arXiv subjects

Wynn C. G. Ho

Publications and source records attributed to Wynn C. G. Ho.

At least 19 recordsLinked to original sources

arXiv2026

Portraits of the young and old in X-ray: New millisecond pulsar detections and updated characterization of young neutron stars

We present analyses of new and archival Chandra and NuSTAR data on two old millisecond pulsars, a young highly-energetic pulsar, and a young neutron star. PSR J1810-0623 is a recently-discovered 4.55 ms radio pulsar in a binary with a possible carbon-oxygen white dwarf companion. We tentatively detect the pulsar for the first time in X-ray using a new short Chandra HRC-I observation. Its faintness suggests a distance more in accord with the further (~1 kpc) of its dispersion measure-inferred distances. PSR J1933-6211 is a 3.54 ms radio pulsar also with a carbon-oxygen white dwarf companion. Our first-time X-ray detection in an archival HRC-S image indicates its X-ray luminosity is similar to that of the 1.9 Msun pulsar PSR J1614-2230. Next, PSR J1813-1749 is a 1-2 kyr pulsar with one of the highest known spin-down luminosities. We measure its spin period of 44.7 ms using 2025 NuSTAR and 2026 Chandra ACIS-S data and update its spin evolution model. We show for the first time the pulse profile of PSR J1813-1749 at 3-79 keV, and we find that the spectra of the pulsar and pulsar wind nebula at these energies did not change significantly between observations in 2018 and 2025. Finally, for the 5-10 kyr neutron star CXOU J182913.1-125113 in the supernova remnant G18.95-1.1, we combine ACIS-I observations from 2009 and 2024 to better measure the neutron star's spectrum.

astro-ph.HE↗

arXiv2026

Vacuum birefringence and the polarized X-ray emission from a radio magnetar

Magnetars are isolated neutron stars with exceptionally strong surface fields exceeding $10^{14}$ G. Their bright X-ray emission probes physical regimes in which quantum electrodynamic (QED) influences radiation propagation. Strong magnetic fields induce polarization-dependent refractive indices in the vacuum; such vacuum birefringence (VB) remains a long-standing but unconfirmed prediction of QED. Here, we report phase- and energy-resolved polarization measurements of the radio-emitting magnetar 1E 1547.0$-$5408 obtained by coordinating X-ray and radio observations from the Imaging X-ray Polarimetry Explorer (IXPE), the Neutron Star Interior Composition ExploreR (NICER), and the Parkes/Murriyang observatory. We detect large polarization degrees (PD) in the thermally-dominant soft X-ray band, reaching phase-averaged values of $65\%$ at 2 keV before substantially decreasing between 2$-$4 keV. At certain rotational phases, the 2$-$3 keV PD rises to nearly $80\%$ while remaining high ($\gtrsim 40\%$) throughout the radio beam crossing. The phase-dependent X-ray and radio polarization angles are both consistent with the rotating vector model, suggesting that the emission geometries track the star's large-scale magnetic field. Collectively, these characteristics challenge standard surface emission models using non-refractive propagation of light to infinity. VB-governed magnetospheric propagation can naturally explain the X-ray polarization signals. Our results represent a significant advance in probing this hallmark prediction of QED, opening a new cosmic window into superstrong-field quantum physics, thereby motivating further observational and theoretical studies concentrating on this domain.

astro-ph.HE↗

arXiv2026

Pulsed radio emission from a Central Compact Object

Located at the centres of supernova remnants, central compact objects (CCOs) are among the most puzzling neutron stars. CCOs are bright in thermal X-rays, yet have evaded detection by major radio telescopes for decades, giving rise to the view that they are intrinsically radio-quiet and possess exceptionally weak magnetic fields. Here we show that the prototypical young CCO 1E 1207.4-5209 is in fact a faint radio pulsar rotating at the 0.4s X-ray period. Analysis of its polarization indicates that the radio beam intersects our line of sight near the magnetic pole, affirming its radio faintness' being intrinsic. Once its supernova remnant dissipates, this source would be misidentified as an apparently gigayear-old pulsar. The CCO's low radio flux density may explain why many supernova remnants lack detectable radio pulsars and suggests a hidden population of young, slowly rotating neutron stars.

