arXiv subjects

Menglei Zhou

Publications and source records attributed to Menglei Zhou.

At least 19 recordsLinked to original sources

arXiv2026

Dense, multi-phase accretion disk atmosphere in the low-luminosity state of black hole transientV4641 Sgr

We present soft X-ray spectroscopy of the black-hole X-ray binary V4641~Sgr with the \textit{XMM-Newton} Reflection Grating Spectrometer (RGS). The RGS spectrum shows narrow emission features from N\,\textsc{vi--vii} and O\,\textsc{vii--viii} superimposed on a partially covered disk blackbody continuum. A blind Gaussian search confirms the presence of significant lines at the expected rest wavelengths. He-like triplet ratios (high $G$, low $R$) and full photoionization modelling both indicate a dense, photoionized plasma. Small redshifted velocities of $\sim 540$--$720\ \mathrm{km\ s^{-1}}$ are suggested, which are consistent with quasi-static or slowly flowing gas away from the observer after accounting for systematics. Photoionization modelling requires two \textsc{xstar} components with an intermediate ionization parameter ($\log\xi \simeq 3.1$) and a low ionization parameter ($\log\xi \simeq 0.36$), respectively. The simultaneous EPIC-pn spectrum suggests highly ionized Fe emission structures, hinting at an additional, more highly ionized component. These results imply the existence of a radially extended, multiphase, and dense disk atmosphere in the source. We compare the source with other X-ray binaries showing similar emission lines. V4641~Sgr shares a similarly high inclination with other sources; however, the presence of low ionization emission lines distinguishes it from the rest.

astro-ph.HE↗

arXiv2026

MTServe: Efficient Serving for Generative Recommendation Models with Hierarchical Caches

Generative recommendation (GR) offers superior modeling capabilities but suffers from prohibitive inference costs due to the repeated encoding of long user histories. While cross-request Key-Value (KV) cache reuse presents a significant optimization opportunity, the massive scale of individual user states creates a storage explosion that far exceeds physical GPU limits. We propose MTServe, a hierarchical cache management system that virtualizes GPU memory by leveraging host RAM as a scalable backup store. To bridge the I/O gap between tiers, MTServe introduces a suite of system-level optimizations, including a hybrid storage layout, an asynchronous data transfer pipeline, and a locality-driven replacement policy. On both public and production datasets, MTServe delivers up to 3.1* speedup while maintaining near-perfect hit ratios (>98.5%).

cs.LG↗

arXiv2026

Bright flare in the obscured state of GRS 1915+105 as seen by NICER and Swift

We report time-resolved NICER and Swift X-ray spectroscopy of a bright flare from the black hole X-ray binary GRS 1915+105 during its obscured state, which is characterized by heavy line-of-sight absorption by dense material with complex geometry. In April 2023, an unexpected flare was detected, with the observed X-ray flux increasing by nearly an order of magnitude relative to the typical obscured-state level. The spectra show pronounced variability, including significant evolution of the Fe K emission features. Time-resolved spectral modeling indicates that the main flare is associated with a combination of enhanced intrinsic emission and reduced obscuration. We further find that neutral and ionized reflection components are subject to distinct absorbers, whose evolving visibility implies a stratified absorber-reflector geometry. These properties are consistent with a re-illumination phase following a failed disk wind. A delayed radio flare detected about 2.5 days later suggests a coupling between accretion and jet activity.

astro-ph.HE↗

arXiv2025

First detection of X-ray polarization from the long-period X-ray pulsar 4U 1954+319

