Optical and infrared flares from a transient Galactic soft gamma-ray repeater
A. J. Castro-Tirado, A. de Ugarte Postigo, J. Gorosabel, M. Jelinek, T. A. Fatkhullin, V. V. Sokolov, P. Ferrero, D. A. Kann, S. Klose, D. Sluse, M. Bremer, J. … 展开作者
A. J. Castro-Tirado, A. de Ugarte Postigo, J. Gorosabel, M. Jelinek, T. A. Fatkhullin, V. V. Sokolov, P. Ferrero, D. A. Kann, S. Klose, D. Sluse, M. Bremer, J. M. Winters, D. Nuernberger, D. Perez-Ramirez, M. A. Guerrero, J. French, G. Melady, L. Hanlon, B. McBreen, F. J. Aceituno, R. Cunniffe, P. Kubanek, S. Vitek, S. Schulze, A. C. Wilson, R. Hudec, J. M. Gonzalez-Perez, T. Shahbaz, S. Guziy, S. B. Pandey, L. Pavlenko, E. Sonbas, S. A. Trushkin, N. N. Bursov, N. A. Nizhelskij, L. Sabau-Graziati
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Version submitted to Nature on 31 Jan 2008. A substantially revised version of this work has been published in Nature, vol. 455 issue 7212 pp 506-509 under the title "Flares from a Galactic magnetar suggest a missing link to dim isolated neutron stars"
Soft gamma-ray repeaters (SGRs) are a rare type of gamma-ray transient sources that are ocasionally detected as bursts in the high-energy sky. They are thought to be produced by magnetars, young neutron stars with very strong magnetic fields of the order of 10^(14-15) G. Only three such objects are known in our Galaxy, and a fourth one is associated with the supernova remnant N49 in the Large Magellanic Cloud. In none of these cases has an optical counterpart to either the gamma-ray flares or the quiescent source been identified. Here we present multi-wavelength observations of a puzzling source, SWIFT J195509+261406, for which we detected more than 40 flaring episodes in the optical band over a time span of 3 days, plus a faint infrared flare 11 days later, after which it returned to quiescence. We propose that SWIFT J195509+261406 is a member of a subgroup of SGRs for which the long-term X-ray emission is transient in nature. Furthermore, it is the first SGR for which bursts have been detected in the optical and near-infrared bands and maybe the link between the "persistent" SGRs and the dim isolated neutron stars.
The light curve, energy characteristics, and localization of a short/hard GRB 051103 burst are considered. Evidence in favor of identifying this event with a giant flare from a soft gamma repeater in the nearby M81 group of interacting galaxies is discussed.
During the last 35 years three giant flares were observed from so-called Soft Gamma Repeaters (SGR's). They are assumed to be associated with star-quakes of pulsars accelerating electrons and, possibly, protons to high energy in the huge magnetic fields as inferred from the observations. Because of this and the observation of non-thermal emission it has been speculated that they may be cosmic ray accelerators producing gamma-rays up to TeV energies. Neutrino telescopes, such as AMANDA and the ANTARES now under construction, could be used as TeV-gamma detectors for very short emissions by measu…
The light curve, energy spectra, energetics, and IPN localization of an exceedingly intense short duration hard spectrum burst, GRB 070201, obtained from Konus-Wind, INTEGRAL (SPI-ACS), and MESSENGER data are presented. The total fluence of the burst and the peak flux are $S = 2.00_{-0.26}^{+0.10} \times 10^{-5}$ erg cm$^{-2}$ and $F_{max} = 1.61_{-0.50}^{+0.29} \times 10^{-3}$ erg cm$^{-2}$ s$^{-1}$. The IPN error box has an area of 446 square arcminutes and covers the peripheral part of the M31 galaxy. Assuming that the source of the burst is indeed in M31 at a distance of 0.78 Mpc, the meas…
We argue that giant flares in SGRs can be associated to the core conversion of an isolated neutron star having a subcritical magnetic field $\sim 10^{12}$ G and a fallback disk around it. We show that, in a timescale of $\lesssim 10^5$ yrs, accretion from the fallback disk can increase the mass of the central object up to the critical mass for the conversion of the core of the star into quark matter. A small fraction of the neutrino-antineutrino emission from the just-converted quark-matter hot core annihilates into $e^+e^-$ pairs above the neutron star surface originating the gamma emission o…
Soft gamma repeaters are high-energy transient sources associated with neutron stars in young supernova remnants. They emit sporadic, short (~ 0.1 s) bursts with soft energy spectra during periods of intense activity. The event of March 5, 1979 was the most intense and the only clearly periodic one to date. Here we report on an even more intense burst on August 27, 1998, from a different soft gamma repeater, which displayed a hard energy spectrum at its peak, and was followed by a ~300 s long tail with a soft energy spectrum and a dramatic 5.16 s period. Its peak and time integrated energy flu…
Two giant flares were observed on 5 March 1979 and 27 August 1998 from the soft gamma-ray repeaters SGR 0526-66 and SGR 1900+14, respectively. The striking similarity between these remarkable bursts strongly implies a common nature. We show that the light curves of the giant bursts may be easily explained in the model where the burst radiation is produced by the bare quark surface of a strange star heated, for example, by impact of a massive comet-like object.
