arXiv subjects

Mark Reynolds

Publications and source records attributed to Mark Reynolds.

At least 19 recordsLinked to original sources

arXiv2026

Galactic Science with Ultra-High Angular Resolution X-ray Imaging

Milli- to micro-arcsecond X-ray imaging will open a new observational regime for Galactic astrophysics by resolving physical scales that are inaccessible to current X-ray observatories. This white paper highlights the science enabled by such capabilities across four broad questions: how particles are accelerated, how stars die, how accretion is fueled, and what populations of X-ray sources inhabit the Galaxy. Ultra-high angular resolution will enable many new studies, such as resolving shocks and jets, measuring proper motions, parallaxes, and binary orbits, and providing secure multiwavelength counterpart identifications in crowded environments such as the Galactic Center and globular clusters. Combined with high-time-resolution observations, these measurements will connect variability and transient events to the physical structures in which they originate, while coordination with gravitational-wave, neutrino, $\gamma$-ray, radio, optical, and infrared facilities will provide spatial information needed to identify and characterize multi-messenger sources. In addition to defining the science cases, this paper presents a curated list of compelling targets spanning various angular resolutions and outlines the complementary specifications necessary to maximize the scientific outcome. Accomplishing these science goals will require an instrument with high angular resolution, precise astrometry, substantial collecting area, sufficient spectral and timing resolution, and high dynamic range imaging capabilities.

astro-ph.HE↗

arXiv2026

XRISM spectroscopy of a crowded Galactic center region -- II. Narrow emission lines in the Black Hole candidate MAXI J1744-294/Swift J174540.2-290037

Narrow, highly ionized X-ray emission lines in black hole low-mass X-ray binaries (BH-LMXBs) are rare and have been observed in only a few sources, during unusual, heavily obscured accretion states. We report on a detailed high-resolution spectral analysis of emission line features from the first XRISM observation of a BH-LMXB candidate in a bright soft state, MAXI J1744-294/Swift J174540.2-290037, in the central parsec region of our galaxy. The source was observed as part of an extensive, coordinated multi-wavelength campaign on its recurring X-ray outburst in early 2025. By carefully modeling the contributions of multiple point sources and diffuse emission within the XRISM/Resolve field of view, and combining these data with broadband X-ray coverage from XMM-Newton and NuSTAR (Paper I), we identified a narrow ($\sigma \sim 500-1000$ km s$^{-1}$), static emission component intrinsic to the system. This component likely arises from a highly ionized (log $\xi \gtrsim 5.5$) photoionized plasma in the inner disk atmosphere, and is accompanied by a weak, narrow Fe I K$\alpha$ line at 6.4 keV. We also detected at least three narrow emission features at atypical energies between 6.7 and 7.1 keV. The lack of corresponding rest-frame atomic transitions points toward highly ionized blueshifted Fe lines with outflow velocities of $-1300$ to $-6000$ km s$^{-1}$, which we model with multiple layers of photoionized or collisional plasma. We explore scenarios in which these unprecedented features are produced by multiple phases in a jet and/or a disk wind, and discuss potential similarities between MAXI J1744- 294 and the exotic microquasar SS 433.

astro-ph.HE↗

arXiv2026

XRISM spectroscopy of a crowded Galactic center region -- I. Disentangling the sources in the field of view

The Galactic center is a complex and crowded region hosting the supermassive black hole Sgr A*, numerous accreting compact objects, and diffuse X-ray emission. This paper presents the first in a series of studies analyzing the XRISM observation of the X-ray transient MAXI J1744-294/Swift J174540.2-290037, located $\sim18''$ from Sgr A*. The observation, conducted in March 2025, along with XMM-Newton and NuSTAR coverage, aimed to investigate the Fe emission features of MAXI J1744-294 during its outburst. However, the region surrounding the source is heavily contaminated by X-ray emission from various diffuse and point sources, including strong line contributions from the supernova remnant Sgr A East and the Galactic center X-ray emission (GCXE). Additionally, the nearby neutron star low-mass X-ray binary (NS-LMXB) AX J1745.6-2901 was also in outburst during the XRISM observation, further complicating the spectral analysis. This study focuses on disentangling the contributions of these overlapping sources by robustly modeling the background contamination and spatial-spectral mixing. We describe the methodologies, region selection, and data reduction techniques applied to the different instruments. Two complementary approaches -- empirical and physical modeling -- are employed to characterize diffuse emission and point-source contributions. The results provide a foundation for the detailed spectral analysis of MAXI J1744-294, AX J1745.6-2901, and the surrounding interstellar medium (ISM), which will be presented in subsequent papers. This study highlights the challenges and robust solutions for analyzing XRISM/Resolve data from crowded regions in conjunction with other X-ray telescope data.