We report the first detection of X-ray polarization with the Imaging X-ray Polarimetry Explorer from the X-ray pulsar (XRP) 4U 1954$+$319. The source belongs to an extremely rare class of systems in which a slowly rotating neutron star accretes from the dense wind of a red supergiant companion. We detect coherent pulsations at $P_{\rm spin}=5.49\pm0.05$ h, which is one of the longest spin periods known among XRPs. While the phase-averaged analysis shows no significant polarization, with a 99% confidence minimum detectable polarization (MDP$_{99}$) of 4.9% in the 2-8 keV band, the phase-resolved analysis shows a single interval at pulse maximum in which the polarization degree (PD) exceeds the MDP$_{99}$, yielding ${\rm PD}=10.2_{-3.0}^{+3.1}$%. The polarization angle (PA) exhibits a smooth $\approx150^{\circ}$ rotation over the pulse, and a joint evaluation of all phase bins yields an overall detection significance of $3.3\sigma$. Using the rotating vector model, we identify a geometric solution that reproduces the observed PA variation. From this model, we infer a phase-independent ${\rm PD}$ of $6.1\pm1.1$% in the 2-8 keV band.

astro-ph.HE↗

arXiv2025

Spin Constraints on 4U 1630-47 via combined Continuum Fitting and Reflection methods: a comparative study using Frequentist and Bayesian statistics

We present a comprehensive Bayesian spectral analysis of the black hole X-ray binary 4U 1630-47 during its 2022 outburst, using simultaneous NICER and NuSTAR observations. Using the traditional frequentist approach, we build our model combining reflection spectroscopy with continuum fitting techniques and analyze the data. In the Bayesian framework, we jointly constrain the black hole's spin, mass, and inclination within a unified framework. Employing nested sampling, we capture parameter degeneracies and rigorously propagate both statistical and systematic uncertainties. Our results yield robust evidence for a high spin, with the exact value still subject to model-dependent systematic uncertainties from both approaches. Our Bayesian analysis infers spin $a^* = 0.96_{-0.03}^{+0.02}$, mass $M_{\rm BH} = 12.19_{-1.46}^{+1.22} \, M_\odot$, and inclination angle $i = 55.75_{-1.37}^{+1.60}$ degrees, obtained via relxillNS flavor of our model. It also demonstrates the power of Bayesian inference in fetching valuable insights into the complex physics of black hole accretion and enabling high-confidence measurements of fundamental parameters.

astro-ph.HE↗

arXiv2025

Probing the emission geometry of the X-ray pulsar 2S 1417$-$624 during a weak outburst with NICER, IXPE, and NuSTAR

We report results from a multi-mission observational campaign of the transient X-ray pulsar 2S~1417$-$624 during its 2025 outburst, using data from NICER, IXPE, and NuSTAR. Phase-averaged and phase-resolved spectroscopy with NICER and NuSTAR reveal that a typical broken power-law model with a high-energy cut-off well describes the broadband spectra. Several spectral parameters, however, show clear and systematic modulations with pulse phase, indicating variations in the physical conditions of the emitting plasma over the neutron star's rotation. IXPE provides the first polarimetric measurements of this source, yielding a phase-averaged polarization degree (PD) of $4.8 \pm 1.2$% and a polarization angle (PA) of ${17}^{\circ} \pm {7}^{\circ}$, both quoted at the $1\sigma$ confidence level. Fitting the phase-resolved PA with the rotating vector model (RVM) gives a magnetic obliquity of $\theta = 69_{-29}^{+13}$ deg, indicating a significantly inclined magnetic geometry that may approach a quasi-orthogonal configuration. In addition, using the unbinned photon-by-photon method, we obtain a PD of $5.9 \pm 1.2$% across the pulse phase, together with a pulsar geometry consistent with that inferred from the binned analysis, assuming the variable PA predicted by the RVM. A simultaneous RVM fit across the three energy bands, 2--5 keV, 5--6 keV, and 6--8 keV, provides the strongest constraints on the geometrical parameters, yielding $\theta = {84}_{-6}^{+4}$ deg. Together, these findings demonstrate pronounced phase-dependent spectral and polarization variability, offering valuable constraints on the geometry and emission processes within the accretion region of this transient X-ray pulsar.