Recent follow-up observations of the 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 was also measured for the first time. Here we show that this radio afterglow gives the first piece of clear evidence that an energetic blast wave sweeps up its surrounding medium and produces a synchrotron afterglow, the same mechanism as established for gamma-ray burst afterglows. The optical afterglow is expected to be intrinsically as bright as $m…
In this short note I discuss the hypothesis that bursting activity of magnetars evolves in time analogously to the glitching activity of normal radio pulsars (i.e. sources are more active at smaller ages), and that the increase of the burst rate follows one of the laws established for glitching radio pulsars. If the activity of soft gamma repeaters decreases in time in the way similar to the evolution of core-quake glitches ($\propto t^{5/2}$), then it is more probable to find the youngest soft gamma repeaters, but the energy of giant flares from these sources should be smaller than observed $…
The Soft Gamma Repeater SGR 1900+14 entered a remarkable phase of activity during the summer of 1998. This activity peaked on August 27, 1998 when a giant periodic gamma-ray flare resembling the famous March 5, 1979 event from SGR 0526-66 was recorded. Two days later (August 29), a strong, bright burst was detected with RXTE and BATSE. This event reveals several similarities to the giant flares of August 27 and March 5, and shows a number of unique features not previously seen in SGR bursts. Unlike typically short SGR bursts, this event features a 3.5 s burst peak that was preceded by an exten…
Recent observations of giant flares from soft gamma-ray repeaters have exhibited multiple 25-150 Hz oscillations. Frequencies in this range are expected for toroidal shear waves in a neutron star (NS) crust, lending support to Duncan's proposal that such modes may be excited in these events. This motivates a reassessment of how these waves reflect the NS structure and what role the magnetic field plays in setting their frequencies. We calculate the eigenfrequencies and eigenfunctions of toroidal oscillations for a realistic NS crust, including a vertical magnetic field at magnetar strengths ($…
We discuss the high enegry afterglow emission (including high energy photons, neutrinos and cosmic rays) following the 2004 December 27 Giant Flare from SGR 1806-20. If the initial outflow is relativistic with a bulk Lorentz factor Γ_0\sim {\rm tens}, the high-energy tail of the synchrotron emission from electrons in the forward shock region gives rise to a prominent sub-GeV emission, if the electron spectrum is hard enough and if the intial Lorentz factor is high enough. This signal could serve as a diagnosis of the initial Lorentz factor of the giant flare outflow. This component is potentia…
We propose that the best sites to search for SGRs outside the Local group are galaxies with active star formation. Different possibilities to observe SGR activity from these sites are discussed. In particular we searched for giant flares from M82, M83, NGC 253, and NGC 4945 in the BATSE data. We present a list of potential candidates, however, in our opinion no good candidates alike giant SGR flares were found. Still, hyperflares similar to the one of 27 December 2004 can be observed from larger distances. From the BATSE data we select 5 candidates coincident with the galaxies Arp 299 and NGC …
We propose that the best sites to search for SGRs outside the Local group are galaxies with active massive star formation. Different possibilities to observe SGR activity from these sites are discussed. In particular we searched for giant flares from nearby galaxies ($\sim 2$ -- 4 Mpc) M82, M83, NGC 253, and NGC 4945 in the BATSE data. No candidates alike giant SGR flares were found. The absence of such detections implies that the rate of giant flares with energy release in the initial spike above $0.5 \cdot 10^{44}$ erg is less then 1/25 yr$^{-1}$ in our Galaxy. However, hyperflares similar t…
