arXiv subjects

Zhongxiang Wang

Publications and source records attributed to Zhongxiang Wang.

At least 55 records · Page 3Linked to original sources

arXiv2021

A possible blazar spectral irregularity case caused by photon--axionlike-particle oscillations

We report detection of a line-like feature in the $γ$-ray spectrum of the blazar B0516$-$621, for which the data obtained with the Large Area Telescope onboard {\it Fermi Gamma-Ray Space Telescope (Fermi)} are analyzed. The feature is at $\sim$7\,GeV and different analyses are conducted to check its real presence. We determine that it has a significance of 2.5--3.0$σ$, and cautiously note the presence of possible systematics in the data which could reduce the significance. This putative feature is too narrow to be explained with radiation processes generally considered for jet emission of blazars. Instead, it could be a signal due to the oscillations between photons and axion-like particles (ALPs) in the source's jet. We investigate this possibility by fitting the spectrum with the photon-ALP oscillation model, and find that the parameter space of ALP mass $m_a\leq 10^{-8}$\,eV and the coupling constant (between photons and ALPs) $g_{aγ}$=1.16--1.48$\times 10^{-10}$\,GeV$^{-1}$ can provide a fit to the line-like feature, while the magnetic field at the emission site of $γ$-rays is fixed at 0.7\,G. The ranges for $m_a$ and $g_{aγ}$ are in tension with those previously obtained from several experiments or methods, but on the other hand in line with some of the others. This spectral-feature case and its possible indication for ALP existence could be checked from similar studies of other blazar systems and also suggest a direction of effort for building future high-energy facilities that would have high sensitivities and spectral resolutions for searching for similar features.

astro-ph.HE↗

arXiv2021

A Radio Quasi-periodic Oscillation of 176 day in the Radio-loud Narrow-line Seyfert 1 Galaxy J0849+5108

We analyze the 11-year long-term light curve of the Radio-loud Narrow-line Seyfert 1 (NLSy1) galaxy J0849+5108 and the nearly simultaneous $γ$-ray data of the source. The data were obtained with the Owens Valley Radio Observatory 40-m telescope at 15~GHz and with the Large Area Telescope onboard Fermi Gamma-ray Space Telescope, respectively. A quasi-periodic oscillation (QPO) signal at a significance of $>$5$σ$ is found in the radio light curve, but no similar modulation is seen in the $γ$-ray light curve. The QPO signal was present for 21 cycles, the longest among the reported radio QPOs, and likely disappeared after 2019 January 16. Different mechanisms proposed to explain the observed QPOs in Active Galactic Nuclei (AGN) are discussed for this QPO case. Either a secular instability in the inner accretion disk or a helical structure in the jet of J0849+5108 may explain the radio QPO, while for the latter scenario the jet has to be collimated up to $\sim$200~pc, similar to that seen in the nearby AGN M87. It will be of interests to keep monitoring the source at radio frequencies, seeing if similar QPO signals would appear again or not.

astro-ph.HE↗

arXiv2020

A Compact X-Ray Emitting Binary in Likely Association with 4FGL J0935.3+0901

4FGL J0935.3+0901 is a gamma-ray source detected by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope. We have conducted detailed analysis of the LAT data for this source and multi-wavelength studies of the source field. Its gamma-ray emission can be described with a power law ($Γ=2.0\pm0.2$) with an exponential cutoff ($E_c=2.9\pm1.6$ GeV), while the flux shows significant long-term variations. From analysis of archival Neil Gehrels Swift X-ray Telescope data, we find only one X-ray source in the LAT's 2$σ$ error region. Within a 3.7" radius error circle of the X-ray source, there is only one optical object down to $r'\sim$23 mag. Time-resolved photometry of the optical object indicates a likely 2.5~hr periodic modulation, while its spectrum shows double-peaked hydrogen and helium emission lines (similar to those seen in accretion discs in low-mass X-ray binaries). Combining these results, we conclude that we have discovered a compact X-ray emitting binary in likely association with 4FGL J0935.3+0901, i.e., a millisecond pulsar (MSP) binary. We discuss the implication of the optical spectral features: this binary could be a transitional MSP system at a sub-luminous disc state, although the other possibility, the binary in a rotation-powered state showing the optical emission lines due to intrabinary interaction processes, can not be excluded. Further observational studies will help determine detailed properties of this candidate MSP binary and thus clarify its current state.

