Radio Afterglow Research Articles

THE GIANT FLARE FROM SGR 1806–20 AND ITS RADIO AFTERGLOW

The multi-wavelength observations of the 2004 December 27 Giant Flare (GF) from SGR 1806–20 and its long-lived radio afterglow are briefly reviewed. The GF appears to have been produced by a dramatic reconfiguration of the magnetic field near the surface of the neutron star, possibly accompanied by fractures in the crust. The explosive release of over 1046 erg (isotropic equivalent) powered a one-sided mildly relativistic outflow. The outflow produced a new expanding radio nebula that is still visible over a year after the GF. Also considered are the constraints on the total energy in the GF, the energy and mass in the outflow, and on the external density, as well as possible implications for short γ-ray bursts and potential signatures in high energy neutrinos, photons, or cosmic rays. Some possible future observations of this and other GFs are briefly discussed.

The interpretation and implication of the afterglow of GRB 060218

The nearby GRB 060216/SN 2006aj was an extremely long, weak and very soft GRB. Itwas peculiar in many aspects. We show here that the x-ray, ultraviolet/optical and radioafterglows of GRB 060218 have to be attributed to different physical processes arising fromdifferent emission regions. From the several components in this burst’s afterglowonly the radio afterglow can be interpreted in terms of the common externalshock model. We infer from the radio that the blast wave’s kinetic energy was∼1050 erg and the circumburst matter had a constant rather than a wind profile.The lack of a ‘jet break’ up to 22 days implies that the outflow was wide,θj>1. Even though the late x-ray afterglow decays normally, it cannot result from an externalshock because of its very steep spectrum. Furthermore, the implied kinetic energy wouldhave produced far too much radio. We suggest that this x-ray afterglow could be attributedto a continued activity of the central engine that within the collapsar scenario could arisefrom fall-back accretion. ‘Central engine afterglow’ may be common in under-luminousGRBs where the kinetic energy of the blast wave is small and the external shock does notdominate over this component. Such under-luminous GRBs might be very common butthey are rarely recorded because they can be detected only from short distances.

An Optically Dark GRB Observed by HETE-2: GRB 051022

Abstract GRB 051022 was detected at 13:07:58 on 2005 October 22 UT by HETE-2. The location of GRB 051022 was determined immediately by the flight localization system. This burst contained multiple pulses, and had a rather long duration of about 190 s. The detections of candidate X-ray and radio afterglows were reported, whereas no optical afterglow was found. Optical spectroscopic observations of the host galaxy revealed a redshift of $z = 0.8$. Using data derived by a HETE-2 observation of the prompt emission, we found absorption of $N_{\mathrm{H}} = (8.8_{-2.9}^{+3.1}) \times 10^{22} \,\mathrm{cm}^{-2}$ and visual extinction of $A_{V} = 49_{-16}^{+17}$ mag in the host galaxy. If this is the case, no detection of any optical transient would be quite reasonable. The absorption derived by Swift XRT observations of the afterglow is fully consistent with those obtained from an early HETE-2 observation of the prompt emission. Our analysis implies an interpretation that the absorbing medium may be outside the external shock at $R \sim 10^{16} \,\mathrm{cm}$, which could be a dusty molecular cloud.

An Energetic Afterglow from a Distant Stellar Explosion

We present the discovery of radio afterglow emission from the high redshift (z=6.295) burst GRB 050904. The peak flux density for this burst is similar to typical low-redshift gamma-ray bursts (GRB). We further show that beyond a redshift of order unity, the flux density of radio afterglows are largely insensitive to redshift, consistent with predictions. By combining the existing X-ray, near-infrared and radio measurements, we derive estimates for the kinetic energy and opening angle of the blast wave, and for the density of the circumburst medium into which it expands. Both the kinetic and radiated energy indicate that GRB 050904 was an unusally energetic burst (10**52 erg). More importantly, we are able to make an in situ measurement of the density structure of the circumburst medium. We conclude that GRB 050904 exploded into a constant density medium with n=680 cm**-3, which is two orders of magnitude above the nominal value for low-redshift GRBs. The next generation of centimeter (EVLA) and millimeter radio instuments (ALMA) will be able to routinely detect events like GRB 050904 and use them to study magnetic fields, and the atomic and molecular gas in the high redshift Universe.