We present a systematic analysis of all the BeppoSAX data of SGR 1900+14. The observations spanning five years show that the source was brighter than usual on two occasions: ~20 days after the August 1998 giant flare and during the 10^5 sec long X-ray afterglow following the April 2001 intermediate flare. In the latter case, we explore the possibility of describing the observed short term softening only with a change of the temperature of a blackbody-like component. In the only BeppoSAX observation performed before the giant flare, the spectrum of the SGR 1900+14 persistent emission was signif…
We present one possible mechanism for the giant flares of the Soft Gamma-Ray Repeaters (SGRs) within the framework of magnetar, i.e., superstrongly magnetized neutron star model, motivated by the positive period increase associated with the August 27 event from SGR 1900+14. From the second-order perturbation analysis of the equilibrium of the magnetic polytrope, we find that there exist different equilibrium states separated by the energy of the giant flares and the shift in the moment of inertia to cause the period increase. This suggests that, if we assume that the global reconfiguration of …
We present a systematic analysis of all the BeppoSAX data of SGR1900+14. The observations spanning five years show that the source was brighter than usual on two occasions: ~20 days after the August 1998 giant flare and during the 10^5 s long X-ray afterglow following the April 2001 intermediate flare. In the latter case, we explore the possibility of describing the observed short term spectral evolution only with a change of the temperature of the blackbody component. In the only BeppoSAX observation performed before the giant flare, the spectrum of the SGR1900+14 persistent emission was sign…
The giant outburst from SGR 1806-20 was observed on 2004 December 27 by many spacecraft. This extremely rare event exhibits a striking similarity to the two giant outbursts thus far observed, on 1979 March 5 from SGR 0526-66 and 1998 August 27 from SGR 1900+14. All the three outbursts start with a short giant radiation pulse followed by a weaker tail. The tail pulsates with the period of neutron star rotation of $\sim$5--8 s, to decay finally in a few minutes. The enormous intensity of the initial pulse proved to be far above the saturation level of the gamma-ray detectors, with the result tha…
In the last few years it has been recognized that two apparently distinct classes of peculiar high-energy sources are actually related and can be explained as young neutron stars with magnetic fields as high as 10e14 - 10e15 Gauss. One of these ''magnetars'', SGR 1806-20, has recently emitted the most powerful giant flare ever recorded. The high-energy observations of SGR 1806-20 carried out with XMM-Newton and INTEGRAL in the past two years showed a long term trend of increasing activity preceding the 2004 December 27 event. INTEGRAL data of this giant flare provided unique evidence for hard …
We report on the results of a ~30 ks Chandra pointing of the soft gamma-ray repeater SGR 1806-20, the first X-ray observation with high spectral resolution performed after the 2004 December 27 Giant Flare. The source was found in a bursting active phase and with a significantly softer spectrum than that of the latest observations before the Giant Flare. The observed flux in the 2-10 keV range was ~2.2 x10^-11 erg/s/cm^2, about 20% lower than that measured three months before the event. This indicates that, although its giant flare was ~100 times more intense than those previously observed in t…
The low energy (<10 keV) X-ray emission of the Soft Gamma-ray Repeater SGR1806-20 has been studied by means of four XMM-Newton observations carried out in the last two years, the latter performed in response to a strong sequence of hard X-ray bursts observed on 2004 October 5. The source was caught in different states of activity: over the 2003-2004 period the 2-10 keV flux doubled with respect to the historical level observed previously. The long term raise in luminosity was accompanied by a gradual hardening of the spectrum, with the power law photon index decreasing from 2.2 to 1.5, and by …
SGR 1806-20 has been observed for more than 2 years with the INTEGRAL satellite. In this period the source went from a quiescent state into a very active one culminating in a giant flare on December 27 2004. Here we report on the properties of all the short bursts detected with INTEGRAL before the giant flare. We derive their number-intensity distribution and confirm the hardness-intensity correlation for the bursts found by Gotz et al. (2004a). Our sample includes a very bright outburst that occurred on October 5 2004, during which over one hundred bursts were emitted in 10 minutes, involving…
