Afterglow Light Curves Research Articles

On the Magnetar Origin of the GRBs Presenting X-Ray Afterglow Plateaus

Abstract The X-ray afterglow plateau emission observed in many gamma-ray bursts (GRBs) has been interpreted as being fueled either by fallback onto a newly formed black hole or by the spin-down luminosity of an ultra-magnetized millisecond neutron star. If the latter model is assumed, GRB X-ray afterglow light curves can be reproduced analytically. We fit a sample of GRB X-ray plateaus, interestingly yielding a distribution in the diagram of magnetic field versus spin period (B–P) consistent with B ∝ P 7/6, which is consistent with GRB expectations of the well-established physics of the spin-up line for accreting Galactic X-ray pulsars. The normalization of the relation that we obtain perfectly matches spin-up line predictions for typical neutron star masses (∼1 M ⊙) and radii (∼10 km), and for mass accretion rates typically expected in GRBs, . Short GRBs with extended emission (SEEs) appear toward the long-period end of the distribution, and long GRBs (LGRBs) toward the short-period end. This result is consistent with expectations from the spin-up limit, where the total accreted mass determines the position of the neutron star in the B–P diagram. The B–P distributions for LGRBs and SEEs are statistically different, further supporting the idea that the fundamental plane relation—a tri-dimensional correlation between the X-ray luminosity at the end of the plateau, the end time of the plateau, and the 1 s peak luminosity in the prompt emission—is a powerful discriminant among those populations. Our conclusions are robust against suppositions regarding the collimation angle of the GRB and the magnetar braking index, which shift the resulting properties of the magnetar parallel to the spin-up line, and strongly support a magnetar origin for GRBs presenting X-ray plateaus.

Detecting the imprint of a kilonova or supernova in short gamma-ray burst afterglows

Context. Short gamma-ray bursts (GRBs) result from mergers of two neutron stars or from collapsars, but probably at a smaller rate. In the first case, a kilonova occurs while in the second case a Type Ic supernova is expected. Aims. Even if future observations of kilonovae in association with gravitational wave events provide better data, detecting a kilonova during an afterglow follow-up would remain useful for exploring the diversity of the kilonova phenomenon. As supernovae produce a weaker gravitational signal, afterglow follow-up will be the only possible method to find one. In this work, we identify the conditions of the burst energy, external density, kilonova mass, supernova luminosity, that are necessary for the detection of a kilonova or supernova in the follow-up of short GRB afterglows. Methods. We have used a simple kilonova model to obtain the peak luminosities and times as a function of mass, expansion velocity and ejected matter opacity. Afterglow light curves are computed for a uniform medium and a stellar wind, in the kilonova and supernova cases, respectively. Results. We represent, using diagrams of the burst kinetic energy vs. density of the external medium, the domains where the kilonova or supernova at maximum is brighter than the afterglow. In the kilonova case we vary the mass, the jet opening angle and the microphysics parameters; for supernovae, we consider SN 98bw-like and ten times dimmer events, and again vary the jet opening angle and the microphysics parameters.

The luminous host galaxy, faint supernova and rapid afterglow rebrightening of GRB 100418A

Context. Long gamma-ray bursts (GRBs) give us the chance to study both their extreme physics and the star-forming galaxies in which they form. Aims. GRB 100418A, at a redshift of z = 0.6239, had a bright optical and radio afterglow, and a luminous star-forming host galaxy. This allowed us to study the radiation of the explosion as well as the interstellar medium of the host both in absorption and emission. Methods. We collected photometric data from radio to X-ray wavelengths to study the evolution of the afterglow and the contribution of a possible supernova (SN) and three X-shooter spectra obtained during the first 60 h. Results. The light curve shows a very fast optical rebrightening, with an amplitude of ∼3 magnitudes, starting 2.4 h after the GRB onset. This cannot be explained by a standard external shock model and requires other contributions, such as late central-engine activity. Two weeks after the burst we detect an excess in the light curve consistent with a SN with peak absolute magnitude MV = −18.5 mag, among the faintest GRB-SNe detected to date. The host galaxy shows two components in emission, with velocities differing by 130 km s−1, but otherwise having similar properties. While some absorption and emission components coincide, the absorbing gas spans much higher velocities, indicating the presence of gas beyond the star-forming regions. The host has a star formation rate of SFR = 12.2 M⊙ yr−1, a metallicity of 12 + log(O/H) = 8.55, and a mass of 1.6 × 109 M⊙. Conclusions. GRB 100418A is a member of a class of afterglow light curves which show a steep rebrightening in the optical during the first day, which cannot be explained by traditional models. Its very faint associated SN shows that GRB-SNe can have a larger dispersion in luminosities than previously seen. Furthermore, we have obtained a complete view of the host of GRB 100418A owing to its spectrum, which contains a remarkable number of both emission and absorption lines.

