arXiv subjects

Y. C. Zou

Publications and source records attributed to Y. C. Zou.

16 recordsLinked to original sources

arXiv2024

RRAT J1913+1330: an extremely variable and puzzling pulsar

Rotating Radio Transients (RRATs) are neutron stars that emit sporadic radio bursts. We detected 1955 single pulses from RRAT J1913+1330 using the 19-beam receiver of the Five-hundred-meter Aperture Spherical Radio Telescope (FAST). These pulses were detected in 19 distinct clusters, with 49.4% of them occurring with a waiting time of one rotation period. The energy distribution of these individual pulses exhibited a wide range, spanning three orders of magnitude, reminiscent of repeating fast radio bursts (FRBs). Furthermore, we observed abrupt variations in pulse profile, width, peak flux, and fluence between adjacent sequential pulses. These findings suggest that this RRAT could be interpreted as a pulsar with extreme pulse-to-pulse modulation. The presence of sequential pulse trains during active phases, along with significant pulse variations in profile, fluence, flux, and width, should be intrinsic to a subset of RRATs. Our results indicate that J1913+1330 represents a peculiar source that shares certain properties with populations of nulling pulsars, giant pulses, and FRBs from different perspectives. The dramatic pulse-to-pulse variation observed in J1913+1330 could be attributed to unstable pair creation above the polar cap region and the variation of the site where streaming pairs emit coherently. Exploring a larger sample of RRATs exhibiting similar properties to J1913+1330 has the potential to significantly advance our understanding of pulsars, RRATs, and FRBs.

astro-ph.HE↗

arXiv2022

Quasi-periodic Eruptions from Helium Envelope of Hydrogen-deficient Stars Stripped by Supermassive Black Holes

Quasi-periodic eruptions (QPEs), which are a new kind of X-ray bursts with a recurrence time of several hours, have been detected from supermassive black holes (SMBHs) in galactic nuclei. Recently, the two QPEs discovered by the \textit{eROSITA} show asymmetric light curves with a fast rise and a slow decline. Current models cannot explain the observational characteristics of QPEs. Here we show that QPEs can be generated from the Roche lobe overflows at each periapsis passage of an evolved star orbiting an SMBH. The properties of the companion stars are constrained via analytic estimations. We find that hydrogen-deficient post-AGB stars are promising candidates. Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code is used to construct the hydrogen-deficient stars which can fulfill the requirements, as obtained through analytical estimates, to produce the properties of QPEs, including the fast-rise and slow-decay light curves, periods, energetics, and rates. Furthermore, the extreme mass ratio $\sim 10^5$ between the SMBH and the donor will lead to a phenomenon called extreme mass-ratio inspiral (EMRI), producing millihertz gravitational waves. These QPEs would be detected as EMRI sources with electromagnetic counterparts for space-based GW detectors, such as Laser Interferometer Space Antenna (LISA) and Tianqin. They would provide a new way to measure the Hubble constant and further test the Hubble constant tension.

astro-ph.HE↗

arXiv2020

Reply to "Rapid $^{14}$C excursion at 3372-3371 BCE not observed at two different locations"

The nuclide $^{14}$C can be produced in the atmosphere by high energy particles and $γ$-rays from high-energy phenomena. Through the carbon cycle, some of $^{14}$CO$_2$ produced in the atmosphere can be retained in annual tree rings. Four events of rapid increase of the $^{14}$C content occurred in AD 775, AD 994, BC 660 and BC 3371 were found. Recently, the data of Jull et al. (2020) was inconsistent with our records around BC 3371. We measured our sample again and found the $^{14}$C records are consistent with the value in Wang et al. (2017). Therefore, our $^{14}$C records are robust. The inconsistency may be caused by the difference of calendar ages for the wood samples, or the physical origin of the event. First, crossdating on ring width can be performed only between trees whose growth has the same environmental conditions. Because the master tree-ring for dendrochronology is lack for Chinese trees. The master tree-ring from California has to be used. Therefore, the calendar ages derived from dendrochronology may be not precise. Second, the $^{14}$C even may be not global. One evidence is the variation of $^{14}$C content around AD 1006. The $^{14}$C contents of Californian trees increase 12\textperthousand~ in two years, while Japanese trees show no $^{14}$C increase.

