arXiv subjects

B. Zhang

Publications and source records attributed to B. Zhang.

At least 73 records · Page 4Linked to original sources

arXiv2022

A hyper flare of a weeks-old magnetar born from a binary-neutron-star merger

Magnetars, a population of isolated neutron stars with ultra-strong magnetic fields of $\sim 10^{14}-10^{15}$ G, have been increasingly accepted to explain a variety of astrophysical transients. A nascent millisecond-period magnetar can release its spin-down energy and power bright sources such as Gamma-ray Bursts (GRBs) and their subsequent X-ray plateaus, Super Luminous Supernovae (SLSNe), and the fast X-ray transients such as CDF-S XT-2. Magnetars with ages of $10^3-10^4$ years have been observed within the Milky Way Galaxy, which are found to power diverse transients with the expense of their magnetic energy, in the form of giant flares and repeated soft-$γ$-ray or hard X-ray bursts and occasionally fast radio bursts (FRBs). Magnetar giant flares were also detected as disguised short GRBs from nearby galaxies . Here we report the identification of a GRB as a hyper flare of magnetar in a nearby galaxy. The magnitude of the hyper flare is about one thousand times brighter than that of a typical magnetar giant flare. A significant $\sim 80$ millisecond period is detected in the decaying light curve. Interpreting this period as the rotation period and given a magnetic field strength typical for a young magnetar, the age of the magnetar is constrained to be only a few weeks. The non-detection of a (superluminous) supernova nor a GRB weeks before the event further constrains that the magnetar is likely born from an off-axis merger event of two neutron stars. Our finding bridges the gap between the hypothetical millisecond magnetars and the observed Galactic magnetars, and points toward a broader channel of magnetar-powered gamma-ray transients.

astro-ph.HE↗

arXiv2022

A repeating fast radio burst associated with a persistent radio source

The dispersive sweep of fast radio bursts (FRBs) has been used to probe the ionized baryon content of the intergalactic medium, which is assumed to dominate the total extragalactic dispersion. While the host galaxy contributions to dispersion measure (DM) appear to be small for most FRBs, in at least one case there is evidence for an extreme magneto-ionic local environment and a compact persistent radio source. Here we report the detection and localization of the repeating FRB 20190520B, which is co-located with a compact, persistent radio source and associated with a dwarf host galaxy of high specific star formation rate at a redshift $z=0.241\pm0.001$. The estimated host galaxy DM $\approx 903^{+72}_{-111}$ pc cm$^{-3}$, nearly an order of magnitude higher than the average of FRB host galaxies, far exceeds the DM contribution of the intergalactic medium. Caution is thus warranted in inferring redshifts for FRBs without accurate host galaxy identifications.

astro-ph.HE↗

arXiv2022

Characterization of Pedestal Burst Instabilities during I-mode to H-mode Transition in the EAST Tokamak

Quasi-periodic Pedestal Burst Instabilities (PBIs), featuring alternative turbulence suppression and bursts, have been clearly identified by various edge diagnostics during I-mode to H-mode transition in the EAST Tokamak. The radial distribution of the phase perturbation caused by PBI shows that PBI is localized in the pedestal. Prior to each PBI, a significant increase of density gradient close to the pedestal top can be clearly distinguished, then the turbulence burst is generated, accompanied by the relaxation of the density profile, and then induces an outward particle flux. The relative density perturbation caused by PBIs is about $6 \sim 8\%$. Statistic analyses show that the pedestal normalized density gradient triggering the first PBI has a threshold value, mostly in the range of $22 \sim 24$, suggesting that a PBI triggering instability could be driven by the density gradient. And the pedestal normalized density gradient triggering the last PBI is about $30 \sim 40$ and seems to increase with the loss power and the chord-averaged density. In addition, the frequency of PBI is likely to be inversely proportional to the chord-averaged density and the loss power. These results suggest that PBIs and the density gradient prompt increase prior to PBIs can be considered as the precursor for controlling I-H transition.