Observational constraints on the structure of gamma-ray burst jets

Motivated by GW170817 we examine constraints that observations put on the angular structure of long gamma-ray burst (GRB) jets. First, the relatively narrow observed distribution of $E_{X}/E_{\gamma}$ (the isotropic equivalent early X-ray afterglow to prompt $\gamma$-ray energy ratio) implies that at any angle that $\gamma$-rays are emitted the Lorentz factor must be high. Specifically, the Lorentz factor of $\gamma$-ray emitting material cannot drop rapidly with angle, and must be $\Gamma(\theta)\gtrsim 50$ even if there are angles for which the gamma-ray received energy is lower by three orders of magnitude compared to the jet core. Second, jets with an angular structure of the $\gamma$-ray emission that over-produce events with a $\gamma$-ray luminosity below the peak of the observed luminosity function are ruled-out. This eliminates models in which the $\gamma$-ray energy angular distribution isn't sufficiently steep and the Lorentz factor distribution isn't sufficiently shallow. Finally, models with a steep structure (e.g. Gaussian) which are detected away from the jet core generate afterglow light-curves that were never observed. We conclude that even if the jet kinetic energy distribution drops continuously with latitude, efficient $\gamma$-ray emission seems to be restricted to material with $\Gamma\gtrsim 50$ and is most likely confined to a narrow region around the core. While our study is confined to long GRBs, where the observed sample is larger and more complete, there are indications that similar conclusions may be applicable also to short GRBs. We discuss the possible implications to the $\gamma$-rays observed in GRB 170817A.

On the Deceleration and Spreading of Relativistic Jets. I. Jet Dynamics

Abstract Jet breaks in gamma-ray burst (GRB) afterglows provide a direct probe of their collimation angle. Modeling a jet break requires an understanding of the jet spreading process, whereby the jet transitions from a collimated outflow into the spherical Sedov–Taylor solution at late times. Currently, direct numerical calculations are the most accurate way to capture the deceleration and spreading process, as analytical models have previously given inaccurate descriptions of the dynamics. Here (in paper I) we present a new, semi-analytical model built empirically by performing relativistic numerical jet calculations and inferring the relationship between the Lorentz factor, opening angle, and shock radius. We then use the analytical model to calculate the Lorentz factor and jet opening angle as a function of the shock radius and compare to the numerical solutions. Our analytic model provides efficient means of computing synthetic GRB afterglow light curves and spectra, which is the focus of paper II.

Off-axis afterglow light curves and images from 2D hydrodynamic simulations of double-sided GRB jets in a stratified external medium

Gamma-ray burst (GRB) jets are narrow, and thus typically point away from us. They are initially ultra-relativistic, causing their prompt $\gamma$-ray and early afterglow emission to be beamed away from us. However, as the jet gradually decelerates its beaming cone widens and eventually reaches our line of sight and the afterglow emission may be detected. Such orphan afterglows were not clearly detected so far. Nevertheless, they should be detected in upcoming optical or radio surveys, and it would be challenging to clearly distinguish between them and other types of transients. Therefore, we perform detailed, realistic calculations of the expected afterglow emission from GRB jets viewed at different angles from the jet's symmetry axis. The dynamics are calculated using 2D relativistic hydrodynamics simulations of jets propagating into different power-law external density profiles, $\rho_{\rm ext}\propto{}R^{-k}$ for $k=0,\,1,\,1.5,\,2$, ranging from a uniform ISM-like medium ($k=0$) to a stratified steady stellar-wind like profile ($k=2$). We calculate radio, optical and X-ray lightcurves, and the evolution of the radio afterglow image size, shape and flux centroid. This may help identify misaligned relativistic jets, whether initially ultra-relativistic and producing a GRB for observers within their beam, or (possibly intrinsically more common) moderately relativistic, in either (i) nearby supernovae Ib/c (some of which are associated with long duration GRBs), or (ii) in binary neutron star mergers, which may produce short duration GRBs, and may also be detected in gravitational waves (e.g. GW$\,$170827/GRB$\,$170817A with a weak prompt $\gamma$-ray emission may harbor an off-axis jet).