astro-ph.HE↗

arXiv2025

Maritime object classification with SAR imagery using quantum kernel methods

Illegal, unreported, and unregulated (IUU) fishing causes global economic losses of 10-25 billion USD annually and undermines marine sustainability and governance. Synthetic Aperture Radar (SAR) provides reliable maritime surveillance under all weather and lighting conditions, but classifying small maritime objects in SAR imagery remains challenging. We investigate quantum machine learning for this task, focusing on quantum kernel methods (QKMs) applied to real and complex SAR chips extracted from the SARFish dataset. We tackle two binary classification problems, the first for distinguishing vessels from non-vessels, and the second for distinguishing fishing vessels from other types of vessels. We compare QKMs applied to real and complex SAR chips against classical Laplacian, RBF, and linear kernels applied to real SAR chips. We restrict the comparison to be between just kernel based models so that the comparison is as fair and meaningful as possible. Using noiseless numerical simulations of the quantum kernels, we find that with the real SAR chips, QKMs are capable of obtaining equal or better performance than the classical kernels in the best case. However, the specific quantum kernel used to encode the complex SAR data overfits and performs poorly. This work presents the first application of QKMs to maritime classification in SAR imagery and offers insight into the potential and current limitations of quantum-enhanced learning for maritime surveillance.

quant-ph↗

arXiv2025

The very faint X-ray transient Swift J174610-290018 at the Galactic center

Very Faint X-ray Transients (VFXTs) are a class of X-ray binary systems that exhibit occasional outbursts with peak X-ray luminosities (L_X< 1e36 erg s^-1) much lower than typical X-ray transients. On 22nd February 2024, during its daily Galactic center monitoring, Swift-XRT detected a VFXT, 7 arcmin from Sgr A* dubbing it Swift J174610--290018. We aim to characterize the outburst that occurred in 2024, and a second, distinct outburst in 2025, to understand the nature and accretion flow properties of this new VFXT. Swift-XRT light curves are used to constrain the duration of the two events. We carried out X-ray spectral analysis exploiting XMM and NuSTAR data. We used Chandra and XMM observations of the last 25 years to constrain the quiescent luminosity of the source. During the 2024 outburst, which lasted about 50 days, the source reached a luminosity in the 2-10 keV band of L_X = 1.2e35 erg s^-1 (assuming it is located at the Galactic center). The 2025 outburst is shorter (about 5 days), and reached L_X = 9e34 erg s^-1. The spectral features of the source include an excess at 6.5-7 keV, which can be associated either with a single reflection line or with the ionized Fe XXV and XXVI lines. The same source was identified in both the XMM and Chandra catalogs of point sources (known as 4XMM J174610.7--290020). During previous detections, the source displayed luminosity levels ranging from L_X= 2e32 to L_X = 3e34 erg s^-1 between 2000 and 2010. Moreover, it exhibited a potential type I X-ray burst in 2004. The analysis of the outbursts and the potential type I burst strongly suggests the neutron star low mass X-ray binary (NS-LMXB) nature of the VFXT. The source can be described by an accretion disk corona (as has been recently proposed by the XRISM/Xtend analysis). This scenario explains the overall low luminosity of this transient and the peculiar iron lines in the spectrum.

astro-ph.HE↗

arXiv2025

A multiwavelength study of the Galactic center black hole candidate MAXI J1744-294