A giant flare from the Soft Gamma-ray Repeater SGR 1806-20 has been detected by several satellites on 2004 December 27. This tremendous outburst, the first one observed from this source, was a hundred times more powerful than the two previous giant flares from SGR 0525-66 and SGR 1900+14. We report the results obtained for this event with the Anticoincidence Shield of the SPI spectrometer on board the INTEGRAL satellite, which provides a high-statistics light curve at E>~80 keV. The flare started with a very strong pulse, which saturated the detector for ~0.7 s, and whose backscattered radiati…
We present a geometrical methodology to interpret the periodical light curves of Soft Gamma Repeaters based on the magnetar model and the numerical arithmetic of the three-dimensional magnetosphere model for the young pulsars. The hot plasma released by the star quake is trapped in the magnetosphere and photons are emitted tangent to the local magnetic field lines. The variety of radiation morphologies in the burst tails and the persistent stages could be well explained by the trapped fireballs on different sites inside the closed field lines. Furthermore, our numerical results suggests that t…
On 2004 December 27, a giant flare from the soft gamma repeater 1806$-$20 was observed. The radiation mechanism of the initial peak of the flare would be controversial. In this letter we point out that very high-energy cosmic rays would be produced in the case that the flare was caused by internal shocks, as is usually considered for gamma-ray bursts. The highest energy of cosmic rays can reach $10^{19}$ eV, if the Lorentz factor of the shocks is sufficiently high. Future observations of cosmic rays will inform us about the mechanism of the giant flare.
We first review on the peculiar characteristics of the bursting and flaring activity of the Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars. We then report on the properties of the SGR 1806-20's Giant Flare occurred on 2004 December 27th, with particular interest on the pre and post flare intensity/hardness correlated variability. We show that these findings are consistent with the picture of a twisted internal magnetic field which stresses the star solid crust that finally cracks causing the giant flare (and the observed torsional oscillations). This crustal fracturing is accompanied by …
We report the discovery of persistent hard X-ray emission extending up to 150 keV from the soft gamma-ray repeater SGR 1806-20 using data obtained with the INTEGRAL satellite in 2003-2004. Previous observations of hard X-rays from objects of this class were limited to short duration bursts and rare transient episodes of strongly enhanced luminosity (``flares''). The emission observed with the IBIS instrument above 20 keV has a power law spectrum with photon index in the range 1.5-1.9 and a flux of 3 milliCrabs, corresponding to a 20-100 keV luminosity of ~10^36 erg s^-1 (for a distance of 15 k…
Giant flares from soft gamma-ray repeaters (SGRs) are one of the most violent phenomena in neutron stars. Quasi-periodic oscillations (QPOs) with frequencies ranging from 18 to 1840 Hz have been discovered in the tails of giant flares from two SGRs, and were ascribed to be seismic vibrations or torsional oscillations of magnetars. Here we propose an alternative explanation for the QPOs in terms of standing sausage mode oscillations of flux tubes in the magnetar coronae. We show that most of the QPOs observed in SGR giant flares could be well accounted for except for those with very high freque…
The soft gamma repeater (SGR) 1900+14 emitted the giant flare on 27 August 1998. Most gamma-ray detectors saturated during the initial spike of the giant flare because of the intense flux. However the plasma particle detector onboard GEOTAIL observed the first 300 ms time profile with a time resolution of 5.577 ms and the initial spike of the giant flare was first resolved. The time profile shows some similarities to that of the SGR 1806-20 giant flare in 2004: the clear exponential decay and the small hump in the decay phase around 300 or 400 ms.