Off-axis emission of short GRB jets from double neutron star mergers and GRB 170817A

The short-duration ($\lesssim2\;$s) GRB 170817A in the nearby ($D=40\;$Mpc) elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the first gravitational wave (GW) detection of a binary neutron-star (NS-NS) merger. It was followed by optical, IR, and UV emission from half a day up to weeks after the event, as well as late time X-ray and radio emission. The early UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of radioactive-decay powered kilonova-like emission. Comparison to kilonova models favors the formation of a short-lived ($\sim1\;$s) hypermassive NS, which is also supported by the $\Delta t\approx1.74\;$s delay between the GW chirp signal and the prompt GRB onset. However, the late onset of the X-ray (8.9$\;$days) and radio (16.4$\;$days) emission, together with the low isotropic equivalent $\gamma$-ray energy output ($E_{\rm\gamma,iso}\approx5\times10^{46}\;$erg), strongly suggest emission from a narrow relativistic jet viewed off-axis. Here we set up a general framework for off-axis GRB jet afterglow emission, comparing analytic and numerical approaches, and showing their general predictions for short-hard GRBs that accompany binary NS mergers. The prompt GRB emission suggests a viewing angle well outside the jet's core, and we compare the afterglow lightcurves expected in such a case to the X-ray to radio emission from GRB 170817A. We fit an afterglow off-axis jet model to the X-ray and radio data and find that the observations are explained by a viewing angle $\theta_{\rm obs}\approx16^\circ-26^\circ$, GRB jet energy $E\sim10^{48.5}-10^{49.5}~{\rm erg}$, and external density $n\sim10^{-5}-10^{-1}~{\rm cm}^{-3}$ for a $\xi_e\sim 0.1$ non-thermal electron acceleration efficiency.

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.

Inhomogeneities in the light curves of gamma-ray bursts afterglow

We discuss the inhomogeneous behavior of gamma-ray burst afterglow light curves in optic. We use well-sampled light curves based on mostly our own observations to find and identify deviations (inhomogeneities) from broken power law. By the inhomogeneous behavior we mean flashes, bumps, slow deviations from power law (wiggles) in a light curve. In particular we report parameters of broken power law, describe phenomenology, compare optical light curves with X-ray ones and classify the inhomogeneities. We show that the duration of the inhomogeneities correlates with their peak time relative to gamma-ray burst (GRB) trigger and the correlation is the same for all types of inhomogeneities.

Gamma-Ray Burst Jet Breaks Revisited

Abstract Gamma-ray Burst (GRB) collimation has been inferred with the observations of achromatic steepening in GRB light curves, known as jet breaks. Identifying a jet break from a GRB afterglow light curve allows a measurement of the jet opening angle and true energetics of GRBs. In this paper, we re-investigate this problem using a large sample of GRBs that have an optical jet break that is consistent with being achromatic in the X-ray band. Our sample includes 99 GRBs from 1997 February to 2015 March that have optical and, for Swift GRBs, X-ray light curves that are consistent with the jet break interpretation. Out of the 99 GRBs we have studied, 55 GRBs are found to have temporal and spectral behaviors both before and after the break, consistent with the theoretical predictions of the jet break models, respectively. These include 53 long/soft (Type II) and 2 short/hard (Type I) GRBs. Only 1 GRB is classified as the candidate of a jet break with energy injection. Another 41 and 3 GRBs are classified as the candidates with the lower and upper limits of the jet break time, respectively. Most jet breaks occur at 90 ks, with a typical opening angle θ j = (2.5 ± 1.0)°. This gives a typical beaming correction factor f b − 1 ∼ 1000 for Type II GRBs, suggesting an even higher total GRB event rate density in the universe. Both isotropic and jet-corrected energies have a wide span in their distributions: log(E γ,iso/erg) = 53.11 with σ = 0.84; log(E K,iso/erg) = 54.82 with σ = 0.56; log(E γ /erg) = 49.54 with σ = 1.29; and log(E K/erg) = 51.33 with σ = 0.58. We also investigate several empirical correlations (Amati, Frail, Ghirlanda, and Liang–Zhang) previously discussed in the literature. We find that in general most of these relations are less tight than before. The existence of early jet breaks and hence small opening angle jets, which were detected in the Swfit era, is most likely the source of scatter. If one limits the sample to jet breaks later than 104 s, the Liang–Zhang relation remains tight and the Ghirlanda relation still exists. These relations are derived from Type II GRBs, and Type I GRBs usually deviate from them.