For the first time in nearly a decade, a bright transient was detected in the central parsec (pc) of the Galaxy. MAXI J1744-294, or -- as it was known in its previous life -- Swift J174540.2-290037, was discovered in outburst by the MAXI telescope in January 2025. We present the results of a broadband, multi-wavelength study of MAXI J1744-294, including data from the NuSTAR, Chandra, XMM-Newton, Swift, and NICER X-ray telescopes, as well as complementary radio and near-infrared observations. We analyze the changing X-ray emission as the outburst evolved from the high/soft to the low/hard state. Using relativistic reflection features in the data, we estimate a spin of $a>0.92$ and viewing inclination $\theta=28^{+3}_{-4}$ deg, consistent with the parameters measured for Swift J174540.2-290037. Based on the spectral and temporal characteristics of MAXI J1744-294, we reaffirm its classification as a candidate black hole (BH) low-mass X-ray binary (LMXB) -- the third candidate BH transient discovered within 20 arcsec of the Galactic supermassive black hole Sgr~A*. This work provides further evidence for a cusp of BH-LMXBs in the central pc of our Galaxy, as argued for in previous observational studies and suggested by analytical and theoretical work. Our ongoing multi-wavelength study, involving a complementary range of observatories and spanning different outburst states, can serve as a model for future time domain astrophysics research.

astro-ph.HE↗

arXiv2025

Unraveling the Nature of the Nuclear Transient AT2020adpi

Transient events associated with supermassive black holes provide rare opportunities to study accretion and the environments of supermassive black holes. We present a multiwavelength study of AT2020adpi (ZTF20acvfraq), a luminous optical/UV transient in the nucleus of the galaxy WISEA J231853.77$-$103505.6 ($z=0.26$) that exhibits the properties of an ambiguous nuclear transient. Near peak, its spectral energy distribution is well described by a power law ($\lambda L_\lambda \propto \lambda^{-\alpha}$, $\alpha = 0.44 \pm 0.04$), with a maximum $g$-band luminosity of $(3.6 \pm 0.6)\times10^{44}$ erg s$^{-1}$, which is consistent with luminous AGN flares. We detect a strong mid-infrared flare ($L_\mathrm{peak}^{\mathrm{MIR}} = (2.3 \pm 0.05)\times10^{44}$ erg s$^{-1}$) delayed by $\sim$210 rest-frame days, indicating a hot dust echo from material at $\sim$0.2 pc. The optical and near-infrared spectra show broad H, He I, [OIII] lines, as well as narrow Fe II, and prominent Mg II, which is a combination not typical of TDEs. Taken together, these features suggest AT2020adpi is an ambiguous nuclear transient, where an accretion episode was triggered by stellar disruption of an accretion disk or instabilities within an active nucleus. This source demonstrates the need for careful multiwavelength analysis to distinguish between extreme AGN variability and TDEs.

astro-ph.HE↗

arXiv2025

The population of NuSTAR Black Hole X-ray Binaries

The spin of a black hole (BH) encodes information about its formation and evolution history. Yet the understanding of the distribution of BH spins in X-ray binaries (XBs), of the models used to measure spin, and of their impact on systematic uncertainties remains incomplete. In this work, we expand on previous analyses of the entire NuSTAR archive of accreting BH XBs. Prior work compiled a sample of 245 spectral fits using the relativistic reflection method for NuSTAR observations of 36 BH systems. Here, we aim to probe two aspects: the connection between BH spin and binary system properties, and the relationships between parameters in the spectral fits. We identify moderate negative correlations between spin uncertainty and both BH mass and system inclination, and a moderate positive correlation with distance. We also point out tentative multidimensional degeneracies between inclination, disk density, Fe abundance, ionization, and the presence or absence of absorption features from ionized outflows linked to disk winds. Lastly, we provide a comprehensive view of the observed distribution of BH spins in XBs, in comparison to spins inferred from gravitational waves. We find that the distribution of BH spins in XBs can be described by a beta distribution with $\alpha=5.66$ and $\beta=1.09$. This data set is highly complex, and the analysis presented here does not fully explore all potential parameter correlations. We make the full data set available in Zenodo to the community to encourage further exploration.

astro-ph.HE↗

arXiv2025

DocSpiral: A Platform for Integrated Assistive Document Annotation through Human-in-the-Spiral