astro-ph.HE↗

arXiv2025

MTGR: Industrial-Scale Generative Recommendation Framework in Meituan

Scaling law has been extensively validated in many domains such as natural language processing and computer vision. In the recommendation system, recent work has adopted generative recommendations to achieve scalability, but their generative approaches require abandoning the carefully constructed cross features of traditional recommendation models. We found that this approach significantly degrades model performance, and scaling up cannot compensate for it at all. In this paper, we propose MTGR (Meituan Generative Recommendation) to address this issue. MTGR is modeling based on the HSTU architecture and can retain the original deep learning recommendation model (DLRM) features, including cross features. Additionally, MTGR achieves training and inference acceleration through user-level compression to ensure efficient scaling. We also propose Group-Layer Normalization (GLN) to enhance the performance of encoding within different semantic spaces and the dynamic masking strategy to avoid information leakage. We further optimize the training frameworks, enabling support for our models with 10 to 100 times computational complexity compared to the DLRM, without significant cost increases. MTGR achieved 65x FLOPs for single-sample forward inference compared to the DLRM model, resulting in the largest gain in nearly two years both offline and online. This breakthrough was successfully deployed on Meituan, the world's largest food delivery platform, where it has been handling the main traffic.

cs.IR↗

arXiv2025

Probing the polarized emission from the accretion-powered pulsar 4U 1907+09 with IXPE

We present observations of the accretion-powered X-ray pulsar (XRP) 4U 1907+09, conducted with the Imaging X-ray Polarimetry Explorer (IXPE) that delivers the first high-quality polarization measurements of this source. 4U 1907+09 was observed twice during its brightest periods close to the periastron. We observe a stronger polarization in the first observation, with the phase-averaged polarization degree (PD) of $6.0 \pm 1.6\%$ and polarization angle (PA) of $69^\circ \pm 8^\circ$. In contrast, the second observation provides weaker constraints on the polarimetric properties, with a PD=$2.2 \pm 1.6\%$ and a PA=$46^\circ \pm 23^\circ$, as determined from the spectral-polarimetric analysis. Combining the data from both observations results in a PD=$3.7 \pm 1.1\%$ and a PA=$63^\circ \pm 9^\circ$. We detect an energy-dependent PA in the phase-averaged analyses with a significance of 1.7 $σ$. In the phase-resolved analyses, we observe a potential PA rotation by approximately $90^\circ$ between adjacent energy bands (4--5 and 5--6 keV) within the single phase bin of 0.25--0.375. We also investigate the influence of short flares on the polarization properties of this source. The results suggest that flares do not significantly affect the energy-phase-dependent PA, implying that the pulsar's geometry remains stable during flare events.

astro-ph.HE↗

arXiv2024

Variable ionized disk wind in MAXI J1803-298 revealed by NICER

We present the results from the NICER observation data of MAXI J1803-298 across the entire 2021 outburst. In the intermediate and soft state, we detect significant absorption lines at $\sim 7.0$ keV and $\sim 6.7$ keV, arising from the X-ray disk wind outflowing with a velocity of hundreds of km per second along our line of sight. The fitting results from photoionized model suggest that the wind is driven by thermal pressure and the mass-loss rate is low. We find a clear transition for iron from predominantly H-like to predominantly He-like during the intermediate-to-soft state transition. Our results indicate this transition for iron is caused by the evolution of the illuminating spectrum and the slow change of the geometric properties of the disk wind together. The coexistence of disk wind and QPOs features in intermediate state is also reported. Our study makes MAXI J1803-298 the first source in which a transition from optical wind to X-ray wind is detected, offering new insights into the evolution of disk winds across an entire outburst and long-term coupling of accretion disks and mass outflows around accreting black holes.