Short GRBs: Opening Angles, Local Neutron Star Merger Rate, and Off-axis Events for GRB/GW Association

Abstract The jet breaks in the afterglow light curves of short gamma-ray bursts (SGRBs), rarely detected so far, are crucial for estimating the half-opening angles of the ejecta (θ j) and hence the neutron star merger rate. In this work, we report the detection of jet decline behaviors in GRB 150424A and GRB 160821B, and find θ j ∼ 0.1 rad. Together with five events reported before 2015 and three others “identified” recently (GRB 050709, GRB 060614, and GRB 140903A), we have a sample consisting of nine SGRBs and one long-short GRB with reasonably estimated θ j. In particular, three Swift bursts in the sample have redshifts z ≤ 0.2, with which we estimate the local neutron star merger rate density to be or if the narrowly beamed GRB 061201 is excluded. Inspired by the typical θ j ∼ 0.1 rad found currently, we further investigate whether the off-beam GRBs (in the uniform jet model) or the off-axis events (in the structured jet model) can significantly enhance the GRB/GW association. For the former, the enhancement is at most moderate, while for the latter the enhancement can be much greater and a high GRB/GW association probability of ∼10% is possible. We also show that the data of GRB 160821B may contain a macronova/kilonova emission component with a temperature of ∼3100 K at ∼3.6 days after the burst and more data are needed to ultimately clarify.

Colour variations in the GRB 120327A afterglow

We present a comprehensive temporal and spectral analysis of the long Swift GRB 120327A afterglow data to investigate the possible causes of the observed early time colour variations. We collected data from various instruments/telescopes in different bands (X-rays, ultra- violet, optical and near-infrared) and determined the shapes of the afterglow early-time light curves. We studied the overall temporal behaviour and the spectral energy distributions from early to late times. The ultra-violet, optical, and near-infrared light curves can be modelled with a single power-law component between 200 and 2e4 s after the burst event. The X-ray light curve shows a canonical steep-shallow-steep behaviour, typical of long gamma-ray bursts. At early times a colour variation is observed in the ultra-violet/optical bands, while at very late times a hint of a re-brightening is visible. The observed early time colour change can be explained as a variation in the intrinsic optical spectral index, rather than an evolution of the optical extinction.

Very High Energy Neutrinos from nearby long GRB Afterglows

Long duration Gamma Ray Bursts (GRBs) are well-motivated sources of Ultra High Energy Cosmic Rays (UHECRs) and neutrinos. During the afterglow phase these particles can be produced as a result of acceleration and interaction there in. We have modeled afterglow spectra and light curves from synchrotron cooling of accelerated electrons. We have fitted data of 17 long GRBs detected within redshift 0.5 in case of the GRB blastwave evolving in a wind and constant density interstellar medium. The afterglow photons can interact with the shock accelerated protons to produce very high energy neutrinos. We have calculated the neutrino flux for photo-pion interactions for all these GRBs. As IceCube have been detecting very high energy neutrinos for the last four years and a larger future extension called Gen 2 is planned, this calculation will help in understanding more about GRB neutrino production. Calculation of flux and estimation of events for Northern Hemisphere GRBs are done for the upcoming neutrino observatory KM3NeT.

Early GRB Afterglows from Reverse Shocks in Ultra-relativistic, Long-lasting Winds

Abstract We develop a model of early Gamma-Ray Burst (GRB) afterglows with dominant X-ray contribution from the reverse shock (RS) propagating in highly relativistic (Lorentz factor γ w ∼ 106) magnetized wind of a long-lasting central engine. The model reproduces, in a fairly natural way, the overall trends and yet allows for variations in the temporal and spectral evolution of early optical and X-ray afterglows. The high energy and the optical synchrotron emission from the RS particles occurs in the fast cooling regime; the resulting synchrotron power L s is a large fraction of the wind luminosity, (L w and σ w are wind power and magnetization). Thus, plateaus—parts of afterglow light curves that show slowly decreasing spectral power—are a natural consequence of the RS emission. Contribution from the forward shock (FS) is negligible in the X-rays, but in the optical both FS and RS contribute similarly: FS optical emission is in the slow cooling regime, producing smooth components, while RS optical emission is in the fast cooling regime, and thus can both produce optical plateaus and account for fast optical variability correlated with the X-rays, e.g., due to changes in the wind properties. We discuss how the RS emission in the X-rays and combined FS and RS emission in the optical can explain many puzzling properties of early GRB afterglows.

