Em Swift Research Articles

Magnetar as Central Engine of Gamma-Ray Bursts: Central Engine–Jet Connection, Wind–Jet Energy Partition, and Origin of Some Ultra-long Bursts

Gamma-ray burst (GRB) central engines and jet production mechanisms are still open questions. Assuming that the shallow decay segments of canonical X-ray afterglow lightcurves of {\em Swift} GRBs are attributed to the magnetic dipole (MD) radiations of newly-born magnetars, we derive the parameters of the magnetars and explore their possible relations to jet and MD wind emission. We show that the magnetar initial spin period ($P_0$) are tightly correlated with the jet energy ($E_{\rm jet}$), which is almost proportional to the wind energy ($E_{\rm wind}$). Our least square fits yield $P_0\propto E^{-0.36\pm 0.03}_{\rm jet}$ and $E_{\rm wind}\propto E^{0.91\pm 0.07}_{\rm jet}$. These relations may imply that a magnetar with faster rotating speed can power a more energetic GRB, and energy partition between the jet and wind may be quasi-universal. Although the $P_0-E_{\rm jet}$ relation is driven by a few sub-energetic GRBs in our sample, our Monte Carlo simulation analysis shows that sample selection biases from instrumental flux limits and contaminations of the bright jet afterglows cannot make this correlation. Within this jet-wind paradigm, we propose that GRB 101225A-like ultra-long GRBs, whose prompt gamma-ray/X-ray lightcurves are featured as a long-lasting plateau with a sharp drop, may be the orphan MD wind emission being due to misalignment of their jet axis to the light of sight. Brief discussion on the orphan MD wind emission and its association with the gravitational wave radiation of newly-born magnetars is presented.

Electromagnetic emission from newly born magnetar spin-down by gravitational-wave and magnetic dipole radiations

A newly-born magnetar is thought to be central engine of some long gamma-ray bursts (GRBs). We investigate the evolution of the electromagnetic (EM) emission from the magnetic dipole (MD) radiation wind injected by spin-down of a newly-born magnetar via both quadrupole gravitational-wave (GW) and MD radiations. We show that the EM luminosity evolves as $L_{\rm em}\propto (1+t/\tau_c)^{\alpha}$, and $\alpha$ is $-1$ and $-2$ in the GW and MD radiation dominated scenarios, respectively. Transition from the GW to MD radiation dominated epoch may show up as a smooth break with slope changing from $-1$ to $-2$. If the magnetar collapses to a black hole before $\tau_c$, the MD radiation should be shut down, then the EM light curve should be a plateau followed by a sharp drop. The expected generic light curve in this paradigm is consistent with the canonical X-ray light curve of {\em Swift} long GRBs. The X-ray emission of several long GRBs are identified and interpreted as magnetar spin-down via GW or MD, as well as constrain the physical parameters of magnetar. The combination of MD emission and GRB afterglows may make the diversity of the observed X-ray light curves. This may interpret the observed chromatic behaviors of the X-ray and optical afterglow light curves and the extremely low detection rate of a jet-like break in the X-ray afterglow light curves of long GRBs.

