GRB 200612A:An Ultralong Gamma-Ray Burst Powered by Magnetar Spinning Down

GRB 200612A could be classified as an ultralong gamma-ray burst due to its prompt emission lasting up to~1020 s and the true timescale of the central engine activity t_(burst)≥4×10^(4) s.The late X-ray light curve with a decay index ofα=7.53 is steeper than the steepest possible decay from an external shock model.We propose that this X-ray afterglow can be driven by dipolar radiation from the magnetar spindown during its early stage,while the magnetar collapsed into the black hole before its spindown,resulting in a very steep decay of the late X-ray light curve.The optical data show that the light curve is still rising after 1.1 ks,suggesting a late onset.We show that GRB 200612A’s optical afterglow light curve is fitted with the forward shock model by Gaussian structured off-axis jet.This is a special case among GRBs,as it may be an ultralong gamma-ray burst powered by a magnetar in an off-axis observation scenario.

Variable Stars in the 50BiN Open Cluster Survey.Ⅲ.NGC 884

Open clusters are the basic building blocks that serve as a laboratory for the study of young stellar populations in the Milky Way.Variable stars in open clusters provide a unique way to accurately probe the internal structure,temporal and dynamical evolutionary stages of individual stars and the host cluster.The most powerful tool for such studies is time-domain photometric observations.This paper follows the route of our previous work,concentrating on a photometric search for variable stars in NGC 884.The target cluster is the companion of NGC869,forming the well-known double cluster system that is gravitationally bound.From the observation run in 2016 November,a total of 9247 B-band CCD images and 8218Ⅴ-band CCD images were obtained.We detected a total of 15 stars with variability in visual brightness,including five Be stars,three eclipsing binaries,and seven of unknown types.Two new variable stars were discovered in this work.We also compared the variable star content of NGC 884 with its companion NGC 869.

X-Ray Properties of PSR J1811-1925 by Nu STAR

We analyzed the spectral properties and pulse profile of PSR J1811-1925,a pulsar located in the center of composite supernova remnant(SNR)G11.2-0.3,by using high timing resolution archival data from the Nuclear Spectroscopic Telescope Array Mission(NuSTAR).Analysis of archival Chandra data over different regions rules out the SNR shell as the site of the hard X-ray emission while spectral analysis indicates that the NuSTAR photons originate in the pulsar and its nebula.The pulse profile exhibits a broad single peak up to 35 keV.The jointed spectrum by combining NuSTAR and Chandra can be well fitted by a power-law model with a photon index ofΓ=1.58±0.04.The integrated flux of jointed spectrum over 1-10 keV is 3.36×10^(-12)erg cm^(-2)s^(-1).The spectrum of pulsar having photon indexΓ=1.33±0.06 and a 1-10 keV flux of 0.91×10^(-12)erg cm^(-2)s^(-1).We also performed the phase-resolved spectral analysis by splitting the whole pulse-on phase into five phase bins.The photon indices of the bins are all around 1.4,indicating that the photon index does not evolve with the phase.

Are There Magnetars in High-mass X-Ray Binaries?

Magnetars form a special population of neutron stars with strong magnetic fields and long spin periods. About 30 magnetars and magnetar candidates known currently are probably isolated, but the possibility that magnetars are in binaries has not been excluded. In this work, we perform spin evolution of neutron stars with different magnetic fields in wind-fed high-mass X-ray binaries and compare the spin period distribution with observations, aiming to find magnetars in binaries. Our simulation shows that some of the neutron stars, which have long spin periods or are in widely-separated systems, need strong magnetic fields to explain their spin evolution. This implies that there are probably magnetars in high-mass X-ray binaries. Moreover, this can further provide a theoretical basis for some unclear astronomical phenomena, such as the possible origin of periodic fast radio bursts from magnetars in binary systems.

Statistical properties of fast radio bursts elucidate their origins:magnetars are favored over gamma-ray bursts

Fast radio bursts(FRBs) are extremely strong radio flares lasting several milliseconds,most of which come from unidentified objects at a cosmological distance.They can be apparently repeating or not.In this paper,we analyzed 18 repeaters and 12 non-repeating FRBs observed in the frequency bands of 400–800 MHz from Canadian Hydrogen Intensity Mapping Experiment(CHIME).We investigated the distributions of FRB isotropic-equivalent radio luminosity,considering the K correction.Statistically,the luminosity distribution can be better fitted by Gaussian form than by power-law.Based on the above results,together with the observed FRB event rate,pulse duration,and radio luminosity,FRB origin models are evaluated and constrained such that the gamma-ray bursts(GRBs) may be excluded for the non-repeaters while magnetars or neutron stars(NSs) emitting the supergiant pulses are preferred for the repeaters.We also found the necessity of a small FRB emission beaming solid angle(about 0.1 sr) from magnetars that should be considered,and/or the FRB association with soft gamma-ray repeaters(SGRs) may lie at a low probability of about 10%.Finally,we discussed the uncertainty of FRB luminosity caused by the estimation of the distance that is inferred by the simple relation between the redshift and dispersion measure(DM).