astro-ph.HE↗

arXiv2019

Property Studies of "Loner" Flares of Gamma-Ray Blazars

We search through $γ$-ray data obtained with the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope and find 24 blazars (or candidates) that have a single clear flare event in their 9.5 year long-term light curves. We define these events as loner flares since each flare stands out significantly above the relatively stable, low-flux light curve. We analyze the LAT data in detail for these 24 sources. The flares in ten of them are primarily due to a single sharp peak, for which we study by fitting with two different analytic functions. The time durations thus determined for the sharp peaks are in a range of 4-25 days. The $γ$-ray spectra of the 24 blazar sources can be described with a power-law or a log-parabola function. We obtain their spectral properties in the flaring and quiescent states, and find that in the flares 16 of the sources have harder emission and three have softer emission while the other five keep the same emission. We discuss a possible correlation between the differences in photon index in the quiescent and flaring states and photon indices in quiescence. In addition, the sharp peak flares seem to have a tendency of having long time durations and hard emission, possibly related to their physical origin in a blazar jet. Studies of more similar flares will help establish these possible features.

astro-ph.HE↗

arXiv2019

The likely Fermi detection of the Be X-ray binary GRO J1008-57: hint for gamma-ray production in neutron-star X-ray binaries

In our search for gamma-ray emission from Be X-ray binaries from analysis of the data obtained with the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope, we find likely detection of GRO J1008-57. The binary has an orbital period of 249.48 days, and it is only significantly detected in its orbital phase 0.8--0.9 (>4 sigma). Further light curve analysis indicates that the detection is probably largely due to an emitting event in one orbital cycle around year 2012--2013, following a giant X-ray outburst of the source. This property of having occasional gamma-ray emitting events is similar to that seen in another high-mass X-ray binary 4U 1036-56. However, models considering possible gamma-ray emission from an accreting neutron star have difficulty in explaining the observed 10^{34} erg s^{-1} luminosity of the source, unless the distance was largely over-estimated. Further observational studies are required, in order to more clearly establish the high-energy emission properties of GRO J1008-57 or similar high-mass X-ray binaries and find clues for understanding how gamma-ray emission is produced from them.

astro-ph.HE↗

arXiv2019

Fermi LAT detection of the supernova remnant SN 1006 revisited: the south-west limb

The data from the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope have recently been updated. We thus re-analyze the LAT data for the supernova remnant (SNR) SN 1006. Two parts of gamma-ray emission from the region is clearly resolved, which correspond to the north-east (NE) and south-west (SW) limbs of the SNR. The former was detected in the previous LAT data (Xing et al. 2016), but the latter is newly detected in this work. The detection of the two limbs are at a 4 sigma significance level, and the spectral results for the NE limb is consistent with those obtained in previous detection analyses. We construct the broadband spectral energy distribution (SED) for the SW limb. Different scenarios are considered for the SED in gamma-ray energies. We conclude very similar to that of the NE limb, the high-energy and very high-energy emission from the SW limb is likely dominated by the leptonic process, in which high-energy electrons accelerated from the shell region of the SNR inverse-Compton scatter background photons to gamma-rays.

astro-ph.HE↗

arXiv2019

Searching for Gamma-Ray Millisecond Pulsars: Selection of Candidates Revisited

We are starting a project to find $γ$-ray millisecond pulsars (MSPs) among the unidentified sources detected by the Large Area Telescope (LAT) onboard the {Fermi Gamma-Ray Space Telescope (Fermi)}, by radio observations. The selection of good candidates from analysis of the LAT data is an important part of the project. Given that there is more than 10 years worth of LAT data and the advent of the newly released LAT 8-year point source list (FL8Y), we have conducted a selection analysis, on the basis of our previous analysis, and report the results here. Setting the requirements for the unidentified sources in FL8Y of Galactic latitudes $|b|>$ 5$^\circ$ and curvature significances $>$3$σ$, there are 202 sources with detection signficances $>$6$σ$. We select 57 relatively bright ones (detection significances $>$15$σ$) and analyze their 10.2 years of LAT data. Their variability is checked to exclude variable sources (likely~blazars), test statistic maps are constructed to avoid contaminated sources, and curvature significances are re-obtained and compared to their $γ$-ray spectra to exclude non-significant sources. In the end, 48 candidates are found. Based on the available information, mostly from multi-wavelength studies, we discuss the possible nature of several of the candidates. Most of these candidates are currently being observed with the 65-meter Shanghai Tian Ma Radio Telescope.