physics.plasm-ph↗

arXiv2021

Search for the metal-weak thick disk from the LAMOST DR5

Based on the data release of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope survey (LAMOST DR5) and the \emph{Gaia} Early Data Release 3 (\emph{Gaia} EDR3), we construct a sample containing 46,109 giant (log~$\emph{g}$ $\leqslant$ 3.5\,dex) stars with heliocentric distance d $\leqslant$ 4 kpc, and the sample is further divided into two groups of the inner ({\RGC} $<$ 8.34 \,kpc) and outer region ({\RGC} $>$ 8.34\,kpc). The {\LZ} distributions of our program stars in the panels with different [Fe/H] and [$α$/Fe] suggest that the thick-disk consists of two distinct components with different chemical compositions and kinematic properties. For the inner region, the metal-weak thick disk (MWTD) contributes significantly when [$α$/Fe] $>+$0.2\,dex and [Fe/H] $<-$0.8\,dex, while the canonical thick-disk (TD) dominates when [Fe/H]$>-$0.8\,dex. However, MWTD clearly appears only when [$α$/Fe] $>+$0.2\,dex and [Fe/H] $<-$1.2\,dex for the outer region, and its proportion is lower than that of the inner region within the same metallicity. Similar result can be obtained from the {\VP} distribution. The high fraction of MWTD in the inner than that in the outer region imply that MWTD may form in the inner disk, and is an observational evidence about the inside-out disk formation scenario.

astro-ph.GA↗

arXiv2021

The Fast Radio Burst FRB 20201124A in a star forming region: constraints to the progenitor and multiwavelength counterparts

We present the results of a multiwavelength campaign of FRB 20201124A, the third closest repeating fast radio burst recently localized in a nearby (z=0.0978) galaxy. Deep VLA observations led to the detection of quiescent radio emission, also marginally visible in X-rays with Chandra. Imaging at 22 GHz allowed us to resolve the source on a scale of $\gtrsim$ 1 arcsec and locate it at the position of the FRB, within an error of 0.2 arcsec. EVN and e-MERLIN observations sampled small angular scales, from 2 to 100 mas, providing tight upper limits on the presence of a compact source and evidence for diffuse radio emission. We argue that this emission is associated with enhanced star formation activity in the proximity of the FRB, corresponding to a star formation rate of $\approx 10\ {\rm M}_\odot {\rm yr}^{-1}$. The surface star formation rate at the location of FRB 20201124A is two orders of magnitude larger than typically observed in other precisely localized FRBs. Such a high SFR is indicative of this FRB source being a new-born magnetar produced from a SN explosion of a massive star progenitor. Upper limits to the X-ray counterparts of 49 radio bursts observed in our simultaneous FAST, SRT and Chandra campaign are consistent with a magnetar scenario.

astro-ph.HE↗

arXiv2021

A bimodal burst energy distribution of a repeating fast radio burst source

The event rate, energy distribution, and time-domain behaviour of repeating fast radio bursts (FRBs) contains essential information regarding their physical nature and central engine, which are as yet unknown. As the first precisely-localized source, FRB 121102 has been extensively observed and shows non-Poisson clustering of bursts over time and a power-law energy distribution. However, the extent of the energy distribution towards the fainter end was not known. Here we report the detection of 1652 independent bursts with a peak burst rate of 122~hr^{-1}, in 59.5 hours spanning 47 days. A peak in the isotropic equivalent energy distribution is found to be ~4.8 x 10^{37} erg at 1.25~GHz, below which the detection of bursts is suppressed. The burst energy distribution is bimodal, and well characterized by a combination of a log-normal function and a generalized Cauchy function. The large number of bursts in hour-long spans allow sensitive periodicity searches between 1 ms and 1000 s. The non-detection of any periodicity or quasi-periodicity poses challenges for models involving a single rotating compact object. The high burst rate also implies that FRBs must be generated with a high radiative efficiency, disfavoring emission mechanisms with large energy requirements or contrived triggering conditions.

astro-ph.HE↗

arXiv2021

Energy and waiting time distributions of FRB 121102 observed by FAST

The energy and waiting time distributions are important properties for understanding the physical mechanism of repeating fast radio bursts (FRBs). Recently, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) detected the largest burst sample of FRB 121102, containing 1652 bursts. We use this sample to investigate the energy and waiting time distributions. The energy count distribution $dN/dE$ at the high-energy range ($>10^{38}$ erg) can be fitted with a single power-law function with an index of $-1.86 ^{+0.02}_{-0.02}$, while the distribution at the low-energy range deviates from the power-law function. An interesting result of Li et al. (2021) is that there is an apparent temporal gap between early bursts (occurring before MJD 58740) and late bursts (occurring after MJD 58740). We find the energy distributions of high-energy bursts at different epochs are inconsistent. The power-law index is $-1.70^{+0.03}_{-0.03}$ for early bursts and $-2.60^{+0.15}_{-0.14}$ for late bursts. For bursts observed in a single day, a linear repetition pattern is found. We use the Weibull function to fit the distribution of waiting time of consecutive bursts. The shape parameter $k = 0.72^{+0.01}_{-0.01}$ and the event rate $r = 736.43^{+26.55}_{-28.90}$ day$ ^{-1} $ are derived. If the waiting times with $δ_t < 28$ s are excluded, the burst behavior can be described by a Poisson process. The best-fitting values of $k$ are slightly different for low-energy ($E < 1.58\times10^{38}$ erg) and high-energy ($E > 1.58\times10^{38}$ erg) bursts.