astro-ph.HE↗

arXiv2026

NICER Magnetar Burst Catalog

In this paper, we present a comprehensive catalog of short bursts from magnetars based on eight years of NICER observations. A total of 1130 bursts were identified from 14 sources, with the sample dominated by SGR 1935+2154, which accounts for 76% of all detected bursts. We analyzed burst durations, spectral properties, and their correlations across multiple sources. Bursts from SGR 1935+2154 exhibit significantly longer durations, with a distribution peak at 316 ms, compared to a peak of 23 ms for bursts from other magnetars. Two μs-scale bursts were detected for the first time, originating from 1E 1048.1-5937 and CXOU J010043.1-721134. Spectral analysis in the 0.5-8 keV range using both blackbody and power-law models shows that bursts with higher fluences have harder spectra. In contrast, correlations between burst duration and spectral parameters are weak or absent. This catalog provides a valuable dataset for studying magnetar short bursts, enabling future modeling efforts and improving our understanding of the diversity and physical mechanisms of magnetar bursts.

astro-ph.HE↗

arXiv2026

New and updated timing models for seven young energetic X-ray pulsars, including the Big Glitcher PSR J0537-6910

We present new timing models and update our previous ones for the rotational evolution of seven young energetic pulsars, including four of the top five in spin-down luminosity Edot among all known pulsars. For each of the six pulsars that were monitored on a regular basis by NICER, their rotation phase-connected timing model covers the entire period of NICER observations, in many cases from 2017-2025. For PSR J0058-7218, which was only identified in 2021, we extend the baseline of its timing model by 3 years and report detections of its first three glitches. The timing model for PSR J0537-6910 over the entire 8 years of NICER monitoring is presented, including a total of 23 glitches; we also report its spin frequency and pulsed spectrum from a 2016 NuSTAR observation. For PSR B0540-69, its complete timing model from 2015-2025 is provided, including a braking index evolution from near 0 to 1.6 during this period. The 8-year timing model for PSR J1412+7922 (also known as Calvera) is reported, which includes a position that is consistent with that measured from imaging. For PSR J1811-1925, we present its 3.5-year timing model. For PSR J1813-1749, its incoherent timing model is extended through early 2025 using new Chandra observations. For PSR J1849-0001, its 7-year timing model is provided, including a position that is consistent with and more accurate than its imaging position and its first glitch that is one of the largest ever measured. Our timing models of these seven X-ray pulsars enable their study at other energies and in gravitational wave data.

astro-ph.HE↗

arXiv2026

Neutron star cooling implications and magnetic field of the Vela Junior central compact object from all XMM-Newton and Chandra spectra

The central compact object (CCO) in the Vela Junior supernova remnant is a young neutron star whose relatively low X-ray flux and small distance suggest it has a mass high enough to activate fast neutrino cooling processes. Here we analyse all XMM-Newton MOS and pn and Chandra ACIS-S spectra of the Vela Junior CCO, with observations taking place over the 9 years from 2001 to 2010. We find that the best-fit flux and spectral model parameters do not vary significantly when treating each observation independently, and therefore we fit all the spectra simultaneously using various spectral models to characterize the predominantly thermal emission from the neutron star surface. Our results indicate the Vela Junior CCO has an atmosphere composed of hydrogen, a hot spot temperature (unredshifted) of 3.5x10^6 K, and a colder surface temperature of (6.6-8.8)x10^5 K. Possible absorption lines at ~0.6 keV and 0.9 keV provide evidence for the first-time of an average surface magnetic field B~3x10^10 G for this CCO, which is similar to the magnetic field of other CCOs. At the accurate new Vela Junior distance of 1.4 kpc, the observed luminosity that is dominated by the hot spot is ~5x10^32 erg s^-1. The luminosity from the rest of the colder surface is (1.3-4.0)x10^32 erg s^-1. The cool luminosity and temperature imply the Vela Junior CCO is indeed colder than many other young neutron stars and probably has a high mass that triggered fast neutrino cooling.

astro-ph.HE↗

arXiv2025

Intertwined birth and death: a Herbig-Haro outflow resolves the distance to Vela Junior