Envelope expansion with core collapse - III. Similarity isothermal shocks in a magnetofluid

We explore magnetohydrodynamic (MHD) solutions for envelope expansions with core collapse (EECC) with isothermal MHD shocks in a quasi-spherical symmetry, and outline potential astrophysical applications of such magnetized shock flows. By including a random magnetic field in a gas medium, we further extend the recent isothermal shock results of Bian & Lou who have unified earlier similarity isothermal shock solutions of Tsai & Hsu, of Shu et al. and of Shen & Lou in a more general framework. MHD shock solutions are divided into three classes according to the downstream characteristics near the core. Class I solutions are those characterized by free-fall collapses towards the core downstream of an MHD shock, while Class II solutions are those characterized by Larson–Penston (LP) type near the core downstream of an MHD shock. Class III solutions are novel, sharing both features of Class I and II solutions with the presence of a sufficiently strong magnetic field as a prerequisite. Various MHD processes may occur within the regime of these isothermal MHD shock similarity solutions, such as submagnetosonic oscillations, free-fall core collapses, radial contractions and expansions. Both possibilities of perpendicular and oblique MHD shocks are analysed. Under the current approximation of MHD EECC (MEECC) solutions, only perpendicular shocks are systematically calculated. These similar MHD shocks propagate at either submagnetosonic or supermagnetosonic constant speeds. We can construct Class I, II and III MHD shocks matching with an isothermal magnetostatic outer envelope or an MHD breeze. We can also construct families of twin MHD shock solutions as well as an ‘isothermal MHD shock’ separating two magnetofluid regions of two different yet constant temperatures. The versatile behaviours of such MHD shock solutions may be utilized to model a wide range of astrophysical problems, including star formation in magnetized molecular clouds, ‘MHD champagne flows’ in H-ii regions around luminous massive OB stars, MHD link between the asymptotic giant branch (AGB) phase and the protoplanetary nebula (pPN) phase with a hot central magnetized white dwarf, relativistic MHD pulsar winds in supernova remnants (SNRs), radio afterglows of soft gamma-ray repeaters (SGRs) and so forth.

Pure and Loaded Fireballs in Soft Gamma‐Ray Repeater Giant Flares

On 2004 December 27, a giant flare from SGR 1806-20 was detected on Earth. Its thermal spectrum and temperature suggest that the flare resulted from an energy release of about 1047 ergs s-1 close to the surface of a neutron star in the form of radiation and/or pairs. This plasma expanded under its own pressure, producing a fireball, and the observed gamma rays escaped once the fireball became optically thin. The giant flare was followed by a bright radio afterglow, with an observable extended size, implying an energetic relativistic outflow. We revisit here the evolution of relativistic fireballs, and we calculate the Lorentz factor and energy remaining in relativistic outflow once the radiation escapes. We show that pairs that arise naturally in a pure pair-radiation fireball do not carry enough energy to account for the observed afterglow. We consider various alternatives and show that if the relativistic outflow that causes the afterglow is related directly to the prompt flare, then the initial fireball must be loaded by baryons or Poynting flux. While we focus on parameters applicable to the giant flare and the radio afterglow of SGR 1806-20, the calculations presented here could be also applicable to gamma-ray bursts (GRBs).

The afterglow and elliptical host galaxy of the short γ-ray burst GRB 050724

Despite a rich phenomenology, gamma-ray bursts (GRBs) are divided into two classes based on their duration and spectral hardness--the long-soft and the short-hard bursts. The discovery of afterglow emission from long GRBs was a watershed event, pinpointing their origin to star-forming galaxies, and hence the death of massive stars, and indicating an energy release of about 10(51) erg. While theoretical arguments suggest that short GRBs are produced in the coalescence of binary compact objects (neutron stars or black holes), the progenitors, energetics and environments of these events remain elusive despite recent localizations. Here we report the discovery of the first radio afterglow from the short burst GRB 050724, which unambiguously associates it with an elliptical galaxy at a redshift z = 0.257. We show that the burst is powered by the same relativistic fireball mechanism as long GRBs, with the ejecta possibly collimated in jets, but that the total energy release is 10-1,000 times smaller. More importantly, the nature of the host galaxy demonstrates that short GRBs arise from an old (> 1 Gyr) stellar population, strengthening earlier suggestions and providing support for coalescing compact object binaries as the progenitors.

The Radio Afterglow of GRB 030329 at Centimeter Wavelengths: Evidence for a Structured Jet or Nonrelativistic Expansion

We present our centimeter wavelength (1.4, 2.3, and 4.8 GHz) light curves of the afterglow of GRB 030329, which were obtained with the Westerbork Synthesis Radio Telescope. Modeling the data according to a collimated afterglow results in a jet-break time of 10 days. This is in accordance with earlier results obtained at higher radio frequencies. However, with respect to the afterglow model, some additional flux at the lower frequencies is present when these light curves reach their maximum after 40-80 days. We show that this additional flux can be modeled with two or more components with progressively later jet breaks. From these results we infer that the jet is in fact a structured or a layered jet, where the ejecta with lower Lorentz factors produce additional flux that becomes visible at late times in the lowest frequency bands. We show that a transition to nonrelativistic expansion of the fireball at late times can also account for the observed flux excess, except for the lowest frequency (1.4 GHz) data.