X-ray flares raising upon magnetar plateau as an implication of a surrounding disk of newborn magnetized neutron star

The X-ray flares have usually been ascribed to long-lasting activities of the central engine of gamma-ray bursts(GRBs),e.g.,fallback accretion.The GRB X-ray plateaus,however,favor a millisecond magnetar central engine.The fallback accretion can be significantly suppressed due to the propeller effect of a magnetar.Therefore,if the propeller regime cannot resist the mass flow onto the surface of the magnetar efficiently,the X-ray flares raising upon the magnetar plateau would be expected.In this work,such peculiar cases are connected to the accretion process of the magnetars,and an implication for magnetar-disc structure is given.We investigate the repeated accretion process with multi-flare GRB 050730,and give a discussion for the accretion-induced variation of the magnetic field in GRB 111209 A.Two or more flares exhibit in the GRB 050730,060607 A and 140304 A;by adopting magnetar mass M=1.4 M_(⊙)and radius R=12 km,the average mass flow rates of the corresponding surrounding disk are 3.53×10^(-4)M_(⊙)s^(-1).4.23×10^(-4)M_(⊙)s^(-1),and 4.33×10^(-4)M_(⊙)s^(-1),and the corresponding average sizes of the magnetosphere are 5.01×10^(6)cm,6.45 x 10^(-6)cm,and 1.09×10^(-7)cm,respectively.A statistic analysis that contains eight GRBs within 12 flares shows that the total mass loading in single flare is~2×10^(-5)M_(⊙).In the lost mass of a disk,there are about 0.1%used to feed a collimated jet.

A Possible γ-Ray Pulsation from PSR J1740-5340B in the Globular Cluster NGC 6397

Recently, a new radio millisecond pulsar(MSP) J1740-5340B, hosted in the globular cluster(GC) NGC 6397,was reported with a 5.78 ms spin period in an eclipsing binary system with a 1.97 days orbital period. Based on a modified radio ephemeris updated by tool tempo2, we analyze the ~15 yr γ-ray data obtained from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope and detect PSR J1740-5340B's γ-ray pulsation at a confidence level of ~4σ with a weighted H-test value of ~26. By performing a phase-resolved analysis, the γ-ray luminosity in on-pulse interval of PSR J1740-5340B is L_(γ)~ 3.8 × 10^(33) erg s^(-1) using NGC 6397's distance of 2.48 kpc. And γ-rays from the on-pulse part of PSR J1740-5340B contribute ~90% of the total observed γ-ray emissions from NGC 6397. No significant γ-ray pulsation of another MSP J1740-5340A in the GC is detected.Considering that the previous four cases of MSPs in GCs, more data in γ-ray, X-ray, and radio are encouraged to finally confirm the γ-ray emissions from MSP J1740-5340B, especially starving for a precise ephemeris.

Unifying neutron star sub-populations in the supernova fallback accretion model

We employ the supernova fallback disk model to simulate the spin evolution of isolated young neutron stars(NSs). We consider the submergence of the NS magnetic fields during the supercritical accretion stage and its succeeding reemergence. It is shown that the evolution of the spin periods and the magnetic fields in this model is able to account for the relatively weak magnetic fields of central compact objects and the measured braking indices of young pulsars. For a range of initial parameters, evolutionary links can be established among various kinds of NS sub-populations including magnetars, central compact objects and young pulsars. Thus, the diversity of young NSs could be unified in the framework of the supernova fallback accretion model.

Magnetar giant flare originating from GRB 200415A:transient GeV emission, time-resolved E_(p)-L_(iso) correlation and implications

Giant flares(GFs)are unusual bursts from soft gamma-ray repeaters(SGRs)that release an enormous amount of energy in a fraction of a second.The afterglow emission of these SGR-GFs or GF candidates is a highly beneficial means of discerning their composition,relativistic speed and emission mechanisms.GRB 200415A is a recent GF candidate observed in a direction coincident with the nearby Sculptor galaxy at 3.5 Mpc.In this work,we searched for transient gamma-ray emission in past observations by Fermi-LAT in the direction of GRB 200415A.These observations confirm that GRB 200415A is observed as a transient GeV source only once.A pure pair-plasma fireball cannot provide the required energy for the interpretation of GeV afterglow emission and a baryonic poor outflow is additionally needed to explain the afterglow emission.A baryonic rich outflow is also viable,as it can explain the variability and observed quasi-thermal spectrum of the prompt emission if dissipation is happening below the photosphere via internal shocks.Using the peak energy(Ep)of the time-resolved prompt emission spectra and their fluxes(Fp),we found a correlation between Ep and Fp or isotropic luminosity Liso for GRB 200415A.This supports the intrinsic nature of Ep-Liso correlation found in SGRs-GFs,hence favoring a baryonic poor outflow.Our results also indicate a different mechanism at work during the initial spike,and that the evolution of the prompt emission spectral properties in this outflow would be intrinsically due to the injection process.