The distance to the Vela Junior supernova remnant (RX J0852.0-4622 or G266.2-1.2) has long remained uncertain, limiting our understanding of its physical properties. Using VLT/MUSE integral field spectroscopy, we uncover chemical and kinematic connections between the nebula surrounding its Central Compact Object (CXOU J085201.4-461753) and the nearby Herbig-Haro outflow of Ve 7-27 (Wray 16-30), indicating a shared nitrogen-rich, Fe-peak-enhanced environment. This link ties stellar birth and death, with the young star Ve 7-27 embedded in material expelled by Vela Junior's massive progenitor, and the remnant's blast wave is expanding through the same medium. Adopting the Gaia-based distance to Ve 7-27, we revise Vela Junior's distance to $1.41\pm0.14$ kpc. At this distance, the remnant's physical radius is $23.3\pm2.3$ pc, and X-ray proper motions of the northwestern rim correspond to shock speeds of $(2.8\pm0.7)\times10^3$ to $(5.6\pm1.5)\times10^3$ km s$^{-1}$. These imply an age of $\sim$1.6-3.3 kyr and a very low ambient density, indicating that Vela Junior is expanding within a highly rarefied wind-blown cavity carved by a massive progenitor -- consistent with the non-detection of strong thermal X-ray emission. This distance update also resolves long-standing inconsistencies, with major implications for its energy budget, particle acceleration efficiency, and compact object evolution.

astro-ph.HE↗

arXiv2025

Verification of Cas A neutron star cooling rate using Chandra HRC-S observations

The young neutron star (NS) in the Cassiopeia A (Cas A) supernova remnant is a fascinating test for theories of NS cooling. Chandra observations have indicated that its surface temperature is declining rapidly, about 2% per decade, using 20 years of data, if a uniform carbon atmosphere is assumed for the NS. This rapid decline may be caused by the neutrons in the NS core transitioning from a normal to a superfluid state. However, most of the Cas A NS observations were performed by the Chandra ACIS detectors, which suffer complicated systematic effects. Here, we test the cooling of the Cas A NS with Chandra HRC data over 25 years. The Chandra HRC detector has independent systematics, serving as a cross-check. Assuming a fixed hydrogen column density ($N_{\rm H}$), we infer the cooling rate of the Cas A NS to be 0.57$^{+0.26}_{-0.27}$% per decade. Allowing the $N_{\rm H}$ to vary with time (as estimated using ACIS data), the cooling rate is 1.11$^{+0.25}_{-0.28}$% per decade. These cooling rates are smaller than measured using ACIS data, implying systematic uncertainties have not been eradicated from either or both datasets. However, we have verified the decline in the absorbed flux from the Cas A NS using an independent instrument, at $>3σ$ level (4.7%$\pm$1.5% over 10 years). Additionally, the weaker cooling rate of Cas A NS inferred from HRC datasets eliminates the tension with the theoretically predicted cooling, and can be explained by the reduced efficiency of the neutrino emission accompanying the Cooper pair breaking and formation process in neutron triplet-state superfluid.

astro-ph.HE↗

arXiv2025

Rapid Spectral Evolution of SGR 1935+2154 During its 2022 Outburst

During the 2022 outburst of SGR 1935+2154, a Fast-Radio-Burst-like event (FRB 20221014A) and X-ray activities occurred between two spin-up glitches, suggesting these glitches may connect to multiwavelength phenomenology. However, the mechanisms altering the magnetar's magnetosphere to enable radio emission remain unclear. This study presents high-cadence NICER and NuSTAR observations revealing spectral changes in burst and persistent emission. Hardness ratio and spectral analysis reveal significant changes during an "intermediate flare" 2.5 hours before FRB 20221014A. This 80-second flare, releasing $>(6.3\pm0.2)\times10^{40}$ erg, coincides with a rapid spectral softening in both burst and persistent emission and a notable decrease in burst occurrence rate. The intermediate flare is bright enough to be detected if placed at a few Mpc, and would appear as a fast X-ray transient. This implies that the connection between magnetar X-ray activity and FRBs can be observed in the local Universe. Post-flare burst spectra peak near 5 keV, resembling the characteristics of the FRB-associated X-ray burst of 2020. Such change persisted for a few hours, implying magnetospheric evolution on similar timescales. However, no radio emission was detected from post-flare bursts, suggesting that FRB emission requires conditions beyond peculiar short bursts. The burst waiting times exhibit a broken power-law distribution, likely resulting from contamination by enhanced persistent emission. Although the bursts appear randomly distributed in the spin phase, the hardness ratio profile as a function of spin phase follows that of the persistent emission, indicating that X-ray bursts originate at low altitudes.

