arXiv subjects

Xiaowan Zhang

Publications and source records attributed to Xiaowan Zhang.

9 recordsLinked to original sources

arXiv2026

When will T Coronae Borealis next erupt as a nova? Constraints from recurrence, orbital phase, and accretion-state evolution

T Coronae Borealis (T CrB) is the nearest symbiotic recurrent nova and is now intensively monitored for its next eruption. We combine three constraints on the eruption time: historical recurrence, orbital phase, and recent accretion-state evolution. Conditioning on no eruption by 2026 July 11, the three effective historical intervals give illustrative survival-conditioned probabilities of 30.2\% for the rest of 2026 and 56.9\% within the following year; these are empirical indicators, not physical prediction probabilities. The four adopted historical eruption phases do not select a unique ignition phase, but form two loose pairs near $ϕ\simeq0.44$ and $ϕ\simeq0.62$, used here only as monitoring windows. The 1946 pre-eruption dip is difficult to explain by either a pure accretion-rate decline or standard dust extinction, and may have involved both accretion restructuring and source-dependent obscuration. If the renewed 2026 decline is the true pre-1946 analogue, an eruption around 2026 December remains plausible. Conversely, if post-2024 brightness remains below the 2014--2023 high state, an accretion-deficit estimate gives an earliest lower limit near 2029 May. The data therefore support conditional monitoring windows, not a unique date. These are conditional diagnostic scenarios rather than competing point predictions; the eventual eruption epoch will test their underlying assumptions.

astro-ph.HE↗

arXiv2026

Lack of Significant Orbital-Phase Locking in the Active Phases of the Recurrent Nova T CrB

T Coronae Borealis (T CrB) is a symbiotic recurrent nova (RN) that exhibits both nova eruptions and long-term active phases resembling superoutbursts and normal outbursts. Motivated by proposed connections between these events and the binary orbit, we test whether the onset, maximum, or termination of the active phases is locked to orbital phase. We use long-term optical $B$- and $V$-band light curves from the American Association of Variable Stars Observers (AAVSO) International Database and historical photometry from the literature. We measure the onset, maximum, and termination times of superoutbursts and normal outbursts and convert these times to orbital phase. We test the resulting circular distributions with Kuiper and Watson statistics. We find no statistically significant orbital-phase locking. The onset phases and maxima are consistent with a uniform phase distribution. The smallest probabilities occur for the 13 measurable termination phases ($p_{\rm MC}=0.043$ for the Kuiper statistic and $p_{\rm MC}=0.048$ for the Watson statistic), but this result is only marginal in an uncorrected $p<0.05$ sense, far from a $3σ$ detection, and insufficient to establish robust phase locking. The four historical nova eruptions likewise do not provide robust evidence for a unique ignition phase once the small sample size, historical date uncertainties, and long-term period changes are considered. The two known secondary eruptions occurred at similar phases, but two events are insufficient to establish an orbital-geometry connection. Overall, the active phases of T CrB appear to be governed primarily by accretion-disk physics rather than by a fixed binary phase.

astro-ph.HE↗

arXiv2026

Analytics for Quality Assurance for Item Pools (AQuAP): Monitoring and Maintaining Item Bank Health in AI-Driven Assessment Systems

The large-scale digitization of educational assessment has made the continuous oversight of item banks both essential and complex. This paper presents Analytics for Quality Assurance for Item Pools (AQuAP), a dashboard environment for monitoring item quality and item bank health. AQuAP supports the operational implementation of the large scale item generation procedures for high-stakes tests as included in the Item Factory, a framework for automated and human-supported test development. The paper describes AQuAP in relationship with the process of item development, outlines the broader metric framework for item-pool quality assurance, and highlights the Effective Bank Size (EBS) as one central indicator of pool vitality. EBS quantifies how many independent test sessions can be constructed before content repetition occurs and, when coupled with exposure and usage metrics, provides insight into item bank security, diversity, and efficiency. We further introduce bank-health metrics, such as maximum exposure, maximum conditional exposure, adjusted effective bank size, and the rarely-administered fraction, all of which extend this picture of item utilization. AQuAP illustrates how operational analytics can translate psychometric concepts into quality assurance tools for high-volume, AI-enabled testing programs. This work is illustrated with the Duolingo English Test (DET) processes.