Constraints on binary neutron star merger product from short GRB observations

Binary neutron star mergers are strong gravitational wave (GW) sources and the leading candidates to interpret short duration gamma-ray bursts (SGRBs). Under the assumptions that SGRBs are produced by double neutron star mergers and that the X-ray plateau followed by a steep decay as observed in SGRB X-ray light curves marks the collapse of a supra-massive neutron star to a black hole (BH), we use the statistical observational properties of {\em Swift} SGRBs and the mass distribution of Galactic double neutron star systems to place constraints on the neutron star equation of state (EoS) and the properties of the post-merger product. We show that current observations already put following interesting constraints: 1) A neutron star EoS with a maximum mass close to a parameterization of $M_{\rm max} = 2.37\,M_\odot (1+1.58\times10^{-10} P^{-2.84})$ is favored; 2) The fractions for the several outcomes of NS-NS mergers are as follows: $\sim40\%$ prompt BHs, $\sim30\%$ supra-massive NSs that collapse to BHs in a range of delay time scales, and $\sim30\%$ stable NSs that never collapse; 3) The initial spin of the newly born supra-massive NSs should be near the breakup limit ($P_i\sim1 {\rm ms}$), which is consistent with the merger scenario; 4) The surface magnetic field of the merger products is typically $\sim 10^{15}$ G; 5) The ellipticity of the supra-massive NSs is $\epsilon \sim (0.004 - 0.007)$, so that strong GW radiation is released post the merger; 6) Even though the initial spin energy of the merger product is similar, the final energy output of the merger product that goes into the electromagnetic channel varies in a wide range from several $10^{49}$ erg to several $10^{52}$ erg, since a good fraction of spin energy is either released in the form of GW or falls into the black hole as the supra-massive NS collapses.

CONSTRAINTS ON THE BULK LORENTZ FACTORS OF GRB X-RAY FLARES

X-ray flares were discovered in the afterglow phase of gamma-ray bursts (GRBs) by the {\em Swift} satellite a decade ago and known as a canonical component in GRB X-ray afterglows. In this paper, we constrain the Lorentz factors of GRB X-ray flares using two different methods. For the first method, we estimate the lower limit on the bulk Lorentz factor with the flare duration and jet break time. In the second method, the upper limit on the Lorentz factor is derived by assuming that the X-ray flare jet has undergone saturated acceleration. We also re-estimate the initial Lorentz factor with GRB afterglow onsets, and find the coefficient of the theoretical Lorentz factor is 1.67 rather than the commonly used 2 for interstellar medium (ISM) and 1.44 for the wind case. We find that the correlation between the limited Lorentz factor and the isotropic radiation energy of X-ray flares in the ISM case is more consistent with that of prompt emission than the wind case in a statistical sense. For a comparison, the lower limit on Lorentz factor is statistically larger than the extrapolation from prompt bursts in the wind case. Our results indicate that X-ray flares and prompt bursts are produced by the same physical mechanism.

THE MILLISECOND MAGNETAR CENTRAL ENGINE IN SHORT GRBs

One favored progenitor model for short duration gamma-ray bursts (SGRBs) is the coalescence of two neutron stars (NS-NS). One possible outcome of such a merger would be a rapidly spinning, strongly magnetized neutron star (known as a millisecond magnetar). These magnetars may be "supra-massive", implying they would collapse to black holes after losing centrifugal support due to magnetic dipole spindown. By systematically analyzing the BAT-XRT light curves of all short GRBs detected by {\em swift}, we test how well the data are consistent with this central engine model of short GRBs. We find that the so-called "extended emission" observed with BAT in some short GRBs are fundamentally the same component as the "internal X-ray plateau" as observed in many short GRBs, which is defined as a plateau in the lightcurve followed by a very rapid drop. Based on how likely a short GRB hosts a magnetar, we characterize the entire {\em Swift} short GRB sample into three categories: the "internal plateau" sample, the "external plateau" sample, and the "no plateau" sample. The derived magnetar surface magnetic field $B_{\rm p}$ and the initial spin period $P_0$ fall into the reasonable range. No GRBs in the internal plateau sample have the total energy exceeding the maximum energy budget of a millisecond magnetar. Assuming that the rapid fall time at the end of the internal plateau is the collapse time of a supra-massive magnetar to a black hole, and applying the measured mass distribution of NS-NS systems in our Galaxy, we constrain the neutron star equation of state (EOS). The data suggest that the NS equation of state is close to the GM1 model, which has a maximum non-rotating NS mass $M_{\rm TOV} \sim 2.37 M_\odot$.