We report evidence for an ~ 80 d periodicity in the X-ray flux of the hard transient pulsar XTE J1946+274. The 1.3-12 keV light curve obtained with the RossiXTE All Sky Monitor shows five regularly spaced flares over a ~ 1 year baseline starting from the outburst onset in Sept. 1998. The first and strongest flare is somewhat longer than the subsequent four flares, which recur in a fairly periodic fashion. This suggests that the profile of the first flare is dominated by the time variability of the Be star ejection episode, while the following four flares are primarily caused by the neutron sta…
We show that the giant flares of soft-gamma ray repeaters (E~10^44 ergs) can push the inner regions of a fall-back disk out to larger radii by radiation pressure, while matter remains bound to the system for plausible parameters. The subsequent relaxation of this pushed-back matter can account for the observed enhanced X-ray emission after the August 27 giant flare of SGR 1900+14. Based on the results of our models, we estimate that the ratio of the fluences of the enhanced X-ray emissions to that of the preceding bursts remains constant for a particular SGR with similar preburst inner disk co…
We report on X-ray observations made with the Rossi X-ray Timing Explorer of the black hole candidate (BHC) transient XTE J1650-500 at the end of its first, and currently only, outburst. By monitoring the source at low luminosities over several months, we found 6 bright ~100 second X-ray flares and long time scale oscillations of the X-ray flux. The oscillations are aperiodic with a characteristic time scale of 14.2 days and an order of magnitude variation in the 2.8-20 keV flux. The oscillations may be related to optical "mini-outbursts" that have been observed at the ends of outbursts for ot…
The transient emission produced behind internal shocks that are driven by overtaking collisions of a magnetized, relativistic outflow is considered. A self-consistent model capable of describing the structure and dynamics of the shocks and the time evolution of the pair-and gamma-ray distribution functions is developed and applied to gamma-ray flares in blazars, in the case in which gamma-ray production is dominated by inverse Compton scattering of external radiation (ERC). The dependence of the flare properties on magnetic field dissipation rate, intensity of ambient radiation, and the thickn…
Recent progress on the nature of short duration Gamma Ray Bursts (GRBs) has shown that a fraction of them originate in the local universe. These systems may well be the result of giant flares from Soft Gamma Repeaters (SGRs) believed to be magnetars (neutron stars with extremely large magnetic fields >= 10^14 G). If these magnetars are formed via the core collapse of massive stars, then it would be expected that the bursts should originate from predominantly young stellar populations. However, correlating the positions of BATSE short bursts with structure in the local universe reveals a correl…
We present a time-resolved spectrophotometric study of the optical variability in the quiescent soft X-ray transient A0620-00. Superimposed on the double-humped continuum lightcurve are the well known flare events which last tens of minutes. Some of the flare events that appear in the continuum lightcurve are also present in the emission line lightcurves. From the Balmer line flux and variations, we find that the persistent emission is optically thin. During the flare event at phase 1.15 the Balmer decrement dropped suggesting either a significant increase in temperature or that the flares are…
Soft gamma-ray repeaters (SGR) are a class of high energy transients whose brief emissions are thought to arise from young and highly magnetized neutron stars. The exact cause for these outbursts and the nature of the energy loss remain unknown. Here we report the discovery of a fading radio source within the localization of the relatively under-studied SGR 1900+14. We argue that this radio source is a short-lived nebula powered by the particles ejected during the intense high energy activity in late August 1998, which included the spectacular gamma-ray burst of August 27. The radio observatio…
We have analyzed {\it Rossi X-ray Timing Explorer} (RXTE) data of the neutron star transient Aquila X-1 obtained during its outbursts in May/June 1999 and September/October 2000. We find that in the early rise of these outbursts, a hard flare in the energy range above 15 keV preceded the soft X-ray peak. The hard X-ray flux of the hard flares at maximum was more than a factor of three stronger than at any other point in the outbursts. The rise of the hard X-ray flare to this maximum, was consistent with a monotonically brightening low/hard state spectrum. After the peak of the hard flare, a sh…