stat.AP↗

arXiv2025

Multi-wavelength study of the pre-eruption dip in the recurrent nova T Coronae Borealis preceding imminent nova eruption

We present a multi-wavelength study of the symbiotic recurrent nova (RN) T Coronae Borealis (T CrB) using Swift Burst Alert Telescope (BAT) / X-Ray Telescope (XRT) / UltraViolet Optical Telescope (UVOT) and American Association of Variable Stars Observers (AAVSO) observations from 2005 to 2025. Our analysis spans quiescent, high, and pre-eruption dip states. We find that brightening amplitudes increase toward shorter wavelengths in both optical and UV bands, while the UV and X-ray fluxes are generally anti-correlated throughout all phases. During the 2023-2024 pre-eruption dip, soft and hard X-rays increased as optical and ultraviolet (UV) brightness declined, consistent with a transition from an optically thick to thin boundary layer driven by a reduction in the accretion rate. We also report, for the first time, a second, lower-amplitude dip occurring between September 2024 and February 2025 following the primary 2023-2024 pre-eruption dip. The observed variability supports an accretion-variation scenario as a unifying explanation for both the high and dip states, and may signal an imminent nova eruption.

astro-ph.HE↗

arXiv2025

Temporal variability and obscuration effects in the X-ray emission of classical nova V339 Delphini (Nova Delphini 2013)

In this study, we present a detailed analysis of public archival soft X-ray data on the classical nova V339 Delphini (Nova Del 2013) during its outburst, obtained using the {\it Chandra} High-Resolution Camera Spectrometer (HRC-S) and Low Energy Transmission Grating (LETG), as well as {\it XMM-Newton} in 2013. The observations, spanning from day 85.2 to day 112.0 after the optical maximum, capture the nova during its luminous supersoft X-ray source (SSS) phase. The spectra reveal numerous absorption features with blue-shifted velocities ranging from $\sim$ 724 to $\sim$ 1474 km s$^{-1}$, with the majority of lines blue-shifted by approximately 1200 km s$^{-1}$. We confirm the presence of a short-period modulation of the X-ray flux with a period of approximately 54 seconds, as well as the drift of this period, which was detected on days 97.0 and 112.0 during the outburst with both {\it XMM-Newton} and {\it Chandra}. This period modulation is transient in nature, with significant variations in amplitude and pulse profile over timescales of a few thousand seconds, likely due to temporary obscuration events that affect the emission from the central hot source. The pulse profiles exhibit substantial deviations from a pure sinusoidal shape, which may be related to the period drift. Additionally, the modulation amplitude shows a possible anti-correlation with the count rates on day 97.0, likely also caused by temporary obscuration events influencing the central source's emission.

astro-ph.HE↗

arXiv2025

Timing and spectral analysis of GK Persei during the 2010 dwarf nova outburst

GK Persei, an old nova and intermediate polar (IP), exhibited a dwarf nova (DN) outburst in 2010. This outburst was extensively observed by the Neil Gehrels Swift Observatory, beginning 1.95 days after the eruption and continuing until 13.9 days before the maximum of the outburst in the optical. In this paper, we present timing and spectral analyses, comparing the results with those of other outbursts. We confirm the spin modulation in the 2 $-$ 10 keV X-ray range with a period of $P_{\rm WD} = 351.325(9)$ s. Additionally, we detected spin modulation in the 0.3 $-$ 2 keV band during the second half of the observations, a feature not seen in the 2015 and 2018 outbursts. This finding suggests that the soft X-ray emission in GK Per may originate partly near the magnetic poles and partly from a wind or circumstellar material.

