arXiv subjects

Mason Ng

Publications and source records attributed to Mason Ng.

At least 37 records · Page 2Linked to original sources

arXiv2025

Pulse profile modelling of the accretion-powered millisecond pulsar SAX J1808.4-3658 using NICER data from its 2019 and 2022 outbursts

Pulse profile modelling is a relativistic ray-tracing technique that has provided constraints on parameters, with a focus on mass and radius, of five rotation-powered millisecond pulsars. While the technique can also be applied to accretion-powered millisecond pulsars (AMPs), this requires accounting for the X-rays from the accretion disc and has only been applied to archival data from the Rossi X-ray Timing Explorer. Here, we apply a previously developed neutron star and accretion disc model to the NICER (Neutron star Interior Composition Explorer) data of the 2019 and 2022 outbursts of SAX J1808.4-3658. We find that a single circular hotspot model is insufficient to explain the data. Modelling with two hotspots and an accretion disc model provides better phase-residuals, but a spectral residual at around 1 keV remains. In contrast, we find a good fit with a flexible background approach, replacing the accretion disk. However, the inferred parameters are not robust due to a degeneracy in the origin of the non-pulsed radiation, which can be caused either by the background or a hotspot that is at least partially in view throughout a full rotation. This work represents an important next step in pulse profile modelling of AMPs by analysing NICER data and underlines the need for more accurate accretion disc and hotspot modelling to achieve robust parameter constraints. We expect the inclusion of higher energy and polarimetric data will provide complementary constraints on inclination, hotspot colatitude, and hotspot size, improving the accuracy of pulse profile modelling of AMPs.

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arXiv2025

First X-ray and radio polarimetry of the neutron star low-mass X-ray binary GX 17+2

We report the first polarimetric results of the neutron star (NS) low-mass X-ray binary (LMXB) Z-source GX 17+2 using the Imaging X-ray Polarimetry Explorer (IXPE) and the Very Large Array (VLA). We find that the X-ray source was polarized at PD = 1.9 +/- 0.3 % (1-sigma errors) with a polarization angle of PA = 11 +/- 4 degree (1-sigma errors). Simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR) observations show that the source was in the normal branch (NB) during our IXPE observations. The X-ray spectro-polarimetry results suggest a source geometry comprising an accretion disk component, a Comptonization component, and a reflection component. The VLA radio polarization study shows a PD = 2.2 +/- 0.2 % with a Faraday-corrected intrinsic polarization angle of 1 +/- 5 degree, which is an indication of the jet axis. Thus, we find the estimated X-ray PA from the source is consistent with the radio PA. We discuss the accretion geometry of the Z-source in light of our X-ray spectro-polarimetry and radio findings.

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arXiv2025

What's the Buzz About GX 13+1? Constraining Coronal Geometry with QUEEN-BEE: A Bayesian Nested Sampling Framework for X-ray Polarization Rotation Analysis

Observations from the Imaging X-ray Polarimetry Explorer (IXPE) have revealed electric vector position angle (EVPA) rotation in several neutron star low-mass X-ray binaries, including the galactic X-ray burster GX 13+1. We developed a novel Bayesian nested sampling framework-"Q-U Event-by-Event Nested sampling for Bayesian EVPA Evolution" (QUEEN-BEE)-to model unbinned Stokes parameters and infer optimal EVPA rotation rates in IXPE data. We then applied this framework to three previous IXPE observations of GX 13+1. In the first observation, QUEEN-BEE recovers a rotation rate of 42+/-4 degrees/day, consistent with prior binned analysis. Energy-binned QUEEN-BEE analysis of this first observation suggests a slab-like coronal geometry, providing the first constraints between slab and shell coronae for this source. We also explore alternative EVPA rotation scenarios in GX 13+1 including variable disk wind behavior. The second observation of this source shows no evidence of rotation, and the third observation shows transient rotating behavior with an EVPA rotation rate when exiting a light curve dip of 170 +20/-40 degrees/day. The results show marginal but consistent increases in the overall measured polarization degree (PD) for epochs where the EVPA rotation is identified. These results demonstrate that QUEEN-BEE can identify evolving polarization signatures in both time- and energy-resolved regimes, even where binned methods fall below detection thresholds. Our findings highlight the diagnostic potential of QUEEN-BEE as a tool for discriminating between competing physical models of coronal geometry and probing disk-wind-related polarization behavior, highlighting the promising potential for application of this framework in a variety of other IXPE observations.