Acquiring structured data from domain-specific, image-based documents such as scanned reports is crucial for many downstream tasks but remains challenging due to document variability. Many of these documents exist as images rather than as machine-readable text, which requires human annotation to train automated extraction systems. We present DocSpiral, the first Human-in-the-Spiral assistive document annotation platform, designed to address the challenge of extracting structured information from domain-specific, image-based document collections. Our spiral design establishes an iterative cycle in which human annotations train models that progressively require less manual intervention. DocSpiral integrates document format normalization, comprehensive annotation interfaces, evaluation metrics dashboard, and API endpoints for the development of AI / ML models into a unified workflow. Experiments demonstrate that our framework reduces annotation time by at least 41\% while showing consistent performance gains across three iterations during model training. By making this annotation platform freely accessible, we aim to lower barriers to AI/ML models development in document processing, facilitating the adoption of large language models in image-based, document-intensive fields such as geoscience and healthcare. The system is freely available at: https://app.ai4wa.com. The demonstration video is available: https://app.ai4wa.com/docs/docspiral/demo.

cs.SE↗

arXiv2025

TimelineKGQA: A Comprehensive Question-Answer Pair Generator for Temporal Knowledge Graphs

Question answering over temporal knowledge graphs (TKGs) is crucial for understanding evolving facts and relationships, yet its development is hindered by limited datasets and difficulties in generating custom QA pairs. We propose a novel categorization framework based on timeline-context relationships, along with \textbf{TimelineKGQA}, a universal temporal QA generator applicable to any TKGs. The code is available at: \url{https://github.com/PascalSun/TimelineKGQA} as an open source Python package.

cs.LO↗

arXiv2023

Systematically Revisiting All NuSTAR Spins of Black Holes in X-Ray Binaries

We extend our recent work on black hole spin in X-ray binary systems to include an analysis of 189 archival NuSTAR observations from 24 sources. Using self-consistent data reduction pipelines, spectral models, and statistical techniques, we report an unprecedented and uniform sample of 36 stellar-mass black hole spin measurements based on relativistic reflection. This treatment suggests that prior reports of low spins in a small number of sources were generally erroneous: our comprehensive treatment finds that those sources tend to harbor black holes with high spin values. Overall, within $1\sigma$ uncertainty, $\sim86\%$ of the sample are consistent with $a \geq 0.95$, $\sim94\%$ of the sample are consistent with $a\geq 0.9$, and $100\%$ is consistent with $a\geq 0.7$ (the theoretical maximum for neutron stars; $a = cJ/GM^{2}$). We also find that the high-mass X-ray binaries (those with A-, B-, or O-type companions) are consistent with $a\geq 0.9$ within the $1\sigma$ errors; this is in agreement with the low-mass X-ray binary population and may be especially important for comparisons to black holes discovered in gravitational wave events. In some cases, different spectra from the same source yield similar spin measurements but conflicting values for the inclination of the inner disk; we suggest that this is due to variable disk winds obscuring the blue wing of the relativistic Fe K emission line. We discuss the implications of our measurements, the unique view of systematic uncertainties enabled by our treatment, and future efforts to characterize black hole spins with new missions.

astro-ph.HE↗

arXiv2023

The Spin of a Newborn Black Hole: Swift J1728.9-3613

The origin and distribution of stellar-mass black hole spins are a rare window into the progenitor stars and supernova events that create them. Swift J1728.9-3613 is an X-ray binary, likely associated with the supernova remnant G351.9-0.9 (Balakrishnan et al. 2023). A NuSTAR X-ray spectrum of this source during its 2019 outburst reveals reflection from an accretion disk extending to the innermost stable circular orbit. Modeling of the relativistic Doppler shifts and gravitational redshifts imprinted on the spectrum measures a dimensionless spin parameter of $a=0.86\pm0.02$ ($1\sigma$ confidence), a small inclination angle of the inner accretion disk $\theta<10$ degrees, and a sub-solar iron abundance in the disk $A_{\rm Fe}<0.84$. This high spin value rules out a neutron star primary at the $5\;\sigma$ level of confidence. If the black hole is located in a still visible supernova remnant, it must be young. Therefore, we place a lower limit on the natal black hole spin of $a>0.82$, concluding that the black hole must have formed with a high spin. This demonstrates that black hole formation channels that leave a supernova remnant, and those that do not (e.g. Cyg X-1), can both lead to high natal spin with no requirement for subsequent accretion within the binary system. Emerging disparities between the population of high-spin black holes in X-ray binaries and the low-spin black holes that merge in gravitational wave events may therefore be explained in terms of different stellar conditions prior to collapse, rather than different environmental factors after formation.