THE STATISTICS OF BAT-TO-XRT FLUX RATIO IN GRBs: EVIDENCE FOR A CHARACTERISTIC VALUE AND ITS IMPLICATIONS

We present the statistics of the ratio, ${\cal R}$, between the prompt and afterglow "plateau" fluxes of GRB. This we define as the ratio between the mean prompt energy flux in {\em Swift} BAT and the {\em Swift} XRT one, immediately following the steep transition between these two states and the beginning of the afterglow stage referred to as the "plateau". Like the distribution of many other GRB observables, the histogram of ${\cal R}$~ is log-normal with maximum at a value ${\cal R}_m \simeq 2,000$, FWHM of about 2 decades and with the entire distribution spanning about 5 decades in the value of ${\cal R}$. We note that the peak of the distribution is close to the proton-to-electron mass ratio $({\cal R}_m \simeq m_p/m_e = 1836)$, as proposed to be the case in an earlier publication, on the basis of a specific model of the GRB dissipation process. It therefore appears that, in addition to the values of the energy of peak luminosity ${E_{\rm pk}\sim m_{e} c^2}$, GRB present us with one more quantity with an apparent characteristic value. The fact that the values of both these quantities (\Ep~and \R) are consistent with the same specific model invoked to account for the efficient conversion of their relativistic proton energies to electrons, argues favorably for its underlying assumptions.

ARE ULTRA-LONG GAMMA-RAY BURSTS DIFFERENT?

The discovery of a number of gamma-ray bursts with duration exceeding 1,000 seconds, in particular the exceptional case of GRB 111209A with a duration of about 25,000 seconds, has opened the question on whether these bursts form a new class of sources, the so called {\em ultra-long} GRBs, or if they are rather the tail of the distribution of the standard long GRB duration. In this Letter, using the long GRB sample detected by {\em Swift}, we investigate on the statistical properties of ultra-long GRBs and compare them with the overall long burst population. We discuss also on the differences observed in their spectral properties. We find that ultra-long GRBs are statistically different from the standard long GRBs with typical burst duration less than 100-500 seconds, for which a Wolf Rayet star progenitor is usually invoked. We interpret this result as an indication that an alternative scenario has to be found in order to explain the ultra-long GRB extreme energetics, as well as the mass reservoir and its size that can feed the central engine for such a long time.

INTERNAL ENERGY DISSIPATION OF GAMMA-RAY BURSTS OBSERVED WITHSWIFT: PRECURSORS, PROMPT GAMMA-RAYS, EXTENDED EMISSION, AND LATE X-RAY FLARES

We jointly analyze the gamma-ray burst (GRB) data observed with BAT and XRT on board the Swift mission to present a global view on the internal energy dissipation processes in GRBs, including precursors, prompt gamma-ray emission, extended soft gamma-ray emission, and late X-ray flares. The Bayesian block method is utilized to analyze the BAT lightcurves to identify various emission episodes. Our results suggest that these emission components likely share a same physical origin, which is repeated activation of the GRB central engine. What we observe in the gamma-ray band may be the tip-of-iceberg of more extended underlying activities. The precursor emission, which is detected in about 10% of {\em Swift} GRBs, is preferably detected in those GRBs that have a massive star core-collapse origin. The soft extended emission (EE) tail, on the other hand, is preferably detected in those GRBs that have a compact star merger origin. Bright X-ray emission is detected during the BAT quiescent phases prior to subsequent gamma-ray peaks, implying that X-ray emission may be detectable prior the BAT trigger time. Future GRB alert instruments with soft X-ray capability would be essential to reveal the early stage of GRB central engine activities, sheding light into jet composition and jet launching mechanism in GRBs.