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arXiv2025

The 2020 Superburst of 4U 1608-522 and its impact on the accretion disk

Superbursts are rare events observed from bursting neutron star low mass X-ray binaries. They are thought to originate from unstable burning of the thick layer of Carbon on the surface of the neutron star, causing the observed X-ray flashes to last several hours. Given their fluence it has long been thought that superbursts may have significant effects on the accretion flow around the neutron star. In this paper, we first present evidence for a new superburst observed from 4U 1608-522 by MAXI during the 2020 outburst, around 00:45 UTC on 16 July 2020. We compare some of the properties of this superburst and the underlying outburst with the events recorded on May 5 2005 by RXTE and most recently in 2025 by MAXI. We then present our spectral analysis of NICER and Insight-HXMT data obtained before and after the 2020 superburst event. Our results indicate that the inner disk temperature and the radius show a systematic evolution in the following few days, which may be related to the superburst. We show that the timescale of the observed evolution can not be governed by viscous timescales unless the viscosity parameter is unrealistically low.

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arXiv2025

X-ray and radio polarimetry of the neutron star low mass X-ray binary GX 13+1

We report the X-ray and radio polarization study of the neutron star (NS) low-mass X-ray binary (LMXB) GX 13+1 using the Imaging X-ray Polarimetry Explorer (IXPE) and Very Large Array (VLA). Simultaneous Neutron Star Interior Composition Explorer (NICER) observations show that the source was in parts of the Z state during our IXPE observations, exhibiting moderate changes in the hardness intensity diagram. The source exhibits X-ray dips in the light curve along with hints of polarization swings between the dip and non-dip states. The X-ray spectro-polarimetry results suggest a source geometry comprising an accretion disk component representing the softer disk emission, along with a blackbody representing the harder emission from the boundary layer (BL) or a spreading layer (SL). We investigate the geometry of GX 13+1 by considering our X-ray and radio polarization findings.

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arXiv2025

CHIME/FRB Discovery of an Unusual Circularly Polarized Long-Period Radio Transient with an Accelerating Spin Period

We report the discovery of CHIME J1634+44, a Long Period Radio Transient (LPT) unique for two aspects: it is the first known LPT to emit fully circularly polarized radio bursts, and it is the first LPT with a significant spin-up. Given that high circular polarization ($>90$\%) has been observed in FRB~20201124A and in some giant pulses of PSR~B1937+21, we discuss the implications of the high circular polarization of CHIME J1634+44 and conclude its emission mechanism is likely to be ``pulsar-like''. While CHIME J1634+44 has a pulse period of 841 s, its burst arrival patterns are indicative of a secondary 4206 s period, probably associated with binary activity. The timing properties suggest it has a significantly negative period derivative of $\dot{P}=-9.03(0.11)\times 10^{-12}$ s s$^{-1}$. Few systems have been known to spin-up, most notably transitional millisecond pulsars and cataclysmic binaries, both of which seem unlikely progenitors for CHIME J1634+44. If the period was only associated with the spin of the object, then the spin up is likely generated by accretion of material from a companion. If, however, the radio pulse period and the orbital period are locked, as appears to be the case for two other LPTs, the spin up of CHIME J1634+44 could be driven by gravitational wave radiation.

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arXiv2025

FRB 20250316A: A Brilliant and Nearby One-Off Fast Radio Burst Localized to 13 parsec Precision

Precise localizations of a small number of repeating fast radio bursts (FRBs) using very long baseline interferometry (VLBI) have enabled multiwavelength follow-up observations revealing diverse local environments. However, the 2--3\% of FRB sources that are observed to repeat may not be representative of the full population. Here we use the VLBI capabilities of the full CHIME Outriggers array for the first time to localize a nearby (40 Mpc), bright (kJy), and apparently one-off FRB source, FRB 20250316A, to its environment on 13-pc scales. We use optical and radio observations to place deep constraints on associated transient emission and the properties of its local environment. We place a $5σ$ upper limit of $L_{\mathrm{9.9~\mathrm{GHz}}} < 2.1\times10^{25}~\mathrm{erg~s^{-1}~Hz^{-1}}$ on spatially coincident radio emission, a factor of 100 lower than any known compact persistent radio source associated with an FRB. Our KCWI observations allow us to characterize the gas density, metallicity, nature of gas ionization, dust extinction and star-formation rate through emission line fluxes. We leverage the exceptional brightness and proximity of this source to place deep constraints on the repetition of FRB 20250316A, and find it is inconsistent with all well-studied repeaters given the non-detection of bursts at lower spectral energies. We explore the implications of a measured offset of 190$\pm20$ pc from the center of the nearest star-formation region, in the context of progenitor channels. FRB 20250316A marks the beginning of an era of routine localizations for one-off FRBs on tens of mas-scales, enabling large-scale studies of their local environments.