astro-ph.HE↗

arXiv2023

The Black Hole Candidate Swift J1728.9$-$3613 and the Supernova Remnant G351.9$-$0.9

A number of neutron stars have been observed within the remnants of the core-collapse supernova explosions that created them. In contrast, black holes are not yet clearly associated with supernova remnants. Indeed, some observations suggest that black holes are ``born in the dark'', i.e. without a supernova explosion. Herein, we present a multi-wavelength analysis of the X-ray transient Swift J1728.9$-$3613, based on observations made with Chandra, ESO-VISTA, MeerKAT, NICER, NuSTAR, Swift, and XMM-Newton. Three independent diagnostics indicate that the system likely harbors a black hole primary. Infrared imaging signals a massive companion star that is broadly consistent with an A or B spectral type. Most importantly, the X-ray binary lies within the central region of the catalogued supernova remnant G351.9$-$0.9. Our deep MeerKAT image at 1.28~GHz signals that the remnant is in the Sedov phase; this fact and the non-detection of the soft X-ray emission expected from such a remnant argue that it lies at a distance that could coincide with the black hole. Utilizing a formal measurement of the distance to Swift J1728.9$-$3613 ($d = 8.4\pm 0.8$ kpc), a lower limit on the distance to G351.9$-$0.9 ($d \geq 7.5$ kpc), and the number and distribution of black holes and supernova remnants within the Milky Way, extensive simulations suggest that the probability of a chance superposition is $<1.7\%$ ($99.7\%$ credible interval). The discovery of a black hole within a supernova remnant would support numerical simulations that produce black holes and remnants, and thus provide clear observational evidence of distinct black hole formation channels. We discuss the robustness of our analysis and some challenges to this interpretation.

astro-ph.HE↗

arXiv2022

A Systematic View of Ten New Black Hole Spins

The launch of NuSTAR and the increasing number of binary black hole (BBH) mergers detected through gravitational wave (GW) observations have exponentially advanced our understanding of black holes. Despite the simplicity owed to being fully described by their mass and angular momentum, black holes have remained mysterious laboratories that probe the most extreme environments in the Universe. While significant progress has been made in the recent decade, the distribution of spin in black holes has not yet been understood. In this work, we provide a systematic analysis of all known black holes in X-ray binary systems (XB) that have previously been observed by NuSTAR, but have not yet had a spin measurement using the "relativistic reflection" method obtained from that data. By looking at all the available archival NuSTAR data of these sources, we measure ten new black hole spins: IGR J17454-2919 -- $a=0.97^{+0.03}_{-0.17}$; GRS 1758-258 -- $a=0.991^{+0.007}_{-0.019}$; MAXI J1727-203 -- $a=0.986^{+0.012}_{-0.159}$; MAXI J0637-430 -- $a=0.97\pm0.02$; Swift J1753.5-0127 -- $a=0.997^{+0.001}_{-0.003}$; V4641 Sgr -- $a=0.86^{+0.04}_{-0.06}$; 4U 1543-47 -- $a=0.98^{+0.01}_{-0.02}$; 4U 1957+11 -- $a=0.95^{+0.02}_{-0.04}$; H 1743-322 -- $a=0.98^{+0.01}_{-0.02}$; MAXI J1820+070 -- $a=0.988^{+0.006}_{-0.028}$ (all uncertainties are at the $1\sigma$ confidence level). We discuss the implications of our measurements on the entire distribution of stellar mass black hole spins in XB, and we compare that with the spin distribution in BBH, finding that the two distributions are clearly in disagreement. Additionally, we discuss the implications of this work on our understanding of how the "relativistic reflection" spin measurement technique works, and discuss possible sources of systematic uncertainty that can bias our measurements.