The Growth, Polarization, and Motion of the Radio Afterglow from the Giant Flare from SGR 1806-20

The extraordinary giant flare (GF) of 2004 December 27 from the soft gamma repeater SGR 1806-20 was followed by a bright radio afterglow. We present an analysis of VLA observations of this radio afterglow from SGR 1806-20, consisting of previously reported 8.5 GHz data covering days 7-20 after the GF, plus new observations at 8.5 and 22 GHz from day 24 to 81. We detect motion in the flux centroid of the afterglow, at an average velocity of (0.26 ± 0.03)c (assuming a distance of 15 kpc) at a position angle of -45°. This motion, in combination with the growth and polarization measurements, suggests an asymmetric outflow, mainly from one side of the magnetar. We find a deceleration in the expansion, from ~9 to <5 mas day-1. The time of deceleration is roughly coincident with the rebrightening in the radio light curve, as expected to result when the ejecta from the GF sweeps up enough of the external medium and transitions from a coasting phase to the Sedov-Taylor regime. The radio afterglow is elongated and maintains a 2 : 1 axis ratio with an average position angle of -40° (north through east), oriented perpendicular to the average intrinsic linear polarization angle.

Diagnosing the Outflow from the SGR 1806−20 Giant Flare with Radio Observations

On 2004 December 27, the soft gamma repeater (SGR) 1806-20 emitted the brightest giant flare (GF) ever detected from an SGR. This burst of energy, which resulted in an (isotropic) energy release ~100 times greater than the only two other known SGR GFs, was followed by a very bright, fading radio afterglow. Extensive follow-up radio observations provided a wealth of information with unprecedented astrometric precision, revealing the temporal evolution of the source size, along with densely sampled light curves and spectra. Here we expand on our previous work on this source, by explaining these observations within one self-consistent dynamical model. In this scenario, the early radio emission is due to the outflow ejected during the GF energizing a thin shell surrounding a preexisting cavity, where the observed steep temporal decay of the radio emission seen beginning on day 9 is attributed to the adiabatic cooling of the shocked shell. The shocked ejecta and external shell move outward together, driving a forward shock into the ambient medium, and are eventually decelerated by a reverse shock. As we show in a separate work by Gelfand and coworkers, the radio emission from the shocked external medium naturally peaks when significant deceleration occurs and then decays relatively slowly. The dynamical modeling of the collision between the ejecta and the external shell, together with the observed evolution of the source size (which is nicely reproduced in our model), suggests that most of the energy in the outflow was in mildly relativistic material, with an initial expansion velocity v/c d15(1 + d)-1/2 ~ 0.7, for a distance of 15d15 kpc to SGR 1806-20. An initially highly relativistic outflow would not have produced a long coasting phase at a mildly relativistic expansion velocity, as was observed.

The afterglow of GRB 050709 and the nature of the short-hard γ-ray bursts

The final chapter in the long-standing mystery of the gamma-ray bursts (GRBs) centres on the origin of the short-hard class of bursts, which are suspected on theoretical grounds to result from the coalescence of neutron-star or black-hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst, GRB 050709, whose accurate position allows us to associate it unambiguously with a star-forming galaxy at redshift z = 0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.

Discovery of Radio Emission from the Transient Anomalous X-Ray Pulsar XTE J1810-197

We report the first detection of radio emission from any anomalous X-ray pulsar (AXP). Data from the Very Large Array (VLA) MAGPIS survey with angular resolution 6'' reveals a point-source of flux density 4.5 {+-} 0.5 mJy at 1.4 GHz at the precise location of the 5.54 s pulsar XTE J1810-197. This is greater than upper limits from all other AXPs and from quiescent states of soft gamma-ray repeaters (SGRs). The detection was made in 2004 January, 1 year after the discovery of XTE J1810-197 during its only known outburst. Additional VLA observations both before and after the outburst yield only upper limits that are comparable to or larger than the single detection, neither supporting nor ruling out a decaying radio afterglow related to the X-ray turn-on. Another hypothesis is that, unlike the other AXPs and SGRs, XTE J1810-197 may power a radio synchrotron nebula by the interaction of its particle wind with a moderately dense environment that was not evacuated by previous activity from this least luminous, in X-rays, of the known magnetars.