We present the results of a LIGO search for short-duration gravitational waves (GWs) associated with Soft Gamma Repeater (SGR) bursts. This is the first search sensitive to neutron star f-modes, usually considered the most efficient GW emitting modes. We find no evidence of GWs associated with any SGR burst in a sample consisting of the 27 Dec. 2004 giant flare from SGR 1806-20 and 190 lesser events from SGR 1806-20 and SGR 1900+14 which occurred during the first year of LIGO's fifth science run. GW strain upper limits and model-dependent GW emission energy upper limits are estimated for indiv…
Supergiant Fast X-ray Transients (SFXTs) are a sub-class of High Mass X-ray Binaries (HMXBs) associated with OB supergiant companions and displaying transient X-ray activity. This behaviour is quite surprising since HMXBs hosting supergiants were known to be persistent sources, until the INTEGRAL discoveries obtained by means of the monitoring of the Galactic plane. We have been performing a monitoring campaign with Swift of four SFXTs with the main aim of characterizing both the long-term behaviour of these transients and the properties during bright outbursts. Here we discuss the properties …
[abridged] We model the X-ray reprocessing from a strong co-rotating flare above an accretion disk in active galactic nuclei. We explore the horizontal structure and evolution of the underlying hot spot. To obtain the spectral evolution seen by a distant observer, we apply a general relativity ray-tracing technique. We concentrate on the energy band around the iron K-line, where the relativistic effects are most pronounced. Persistent flares lasting for a significant fraction of the orbital time scale and short, transient flares are considered. In our time-resolved analysis, the spectra record…
With a redshift of z=6.295, GRB 050904 is the most distant gamma-ray burst ever discovered. It was an energetic event at all wavelengths and the afterglow was observed in detail in the near-infrared bands. We gathered all available optical and NIR afterglow photometry of this GRB to construct a composite NIR light curve spanning several decades in time and flux density. Transforming the NIR light curve into the optical, we find that the afterglow of GRB 050904 was more luminous at early times than any other GRB afterglow in the pre-\emph{Swift} era, making it at these wavelengths the most lumi…
The supergiant fast X-ray transient IGR J08408-4503 was discovered by INTEGRAL on May 15, 2006, during a bright flare. The source shows sporadic recurrent short bright flares, reaching a peak luminosity of 10^36 erg s^-1 within less than one hour. The companion star is HD 74194, an Ob5Ib(f) supergiant star located at 3 kpc in the Vela region. We report the light curves and broad-band spectra (0.1-200 keV) of all the three flares of IGR J08408-4503 detected up to now based on INTEGRAL and Swift data. The flare spectra are well described by a power-law model with a high energy cut-off at ~15 keV…
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 one order of magnitude more energetic than those of the two other SGR giant flares. Starting from about one week after the burst, a very bright ($\sim 80$ mJy), fading radio afterglow was detected. Follow-up observations revealed the multi-frequency 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 mod…
Compact steady sources of X-ray emission have been detected at the positions of at least two soft gamma-ray repeaters (SGRs). These sources have been interpreted as synchrotron nebulae powered by the neutron star that is causing the bursts. We explore a plerion model for the sources surrounding SGRs where the steady observed emission is powered by the SGR bursts rather than by the spin-down of a pulsar. In this case there is no limit on the neutron star magnetic field. We find that the synchrotron lifetime of the particles injected into the plerion around SGR1806-20 is long enough to smear out…
We report on spectral and temporal results of the 40-700 keV observations, obtained with the Gamma-Ray Burst Monitor (GRBM) on board BeppoSAX, of the two large flares from the Soft Gamma-ray Repeater SGR1900+14 occurred on August 27, 1998 and April 18, 2001. From their intensity, fluence and duration, the first one was classified as "giant" and the second as "intermediate". The spectral results have been obtained with an improved response function of the GRBM. We find that the two events have similar spectral properties, but different temporal properties. The major difference concerns the time…
We report multifrequency radio observations of XTE J1550-564 obtained with the Molonglo Observatory Synthesis Telescope and the Australia Telescope Compact Array at the time of its discovery and subsequent hard and soft X-ray outburst in 1998 September. A large radio flare was observed, peaking about 1.8 days after the X-ray flare. In addition, we present Australian Long Baseline Array images obtained shortly after the maximum of the radio flare which show evolving structure. The apparent separation velocity of the two outermost components is v>2c.