astro-ph.HE↗

arXiv2024

The 2018 outburst of MAXI J1820+070 as seen by Insight-HXMT

We present an analysis of the whole 2018 outburst of the black hole X-ray binary MAXI J1820+070 with Insight-HXMT data. We focus our study on the temporal evolution of the parameters of the source. We employ two different models to fit the disk's thermal spectrum: the Newtonian model DISKBB and the relativistic model NKBB. These two models provide different pictures of the source in the soft state. With DISKBB, we find that the inner edge of the disk is close to the innermost stable circular orbit of a fast-rotating black hole and the corona changes geometry from the hard to the soft state. With NKBB, we find that the disk is truncated in the soft state and that the coronal geometry does not change significantly during the whole outburst. However, the model with NKBB can predict an untruncated disk around a fast-rotating black hole if we assume that the disk inclination angle is around $30^\circ$ (instead of $\sim 60^\circ$, which is the inclination angle of the jet and is usually adopted as the disk inclination angle in the literature) and we employ a high-density reflection model. In such a case, we measure a high value of the black hole spin parameter with observations in the soft state, in agreement with the high spin value found from the analysis of the reflection features and in disagreement with the low spin value found by previous continuum-fitting method measurements with the disk inclination angle set to the value of the jet inclination angle.

astro-ph.HE↗

arXiv2024

An IXPE-Led X-ray Spectro-Polarimetric Campaign on the Soft State of Cygnus X-1: X-ray Polarimetric Evidence for Strong Gravitational Lensing

We present the first X-ray spectropolarimetric results for Cygnus X-1 in its soft state from a campaign of five IXPE observations conducted during 2023 May-June. Companion multiwavelength data during the campaign are likewise shown. The 2-8 keV X-rays exhibit a net polarization degree PD=1.99%+/-0.13% (68% confidence). The polarization signal is found to increase with energy across IXPE's 2-8 keV bandpass. The polarized X-rays exhibit an energy-independent polarization angle of PA=-25.7+/-1.8 deg. East of North (68% confidence). This is consistent with being aligned to Cyg X-1's AU-scale compact radio jet and its pc-scale radio lobes. In comparison to earlier hard-state observations, the soft state exhibits a factor of 2 lower polarization degree, but a similar trend with energy and a similar (also energy-independent) position angle. When scaling by the natural unit of the disk temperature, we find the appearance of a consistent trendline in the polarization degree between soft and hard states. Our favored polarimetric model indicates Cyg X-1's spin is likely high (a* above ~0.96). The substantial X-ray polarization in Cyg X-1's soft state is most readily explained as resulting from a large portion of X-rays emitted from the disk returning and reflecting off the disk surface, generating a high polarization degree and a polarization direction parallel to the black hole spin axis and radio jet. In IXPE's bandpass, the polarization signal is dominated by the returning reflection emission. This constitutes polarimetric evidence for strong gravitational lensing of X-rays close to the black hole.

astro-ph.HE↗

arXiv2024

Testing General Relativity with Black Hole X-Ray Data and ABHModels

The past 10 years have seen tremendous progress in our capability of testing General Relativity in the strong field regime with black hole observations. 10 years ago, the theory of General Relativity was almost completely unexplored in the strong field regime. Today, we have gravitational wave data of the coalescence of stellar-mass black holes, radio images of the supermassive black holes SgrA$^*$ and M87$^*$, and high-quality X-ray data of stellar-mass black holes in X-ray binaries and supermassive black holes in active galactic nuclei. In this manuscript, we will review current efforts to test General Relativity with black hole X-ray data and we will provide a detailed description of the public codes available on ABHModels.

astro-ph.IM↗

arXiv2022

Testing regular black holes with X-ray and GW data

The presence of spacetime singularities in physically relevant solutions of the Einstein Equations is normally interpreted as a symptom of the breakdown of classical general relativity at very high densities/curvatures. However, despite significant efforts in the past decades, we do not have yet any robust theoretical framework to solve the problem of spacetime singularities. In this context, the past few years have seen an increasing interest in the study of phenomenological scenarios to describe singularity-free black holes, gravitational collapses, and cosmological models. In the present work, we consider the recent proposal by Mazza, Franzin & Liberati for a rotating regular black hole and we measure their regularization parameter $l$ from the available X-ray and gravitational wave black hole data. For $l = 0$, we recover the singular Kerr solution of general relativity, while for $l \neq 0$ we can have a regular black hole or a regular wormhole. Our analysis shows that the available data are consistent with a vanishing regularization parameter $l$ and we can constrain its value. From a NuSTAR spectrum of the Galactic black hole in EXO 1846-031, we find $l/M < 0.49$ (90% CL). From the gravitational wave event GW190707A, we find $l/M < 0.72$ (90% CL).