astro-ph.HE↗

arXiv2019

FERMI observation of the jets of the microquasar SS433

We report our analysis of the data obtained with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi) for the SS 433/W50 region. The total data length is ten years. Different from the previous results reported for this region (for which an old version of database was used), we show that excess emission is detected around the w1 region in the western lobe of the jets from SS 433. The region is bright at X-rays due to the interaction between the jet and the ambient medium. This detection also matches well the recent results of the very-high-energy detection of SS 433/W50 with the High Altitude Water Cherenkov (HAWC). However, the eastern regions that are slightly brighter in HAWC's observation are not detected in the Fermi data. Constructing the broad-band spectral energy distribution (SED) for the western region, we compare with the HAWC results for the eastern regions and discuss the possible origin of the emission. A leptonic scenario can provide a fit to the Fermi GeV spectrum and HAWC TeV detection, where the former and latter are due to the synchrotron radiation and inverse-Compton process respectively. However, the model can not explain the X-ray and radio emission from the region simultaneously, which thus requires further observational and theoretical studies of the region in order to clarify the reasons.

astro-ph.HE↗

arXiv2018

A 34.5 day quasi-periodic oscillation in gamma-ray emission from the blazar PKS 2247-131

Since 2016 October, the active galaxy PKS 2247-131 has undergone a gamma-ray outburst, which we studied using data obtained with the Fermi Gamma-ray Space Telescope. The emission arises from a relativistic jet in PKS 2247-131, as an optical spectrum only shows a few weak absorption lines, typical of the BL Lacertae sub-class of the blazar class of active galactic nuclei. Here we report a ~34.5 day quasi-periodic oscillation (QPO) in the emission after the initial flux peak of the outburst. Compared to one-year time-scale QPOs, previously identified in blazars in Fermi energies, PKS 2247-131 exhibits the first clear case of a relatively short, month-like oscillation. We show that this QPO can be explained in terms of a helical structure in the jet, where the viewing angle to the dominant emission region in the jet undergoes periodic changes. The time scale of the QPO suggests the presence of binary supermassive black holes in PKS 2247-131.

astro-ph.HE↗

arXiv2018

A matching-based heuristic algorithm for school bus routing problems

School bus planning problem (SBPP) has drawn much research attention due to the huge costs of school transportation. In the literature, the SBPP is usually decomposed into the routing and scheduling subproblems due to its complexity. Because of the nature of the decomposition, even if all the subproblems are solved to optimality, the final solution may not be as good as the solution from the integrated model. In this paper, we present a new approach that incorporates the scheduling information (namely the trip compatibility) into the routing stage such that the interrelationship between the subproblems is still considered even in the decomposed problems. A novel two-step heuristic adopting the trip compatibility idea is presented to solve the school bus routing problem. The first step finds an initial solution using an iterative minimum cost matching-based insertion heuristic. Then, the initial trips are improved using a Simulated Annealing and Tabu Search hybrid method. Experiments were conducted on randomly generated problems and benchmark problems in the literature. The result shows that our two-step heuristic improves existing solutions up to 25% on the benchmark problems.

cs.DS↗

arXiv2018

Searching for $γ$-ray Counterparts to Very Faint X-Ray Transient Neutron Star binaries

Very faint X-ray transients (VFXTs) are a group of X-ray binaries with low luminosities, the peak X-ray luminosities during their outbursts being only 10$^{34}$--10$^{36}$ erg\,s$^{-1}$. Using the $γ$-ray data obtained with the Large Area Telescope (LAT) onboard {\it Fermi Gamma-Ray Space Telescope}, we investigate their possible nature of containing rotation-powered pulsars, or more specifically being transitional millisecond pulsars (MSPs). Among more than 40 known VFXTs, we select 12 neutron star systems. We analyze the LAT data for the fields of the 12 VFXTs in 0.2--300 GeV energy range, but do not find any counterparts likely detected by {\it Fermi}. We obtain the luminosity upper limits for the 12 sources. While the distances to the sources are largely uncertain, the upper limits are comparable to the luminosities of two transitional systems PSR J1023$-$0038 and XSS J12270$-$4859. From our study, we conclude that no evidence is found at $γ$-rays for the suggestion that some of VFXTs could contain rotation-powered MSPs (or be transitional MSP systems).