RADIAL ANGULAR MOMENTUM TRANSFER AND MAGNETIC BARRIER FOR SHORT-TYPE GAMMA-RAY-BURST CENTRAL ENGINE ACTIVITY

Soft extended emission (EE) following initial hard spikes up to 100 seconds was observed with {\em Swift}/BAT for about half of short-type gamma-ray bursts (SGRBs). This challenges the conversional central engine models of SGRBs, i.e., compact star merger models. In the framework of the black hole-neutron star merger models, we study the roles of the radial angular momentum transfer in the disk and the magnetic barrier around the black hole for the activity of SGRB central engines. We show that the radial angular momentum transfer may significantly prolong the lifetime of the accretion process and multiple episodes may be switched by the magnetic barrier. Our numerical calculations based on the models of the neutrino-dominated accretion flows suggest that the disk mass is critical for producing the observed EE. In case of the mass being $\sim 0.8M_{\odot}$, our model can reproduce the observed timescale and luminosity of both the main and EE episodes in a reasonable parameter set. The predicted luminosity of the EE component is lower than the observed EE with about one order of magnitude and the timescale is shorter than 20 seconds if the disk mass being $\sim 0.2M_{\odot}$. {\em Swift}/BAT-like instruments may be not sensitive enough to detect the EE component in this case. We argue that the EE component would be a probe for merger process and disk formation for compact star mergers.

RADIO AND X-RAY OBSERVATIONS OF SN 2006jd: ANOTHER STRONGLY INTERACTING TYPE IIn SUPERNOVA

We report four years of radio and X-ray monitoring of the Type IIn supernova SN 2006jd at radio wavelengths with the Very Large Array, Giant Metrewave Radio Telescope and Expanded Very Large Array; at X-ray wavelengths with {\em Chandra}, {\em XMM-Newton} and {\em Swift}-XRT. We assume that the radio and X-ray emitting particles are produced by shock interaction with a dense circumstellar medium. The radio emission shows an initial rise that can be attributed to free-free absorption by cool gas mixed into the nonthermal emitting region; external free-free absorption is disfavored because of the shape of the rising light curves and the low gas column density inferred along the line of sight to the emission region. The X-ray luminosity implies a preshock circumstellar density $\sim 10^6$ cm$^{-3}$ at a radius $r\sim 2\times 10^{16}$ cm, but the column density inferred from the photoabsorption of X-rays along the line of sight suggests a significantly lower density. The implication may be an asymmetry in the interaction. The X-ray spectrum shows Fe line emission at 6.9 keV that is stronger than is expected for the conditions in the X-ray emitting gas. We suggest that cool gas mixed into the hot gas plays a role in the line emission. Our radio and X-ray data both suggest the density profile is flatter than $r^{-2}$ because of the slow evolution of the unabsorbed emission.

A RADIO-SELECTED SAMPLE OF GAMMA-RAY BURST AFTERGLOWS

We present a catalog of radio afterglow observations of gamma-ray bursts (GRBs) over a 14 year period from 1997 to 2011. Our sample of 304 afterglows consists of 2995 flux density measurements (including upper limits) at frequencies between 0.6 GHz and 660 GHz, with the majority of data taken at 8.5 GHz frequency band (1539 measurements). We use this dataset to carry out a statistical analysis of the radio-selected sample. The detection rate of radio afterglows has stayed unchanged almost at 31% before and after the launch of the {\em Swift} satellite. The canonical long-duration GRB radio light curve at 8.5 GHz peaks at 3-6 days in the source rest frame, with a median peak luminosity of $10^{31}$ erg s$^{-1}$ Hz$^{-1}$. The peak radio luminosities for short-hard bursts, X-ray flashes and the supernova-GRB classes are an order of magnitude or more fainter than this value. There are clear relationships between the detectability of a radio afterglow and the fluence or energy of a GRB, and the X-ray or optical brightness of the afterglow. However, we find few significant correlations between these same GRB and afterglow properties and the peak radio flux density. We also produce synthetic light curves at centimeter (cm) and millimeter (mm) bands using a range of blastwave and microphysics parameters derived from multiwavelength afterglow modeling, and we use them to compare to the radio sample. Finding agreement, we extrapolate this behavior to predict the cm and mm behavior of GRBs observed by the Expanded Very Large Array and the Atacama Large Millimeter Array.