astro-ph.HE↗

arXiv2024

High resolution X-ray spectra of the compact binary supersoft X-ray source CAL 87

In this study, we present an analysis of the archival X-ray data of the eclipsing supersoft X-ray binary CAL 87 observed with the {\it Chandra} Advanced CCD Imaging Spectrometer (ACIS) camera and Low Energy Transmission Grating (LETG) in 2001 August and with {\it XMM-Newton} in 2003 April. The high resolution X-ray spectra are almost unchanged in the two different dates. The average unabsorbed X-ray luminosity during the exposure was 4.64$-$5.46$\times10^{36}$ ergs s$^{-1}$ in 2001 and 4.54$-$4.82 $\times10^{36}$ ergs s$^{-1}$ in 2003, with prominent and red-shifted emission lines, mostly of nitrogen, oxygen, iron and argon, contributing to at least 30\% of the X-ray flux. The continuum X-ray flux is at least an order of magnitude too small for a hot, hydrogen burning WD. However, the continuum flux is consistent with Thomson-scattering reflecting about 5\% of the light of a hydrogen burning WD with effective temperature of 800,000 K, and a mass of $\sim$ 1.2 M$_\odot$. It has been noted before that a large Thomson-scattering corona explains the X-ray eclipse of CAL 87, in which the eclipsed region is found to be of size the order of a solar radius. The emission lines originate in an even more extended region, beyond the eclipsed central X-ray source; the emission spectrum is very complex, with unusual line ratios.

astro-ph.HE↗

arXiv2024

Swift monitoring of GK Persei during the 2018 dwarf nova outburst

The old nova and intermediate polar (IP) GK Persei underwent one of its recurrent dwarf nova (DN) outbursts in 2018. We proposed monitoring it in UV and X-rays with the Neil Gehrels Swift Observatory, starting less than six days after the eruption, until 16 days after the eruption ended. For the first time we could follow the decay to minimum light UV and X-rays. We present the timing and spectral analysis, comparing the results with the previous outbursts and with the quiescent status. We confirm the spin modulation in X-rays with a period 351.325(9) s, only in the 2-10 keV range. The period was not detected in the 0.3-2 keV range and in the UV band, suggesting that the soft portion of the X-ray spectrum in GK Per does not originate near the poles, but in a wind or circumstellar material. The amplitude of the modulation was less prominent than in 2015, a fact that seems correlated with a lower average mass accretion rate. The spectral fits are consistent with a mass accretion rate increasing by a factor of 2 from rise to maximum and decreasing during the return to minimum, following the trend of the modulation amplitude. The maximum plasma temperature is higher than the Swift XRT energy range of 0.3-10 keV, thus it is not well constrained, but our spectral fits indicate that it may have varied irregularly during the outburst.

astro-ph.HE↗

arXiv2013

A New Strategy of Cost-Free Learning in the Class Imbalance Problem

In this work, we define cost-free learning (CFL) formally in comparison with cost-sensitive learning (CSL). The main difference between them is that a CFL approach seeks optimal classification results without requiring any cost information, even in the class imbalance problem. In fact, several CFL approaches exist in the related studies, such as sampling and some criteria-based pproaches. However, to our best knowledge, none of the existing CFL and CSL approaches are able to process the abstaining classifications properly when no information is given about errors and rejects. Based on information theory, we propose a novel CFL which seeks to maximize normalized mutual information of the targets and the decision outputs of classifiers. Using the strategy, we can deal with binary/multi-class classifications with/without abstaining. Significant features are observed from the new strategy. While the degree of class imbalance is changing, the proposed strategy is able to balance the errors and rejects accordingly and automatically. Another advantage of the strategy is its ability of deriving optimal rejection thresholds for abstaining classifications and the "equivalent" costs in binary classifications. The connection between rejection thresholds and ROC curve is explored. Empirical investigation is made on several benchmark data sets in comparison with other existing approaches. The classification results demonstrate a promising perspective of the strategy in machine learning.

cs.LG↗