astro-ph.HE↗

arXiv2021

A Peculiarly Short-duration Gamma-Ray Burst from Massive Star Core Collapse

Gamma-ray bursts (GRBs) have been phenomenologically classified into long and short populations based on the observed bimodal distribution of duration. Multi-wavelength and multi-messenger observations in recent years have revealed that in general long GRBs originate from massive star core collapse events, whereas short GRBs originate from binary neutron star mergers. It has been known that the duration criterion is sometimes unreliable, and multi-wavelength criteria are needed to identify the physical origin of a particular GRB. Some apparently long GRBs have been suggested to have a neutron star merger origin, whereas some apparently short GRBs have been attributed to genuinely long GRBs whose short, bright emission is slightly above the detector's sensitivity threshold. Here we report the comprehensive analysis of the multi-wavelength data of a bright short GRB 200826A. Characterized by a sharp pulse, this burst shows a duration of 1 second and no evidence of an underlying longer-duration event. Its other observational properties such as its spectral behaviors, total energy, and host galaxy offset, are, however, inconsistent with those of other short GRBs believed to originate from binary neutron star mergers. Rather, these properties resemble those of long GRBs. This burst confirms the existence of short duration GRBs with stellar core-collapse origin, and presents some challenges to the existing models.

astro-ph.HE↗

arXiv2021

Accurate flux calibration of GW170817: is the X-ray counterpart on the rise?

X-ray emission from the gravitational wave transient GW170817 is well described as non-thermal afterglow radiation produced by a structured relativistic jet viewed off-axis. We show that the X-ray counterpart continues to be detected at 3.3 years after the merger. Such long-lasting signal is not a prediction of the earlier jet models characterized by a narrow jet core and a viewing angle of about 20 deg, and is spurring a renewed interest in the origin of the X-ray emission. We present a comprehensive analysis of the X-ray dataset aimed at clarifying existing discrepancies in the literature, and in particular the presence of an X-ray rebrightening at late times. Our analysis does not find evidence for an increase in the X-ray flux, but confirms a growing tension between the observations and the jet model. Further observations at radio and X-ray wavelengths would be critical to break the degeneracy between models.

astro-ph.HE↗

arXiv2021

Time Domain Astronomy with the THESEUS Satellite

THESEUS is a medium size space mission of the European Space Agency, currently under evaluation for a possible launch in 2032. Its main objectives are to investigate the early Universe through the observation of gamma-ray bursts and to study the gravitational waves electromagnetic counterparts and neutrino events. On the other hand, its instruments, which include a wide field of view X-ray (0.3-5 keV) telescope based on lobster-eye focusing optics and a gamma-ray spectrometer with imaging capabilities in the 2-150 keV range, are also ideal for carrying out unprecedented studies in time domain astrophysics. In addition, the presence onboard of a 70 cm near infrared telescope will allow simultaneous multi-wavelegth studies. Here we present the THESEUS capabilities for studying the time variability of different classes of sources in parallel to, and without affecting, the gamma-ray bursts hunt.

astro-ph.IM↗

arXiv2021

HXMT Identification of a non-thermal X-ray burst from SGR J1935+2154 and with FRB 200428

Fast radio bursts (FRBs) are short pulses observed in radio band from cosmological distances. One class of models invoke soft gamma-ray repeaters (SGRs), or magnetars, as the sources of FRBs. Some radio pulses have been observed from some magnetars, however, no FRB-like events had been detected in association any magnetar burst, including one giant flare. Recently, a pair of FRB-like bursts (FRB 200428 hereafter) separated by milliseconds (ms) were detected from the general direction of the Galactic magnetar SGR J1935+2154. Here we report the detection of a non-thermal X-ray burst in the 1-250 keV energy band with the Insight-HXMT satellite, which we identify as emitted from SGR J1935+2154. The burst showed two hard peaks with a separation of 34 ms, broadly consistent with that of the two bursts in FRB 200428. The delay time between the double radio and X-ray peaks is about 8.57 s, fully consistent with the dispersion delay of FRB 200428. We thus identify the non-thermal X-ray burst is associated with FRB 200428 whose high energy counterpart is the two hard peaks in X-ray. Our results suggest that the non-thermal X-ray burst and FRB 200428 share the same physical origin in an explosive event from SGR J1935+2154.