astro-ph.HE↗

arXiv2021

Five new hot-Jupiter transits investigated with Swift-UVOT

Short wavelength exoplanet transit measurements have been used to probe mass-loss in exoplanet atmospheres. We present the Swift-UVOT transit light curves for five hot Jupiters orbiting UV-bright F-type stars: XO-3, KELT-3, WASP-3, WASP-62, and HAT-P-6. We report one positive transit detection of XO-3b and one marginal detection of KELT-3b. We place upper limits on the remaining three transit depths. The planetary radii derived from the NUV transit depths of both potential detections are 50-100% larger than their optical radius measurements. We examine the ratio $R_{\rm NUV}/R_{\rm opt}$ for trends as a function of estimated mass-loss rate, which we derive from X-ray luminosity obtained from the Swift-XRT, or XMM-Newton in the case of WASP-62. We find no correlation between the energy-limited photoevaporative mass-loss rate and the $R_{\rm NUV}/R_{\rm opt}$ ratio. We also search for trends based on the equilibrium temperature of the hot Jupiters. We find a possible indication of a transition in the $R_{\rm NUV}/R_{\rm opt}$ ratio around $T_{\rm eq} = 1700~{\rm K}$, analogous to the trends found for NIR water features in transmission spectra. This might be explained by the formation of extended cloud decks with silicate particles $\leq 1~\mu{\rm m}$. We demonstrate that the Swift-UVOT filters could be sensitive to absorption from aerosols in exoplanet atmospheres.

astro-ph.EP↗

arXiv2020

Scene Gated Social Graph: Pedestrian Trajectory Prediction Based on Dynamic Social Graphs and Scene Constraints

Pedestrian trajectory prediction is valuable for understanding human motion behaviors and it is challenging because of the social influence from other pedestrians, the scene constraints and the multimodal possibilities of predicted trajectories. Most existing methods only focus on two of the above three key elements. In order to jointly consider all these elements, we propose a novel trajectory prediction method named Scene Gated Social Graph (SGSG). In the proposed SGSG, dynamic graphs are used to describe the social relationship among pedestrians. The social and scene influences are taken into account through the scene gated social graph features which combine the encoded social graph features and semantic scene features. In addition, a VAE module is incorporated to learn the scene gated social feature and sample latent variables for generating multiple trajectories that are socially and environmentally acceptable. We compare our SGSG against twenty state-of-the-art pedestrian trajectory prediction methods and the results show that the proposed method achieves superior performance on two widely used trajectory prediction benchmarks.

cs.CV↗

arXiv2020

Top-k Socio-Spatial Co-engaged Location Selection for Social Users

With the advent of location-based social networks, users can tag their daily activities in different locations through check-ins. These check-in locations signify user preferences for various socio-spatial activities and can be used to build their profiles to improve the quality of services in some applications such as recommendation systems, advertising, and group formation. To support such applications, in this paper, we formulate a new problem of identifying top-k Socio-Spatial co-engaged Location Selection (SSLS) for users in a social graph, that selects the best set of k locations from a large number of location candidates relating to the user and her friends. The selected locations should be (i) spatially and socially relevant to the user and her friends, and (ii) diversified in both spatially and socially to maximize the coverage of friends in the spatial space. This problem has been proved as NP-hard. To address the challenging problem, we first develop a branch-and-bound based Exact solution by designing some pruning strategies based on the derived bounds on diversity. To make the solution scalable for large datasets, we also develop an approximate solution by deriving the relaxed bounds and advanced termination rules to filter out insignificant intermediate results. To further accelerate the efficiency, we present one fast exact approach and a meta-heuristic approximate approach by avoiding the repeated computation of diversity at the running time. Finally, we have performed extensive experiments to evaluate the performance of our proposed models and algorithms against the adapted existing methods using four real-world large datasets.

cs.SI↗

arXiv2020

Take a NAP: Non-Autoregressive Prediction for Pedestrian Trajectories

Pedestrian trajectory prediction is a challenging task as there are three properties of human movement behaviors which need to be addressed, namely, the social influence from other pedestrians, the scene constraints, and the multimodal (multiroute) nature of predictions. Although existing methods have explored these key properties, the prediction process of these methods is autoregressive. This means they can only predict future locations sequentially. In this paper, we present NAP, a non-autoregressive method for trajectory prediction. Our method comprises specifically designed feature encoders and a latent variable generator to handle the three properties above. It also has a time-agnostic context generator and a time-specific context generator for non-autoregressive prediction. Through extensive experiments that compare NAP against several recent methods, we show that NAP has state-of-the-art trajectory prediction performance.

cs.CV↗