we show that the strangeon-SQM mixed phase is not preferred if the charge-neutrality condition is imposed at the strangeon-SQM transition region.

nucl-th↗

arXiv2023

Broadband X-ray timing and spectral characteristics of the accretion-powered millisecond X-ray pulsar MAXI J1816$-$195

We studied the broadband X-ray timing and spectral behaviors of the newly confirmed accreting millisecond X-ray pulsar MAXI J1816$-$195 during its 2022 outburst. We used the data from Insight-HXMT ME/HE, NICER and NuSTAR which cover the energy range between 0.8$-$210 keV. A coherent timing analysis of solely Insight-HXMT HE data across the full outburst revealed a complex behavior of the timing residuals, also prominently visible in independent Insight-HXMT ME and NICER data, particularly at rising part of the outburst and at the very end in NICER data. Therefore, we broke down the full outburst into a (noisy) rising part, covering only about five days from MJD 59737.0 to 59741.9, and a decaying part lasting for 19 days across MJD 59741.9$-$59760.6. Fitting for the decaying part a timing model including a frequency $ν$ and frequency time derivative $\dotν$ component yielded a value of $(+9.0\pm2.1)\times10^{-14}~{\rm Hz~s^{-1}}$ for $\dotν$, which could be interpreted as a spin-up under our model assumptions. We detected the X-ray pulsations up to $\sim$95 keV in a combination of Insight-HXMT HE observations. The pulse profiles were quite stable over the whole outburst and could be well described by a truncated Fourier series using two harmonics, the fundamental and the first overtone. Both components kept alignment in the range 0.8$-$64 keV. The joint and time-averaged NICER and Insight-HXMT spectra in the energy range 1$-$150 keV were well fitted by the absorbed Comptonization model compps plus disk blackbody with two additional Gaussian components. Using the bolometric flux and spin-up values both evaluated during the decay phase, we determined a magnetic field strength of $(0.2-2)\times10^8$ G for MAXI J1816$-$195.

astro-ph.HE↗

arXiv2023

Repeating FRBs reveal the secret of pulsar magnetospheric activity

The puzzling mechanism of coherent radio emission remains unknown, but fortunately, repeating fast radio bursts (FRBs) provide a precious opportunity, with extremely bright subpulses created in a clear and vacuum-like pulsar magnetosphere. FRBs are millisecond-duration signals that are highly dispersed at distant galaxies but with uncertain physical origin(s). Coherent curvature radiation by bunches has already been proposed for repeating FRBs. The charged particles are created during central star's quakes, which can form bunches streaming out along curved magnetic field lines, so as to trigger FRBs. The nature of narrow-band radiation with time-frequency drifting can be a natural consequence that bunches could be observed at different times with different curvatures. Additionally, high linear-polarization can be seen if the line of sight is confined to the beam angle, whereas the emission could be highly circular-polarized if off-beam. It is also discussed that pulsar surface may be full of small hills (i.e., zits) which would help producing bulk of energetic bunches for repeating FRBs as well as for rotation-powered pulsars.

astro-ph.HE↗