SELECTION EFFECTS ON THE OBSERVED REDSHIFT DEPENDENCE OF GAMMA-RAY BURST JET OPENING ANGLES

Apparent redshift dependence of the jet opening angles ($\theta_{\rm j}$) of gamma-ray bursts (GRBs) is observed from current GRB sample. We investigate whether this dependence can be explained with instrumental selection effects and observational biases by a bootstrapping method. Assuming that (1) the GRB rate follows the star formation history and the cosmic metallicity history and (2) the intrinsic distributions of the jet-corrected luminosity ($L_{\rm \gamma}$) and $\theta_{\rm j}$ are a Gaussian or a power-law function, we generate a mock {\em Swift}/BAT sample by considering various instrumental selection effects, including the flux threshold and the trigger probability of BAT, the probabilities of a GRB jet pointing to the instrument solid angle and the probability of redshift measurement. Our results well reproduce the observed $\theta_{\rm j}-z$ dependence. We find that in case of $L_{\gamma}\propto \theta_{\rm j}^2$ good consistency between the mock and observed samples can be obtained, indicating that both $L_{\rm \gamma}$ and $\theta_{\rm j}$ are degenerate for a flux-limited sample. The parameter set $(L_{\rm \gamma}, \theta_{\rm j})=(4.9\times 10^{49} \rm {erg\ s}^{-1},\ 0.054 {rad})$ gives the best consistency for the current {\em Swift} GRB sample. Considering the beaming effect, the derived intrinsic local GRB rate accordingly is $2.85\times 10^2$ Gpc$^{-3}$ yr$^{-1}$, inferring that $\sim 0.59%$ of Type Ib/c SNe may be accompanied by a GRB.

THE 2009 OUTBURST OF MAGNETAR 1E 1547--5408: PERSISTENT RADIATIVE AND BURST PROPERTIES

The magnetar 1E~1547$-$5408 recently exhibited two periods of outburst, beginning on 2008 October 3 and 2009 January 22. Here we present an analysis of the persistent radiative evolution and a statistical study of the burst properties during the 2009 outburst using the {\em Swift} X-ray Telescope (XRT). We find that the 1--10 keV flux increased by a factor of $\sim500$ and hardened significantly, peaking $\sim6$ hours after the onset of the outburst. The observed pulsed fraction exhibited an anti-correlation with phase-averaged flux. Properties of the several hundred X-ray bursts during the 2009 outburst were determined and compared to those from other magnetar outburst events. We find that the peaks of the bursts occur randomly in phase but that the folded counts that compose the bursts exhibit a pulse which is misaligned with the persistent pulse phase. We also report a correlation between burst hardness and flux. We compare the hardness-flux evolution of the persistent emission of both outbursts to those from other magnetars and find that although there does exist an overall trend, the degree of hardening for a given increase in flux is not uniform from source to source. These results are discussed in the context of previous results and within the magnetar model.

Probing the nature of high-z short GRB 090426 with its early optical and X-ray afterglows

GRB 090426 is a short duration burst detected by Swift ($T_{90}\sim 1.28$ s in the observer frame, and $T_{90}\sim 0.33$ s in the burst frame at $z=2.609$). Its host galaxy properties and some $\gamma$-ray related correlations are analogous to those seen in long duration GRBs, which are believed to be of a massive-star origin (so-called Type II GRBs). We present the results of its early optical observations with the 0.8-m TNT telescope at Xinglong observatory, and the 1-m LOAO telescope at Mt. Lemmon Optical Astronomy Observatory in Arizona. Our well-sampled optical afterglow lightcurve covers from $\sim 90$ seconds to $\sim 10^4$ seconds post the GRB trigger. It shows two shallow decay episodes that are likely due to energy injection, which end at $\sim 230$ seconds and $\sim 7100$ seconds, respectively. The decay slopes post the injection phases are consistent with each other ($\alpha\simeq 1.22$). The X-ray afterglow lightcurve appears to trace the optical, although the second energy injection phase was missed due to visibility constraints introduced by the {\em Swift} orbit. The X-ray spectral index is $\beta_X\sim 1.0$ without temporal evolution. Its decay slope is consistent with the prediction of the forward shock model. Both X-ray and optical emission is consistent with being in the same spectral regime above the cooling frequency ($\nu_c$). The fact that $\nu_c$ is below the optical band from the very early epoch of the observation provides a constraint on the burst environment, which is similar to that seen in classical long duration GRBs. We therefore suggest that death of a massive star is the possible progenitor of this short burst.