Exploring the Behaviour of Long Gamma-Ray Bursts with Intrinsic Afterglow Correlations: log L200s−α>200s

In these proceedings, we summarise the exploration so far of the relationship between the afterglow luminosity (measured at rest frame 200s; log L 200 s ) and average afterglow decay rate (measured from rest frame 200s onwards, α > 200 s ) of long duration Gamma-ray Bursts (GRBs), first reported in the optical/UV light curves of GRB afterglows. We show that this correlation is also present in the X-ray afterglows of GRBs as observed by Swift-XRT. We explore how the parameters of the correlation observed in both the X-ray and optical/UV light curves relate to each other and the prompt emission phase and whether these correlations are consistent with predictions of the standard afterglow model. We find that the observed correlations are consistent with a common underlying physical mechanism producing GRBs and their afterglows regardless of the detailed temporal behaviour. However, a basic afterglow model has difficulty explaining all the observed correlations. This leads us to briefly discuss alternative more complex models.

VERY BRIGHT PROMPT AND REVERSE SHOCK EMISSION OF GRB 140512A

ABSTRACT We report our observations of very bright prompt optical and reverse shock (RS) optical emission of GRB 140512A and analyze its multi-wavelength data observed with the Swift and Fermi missions. It is found that the joint optical–X-ray–gamma-ray spectrum with our first optical detection (R = 13.09 mag) at s during the second episode of the prompt gamma-rays can be fit by a single power law with an index of −1.32 ± 0.01. Our empirical fit to the afterglow light curves indicates that the observed bright optical afterglow with R = 13.00 mag at the peak time is consistent with predictions of the RS and forward shock (FS) emission of external shock models. A joint optical–X-ray afterglow spectrum is well fit with an absorbed single power law, with an index evolving with time from −1.86 ± 0.01 at the peak time to −1.57 ± 0.01 at a late epoch, which could be due to the evolution of the ratio of the RS to FS emission fluxes. We fit the light curves with standard external models, and derive the physical properties of the outflow. It is found that the ratio is 8187, indicating a high magnetization degree in the RS region. Measuring the relative radiation efficiency with , we have R e = 0.02, implying that the radiation efficiency of the RS is much lower than that in FS. We also show that the R B of GRBs 990123, 090102, and 130427A are similar to that of GRB 140512A and their apparent difference may be mainly attributed to the difference of the jet kinetic energy, initial Lorentz factor, and medium density among them.

GRB 110530A: PECULIAR BROAD BUMP AND DELAYED PLATEAU IN EARLY OPTICAL AFTERGLOWS

ABSTRACT We report our very early optical observations of GRB 110530A and investigate its jet properties together with its X-ray afterglow data. A peculiar broad onset bump followed by a plateau is observed in its early R band afterglow light curve. The optical data in the other bands and the X-ray data are well consistent with the temporal feature of the R band light curve. Our joint spectral fits of the optical and X-ray data show that they are in the same regime, with a photon index of ∼1.70. The optical and X-ray afterglow light curves are well fitted with the standard external shock model by considering a delayed energy injection component. Based on our modeling results, we find that the radiative efficiency of the gamma-ray burst jet is and the magnetization parameter of the afterglow jet is with a derived extremely low (the ratio of shock energy to the magnetic field) of . These results indicate that the jet may be matter dominated. A discussion on delayed energy injection from the accretion of the late fall-back material of its pre-supernova star is also presented.