gr-qc↗

arXiv2022

Testing the Kerr black hole hypothesis with GRS 1716-249 by combining the continuum-fitting and the iron-line methods

The continuum-fitting and the iron-line methods are currently the two leading techniques for measuring the spins of accreting black holes. In the past few years, these two methods have been developed for testing fundamental physics. In the present work, we employ state-of-the-art models to test black holes through the continuum-fitting and the iron-line methods and we analyze three NuSTAR observations of the black hole binary GRS 1716-249 during its outburst in 2016-2017. In these three observations, the source was in a hard-intermediate state and the spectra show both a strong thermal component and prominent relativistic reflection features. Our analysis confirms the Kerr nature of the black hole in GRS 1716-249 and provides quite stringent constraints on possible deviations from the predictions of general relativity.

astro-ph.HE↗

arXiv2022

Testing the Kerr black hole hypothesis with the continuum-fitting and the iron line methods: the case of GRS 1915+105

The continuum-fitting and the iron line methods are currently the two leading techniques for probing the strong gravity region around accreting black holes. In the present work, we test the Kerr black hole hypothesis with the stellar-mass black hole in GRS 1915+105 by analyzing five disk-dominated RXTE spectra and one reflection-dominated Suzaku spectrum. The combination of the constraints from the continuum-fitting and the iron line methods has the potential to provide more stringent tests of the Kerr metric. Our constraint on the Johannsen deformation parameter $α_{13}$ is $-0.15 < α_{13} < 0.14$ at 3$σ$, where the Kerr metric is recovered when $α_{13} = 0$.

astro-ph.HE↗

arXiv2021

Testing General Relativity with NuSTAR data of Galactic Black Holes

Einstein's theory of General Relativity predicts that the spacetime metric around astrophysical black holes is described by the Kerr solution. In this work, we employ state-of-the-art in relativistic reflection modeling to analyze a selected set of NuSTAR spectra of Galactic black holes to obtain the most robust and precise constraints on the Kerr black hole hypothesis possible today. Our constraints are much more stringent than those from other electromagnetic techniques, and with some sources we get stronger constraints than those currently available from gravitational waves.

astro-ph.HE↗

arXiv2021

Probing the near-horizon region of Cygnus X-1 with Suzaku and NuSTAR

Astrophysical black holes are ideal laboratories for testing Einstein's theory of general relativity in the strong field regime. In this manuscript, we present an analysis of Suzaku and NuSTAR spectra of the black hole binary Cygnus X-1 using RELXILL_NK. Unlike our previous study on Cygnus X-1 with NuSTAR data, here we are able to constrain the Johannsen deformation parameter $α_{13}$. However, despite the high energy resolution near the iron line provided by Suzaku, our constraints on the Kerr metric from Cygnus X-1 are not very stringent in comparison with those that have been obtained from other sources, confirming that Cygnus X-1 is quite a complicated source.

astro-ph.HE↗

arXiv2021

Testing the Kerr Black Hole Hypothesis with GX 339-4 by a combined analysis of its thermal spectrum and reflection features

We analyze simultaneous observations with NuSTAR and Swift of the black hole binary GX 339-4 in which we clearly detect both a strong thermal component and prominent relativistic reflection features. We employ NKBB and RELXILL_NK, which are the state of the art of, respectively, the thermal and reflection models to test the Kerr black hole hypothesis. We obtain unprecedented precise measurements of the Johannsen deformation parameter $α_{13}$: $α_{13} = -0.012_{-0.039}^{+0.011}$ (lamppost coronal geometry) and $α_{13} = -0.010_{-0.018}^{+0.024}$ (broken power-law emissivity profile) at a 90% confidence level, where the Kerr metric corresponds to $α_{13} = 0$. We investigate the systematic uncertainties by fitting the data with different models.

astro-ph.HE↗