astro-ph.SR↗

arXiv2019

A bright electromagnetic counterpart to extreme mass ratio inspirals

The extreme mass ratio inspiral (EMRI), defined as a stellar-mass compact object inspiraling into a supermassive black hole (SMBH), has been widely argued to be a low-frequency gravitational wave (GW) source. EMRIs providing accurate measurements of black hole mass and spin, are one of the primary interests for Laser Interferometer Space Antenna (LISA). However, it is usually believed that there are no electromagnetic (EM) counterparts to EMRIs. Here we show a new formation channel of EMRIs with tidal disruption flares as EM counterparts. In this scenario, flares can be produced from the tidal stripping of the helium (He) envelope of a massive star by an SMBH. The left compact core of the massive star will evolve into an EMRI. We find that, under certain initial eccentricity and semimajor axis, the GW frequency of the inspiral can enter LISA band within 10 $\sim$ 20 years, which makes the tidal disruption flare an EM precursor to EMRI. Although the event rate is just $2\times 10^{-4}~\rm Gpc^{-3}yr^{-1}$, this association can not only improve the localization accuracy of LISA and help to find the host galaxy of EMRI, but also serve as a new GW standard siren for cosmology.

astro-ph.HE↗

arXiv2018

Precise peculiar velocities from gravitational waves accompanied by electromagnetic signals and cosmological applications

Peculiar velocities are a precious tool to study the large-scale distribution of matter in the local universe and test cosmological models. However, present measurements of peculiar velocities are based on empirical distance indicators, which introduce large error bars. Here we present a new method to measure the peculiar velocities, by directly estimating luminosity distances through waveform signals from inspiralling compact binaries and measuring redshifts from electromagnetic (EM) counterparts. In the future, with the distance uncertainty of GW events reducing to $0.1$ per cent by future GW detectors, the uncertainty of the peculiar velocity can be reduced to $10$ km/s at 100 mega parsecs. We find that dozens of GW events with EM counterparts can provide a Hubble constant $H_0$ uncertainty of $0.5\%$ and the growth rate of structure with a $0.6\%$ precision in the third-generation ground-base GW detectors, which can reconcile the $H_0$ tension and determine the origins for cosmic accelerated expansion.

astro-ph.CO↗

arXiv2018

What happens if measure the electron spin twice?

The mainstream textbooks of quantum mechanics explains the quantum state collapses into an eigenstate in the measurement, while other explanations such as hidden variables and multi-universe deny the collapsing. Here we propose an ideal thinking experiment on measuring the spin of an electron with 3 steps. It is simple and straightforward, in short, to measure a spin-up electron in x-axis, and then in z-axis. Whether there is a collapsing predicts different results of the experiment. The future realistic experiment will show the quantum state collapses or not in the measurement.

physics.gen-ph↗

arXiv2017

A rapid cosmic-ray increase in BC 3372-3371 from ancient buried tree rings in China

Cosmic rays interact with the Earth's atmosphere to produce $^{14}$C, which can be absorbed by trees. Therefore, rapid increases of $^{14}$C in tree rings can be used to probe previous cosmic-ray events. By this method, three $^{14}$C rapidly increasing events have been found. Plausible causes of these events include large solar proton events, supernovae or short gamma-ray bursts. However, due to the lack of measurements of $^{14}$C by year, the occurrence frequency of such $^{14}$C rapidly increasing events is poorly known. In addition, rapid increases may be hidden in the IntCal13 data with five-year resolution. Here we report the result of $^{14}$C measurements using an ancient buried tree during the period between BC 3388 and 3358. We find a rapid increase of about 9\textperthousand~ in the $^{14}$C content from BC 3372 to BC 3371. We suggest that this event could originate from a large solar proton event.

astro-ph.HE↗

arXiv2013

Discovery of the broad-lined Type Ic SN 2013cq associated with the very energetic GRB 130427A