astro-ph.HE↗

arXiv2025

X-ray polarization of the magnetar 1E 1841-045

We report on IXPE and NuSTAR observations beginning forty days after the 2024 outburst onset of magnetar 1E 1841-045, marking the first IXPE observation of a magnetar in an enhanced state. Our spectropolarimetric analysis indicates that both a blackbody (BB) plus double power-law (PL) and a double blackbody plus power-law spectral model fit the phase-averaged intensity data well, with a hard PL tail ($Γ$=1.19 and 1.35, respectively) dominating above $\approx 5$ keV. For the former model, we find the soft PL (the dominant component at soft energies) exhibits a polarization degree (PD) of $\approx 30\%$ while the hard PL displays a PD of $\approx 40\%$. Similarly, the cool BB of the 2BB+PL model possesses a PD of $\approx 15\%$ and a hard PL PD of $\approx 57\%$. For both models, each component has a polarization angle (PA) compatible with celestial north. Model-independent polarization analysis supports these results, wherein the PD increases from$ \approx 15\%$ to $\approx 70\%$ in the 2-3 keV and 6-8 keV ranges, respectively, while the PA remains nearly constant. We find marginal evidence for phase-dependent variability of the polarization properties, namely a higher PD at phases coinciding with the hard X-ray pulse peak. We compare the hard X-ray PL to the expectation from resonant inverse Compton scattering (RICS) and secondary pair cascade synchrotron radiation from primary high-energy RICS photons; both present reasonable spectropolarimetric agreement with the data, albeit, the latter more naturally. We suggest that the soft PL X-ray component may originate from a Comptonized corona in the inner magnetosphere.

astro-ph.HE↗

arXiv2024

A NICER View of PSR J1231$-$1411: A Complex Case

Recent constraints on neutron star mass and radius have advanced our understanding of the equation of state (EOS) of cold dense matter. Some of them have been obtained by modeling the pulses of three millisecond X-ray pulsars observed by the Neutron Star Interior Composition Explorer (NICER). Here, we present a Bayesian parameter inference for a fourth pulsar, PSR J1231$-$1411, using the same technique with NICER and XMM-Newton data. When applying a broad mass-inclination prior from radio timing measurements and the emission region geometry model that can best explain the data, we find likely converged results only when using a limited radius prior. If limiting the radius to be consistent with the previous observational constraints and EOS analyses, we infer the radius to be $12.6 \pm 0.3$ km and the mass to be $1.04_{-0.03}^{+0.05}$ $M_\odot$, each reported as the posterior credible interval bounded by the $16\,\%$ and $84\,\%$ quantiles. If using an uninformative prior but limited between $10$ and $14$ km, we find otherwise similar results, but $R_{\mathrm{eq}} = 13.5_{-0.5}^{+0.3}$ km for the radius. In both cases, we find a nonantipodal hot region geometry where one emitting spot is at the equator or slightly above, surrounded by a large colder region, and where a noncircular hot region lies close to southern rotational pole. If using a wider radius prior, we only find solutions that fit the data significantly worse. We discuss the challenges in finding the better fitting solutions, possibly related to the weak interpulse feature in the pulse profile.

astro-ph.HE↗

arXiv2024

The Radius of the High-mass Pulsar PSR J0740+6620 with 3.6 yr of NICER Data

We report an updated analysis of the radius, mass, and heated surface regions of the massive pulsar PSR J0740+6620 using Neutron Star Interior Composition Explorer (NICER) data from 2018 September 21 to 2022 April 21, a substantial increase in data set size compared to previous analyses. Using a tight mass prior from radio timing measurements and jointly modeling the new NICER data with XMM-Newton data, the inferred equatorial radius and gravitational mass are $12.49_{-0.88}^{+1.28}$ km and $2.073_{-0.069}^{+0.069}$ $M_\odot$ respectively, each reported as the posterior credible interval bounded by the $16\,\%$ and $84\,\%$ quantiles, with an estimated systematic error $\lesssim 0.1$ km. This result was obtained using the best computationally feasible sampler settings providing a strong radius lower limit but a slightly more uncertain radius upper limit. The inferred radius interval is also close to the $R=12.76_{-1.02}^{+1.49}$ km obtained by Dittmann et al., when they require the radius to be less than $16$ km as we do. The results continue to disfavor very soft equations of state for dense matter, with $R<11.15$ km for this high-mass pulsar excluded at the $95\,\%$ probability. The results do not depend significantly on the assumed cross-calibration uncertainty between NICER and XMM-Newton. Using simulated data that resemble the actual observations, we also show that our pipeline is capable of recovering parameters for the inferred models reported in this paper.