Transient Phenomena from Gas Erupting Neutron Stars

Starquakes probably occur in rapidly spinning or ultra high field neutron stars. In this article, we argue that highly compressed gas containing electron–positron pairs could evaporate and erupt from inside the neutron star when a crack forms and then heals during a starquake. Under the influence of the existing oscillation modes of the star, the crack may have sufficiently large size and long lifetime. An appropriate amount of gas can erupt into the magnetosphere with relativistic and nearly uniform velocity, producing various transient and bursting phenomena.

Spindown of magnetars:quantum vacuum friction?

Magnetars are proposed to be peculiar neutron stars which could power their X-ray radiation by super-strong magnetic fields as high as > 10^(14) G.However,no direct evidence for such strong fields has been obtained till now,and the recent discovery of low magnetic field magnetars even indicates that some more efficient radiation mechanism than magnetic dipole radiation should be included.In this paper,quantum vacuum friction(QVF) is suggested to be a direct consequence of super-strong surface fields,therefore the magnetar model could then be tested further through QVF braking.The high surface magnetic field of a pulsar interacting with the quantum vacuum results in a significantly high spindown rate(P).It is found that a QVF dominates the energy loss of pulsars when the pulsar's rotation period and its first derivative satisfy the relationship P^3P > 0.63 ×10^(-16)ξ^(-4) s^2,whereξ is the ratio of the surface magnetic field over the dipole magnetic field.In the "QVF + magnetodipole" joint braking scenario,the spindown behavior of magnetars should be quite different from that in the pure magnetodipole model.We are expecting these results could be tested by magnetar candidates,especially low magnetic field cases,in the future.

Fast Radio Bursts as Crustal Dynamical Events Induced by Magnetic Field Evolution in Young Magnetars

We revisit in this work a model for repeating Fast Radio Bursts based of the release of energy provoked by the magnetic field dynamics affecting a magnetar’s crust.We address the basics of such a model by solving the propagation of the perturbation approximately,and quantify the energetics and the radiation by bunches of charges in the so-called charge starved region in the magnetosphere.The(almost)simultaneous emission of newly detected X-rays from SGR 1935+2154 is tentatively associated with a reconnection behind the propagation.The strength of f-mode gravitational radiation excited by the event is quantified,and more detailed studies of the nonlinear(spiky)soliton solutions are suggested.

A note on the puzzling spindown behavior of the Galactic center magnetar SGR J1745-2900

SGR J1745-2900 is a magnetar near the Galactic center. X-ray observations of this source found a decreasing X-ray luminosity accompanied by an enhanced spindown rate. This negative correlation between X-ray luminosity and spindown rate is hard to understand. The wind braking model of magnetars is employed to explain this puzzling spindown behavior. During the release of magnetic energy of magnetars,a system of particles may be generated. Some of these particles remain trapped in the magnetosphere and may contribute to the X-ray luminosity. The rest of the particles can flow out and take away the rotational energy of the central neutron star. A smaller polar cap angle will cause the decrease of X-ray luminosity and enhanced spindown rate of SGR J1745-2900. This magnetar is shortly expected to have a maximum spindown rate.

On the possible wind nebula of magnetar Swift J1834.9-0846:a magnetism-powered synchrotron nebula

Recently,the magnetar Swift J1834.9—0846 has been reported to have a possible wind nebula.It is shown that both the magnetar and its wind nebula are understandable in the wind braking scenario.The magnetar's rotational energy loss rate is not enough to power the particle luminosity.The required particle luminosity should be about 10^(36) erg s^(-1) to 10^(38) ergs^(-1).It is obtained in three different approaches:considering wind braking of Swift J1834.9-0846;the spectral and spatial observations of the wind nebula;and an empirical upper bound on wind nebula X-ray luminosity.The nebula magnetic field is about 10^(-4)G.The possible wind nebula of Swift J1834.9-0846 should be a magnetar wind nebula.It is powered by the magnetic energy released from the magnetar.