GAMMA-RAY BURST REVERSE SHOCK EMISSION IN EARLY RADIO AFTERGLOWS

ABSTRACT Reverse shock (RS) emission from gamma-ray bursts is an important tool in investigating the nature of the ejecta from the central engine. If the magnetization of the ejecta is not high enough to suppress the RS, a strong RS emission component, usually peaking in the optical/IR band early on, would provide an important contribution to early afterglow light curve. In the radio band, synchrotron self-absorption may suppress early RS emission and also delay the RS peak time. In this paper, we calculate the self-absorbed RS emission in the radio band under different dynamical conditions. In particular, we stress that the RS radio emission is subject to self-absorption in both RSs and forward shocks (FSs). We calculate the ratio between the RS to FS flux at the RS peak time for different frequencies, which is a measure of the detectability of the RS emission component. We then constrain the range of physical parameters for a detectable RS, in particular the role of magnetization. We notice that unlike optical RS emission which is enhanced by moderate magnetization, moderately magnetized ejecta do not necessarily produce a brighter radio RS due to the self-absorption effect. For typical parameters, the RS emission component would not be detectable below 1 GHz unless the medium density is very low (e.g., n < 10−3 cm−3 for the interstellar medium and A * < 5 × 10−4 for wind). These predictions can be tested using the afterglow observations from current and upcoming radio facilities such as the Karl G. Jansky Very Large Array, the Low-Frequency Array, the Five Hundred Meter Aperture Spherical Telescope, and the Square Kilometer Array.

ELECTROMAGNETIC EMISSION FROM LONG-LIVED BINARY NEUTRON STAR MERGER REMNANTS. II. LIGHT CURVES AND SPECTRA

ABSTRACT Recent observations indicate that in a large fraction of binary neutron star (BNS) mergers a long-lived neutron star (NS) may be formed rather than a black hole. Unambiguous electromagnetic (EM) signatures of such a scenario would strongly impact our knowledge on how short gamma-ray bursts (SGRBs) and their afterglow radiation are generated. Furthermore, such EM signals would have profound implications for multimessenger astronomy with joint EM and gravitational-wave (GW) observations of BNS mergers, which will soon become reality thanks to the ground-based advanced LIGO/Virgo GW detector network. Here we explore such EM signatures based on the model presented in a companion paper, which provides a self-consistent evolution of the post-merger system and its EM emission up to ∼107 s. Light curves and spectra are computed for a wide range of post-merger physical properties. We present X-ray afterglow light curves corresponding to the “standard” and the “time-reversal” scenario for SGRBs (prompt emission associated with the merger or with the collapse of the long-lived NS). The light curve morphologies include single and two-plateau features with timescales and luminosities that are in good agreement with Swift observations. Furthermore, we compute the X-ray signal that should precede the SGRB in the time-reversal scenario, the detection of which would represent smoking-gun evidence for this scenario. Finally, we find a bright, highly isotropic EM transient peaking in the X-ray band at ∼102–104 s after the BNS merger with luminosities of L X ∼ 1046–1048 erg s−1. This signal represents a very promising EM counterpart to the GW emission from BNS mergers.

MULTI-WAVELENGTH OBSERVATIONS OF GRB 111228A AND IMPLICATIONS FOR THE FIREBALL AND ITS ENVIRONMENT

ABSTRACT Observations of very early multi-wavelength afterglows are critical to reveal the properties of the radiating fireball and its environment as well as the central engine of gamma-ray bursts (GRBs). We report our optical observations of GRB 111228A from 95 s to about 50 hr after the burst trigger and investigate its properties of the prompt gamma-rays and the ambient medium using our data and the data from the Swift and Fermi missions. Our joint optical and X-ray spectral fits to the afterglow data show that the ambient medium features a low dust-to-gas ratio. Incorporating the energy injection effect, our best fit to the afterglow light curves with the standard afterglow model via the Markov Chain Monte Carlo technique shows that ϵ e = ( 6.9 ± 0.3 ) × 10 − 2 ?> , ϵ B = ( 7.73 ± 0.62 ) × 10 − 6 , E K = ( 6.32 ± 0.86 ) × 10 53 erg ?> , n = 0.100 ± 0.014 ?> cm−3. The low medium density likely implies that the afterglow jet may be in a halo or in a hot ISM. A chromatic shallow decay segment observed in the optical and X-ray bands is well explained with the long-lasting energy injection from the central engine, which would be a magnetar with a period of about 1.92 ms inferred from the data. The E p of its time-integrated prompt gamma-ray spectrum is ∼26 KeV. Using the initial Lorentz factor ( Γ 0 = 476 − 237 + 225 ?> ) derived from our afterglow model fit, it is found that GRB 111228A satisfies the L iso − E p , z − Γ 0 ?> relation and bridges the typical GRBs and low luminosity GRBs in this relation.