astro-ph.HE↗

arXiv2024

First detection of X-ray pulsations and spectrum of the high Galactic latitude pulsar PSR J0837-2454 and direct Urca cooling implications

PSR J0837-2454 is a young 629 ms radio pulsar whose uncertain distance has important implications. A large distance would place the pulsar far out of the Galactic plane and suggest it is the result of a runaway star, while a short distance would mean the pulsar is extraordinarily cold. Here we present further radio observations and the first deep X-ray observation of PSR J0837-2454. Data from the Parkes Murriyang telescope show flux variations over short and long timescales and also yield an updated timing model, while the position and proper motion (and, less strongly, parallax) of the pulsar are constrained by a number of low-significance detections with the Very Long Baseline Array. XMM-Newton data enable detection of X-ray pulsations for the first time from this pulsar and yield a spectrum that is thermal and blackbody-like, with a cool blackbody temperature ~70 eV or atmosphere temperature ~50 eV, as well as a small hotspot. The spectrum also indicates the pulsar is at a small distance of <~1 kpc, which is compatible with the marginal VLBA parallax constraint that favours a distance of >~330 pc. The low implied luminosity (~7.6x10^31 erg s^-1 at 0.9 kpc) suggests PSR J0837-2454 has a mass high enough that fast neutrino emission from direct Urca reactions operates in this young star and points to a nuclear equation of state that allows for direct Urca reactions at the highest densities present in neutron star cores.

astro-ph.HE↗

arXiv2024

A growing braking index and spin-down swings for the pulsar PSR B0540-69

The way pulsars spin down is not understood in detail, but a number of possible physical mechanisms produce a spin-down rate that scales as a power of the rotation rate ($\dotν\propto-ν^n$), with the power-law index $n$ called the braking index. PSR B0540-69 is a pulsar that in 2011, after 16 years of spinning down with a constant braking index of 2.1, experienced a giant spin-down change and a reduction of its braking index to nearly zero. Here, we show that following this episode the braking index monotonically increased during a period of at least four years and stabilised at ~1.1. We also present an alternative interpretation of a more modest rotational irregularity that occurred in 2023, which was modelled as an anomalous negative step of the rotation rate. Our analysis shows that the 2023 observations can be equally well described as a transient swing of the spin-down rate (lasting ~65 days), and the Bayesian evidence indicates that this model is strongly preferred.

astro-ph.HE↗

arXiv2024

A NICER View of the Nearest and Brightest Millisecond Pulsar: PSR J0437$\unicode{x2013}$4715

We report Bayesian inference of the mass, radius and hot X-ray emitting region properties - using data from the Neutron Star Interior Composition ExploreR (NICER) - for the brightest rotation-powered millisecond X-ray pulsar PSR J0437$\unicode{x2013}$4715. Our modeling is conditional on informative tight priors on mass, distance and binary inclination obtained from radio pulsar timing using the Parkes Pulsar Timing Array (PPTA) (Reardon et al. 2024), and we use NICER background models to constrain the non-source background, cross-checking with data from XMM-Newton. We assume two distinct hot emitting regions, and various parameterized hot region geometries that are defined in terms of overlapping circles; while simplified, these capture many of the possibilities suggested by detailed modeling of return current heating. For the preferred model identified by our analysis we infer a mass of $M = 1.418 \pm 0.037$ M$_\odot$ (largely informed by the PPTA mass prior) and an equatorial radius of $R = 11.36^{+0.95}_{-0.63}$ km, each reported as the posterior credible interval bounded by the 16% and 84% quantiles. This radius favors softer dense matter equations of state and is highly consistent with constraints derived from gravitational wave measurements of neutron star binary mergers. The hot regions are inferred to be non-antipodal, and hence inconsistent with a pure centered dipole magnetic field.