Long-duration gamma-ray bursts (GRBs) at z < 1 are in most cases found to be accompanied by bright, broad-lined Type Ic supernovae (SNe Ic-BL). The highest-energy GRBs are mostly located at higher redshifts, where the associated SNe are hard to detect observationally. Here we present early and late observations of the optical counterpart of the very energetic GRB 130427A. Despite its moderate redshift z = 0.3399+/-0.0002, GRB 130427A is at the high end of the GRB energy distribution, with an isotropic-equivalent energy release of Eiso ~ 9.6x10^53 erg, more than an order of magnitude more energetic than other GRBs with spectroscopically confirmed SNe. In our dense photometric monitoring, we detect excess flux in the host-subtracted r-band light curve, consistent with what expected from an emerging SN, ~0.2 mag fainter than the prototypical SN 1998bw. A spectrum obtained around the time of the SN peak (16.7 days after the GRB) reveals broad undulations typical of SNe Ic-BL, confirming the presence of a SN, designated SN 2013cq. The spectral shape and early peak time are similar to those of the high expansion velocity SN 2010bh associated with GRB 100316D. Our findings demonstrate that high-energy long-duration GRBs, commonly detected at high redshift, can also be associated with SNe Ic-BL, pointing to a common progenitor mechanism.

astro-ph.HE↗

arXiv2013

Long-term X-ray emission from Swift J1644+57

The X-ray emission from Swift J1644+57 is not steadily decreasing instead it shows multiple pulses with declining amplitudes. We model the pulses as reverse shocks from collisions between the late ejected shells and the externally shocked material, which is decelerated while sweeping the ambient medium. The peak of each pulse is taken as the maximum emission of each reverse shock. With a proper set of parameters, the envelope of peaks in the light curve as well as the spectrum can be modelled nicely.

astro-ph.HE↗

arXiv2009

Magnetically Torqued Neutrino-Dominated Accretion Flows for Gamma-ray Bursts

Recent observations and theoretical work on gamma-ray bursts (GRBs) favor the central engine model of a Kerr black hole (BH) surrounded by a magnetized neutrino-dominated accretion flow (NDAF). The magnetic coupling between the BH and disk through a large-scale closed magnetic field exerts a torque on the disk, and transports the rotational energy from the BH to the disk. We investigate the properties of the NDAF with this magnetic torque. For a rapid spinning BH, the magnetic torque transfers enormous rotational energy from BH into the inner disk. There are two consequences: (i) the luminosity of neutrino annihilation is greatly augmented; (ii) the disk becomes thermally and viscously unstable in the inner region, and behaves S-Shape of the surface density versus accretion rate. It turns out that magnetically torqued NDAF can be invoked to interpret the variability of gamma-ray luminosity. In addition, we discuss the possibility of restarting the central engine to produce the X-ray flares with required energy.

astro-ph.HE↗

arXiv2008

Clues from the prompt emission of GRB 080319B

The extremely bright optical flash that accompanied GRB 080319B suggested, at first glance, that the prompt $γ$-rays in this burst were produced by Synchrotron self Compton (SSC). We analyze here the observed optical and $γ$ spectrum. We find that the very strong optical emission poses, due to self absorption, very strong constraints on the emission processes and put the origin of the optical emission at a very large radius, almost inconsistent with internal shock. Alternatively it requires a very large random Lorentz factor for the electrons. We find that SSC could not have produced the prompt $γ$-rays. We also show that the optical emission and the $γ$ rays could not have been produced by synchrotron emission from two populations of electron within the same emitting region. Thus we must conclude that the optical and the $γ$-rays were produced in different physical regions. A possible interpretation of the observations is that the $γ$-rays arose from internal shocks but the optical flash resulted from external shock emission. This would have been consistent with the few seconds delay observed between the optical and $γ$-rays signals.

astro-ph↗

arXiv2007

High Efficiency of Gamma-Ray Bursts Revisited

Using the conservation of energy and momentum during collisions of any two shells, we consider the efficiency of gamma-ray bursts by assuming that the ejecta from the central engine are equally massive and have the same Lorentz factors. We calculate the efficiency and the final Lorentz factor of the merged whole shell for different initial diversities of Lorentz factors and for different microscopic radiative efficiency. As a result, a common high efficiency in the range of 0.1 to 0.9 is considerable, and a very high value near 100% is also reachable if the diversity of the Lorentz factors is large enough.

astro-ph↗

arXiv2007

Is GRB 050904 a super-long burst?