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arXiv2025

Discovery and Localization of the Swift-Observed FRB 20241228A in a Star-forming Host Galaxy

On 2024 December 28, CHIME/FRB detected the thus-far non-repeating FRB 20241228A with a real-time signal-to-noise ratio of $>50$. Approximately 112~s later, the X-ray Telescope onboard the Neil Gehrels Swift Observatory was on source, the fastest follow-up to-date of a non-repeating FRB (Tohuvavohu et al. in prep.). Using CHIME/FRB and two of the three CHIME/FRB Outriggers, we obtained a Very Long Baseline Interferometry localization for FRB 20241228A with a 1$σ$ confidence ellipse of 11$^{\prime\prime}$ by 0.2$^{\prime\prime}$. This represents the first published localization using both the CHIME-KKO and CHIME-GBO Outriggers. We associate FRB 20241228A with a star-forming galaxy at a redshift of $z = 0.1614\pm0.0002$. The persistent X-ray luminosity limit at this source's location and distance is $<1.2 \times 10^{43}$ erg s$^{-1}$ in the $0.3-10$ keV band, the most stringent limit of any non-repeating FRB to-date (Tohuvavohu et al. in prep.). The stellar mass ($\sim 2.6 \times 10^{10}\,M_{\odot}$) and star formation rate ($\sim 2.9\,M_{\odot}$~yr$^{-1}$) of the host galaxy of FRB 20241228A are consistent with the broader FRB host galaxy population. We measure significant scattering ($\sim$1ms) and scintillation ($\sim$20 kHz at 600 MHz) along the line of sight to this source, and suggest the scintillation screen is Galactic while the scattering screen is extragalactic. FRB 20241228A represents an exciting example of a new era in which we can harness VLBI-localizations and rapid high-energy follow-up to probe FRB progenitors.

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arXiv2025

X-ray and radio polarimetry of the neutron star low-mass X-ray binary 4U 1728-34

We report the first X-ray and radio polarimetric results of the neutron star (NS) low-mass X-ray binary (LMXB) atoll-source 4U 1728-34 using the Imaging X-ray Polarimetry Explorer (IXPE) and Australia Telescope Compact Array (ATCA). We discovered that the X-ray source was polarized at PD = 1.9 +/- 1.0% with a polarization angle of PA = -41 +/- 16 degree. Simultaneous Neutron Star Interior Composition Explorer (NICER) observations show that the source was in a relatively hard state, marking it as the first IXPE observation of an NS atoll source in the hard state. We do not detect any significant linear polarization (LP) in the radio band, with a 3-sigma upper limit of 2% at 5.5 GHz and 1.8% at 9 GHz. Combining the radio datasets provides the deepest upper limits on the radio polarization at < 1.5% on the linear and circular polarization (measured at 7.25 GHz). The X-ray polarimetric results suggest a source geometry with a Comptonization component possibly attributed to a boundary layer (BL) emission or BL emission reflected off the disk, consistent with the other NS atoll sources.

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arXiv2025

The CHIME/FRB Discovery of the Extremely Active Fast Radio Burst Source FRB 20240114A

Among the thousands of observed fast radio bursts (FRBs), a few sources exhibit exceptionally high burst activity observable by many telescopes across a broad range of radio frequencies. Almost all of these highly active repeaters have been discovered by CHIME/FRB, due to its daily observations of the entire Northern sky as a transit radio telescope. FRB 20240114A is a source discovered and reported by CHIME/FRB to the community in January 2024; given its low declination, even the detection of a few bursts hints at a high burst rate. Following the community announcement of this source as a potentially active repeater, it was extensively followed up by other observatories and has emerged as one of the most prolific FRB repeaters ever observed. This paper presents the five bursts CHIME/FRB observed from FRB 20240114A, with channelized raw voltage data saved for two bursts. We do not observe changes in the DM of the source greater than ~1.3 pc cm$^{-3}$ in our observations over nearly a year baseline. We find an RM of ~ +320 rad m$^{-2}$. We do not find evidence for scattering at the level of < 0.3 ms in the bursts, and we find no evidence for astrophysical scintillation. In our observations of FRB 20240114A, we see a burst rate ~49x higher than the median burst rate of apparent non-repeaters also discovered by CHIME/FRB. Each discovery of highly active FRBs provides a valuable opportunity to investigate whether there is a fundamental difference between repeating and apparently non-repeating sources.