Radio and Optical Follow‐up Observations of a Uniform Radio Transient Search: Implications for Gamma‐Ray Bursts and Supernovae

We had previously reported on a survey for radio transients, used to set an upper limit on the number of orphan gamma-ray burst (GRB) radio afterglows, and thus a lower limit on the typical GRB beaming factor. Here we report radio and optical follow-up observations of these possible transients, achieving the first full characterization of the transient radio sky. We find that only two source are likely to be real radio transients, an optically obscured radio supernova (SN) in the nearby galaxy NGC 4216, and a source not associated with a bright host galaxy, which is too radio luminous to be a GRB afterglow. We speculate that this may be a flare from a peculiar active galactic nucleus, or a burst from an unusual Galactic compact object. We place an upper limit of 65 radio transients above 6 mJy over the entire sky at the 95% confidence level. The implications are as follows. First, we derive a limit on the typical beaming of GRBs; we find f_b^{-1} >~ 60, ~5 times higher than our earlier results. Second, we impose an upper limit on the rate of events that eject >~ 10^{51} erg in unconfined relativistic ejecta, whether or not accompanied by detectable emission in wavebands other than the radio. Our estimated rate, <=1000/y/Gpc, is about two orders of magnitude smaller than the rate of core-collapse SNe (and type Ib/c events in particular), indicating that only a minority of such events eject significant amounts of relativistic material, which are required by fireball models of long-soft GRBs. Finally, we show that future wider and/or deeper radio variability surveys are expected to detect numerous orphan radio GRB afterglows. Our survey also illustrates the great potential of sensitive surveys with new instruments to revolutionize the study of nearby SNe (abridged).

Constraints on the Distance to SGR 1806-20 from H i Absorption

The giant flare detected from the magnetar SGR 1806-20 on 2004 December 27 had a fluence more than 100 times higher than the only two other soft gamma repeater (SGR) flares ever recorded. Whereas the fluence is independent of distance, an estimate for the luminosity of the burst depends on the source's distance, which has previously been argued to be ~15 kpc. The burst produced a bright radio afterglow, against which Cameron et al. have measured an H I absorption spectrum. This has been used to propose a revised distance to SGR 1806-20 of between 6.4 and 9.8 kpc. Here we analyze this absorption spectrum and compare it both to H I emission data from the Southern Galactic Plane Survey and to archival 12CO survey data. We confirm ~6 kpc as a likely lower limit on the distance to SGR 1806-20, but argue that it is difficult to place an upper limit on the distance to SGR 1806-20 from the H I data currently available. The previous value of ~15 kpc thus remains the best estimate of the distance to the source.

The Radio Afterglow and Host Galaxy of the Dark GRB 020819

Of the fourteen gamma-ray bursts (GRBs) localized to better than 2' radius with the SXC on HETE-2, only two lack optical afterglow detections, and the high recovery rate among this sample has been used to argue that the fraction of truly dark bursts is ~10%. While a large fraction of earlier dark bursts can be explained by the failure of ground-based searches to reach appropriate limiting magnitudes, suppression of the optical light of these SXC dark bursts seems likely. Here we report the discovery and observation of the radio afterglow of GRB 020819, an SXC dark burst, which enables us to identify the likely host galaxy (probability of 99.2%) and hence the redshift (z=0.41) of the GRB. The radio light curve is qualitatively similar to that of several other radio afterglows, and may include an early-time contribution from the emission of the reverse shock. The proposed host is a bright R = 19.5 mag barred spiral galaxy, with a faint R ~ 24.0 mag "blob'' of emission, 3" from the galaxy core (16 kpc in projection), that is coincident with the radio afterglow. Optical photometry of the galaxy and blob, beginning 3 hours after the burst and extending over more than 100 days, establishes strong upper limits to the optical brightness of any afterglow or associated supernova. Combining the afterglow radio fluxes and our earliest R-band limit, we find that the most likely afterglow model invokes a spherical expansion into a constant-density (rather than stellar wind-like) external environment; within the context of this model, a modest local extinction of A_V ~ 1 mag is sufficient to suppress the optical flux below our limits.