GRB 070518: a gamma-ray burst with optically dim luminosity

We present our optical observations of {\em Swift} GRB 070518 afterglow obtained at the 0.8-m Tsinghua University-National Astronomical Observatory of China telescope (TNT) at Xinglong Observatory. Our follow-up observations were performed from 512 sec after the burst trigger. With the upper limit of redshift $\sim$0.7, GRB 070518 is found to be an optically dim burst. The spectra indices $\beta_{ox}$ of optical to X-ray are slightly larger than 0.5, which implies the burst might be a dark burst. The extinction $A_{V}$ of the host galaxy is 3.2 mag inferred from the X-ray hydrogen column density with Galactic extinction law, and 0.3 mag with SMC extinction law. Also, it is similar to three other low-redshift optically dim bursts, which belong to XRR or XRF, and mid-term duration($T_{90}<10$, except for GRB 070419A, $T_{90}$=116s). Moreover, its $R$ band afterglow flux is well fitted by a single power-law with an index of 0.87. The optical afterglow and the X-ray afterglow in the normal segment might have the same mechanism, as they are consistent with the prediction of the classical external shock model. Besides, GRB 070518 agrees with Amati relation under reasonable assumptions. The Ghirlanda relation is also tested with the burst.

Mythbusting Publishing: Questioning the ‘Runaway Popularity’ of Published Biography and Other Life Writing

Mythbusting Publishing: Questioning the ‘Runaway Popularity’ of Published Biography and Other Life Writing

Temporal Profiles and Spectral Lags of XRF 060218

The spectral and temporal properties of the non-thermal emission ofthe nearby XRF 060218 in 0.3-150 keV band are studied. We show that both the spectral energy distribution and the light curve properties suggest the same origin of the non-thermal emission detected by {\em Swift} BAT and XRT. This event has the longest pulse duration and spectral lag observed to date among the known GRBs. The pulse structure and its energy dependence are analogous to typical GRBs. By extrapolating the observed spectral lag to the {\em CGRO/BATSE} bands we find that the hypothesis that this event complies with the same luminosity-lag relation with bright GRBs cannot be ruled out at $2\sigma$ significance level. These intriguing facts, along with its compliance with the Amati-relation, indicate that XRF 060218 shares the similar radiation physics as typical GRBs.

Constraining Ω M and Dark Energy with Gamma-Ray Bursts

An $E_{\gamma,{\rm jet}}\propto {E'_p}^{1.5}$ relationship with a small scatter for current $\gamma$-ray burst (GRB) data was recently reported, where $E_{\gamma,{\rm jet}}$ is the beaming-corrected $\gamma$-ray energy and $E'_p$ is the $\nu F_\nu$ peak energy in the local observer frame. By considering this relationship for a sample of 12 GRBs with known redshift, peak energy, and break time of afterglow light curves, we constrain the mass density of the universe and the nature of dark energy. We find that the mass density $\Omega_M=0.35\pm^{0.15}_{0.15}$ (at the $1\sigma$ confident level) for a flat universe with a cosmological constant, and the $w$ parameter of an assumed static dark-energy equation of state $w=-0.84\pm^{0.57}_{0.83}$ ($1\sigma$). Our results are consistent with those from type Ia supernovae. A larger sample established by the upcoming {\em Swift} satellite is expected to provide further constraints.