In December 2004, the soft gamma-ray repeater SGR 1806-20 emitted the most powerful giant flare ever observed. This probably involved a large-scale rearrangement of the magnetosphere leading to observable variations in the properties of its X-ray emission. Here we present the results of the first Suzaku observation of SGR 1806-20, together with almost simultaneous observations with XMM-Newton and INTEGRAL. The source seems to have reached a state characterized by a flux close to the pre-flare level and by a relatively soft spectrum. Despite this, SGR 1806-20 remained quite active also after th…
The Solar Optical Telescope (SOT) on board Hinode satellite observed an X3.4 class flare on 2006 December 13. Typical two-ribbon structure was observed, not only in the chromospheric CaII H line but also in G-band and FeI 6302A line. The high-resolution, seeing-free images achieved by SOT revealed, for the first time, the sub-arcsec fine structures of the "white light" flare. The G-band flare ribbons on sunspot umbrae showed a sharp leading edge followed by a diffuse inside, as well as previously known core-halo structure. The underlying structures such as umbral dots, penumbral filaments and …
We report on the discovery of a transient X-ray pulsar, AX J1841.0-0536, serendipitously found in the Scutum arm region with the ASCA in two separate observations. The X-ray flux is very faint at the beginning, but exhibits two flares in the second observation. The flare flux increases by a factor 10 within only about 1 hr. Coherent pulsations with a period of 4.7394 plus/minus 0.0008 s were detected in the brightest flare phase. The X-ray spectra in the quiescent and flare phases were fitted with an absorbed power-law model with a photon index about 1 plus a narrow Gaussian line at the center…
I summarize the recent advances in our understanding of the Soft Gamma Repeaters: in particular their spin behavior, persistent emission and hyper-Eddington outbursts. The giant flares on 5 March 1979 and 27 August 1998 provide compelling physical evidence for magnetic fields stronger than 10 B_{QED} = 4.4 x 10^{14} G, consistent with the rapid spindown detected in two of these sources. The persistent X-ray emission and variable spindown of the 6-12 s Anomalous X-ray Pulsars are compared and contrasted with those of the SGRs, and the case made for a close connection between the two types of so…
The simultaneous presence of a strong quasi periodic oscillation of period of about 10 seconds in the optical and X-ray lightcurves of the X-ray transient XTE J1118+480 suggests that a significant fraction of the optical flux originates from the inner part of the accretion flow, where most of the X-rays are produced. We present a model of magnetic flares in an accretion disc corona where thermal cyclo-synchrotron emission contributes significantly to the optical emission, while the X-rays are produced by inverse Compton scattering of the soft photons produced by dissipation in the underlying d…
There is increasing evidence of a local population of short duration Gamma-ray Bursts (sGRB), but it remains to be seen whether this is a separate population to higher redshift bursts. Here we choose plausible Luminosity Functions (LF) for both neutron star binary mergers and giant flares from Soft Gamma Repeaters (SGR), and combined with theoretical and observed Galactic intrinsic rates we examine whether a single progenitor model can reproduce both the overall BATSE sGRB number counts and a local population, or whether a dual progenitor population is required. Though there are large uncertai…
Thanks to INTEGRAL's long exposures of the Galactic Plane, the two brightest Soft Gamma-Ray Repeaters, SGR 1806-20 and SGR 1900+14, have been monitored and studied in detail for the first time at hard-X/soft gamma rays. This has produced a wealth of new scientific results, which we will review here. Since SGR 1806-20 was particularly active during the last two years, more than 300 short bursts have been observed with INTEGRAL. and their characteristics have been studied with unprecedented sensitivity in the 15-200 keV range. A hardness-intensity anticorrelation within the bursts has been disco…
A radio afterglow was detected following the 1998 August 27 giant flare from the soft gamma repeater (SGR) 1900+14. Its short-lived behavior is quite different from the radio nebula of SGR 1806-20, but very similar to radio afterglows from classic gamma-ray bursts (GRBs). Motivated by this, we attempt to explain it with the external shock model as invoked in the standard theory of GRB afterglows. We find that the light curve of this radio afterglow is not consistent with the forward shock emission of an ultra-relativistic outflow, which is suggested to be responsible for the initial hard spike…
The data collected by the Pierre Auger Observatory are analyzed to search for coincidences between the arrival directions of high-energy cosmic rays and the positions in the sky of astrophysical transients. Special attention is directed towards gamma ray observations recorded by NASA's Swift mission, which have an angular resolution similar to that of the Auger surface detectors. In particular, we check our data for evidence of a signal associated with the giant flare that came from the soft gamma repeater 1806-20 on December 27, 2004.