astro-ph.HE↗

arXiv2024

A More Precise Measurement of the Radius of PSR J0740+6620 Using Updated NICER Data

PSR J0740+6620 is the neutron star with the highest precisely determined mass, inferred from radio observations to be $2.08\pm0.07\,\rm M_\odot$. Measurements of its radius therefore hold promise to constrain the properties of the cold, catalyzed, high-density matter in neutron star cores. Previously, Miller et al. (2021) and Riley et al. (2021) reported measurements of the radius of PSR J0740+6620 based on Neutron Star Interior Composition Explorer (NICER) observations accumulated through 17 April 2020, and an exploratory analysis utilizing NICER background estimates and a data set accumulated through 28 December 2021 was presented in Salmi et al. (2022). Here we report an updated radius measurement, derived by fitting models of X-ray emission from the neutron star surface to NICER data accumulated through 21 April 2022, totaling $\sim1.1$ Ms additional exposure compared to the data set analyzed in Miller et al. (2021) and Riley et al. (2021), and to data from X-ray Multi-Mirror (XMM-Newton) observations. We find that the equatorial circumferential radius of PSR J0740+6620 is $12.92_{-1.13}^{+2.09}$ km (68% credibility), a fractional uncertainty $\sim83\%$ the width of that reported in Miller et al. (2021), in line with statistical expectations given the additional data. If we were to require the radius to be less than 16 km, as was done in Salmi et al. (2024), then our 68% credible region would become $R=12.76^{+1.49}_{-1.02}$ km, which is close to the headline result of Salmi et al. (2024). Our updated measurements, along with other laboratory and astrophysical constraints, imply a slightly softer equation of state than that inferred from our previous measurements.

astro-ph.HE↗

arXiv2024

The Magnificent Seven X-ray Isolated Neutron Stars Revisited. I. Improved Timing Solutions and Pulse Profile Analysis

We present the first systematic X-ray pulse timing analysis of the six members of the so-called "Magnificent Seven" nearby thermally-emitting isolated neutron stars (XINS) with detected pulsations. Using the extensive collection of archival XMM-Newton, Chandra, and NICER observations spanning over two decades, we obtain the first firm measurement of the spin-down rate for RX J2143.0+0654, while for the rest we improve upon previously published spin ephemerides and extend them by up to an additional decade. Five of the XINS follow steady spin-down with no indication of major anomalies in their long-term timing behavior; the notable exception is RX J0720.4-3125, for which, in addition to confirming the previously identified glitch, we detect a second spin derivative. The high quality folded X-ray pulse profiles produced with the updated timing solutions exhibit diverse and complex morphologies, as well as striking energy dependence. These peculiarities cannot be readily explained by blackbody-like isotropic emission and simple hot spot configurations, hinting at the presence of complex multi-temperature surface heat distributions and highly anisotropic radiation patterns, such as may arise from a strongly magnetized atmospheric layer.

astro-ph.HE↗

arXiv2024

Gravitational waves from glitch-induced f-mode oscillations in quark and neutron stars

Matter in compact stars is dense enough that transient events within the star could have sufficiently high energies to produce detectable gravitational waves (GWs). These GWs could be used to constrain the equation of state (EoS) for matter in the star and could reveal that there is more than one type of EoS at play in the population, implying that multiple types of compact stars exist. One of these types could be quark stars, composed almost entirely of stable quark matter, and observing GWs is a way to test for the strange matter EoS. Here we explore the possibility that, if fundamental (f-) mode oscillations in pulsars are induced by a pulsar glitch, then these oscillations might produce detectable GWs. We use the existing population of pulsars and their glitches, as well as a much larger synthesized population, along with 15 EoSs (8 for neutron stars and 7 for quark stars) to generate frequencies, damping times, and GW strengths for each. We find that of the EoSs examined, all quark star EoSs produce narrower distributions of f-mode frequency than neutron star EoSs. This result, along with other elements of the data, could be used to differentiate between GWs (or other signals from f-modes) originating from neutron stars and quark stars and thus could confirm the existence of quark stars. We also find that GW astronomy is a potentially viable method for detecting a larger population of pulsars which are not observable electromagnetically and that future GW observatories have the possibility to greatly expand this capability.

gr-qc↗