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arXiv2025

X-ray polarization study of the neutron star low-mass X-ray binary GX 349+2

We report the first X-ray polarimetric results of the neutron star (NS) low-mass X-ray binary (LMXB) Z-source GX 349+2 using the Imaging X-ray Polarimetry Explorer (IXPE). We discovered that the X-ray source was polarized at PD = 1.1 +/- 0.3% (1-sigma errors) with a polarization angle of PA = 32 +/- 6 degree (1-sigma errors). Simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR) observations show that the source transitioned through the normal branch (NB), flaring branch (FB), and soft apex (SA) of the Z-track during our IXPE observations. The X-ray spectro-polarimetry results suggest a source geometry comprising an accretion disk component, a blackbody representing the emission from the NS surface, and a Comptonized component. We discuss the accretion geometry of the Z source in light of the spectro-polarimetric results.

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arXiv2025

X-ray and Radio Campaign of the Z-source GX 340+0 II: the X-ray polarization in the normal branch

We present the first X-ray polarization measurement of the neutron star low-mass X-ray binary and Z-source, GX 340$+$0, in the normal branch (NB) using a 200 ks observation with the Imaging X-ray Polarimetric Explorer (IXPE). This observation was performed in 2024 August. Along with IXPE, we also conducted simultaneous observations with NICER, AstroSat, Insight-HXMT, ATCA, and GMRT to investigate the broadband spectral and timing properties in the X-ray and radio wavelengths. During the campaign, the source traced a complete Z-track during the IXPE observation but spent most of the time in the NB. We measure X-ray polarization degree (PD) of $1.22\pm0.25\%$ in the 2-8 keV energy band with a polarization angle (PA) of $38\pm6^\circ$. The PD in the NB is observed to be weaker than in the horizontal branch (HB) but aligned in the same direction. The PD of the source exhibits a marginal increase with energy while the PA shows no energy dependence. The joint spectro-polarimetric modeling is consistent with the observed X-ray polarization originating from a single spectral component from the blackbody or the Comptonized emission while the disk emission does not contribute towards the X-ray polarization. GMRT observations at 1.26 GHz during HB had a tentative detection at 4.5$\pm$0.7 mJy while ATCA observations a day later during the NB detected the source at 0.70$\pm$0.05 mJy and 0.59$\pm$0.05 mJy in the 5.5 & 9 GHz bands, respectively, suggesting an evolving jet structure depending on the Z-track position.

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arXiv2025

X-ray spectro-polarimetry analysis of the weakly magnetized neutron star X-ray binary GX 9+1

We present an X-ray spectro-polarimetric study of the weakly magnetized neutron star low-mass X-ray binary GX 9+1, utilizing data from the Imaging X-ray Polarimetry Explorer (IXPE), alongside simultaneous NuSTAR, NICER, and INTEGRAL observations. GX 9+1, located in the Galactic bulge, is a persistently bright Atoll source known for its spectral variability along the color-color diagram. Our spectral analysis during the soft state confirms emission dominated by a soft blackbody and thermal Comptonization components, with no evidence of a hard X-ray tail. These observations suggest a relatively low-inclination system (23 deg < i < 46 deg) with a weak reflection component, consistent with emission from the accretion disk and neutron star boundary layer. Spectro-polarimetric analysis reveals no significant polarization in the 2-8 keV range, with a 3-sigma upper limit for the polarization degree of 1.9%. However, marginal evidence of polarization was detected in the 2-3 keV band at the 95.5% confidence level (2-sigma), suggesting potential contributions from scattering effects in the individual spectral components (disk, reflection, and Comptonization) that could cancel each other out due to the different orientations of their polarization angles. This behavior aligns with other Atoll sources observed by IXPE, which typically exhibit lower and less variable polarization degrees compared to Z-class sources.