The Discovery of the Optical and Near‐IR Afterglows of the FirstSwiftGamma‐Ray Bursts

We present optical and near-infrared searches for afterglow emission from the first four Swift bursts with accurate positions from the X-Ray Telescope (XRT). Using telescopes at Las Campanas, Keck, and Palomar observatories, we rapidly identified and followed up afterglows for three of the four bursts and subsequently identified the redshift of GRB 050126 (z = 1.290). In three cases the burst positions were also observed with the Very Large Array, but no radio afterglow emission was detected. The optical/near-IR afterglows are fainter than about 70% of all afterglows detected to date, with GRB 050126 being the faintest, and were identified thanks to accurate and rapid positions from the XRT and rapid response with ≳1 m telescopes. This suggests that the fraction of dust-obscured bursts is small, ≲20% when combined with afterglows localized by the HETE-2 Soft X-ray Camera. The X-ray fluxes are typical of the known population, with the exception of GRB 050126, which has the faintest X-ray afterglow to date (normalized to Δt = 10 hr) and was detected thanks to a response time of only 130 s after the burst. Finally, we find that all three optical/near-IR afterglows are located ≲2'' away from the nominal XRT positions, suggesting that the XRT is capable of delivering highly accurate positions, which will revolutionize afterglow studies.

A Two-Component Explosion Model for the Giant Flare and Radio Afterglow from SGR 1806-20

The brightest giant flare from the soft γ-ray repeater (SGR) 1806-20 was detected on 2004 December 27. The isotropic-equivalent energy release of this burst is at least 1 order of magnitude more energetic than those of the two other SGR giant flares. Starting from about 1 week after the burst, a very bright (~80 mJy), fading radio afterglow was detected. Follow-up observations revealed the multifrequency light curves of the afterglow and the temporal evolution of the source size. Here we show that these observations can be understood in a two-component explosion model. In this model, one component is a relativistic collimated outflow responsible for the initial giant flare and the early afterglow, and the other component is a subrelativistic wider outflow responsible for the late afterglow. We also discuss the triggering mechanisms of these two components within the framework of the magnetar model.

An expanding radio nebula produced by a giant flare from the magnetar SGR 1806–20

Soft gamma-ray repeaters (SGRs) are 'magnetars', a small class of slowly spinning neutron stars with extreme surface magnetic fields, B approximately 10(15) gauss (refs 1 , 2 -3). On 27 December 2004, a giant flare was detected from the magnetar SGR 1806-20 (ref. 2), only the third such event recorded. This burst of energy was detected by a variety of instruments and even caused an ionospheric disturbance in the Earth's upper atmosphere that was recorded around the globe. Here we report the detection of a fading radio afterglow produced by this outburst, with a luminosity 500 times larger than the only other detection of a similar source. From day 6 to day 19 after the flare from SGR 1806-20, a resolved, linearly polarized, radio nebula was seen, expanding at approximately a quarter of the speed of light. To create this nebula, at least 4 x 10(43) ergs of energy must have been emitted by the giant flare in the form of magnetic fields and relativistic particles.

An Energetic Blast Wave from the 2004 December 27 Giant Flare of the Soft Gamma-Ray Repeater SGR 1806-20

Recent follow-up observations of the 2004 December 27 giant flare of SGR 1806 - 20 have detected a multiple-frequency radio afterglow from 240 MHz to 8.46 GHz, extending in time from a week to about a month after the flare. The angular size of the source has also been measured for the first time. Here we show that this radio afterglow provides the first clear evidence of an energetic blast wave sweeping up its surrounding medium and producing a synchrotron afterglow, the same mechanism as has been established for gamma-ray burst afterglows. The optical afterglow is expected to have been intrinsically as bright as m(R) similar or equal to 13 at t less than or similar to 0.1 days after the flare, but very heavy extinction due to the low Galactic latitude of the source would have made detection difficult. Rapid infrared follow-up observations of giant flares are therefore crucial for low-latitude soft gamma-ray repeaters (SGRs), while for high-latitude SGRs ( e. g., SGR 0526 - 66), rapid follow-up should result in the identification of possible optical afterglows. Rapid multiwavelength follow-up will also provide more detailed information on the early evolution of the fireball, as well as its composition.

Radio Afterglows of Gamma‐Ray Bursts and Hypernovae at High Redshift and Their Potential for 21 Centimeter Absorption Studies

We investigate the radio afterglows of gamma-ray bursts (GRBs) and hypernovae (HNe) at high redshifts and quantify their detectability, as well as their potential usefulness for 21 cm absorption line studies of the intergalactic medium (IGM) and intervening structures. We examine several sets of source and environment model parameters that are physically plausible at high redshifts. The radio afterglows of GRBs would be detectable out to z ~ 30, while the energetic HNe could be detectable out to z ~ 20 even by the current Very Large Array (VLA). We find that the 21 cm absorption line due to the diffuse neutral IGM is difficult to detect even by the proposed Square Kilometer Array (SKA), except for highly energetic sources. We also find that the 21 cm line due to collapsed gas clouds with high optical depth may be detected on rare occasions.