astro-ph.HE↗

arXiv2021

Influence of Micro-turbulence on Neoclassical Tearing Mode Onset

Direct evidence of micro-turbulence effect on the onset of neoclassical tearing mode (NTM) is reported for the first time in this letter. A puzzling positive correlation between critical width of seed island of NTM and normalized plasma pressure beta_p is first observed employing a novel method for clearly separating the processes of seed island and the onset of NTM in the EAST tokamak. Different from the methods developed before, the width of the seed island is well controlled by slowly ramping up the current in resonant magnetic perturbation coils. It is revealed that the positive correlation is mainly attributed to the enhancement of perpendicular transport by micro-turbulence, which overcomes the destabilizing effect of beta_p on the onset of NTM. Reduced magnetohydrodynamics (MHD) modeling well reproduced the two states of nonlinear bifurcations observed in this experiment by including the finite transport effect. This result provides a new route for understanding multi-scale interaction in plasma physics.

physics.plasm-ph↗

arXiv2021

DES Y1 results: Splitting growth and geometry to test $Λ$CDM

We analyze Dark Energy Survey (DES) data to constrain a cosmological model where a subset of parameters -- focusing on $Ω_m$ -- are split into versions associated with structure growth (e.g. $Ω_m^{\rm grow}$) and expansion history (e.g. $Ω_m^{\rm geo}$). Once the parameters have been specified for the $Λ$CDM cosmological model, which includes general relativity as a theory of gravity, it uniquely predicts the evolution of both geometry (distances) and the growth of structure over cosmic time. Any inconsistency between measurements of geometry and growth could therefore indicate a breakdown of that model. Our growth-geometry split approach therefore serves as both a (largely) model-independent test for beyond-$Λ$CDM physics, and as a means to characterize how DES observables provide cosmological information. We analyze the same multi-probe DES data as arXiv:1811.02375 : DES Year 1 (Y1) galaxy clustering and weak lensing, which are sensitive to both growth and geometry, as well as Y1 BAO and Y3 supernovae, which probe geometry. We additionally include external geometric information from BOSS DR12 BAO and a compressed Planck 2015 likelihood, and external growth information from BOSS DR12 RSD. We find no significant disagreement with $Ω_m^{\rm grow}=Ω_m^{\rm geo}$. When DES and external data are analyzed separately, degeneracies with neutrino mass and intrinsic alignments limit our ability to measure $Ω_m^{\rm grow}$, but combining DES with external data allows us to constrain both growth and geometric quantities. We also consider a parameterization where we split both $Ω_m$ and $w$, but find that even our most constraining data combination is unable to separately constrain $Ω_m^{\rm grow}$ and $w^{\rm grow}$. Relative to $Λ$CDM, splitting growth and geometry weakens bounds on $σ_8$ but does not alter constraints on $h$.

astro-ph.CO↗

arXiv2021

Trigonometric Parallaxes of Four Star-forming Regions in the Distant Inner Galaxy

We have measured trigonometric parallaxes for four water masers associated with distant massive young stars in the inner regions of the Galaxy using the VLBA as part of the BeSSeL Survey. G026.50$+$0.28. is located at the near end of the Galactic bar, perhaps at the origin of the Norma spiral arm. G020.77$-$0.05 is in the Galactic Center region and is likely associated with a far-side extension of the Scutum arm. G019.60$-$0.23 and G020.08$-$0.13 are likely associated and lie well past the Galactic Center. These sources appear to be in the Sagittarius spiral arm, but an association with the Perseus arm cannot be ruled out.

astro-ph.GA↗

arXiv2020

Swift Multiwavelength Follow-up of LVC S200224ca and the Implications for Binary Black Hole Mergers