astro-ph.HE↗

arXiv2018

SCDA: School Compatibility Decomposition Algorithm for Solving the Multi-School Bus Routing and Scheduling Problem

Safely serving the school transportation demand with the minimum number of buses is one of the highest financial goals of school transportation directors. To achieve that objective, a good and efficient way to solve the routing and scheduling problem is required. Due to the growth of the computing power, the spotlight has been shed on solving the combined problem of the school bus routing and scheduling problem. We show that an integrated multi-school bus routing and scheduling can be formulated with the help of trip compatibility. A novel decomposition algorithm is proposed to solve the integrated model. The merit of this integrated model and the decomposition method is that with the consideration of the trip compatibility, the interrelationship between the routing and scheduling sub-problems will not be lost in the process of decomposition. Results show the proposed decomposed problem could provide the solutions using the same number of buses as the integrated model in much shorter time (as little as 0.6%) and that the proposed method can save up to 26% number of buses from existing research.

math.OC↗

arXiv2018

Superorbital modulation at GeV energies in the gamma-ray binary LS I +61 303

We report the results from our analysis of 8 years of the data for the gamma-ray binary LS I +61 303, obtained with the Large Area Telescope onboard the Fermi Gamma-Ray Space Telescope. We find a significant dip around the binary's periastron in the superorbital light curves, and by fitting the light curves with a sinusoidal function, clear phase shifts are obtained. The superorbital modulation seen in the binary has been long known and different scenarios have been proposed. Based on our results, we suggest that the circumstellar disk around the Be companion of this binary has an elliptical shape and the major axis of the disk rotates at the superorbital period of 1667 days. As a result, the density of the ambient material around the compact star of the binary changes along the binary orbit over the superorbital period, causing the phase shifts in the modulation, and around periastron, the compact star probably enters the disk, causing the appearance of the dip. Numerical simulations may be conducted in order to study the detailed physical processes and verify our suggestion.

astro-ph.HE↗

arXiv2018

A methodology for calculating the latency of GPS-probe data

Crowdsourced GPS probe data has been gaining popularity in recent years as a source for real-time traffic information. Efforts have been made to evaluate the quality of such data from different perspectives. A quality indicator of any traffic data source is latency that describes the punctuality of data, which is critical for real-time operations, emergency response, and traveler information systems. This paper offers a methodology for measuring the probe data latency, with respect to a selected reference source. Although Bluetooth re-identification data is used as the reference source, the methodology can be applied to any other ground-truth data source of choice (i.e. Automatic License Plate Readers, Electronic Toll Tag). The core of the methodology is a maximum pattern matching algorithm that works with three different fitness objectives. To test the methodology, sample field reference data were collected on multiple freeways segments for a two-week period using portable Bluetooth sensors as ground-truth. Equivalent GPS probe data was obtained from a private vendor, and its latency was evaluated. Latency at different times of the day, the impact of road segmentation scheme on latency, and sensitivity of the latency to both speed slowdown, and recovery from slowdown episodes are also discussed.

stat.AP↗

arXiv2018

A cross-vendor and cross-state analysis of the GPS-probe data latency

Crowdsourced GPS probe data has become a major source of real-time traffic information applications. In addition to traditional traveler advisory systems such as dynamic message signs (DMS) and 511 systems, probe data is being used for automatic incident detection, Integrated Corridor Management (ICM), end of queue warning systems, and mobility-related smartphone applications. Several private sector vendors offer minute by minute network-wide travel time and speed probe data. The quality of such data in terms of deviation of the reported travel time and speeds from ground-truth has been extensively studied in recent years, and as a result concerns over the accuracy of probe data has mostly faded away. However, the latency of probe data, defined as the lag between the time that disturbance in traffic speed is reported in the outsourced data feed, and the time that the traffic is perturbed, has become a subject of interest. The extent of latency of probe data for real-time applications is critical, so it is important to have a good understanding of the amount of latency and its influencing factors. This paper uses high-quality independent Bluetooth/Wi-Fi re-identification data collected on multiple freeway segments in three different states, to measure the latency of the vehicle probe data provided by three major vendors. The statistical distribution of the latency and its sensitivity to speed slowdown and recovery periods are discussed.