Non-detection of pulsed radio emission from magnetar Swift J1834.9-0846: constraint on the fundamental plane of magnetar radio emission

The magnetar Swift J1834.9-0846 is observed using the Nanshan 25 m radio telescope. No pulsed radio emission is detected. The upper limit on the pulsed radio emission from this source is 0.5 mJy. According to the "fundamental plane" for radio magnetars, this source should have radio emission. Therefore, our results put constraints on the existence of a fundamental plane of magnetar radio emission. We argue that a magnetar's ability to emit radio emission may have little to do with the spin down luminosity and is related to the magnetar's X-ray luminosity. The only necessary condition is a relatively low X-ray luminosity.

The Remnant of Neutron Star-White Dwarf Merger and the Repeating Fast Radio Bursts

Fast radio bursts (FRBs) at cosmological distances still hold concealed physical origins. Previously Liu (2018) proposes a scenario that the collision between a neutron star (NS) and a white dwarf (WD) can be one of the progenitors of non-repeating FRBs and notices that the repeating FRBs can also be explained if a magnetar formed after such NS-WD merger. In this paper, we investigate this channel of magnetar formation in more detail. We propose that the NS-WD post-merger, after cooling and angular momentum redistribution, may collapse to either a black hole or a new NS or even remains as a hybrid WDNS, depending on the total mass of the NS and WD. In particular, the newly formed NS can be a magnetar if the core of the WD collapsed into the NS while large quantities of degenerate electrons of the WD compressed to the outer layers of the new NS. A strong magnetic field can be formed by the electrons and positive charges with different angular velocities induced by the differential rotation of the newborn magnetar. Such a magnetar can power the repeating FRBs by the magnetic reconnections due to the crustal movements or starquakes.

Pulsating magneto-dipole radiation of a quaking neutron star powered by energy of Alfvn seismic vibrations

We compute the characteristic parameters of the magneto-dipole radiation of a neutron star undergoing torsional seismic vibrations under the action of Lorentz restoring force about an axis of a dipolar magnetic field experiencing decay.After a brief outline of the general theoretical background of the model of a vibration-powered neutron star,we present numerical estimates of basic vibration and radiation characteristics,such as frequency,lifetime and luminosity,and investigate their time dependence on magnetic field decay.The presented analysis suggests that a gradual decrease in frequencies of pulsating high-energy emission detected from a handful of currently monitored AXP/SGR-like X-ray sources can be explained as being produced by the vibration-powered magneto-dipole radiation of quaking magnetars.

Searching for Variable Stars in and around Open Clusters

We present a snapshot of our recent results of a variable star survey in1 degree fields around three open clusters: NGC 188, NGC 7789 and M67. A totalnumber of 39 variable stars are newly discovered, including 22 W UMa stars, 10EA (Algol) type binaries, one RR-lyr and one RRd pulsator, and five unclassifiedvariables.

SMA molecular line survey towards the massive star-forming region G10.6-0.4in W31complex

Line surveys of complex molecules with millimeter and sub-millimeter telescopes are important for probing the physical and chemical environments of massive star forming regions(MSFRs).We present a molecular line survey with the Submillimeter Array(SMA) in the frequency ranges of 220.3–222.3 GHz and 230.3–232.3 GHz toward G10.6-0.4, the brightest star forming core in the W31 complex. Ninety-nine transitions from 22 molecular species and their isotopologues are identified. The moment 0 images of typical molecules show a compact core which is concentrated at the continuum peak position. Based on the local thermodynamic equilibrium assumption, the molecular line data are modeled. The rotational temperatures of those molecular species range from 96 to 178 K and their column densities range from 2.0×10^(14) to 3.7×10^(17) cm^(-2). The observational data suggest that all complex molecules are located in a warm environment. Chemical environments of the molecules are discussed. We compared molecular abundances and gas temperatures in G10.6-0.4 with those in other MSFRs, and found that gas temperatures and fractional abundances of specific molecules in G10.6-0.4 are similar to the typical MSFR W51 North, suggesting that there are similar physical and chemical environments in these two MSFRs.

Probing star formation and feedback using CCOSMA and archival data in the CFG028.68–0.28 quasi-sinusoidal filament

We have performed a multi-wavelength study toward a quasi-sinusoidal filament(CFG028.68–0.28). A new large-scale ^12CO J = 3-2 map was obtained from the China-Cologne Observation for Sub Millimeter Astronomy(CCOSMA) 3m radio telescope. Based on the ATLASGAL catalog, we have identified 27 dust clumps in the filament. Through the relationship between the mass and radius of these clumps, 67% of these clumps are dense and massive enough to potentially form massive stars. The obtained CFE is ~11% in the filament. The filament has a linear mass density of ~305 M⊙pc^-1, which is smaller than its critical mass to length ratio. This suggests that the external pressure from the neighboring H Ⅱ regions may help prevent the filament from dispersing under the effects of turbulence. Comparing the energy injection from outflows and H Ⅱ regions in the filament, the ionization feedback from the H Ⅱ regions can help maintain the observed turbulence.