By considering synchrotron radiative process in the internal shock model and assuming that all internal shocks are nearly equally energetic, we analyze the gamma-ray burst (GRB) emission at different radii corresponding to different observed times. We apply this model to GRB 050904 and find that our analytical results can provide a natural explanation for the multi-band observations of GRB 050904. This suggests that the X-ray flare emission and the optical emission of this burst could have originated from internal shocks being due to collisions among nearly-equally-energetic shells ejected from the central engine. Thus GRB 050904 appears to be a burst with super-long central engine activity.

astro-ph↗

arXiv2006

Estimation of the detectability of optical orphan afterglows

By neglecting sideways expansion of gamma-ray burst (GRB) jets and assuming their half-opening angle distribution, we estimate the detectability of orphan optical afterglows. This estimation is carried out by calculating the durations of off-axis optical afterglows whose flux density exceeds a certain observational limit. We show that the former assumption leads to more detectable orphans, while the latter suppresses the detectability strongly compared with the model with half-opening angle $θ_j=0.1$. We also considered the effects of other parameters, and find that the effects of the ejecta energy $E_j$ and post-jet-break temporal index $-α_2$ are important but that the effects of the electron-energy distribution index $p$, electron energy equipartition factor $ε_e$, and environment density $n$ are insignificant. If $E_j$ and $α_2$ are determined by other methods, one can constrain the half-opening angle distribution of jets by observing orphan afterglows. Adopting a set of "standard" parameters, the detectable rate of orphan afterglows is about $1.3\times 10^{-2} {deg}^{-2} {yr}^{-1}$, if the observed limiting magnitude is 20 in R-band.

astro-ph↗

arXiv2006

Tail emission from a ring-like jet: its application to shallow decays of early afterglows and to GRB 050709

Similar to the pulsar, the magnetic axis and the spin axis of the gamma-ray burst source may not lie on the same line. This may cause a ring-like jet due to collimation of the precessing magnetic axis. We analyze the tail emission from such a jet, and find that it has a shallow decay phase with temporal index equal to -1/2 if the Lorentz factor of the ejecta is not very high. This phase is consistent with the shallow decay phase of some early X-ray afterglow detected by {\it{swift}}. The ring-like jet has a tail cusp with sharp rising and very sharp decay. This effect can provide an explanation for the re-brightening and sharp decay of the X-ray afterglow of GRB 050709.

astro-ph↗

arXiv2005

Early Afterglows in Wind Environments Revisited

When a cold shell sweeps up the ambient medium, a forward shock and a reverse shock will form. We analyze the reverse-forward shocks in a wind environment, including their dynamics and emission. An early afterglow is emitted from the shocked shell, e.g., an optical flash may emerge. The reverse shock behaves differently in two approximations: relativistic and Newtonian cases, which depend on the parameters, e.g., the initial Lorentz factor of the ejecta. If the initial Lorentz factor is much less than $114 E_{53}^{1/4} Δ_{0,12}^{-1/4} A_{*,-1}^{-1/4}$, the early reverse shock is Newtonian. This may take place for the wider of a two-component jet, an orphan afterglow caused by a low initial Lorentz factor, and so on. The synchrotron self absorption effect is significant especially for the Newtonian reverse shock case, since the absorption frequency $ν_a$ is larger than the cooling frequency $ν_c$ and the minimum synchrotron frequency $ν_m$ for typical parameters. For the optical to X-ray band, the flux is nearly unchanged with time during the early period, which may be a diagnostic for the low initial Lorentz factor of the ejecta in a wind environment. We also investigate the early light curves with different wind densities, and compare them with these in the ISM model.

astro-ph↗