We have analyzed Rossi X-ray Timing Explorer Proportional Counter Array observations of the pulsed emission from SGR 1900+14 during September 1996, June - October 1998, and early 1999. Using these measurements and results reported elsewhere, we construct a period history of this source for 2.5 years. We find significant deviations from a steady spin-down trend during quiescence and the burst active interval. Burst and Transient Source Experiment observations of the burst emission are presented and correlations between the burst activity and spin-down rate of SGR 1900+14 are discussed. We find …
The time dependent synchrotron emission from relativistic jets, and the relation between the synchrotron and ERC emission is considered within the framework of the radiative front model. The timescale and profile of the optically thin emission are shown to be determined, in this model, by the shock formation radius, the thickness of expelled fluid slab and the variation of the front's parameters due to its transverse expansion. For a range of reasonable conditions, a variety of flare shapes can be produced, varying from roughly symmetric with exponential rises and decays, as often seen in blaz…
Variability is a common characteristic of magnetically active stars. Flaring variability is usually interpreted as the observable consequence of transient magnetic reconnection processes happening in the stellar outer atmosphere. Stellar flares have been observed now across 11 decades in wavelength/frequency/energy; such a large span implies that a range of physical processes takes place during such events. Despite the fact that stellar radio flares have long been recognized and studied, key unanswered questions remain. I will highlight what, in my opinion, are some of these questions. I will …
The sky region containing the soft gamma-ray repeater SGR 1627-41 has been observed three times with XMM-Newton in February and September 2004. SGR 1627-41 has been detected with an absorbed flux of ~9x10^{-14} erg cm^{-2} s^{-1} (2-10 keV). For a distance of 11 kpc, this corresponds to a luminosity of ~3x10^{33} erg s^{-1}, the smallest ever observed for a Soft Gamma Repeater and possibly related to the long period of inactivity of this source. The observed flux is smaller than that seen with Chandra in 2001-2003, suggesting that the source was still fading and had not yet reached a steady qu…
After a couple of years of quiescence, the soft gamma repeater SGR 1900+14 suddenly reactivated on 18 April 2001, with the emission of a very intense, long and modulated flare, only second in intensity and duration to the 27 August 1998 giant flare. BeppoSAX caught the large flare with its Gamma Ray Burst Monitor and with one of the Wide Field Cameras. The Wide Field Cameras also detected X-ray bursting activity shortly before the giant flare. A target of opportunity observation was started only 8 hours after the large flare with the Narrow Field Instruments, composed of two 60-ks long pointin…
On December 27th 2004, a giant gamma flare from the Soft Gamma-ray Repeater 1806-20 saturated many satellite gamma-ray detectors. This event was by more than two orders of magnitude the brightest cosmic transient ever observed. If the gamma emission extends up to TeV energies with a hard power law energy spectrum, photo-produced muons could be observed in surface and underground arrays. Moreover, high-energy neutrinos could have been produced during the SGR giant flare if there were substantial baryonic outflow from the magnetar. These high-energy neutrinos would have also produced muons in an…