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arXiv2025

A Comprehensive Study of Thermonuclear X-ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation

We present the results obtained from timing and spectral studies of 15 thermonuclear X-ray bursts from 4U 1820-30 observed with the Neutron Star Interior Composition Explorer (NICER) during its five years of observations between 2017-2022. All bursts showed clear signs of photospheric radius expansion, where the neutron star (NS) photosphere expanded more than 50 km above the surface. One of the bursts produced a super-expansion with a blackbody emission radius of 902 km for the first time with NICER. We searched for burst oscillations in all 15 bursts and found evidence of a coherent oscillation at 716 Hz in a burst, with a 2.9$σ$ detection level based on Monte Carlo simulations. If confirmed with future observations, 4U 1820-30 would become the fastest-spinning NS known in X-ray binary systems. The fractional rms amplitude of the candidate burst oscillation was found to be 5.8% in the energy range of 3-10 keV. Following the variable persistent model from burst time-resolved spectroscopy, an anti-correlation is seen between the maximum scaling factor value and the (pre-burst) persistent flux. We detected a low value of ionization at the peak of each burst based on reflection modeling of burst spectra. A partially interacting inner accretion disk or a weakly ionized outer disk may cause the observed ionization dip during the photospheric radius expansion phase.

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arXiv2025

Parameter constraints for accreting millisecond pulsars with synthetic NICER data

Pulse profile modelling (PPM) is a technique for inferring mass, radius and hotspot properties of millisecond pulsars. PPM is now regularly used for analysis of rotation-powered millisecond pulsars (RMPs) with data from the Neutron Star Interior Composition ExploreR (NICER). Extending PPM to accreting millisecond pulsars (AMPs) is attractive, because they are a different source class featuring bright X-ray radiation from hotspots powered by accretion. In this paper, we present a modification of one of the PPM codes, X-PSI, so that it can be used for AMPs. In particular, we implement a model of an accretion disc and atmosphere model appropriate for the hotspots of AMPs, and improve the overall computational efficiency. We then test parameter recovery with synthetic NICER data in two scenarios with reasonable parameters for AMPs. We find in the first scenario, where the hotspot is large, that we are able to tightly and accurately constrain all parameters including mass and radius. In the second scenario, which is a high inclination system with a smaller hotspot, we find degeneracy between a subset of model parameters and a slight bias in the inferred mass and radius. This analysis of synthetic data lays the ground work for future analysis of AMPs with NICER data. Such an analysis could be complemented by future (joint) analysis of polarization data from the Imaging X-ray Polarimetry Explorer (IXPE).

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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.

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arXiv2025

Exploring polarization and geometry in the X-ray pulsar 4U 1538-52

The Imaging X-ray Polarimetry Explorer (IXPE) observations of accreting X-ray pulsars (XRPs) continue to provide novel insights into the physics and geometry of these sources. We present the first X-ray polarimetric study of the persistent wind-fed XRP 4U 1538-52, based on five IXPE observations totaling 360 ks, conducted in March and October 2024. We detect marginally significant polarization in the combined data set in the full 2--8 keV energy band, with a polarization degree (PD) of 3.0+-1.1% and polarization angle (PA) of -18 degrees. The energy-resolved analysis shows a clear energy dependence of the polarization properties, with a remarkable ~70 degrees switch in PA between low and high energies. Similarly, the pulse phase-resolved spectro-polarimetric analysis reveals different signatures at low and high energies. At low (2--3 keV) energies, the PD ranges between ~2% and ~18%, with the PA varying between -16 and 70 degrees. At higher (4--8 keV) energies, the PD varies between ~3% and ~12%, with a drastically different PA behavior. Fitting the rotating vector model to the pulse phase dependence of the PA at the lower energies, we constrain the geometric configuration of the pulsar. The analysis favors a high spin-axis inclination of >50 which agrees with both previous pulse-phase-dependent spectral fitting of the cyclotron line region and the known high orbital inclination of the binary system. The magnetic obliquity is estimated to be 30 degrees and the spin position angle to be 19 degrees. A sharp switch in PA around 3 keV presents a particular theoretical challenge, as it is not consistent with the right-angle switch that was only seen in one other pulsar Vela X-1.

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arXiv2025

NICER observations of type-I X-ray bursts from the ultra-compact X-ray binary M15 X-2

Type-I X-ray bursts are thermonuclear explosions caused by the unstable burning of accreted material on the surface of neutron stars. We report the detection of seven type-I X-ray bursts from the ultracompact X-ray binary M15 X-2 observed by the Neutron Star Interior Composition Explorer (NICER) during its 2022 outburst. We found all the bursts occurred in the soft state and exhibited similar light curve profiles, with no cases of photospheric radius expansion. Time-resolved spectroscopy showed clear deviations from the blackbody model during the first ten seconds of all the bursts. The fits were improved by using the enhanced persistent emission method, which we interpret as evidence of burst-disk interaction. We compared the performance of these models against a neutron star atmosphere model and found it made no significant improvements. After analyzing the burst rise times and fuel composition, we propose that these bursts were powered by the burning of pure helium, confirming the ultracompact nature of the source.

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