On 2020 February 24, during their third observing run ("O3"), the Laser Interferometer Gravitational-wave Observatory and Virgo Collaboration (LVC) detected S200224ca: a candidate gravitational wave (GW) event produced by a binary black hole (BBH) merger. This event was one of the best-localized compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72 deg$^2$), and so the Neil Gehrels Swift Observatory performed rapid near-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately 79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBH event most thoroughly followed-up in near-UV (u-band) and X-ray to date. No likely EM counterparts to the GW event were found by the Swift BAT, XRT, or UVOT, nor by other observatories. Here we report on the results of our searches for an EM counterpart, both in the BAT data near the time of the merger, and in follow-up UVOT/XRT observations. We also discuss the upper limits we can place on EM radiation from S200224ca, and the implications these limits have on the physics of BBH mergers. Namely, we place a shallow upper limit on the dimensionless BH charge, $\hat{q} < 1.4 \times10^{-4}$, and an upper limit on the isotropic-equivalent energy of a blast wave $E < 4.1\times10^{51}$ erg (assuming typical GRB parameters).

astro-ph.HE↗

arXiv2020

The Optical Luminosity-Time Correlation for More Than 100 Gamma-Ray Burst Afterglows

Gamma-Ray Bursts (GRBs) are fascinating events due to their panchromatic nature. Their afterglow emission is observed from sub-TeV energies to radio wavelengths. We investigate GRBs that present an optical plateau, leveraging on the resemblance with the X-ray plateau shown in many GRB light curves (LCs). We comprehensively analyze all published GRBs with known redshifts and optical plateau observed mostly by the Neil Gehrels Swift Observatory (Swift). We fit 267 optical LCs and show the existence of the plateau in 102 cases, which is the largest compilation so far of optical plateaus. For 56 Swift GRBs with optical and X-ray plateaus, we compare the rest-frame end time at both wavelengths (T*_opt , T*_X), and conclude that the plateau is achromatic between T*_opt and T*_X. We also confirm the existence of the two-dimensional relations between T*_opt and the optical luminosity at the end of the plateau emission, which resembles the same luminosity-time correlation in X-rays (Dainotti et al. 2013). The existence of this optical correlation has been demonstrated for the largest sample of optical plateaus in the literature to date. The squared scatter in this optical correlation is smallest for the subset of the Gold GRBs with a decrease in the scatter equivalent to 52.4% when compared to the scatter of the entire GRB sample.

astro-ph.HE↗

arXiv2020

FRB131104 Swift BAT data revisited: No evidence of a gamma-ray counterpart

We present a re-analysis of the Swift Burst Alert Telescope (BAT) data around the radio detection of FRB 131104. Possible evidence of a gamma-ray counterpart was presented by DeLaunay et al. However, based on our analysis using all the available BAT data, no significant emission is found in either the temporal or the image domain. We place a 5~sigma fluence upper limit of 3.3 $\times$ 10$^{-6}$ erg cm$^{-2}$ and 2.7 $\times$ 10$^{-6}$ erg cm$^{-2}$ (15-150 keV) with an integration time of 300 s assuming a simple power-law spectrum with photon index of -1.2 and -2.0, respectively. Our result does not support the association of this FRB with a high-energy counterpart, in agreement with growing observational evidence that most FRBs are not associated to catastrophic events such as gamma-ray bursts.

astro-ph.HE↗

arXiv2020

No pulsed radio emission during a bursting phase of a Galactic magnetar

Fast radio bursts (FRBs) are mysterious millisecond-duration radio transients of unknown origin observed at extragalactic distances. It has been long speculated that magnetars are the engine powering repeating bursts from FRB sources, but no convincing evidence has been collected so far\cite{sun19}. Recently, the Galactic magnetar SGR J1935+2154 entered an active phase by emitting intense soft Gamma-ray bursts. One FRB-like event with two peaks (FRB 200428) and a luminosity slightly lower than the faintest extragalactic FRBs was detected from the source, in association with a soft Gamma-ray / hard X-ray flare. Here we report an eight-hour targeted radio observational campaign comprising four sessions and assisted by multi-wavelength (optical and hard X-rays) data. During the third session, 29 soft Gamma-ray repeater (SGR) bursts were detected in Gamma-ray energies. Throughout the observing period, we detected no single dispersed pulsed emission coincident with the arrivals of SGR bursts, but unfortunately we were not observing when the FRB was detected. The non-detection places a fluence upper limit that is eight orders of magnitude lower than the fluence of FRB 200428. Our results suggest that FRB -- SGR burst associations are rare. FRBs may be highly relativistic and geometrically beamed, or FRB-like events associated with SGR bursts may have narrow spectra and characteristic frequencies outside the observed band. It is also possible that the physical conditions required to achieve coherent radiation in SGR bursts are difficult to satisfy, and that only under extreme conditions could an FRB be associated with an SGR burst.

astro-ph.HE↗