stat.AP↗

arXiv2017

School bus routing by maximizing trip compatibility

School bus planning is usually divided into routing and scheduling due to the complexity of solving them concurrently. However, the separation between these two steps may lead to worse solutions with higher overall costs than that from solving them together. When finding the minimal number of trips in the routing problem, neglecting the importance of trip compatibility may increase the number of buses actually needed in the scheduling problem. This paper proposes a new formulation for the multi-school homogeneous fleet routing problem that maximizes trip compatibility while minimizing total travel time. This incorporates the trip compatibility for the scheduling problem in the routing problem. Since the problem is inherently just a routing problem, finding a good solution is not cumbersome. To compare the performance of the model with traditional routing problems, we generate eight mid-size data sets. Through importing the generated trips of the routing problems into the bus scheduling (blocking) problem, it is shown that the proposed model uses up to 13% fewer buses than the common traditional routing models.

math.OC↗

arXiv2017

High energy properties of the flat spectrum radio quasar 4C 50.11

We investigate the $γ$-ray and X-ray properties of the flat spectrum radio quasar (FSRQ) \4c50 at redshift $z= 1.517$. The {\it Fermi} Large Area Telescope (LAT) data indicate that this source was in an active state since 2013 July. During the active period, the $γ$-ray flux increased by more than a factor of three and two distinct flares were detected with the variability timescale as short as several hours. The $γ$-ray spectra can be well fitted by a log-parabola. From the fitting, we find a correlation between the peak energy and spectral curvature for the $γ$-ray spectra, which is the first time seen in $γ$-ray emission from a blazar. The {\it Swift} X-ray Telescope (XRT) data show that the source was variable at X-ray energies, but no evidence shows flux or spectral changes related to the $γ$-ray activity. The broad-band X-ray spectrum obtained with {\it Swift} XRT and {\it NuSTAR} is well described by a broken PL model, with an extremely hard spectrum ($Γ_{1} \sim 0.1$) below the break energy, $E_{\rm break} \sim 2.1 {\rm keV}$, and $Γ_{2} \sim 1.5$ above the break energy. The spectral steepening below $\sim 3$ keV is likely due to the low energy cut-off in the energy distribution of the photon-emitting electron population. Both the $γ$-ray and X-ray emission appear harder when brighter. The broad-band spectral energy distribution (SED) is constructed for the source, and we provide a model fit to the SED. Our modeling suggests that the emission region should be outside the broad line region, and the properties of the region indicate a jet with 42\% of the Eddington power during the active state.

astro-ph.HE↗

arXiv2017

Long-term Spectral Variability of the Ultra-luminous X-ray source Holmberg IX X--1

We investigate the long-term spectral variability in the ultra-luminous X-ray source Holmberg IX X--1. By analyzing the data from eight {\it Suzaku} and 13 {\it XMM-Newton} observations conducted between 2001 and 2015, we perform a detailed spectral modeling for all spectra with simple models and complex physical models. We find that the spectra can be well explained by a disc plus thermal Comptonization model. Applying this model, we unveil correlations between the X-ray luminosity ($L_{\rm X}$) and the spectral parameters. Among the correlations, a particular one is the statistically significant positive correlation between $L_{\rm X}$ and the photon index ($Γ$), while at the high luminosities of $> 2\times10^{40}\,{\rm~erg\ s}^{-1}$, the source becomes marginally hard and that results a change in the slope of the $Γ- L_{\rm X}$ correlation. Similar variability behavior is observed in the optical depth of the source around $L_{\rm X} \sim 2\times10^{40}\,{\rm~erg\ s}^{-1}$ as the source becomes more optically thick. We consider the scenario that a corona covers the inner part of the disc, and the correlations can be explained as to be driven by the variability of seed photons from the disc input into the corona. On the basis of the disc-corona model, we discuss the physical processes that are possibly indicated by the variability of the spectral parameters. Our analysis reveals the complex variability behavior of Holmberg IX X--1 and the variability mechanism is likely related to the geometry of the X-ray emitting regions.

astro-ph.HE↗