Giant Flare Research Articles

GRB 070201: A Possible Soft Gamma‐Ray Repeater in M31

The gamma-ray burst (GRB) 070201 was a bright short-duration hard-spectrum GRB detected by the Inter-Planetary Network (IPN). Its error quadrilateral, which has an area of 0.124 sq. deg, intersects some prominent spiral arms of the nearby M31 (Andromeda) galaxy. Given the properties of this GRB, along with the fact that LIGO data argues against a compact binary merger origin in M31, this GRB is an excellent candidate for an extragalactic Soft Gamma-ray Repeater (SGR) giant flare, with energy of 1.4x10^45 erg. Analysis of ROTSE-IIIb visible light observations of M31, taken 10.6 hours after the burst and covering 42% of the GRB error region, did not reveal any optical transient down to a limiting magnitude of 17.1. We inspected archival and proprietary XMM-Newton X-ray observations of the intersection of the GRB error quadrilateral and M31, obtained about four weeks prior to the outburst, in order to look for periodic variable X-ray sources. No SGR or Anomalous X-ray Pulsar (AXP) candidates (periods in range 1 to 20 s) were detected. We discuss the possibility of detecting extragalactic SGRs/AXPs by identifying their periodic X-ray light curves. Our simulations suggest that the probability of detecting the periodic X-ray signal of one of the known Galactic SGRs/AXPs, if placed in M31, is about 10% (50%), using 50 ks (2 Ms) XMM-Newton exposures.

Ionospheric disturbances caused by SGR 1900+14 giant gamma ray flare in 1998: Constraints on the energy spectrum of the flare

On 27 August 1998, the Soft Gamma Ray Repeater (SGR) 1900+14, which is an exotic neutron star located near the Galactic Center, produced a giant flare. Gamma rays from the giant flare unusually ionized the lower ionosphere, and the ionospheric disturbance was detected as a large‐amplitude change of the VLF signal whose propagation distance is relatively short (870 km). The peak flux of the flare was so huge that it saturated all the gamma ray detectors on the spacecraft; consequently, the flux and the spectrum during the most intense period was poorly determined. Tanaka et al. (2007) have recently derived the accurate peak flux of the flare from the Geotail data. By means of model calculations based on this accurate estimation and their comparisons with the short‐distance VLF data, we have found that the spectrum during the most intense period was one temperature (kT = 240 keV) optically thin thermal Bremsstrahlung (OTTB). This result provides us with a clue to reveal the emission and triggering mechanisms of the giant flare.

Very-High-Energy Gamma Rays from a Distant Quasar: How Transparent Is the Universe?

The atmospheric Cherenkov gamma-ray telescope MAGIC, designed for a low-energy threshold, has detected very-high-energy gamma rays from a giant flare of the distant Quasi-Stellar Radio Source (in short: radio quasar) 3C 279, at a distance of more than 5 billion light-years (a redshift of 0.536). No quasar has been observed previously in very-high-energy gamma radiation, and this is also the most distant object detected emitting gamma rays above 50 gigaelectron volts. Because high-energy gamma rays may be stopped by interacting with the diffuse background light in the universe, the observations by MAGIC imply a low amount for such light, consistent with that known from galaxy counts.

On the Nature of Quasi‐periodic Oscillations in the Tail of Soft Gamma Repeater Giant Flares

A model is presented for the quasiperiodic component of magnetar emission during the tail phase of giant flares. The model invokes modulation of the particle number density in the magnetosphere. The magnetospheric currents are modulated by torsional motion of the surface and we calculate that the amplitude of neutron star surface oscillation should be ~1% of the NS radius in order to produce the observed features in the power spectrum. Using an axisymmetric analytical model for structure of the magnetosphere of an oscillating NS, we calculate the angular distribution of the optical depth to the resonant Compton scattering. The anisotropy of the optical depth may be why QPO are observed only at particular rotational phases.

A Giant Flare from a Soft Gamma Repeater in the Andromeda Galaxy (M31)

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 measured values of the fluence $S$ and maximum flux $F_{max}$ correspond to a total energy of $Q = 1.5 \times 10^{45}$ erg, and a maximum luminosity $L = 1.2 \times 10^{47}$ erg s$^{-1}$. These data are in good agreement with the corresponding characteristics of the previously observed giant flares from other soft gamma repeaters. The evidence for the identification of this event as a giant flare from a soft gamma repeater in the M31 galaxy is presented.

Synchrotron Self‐Compton Analysis of TeV X‐Ray‐Selected BL Lacertae Objects

We introduce a methodology for analysis of multiwavelength data from X-ray-selected BL Lac (XBL) objects detected in the TeV regime. By assuming that the radio-through-X-ray flux from XBLs is nonthermal synchrotron radiation emitted by isotropically distributed electrons in the randomly oriented magnetic field of a relativistic blazar jet, we obtain the electron spectrum. This spectrum is then used to deduce the synchrotron self-Compton (SSC) spectrum as a function of the Doppler factor, magnetic field, and variability timescale. The variability timescale is used to infer the comoving blob radius from light-travel time arguments, leaving only two parameters. With this approach, we accurately simulate the synchrotron and SSC spectra of flaring XBLs in the Thomson through Klein-Nishina regimes. Photoabsorption by interactions with internal jet radiation and the intergalactic background light (IBL) is included. Doppler factors, magnetic fields, and absolute jet powers are obtained by fitting the H.E.S.S. and Swift data of the recent giant TeV flare observed from PKS 2155–304. For the H.E.S.S. and Swift data from 2006 July 28 and 30, respectively, Doppler factors ≳60 and absolute jet powers ≳1046 ergs s−1 are required for a synchrotron/SSC model to give a good fit to the data, for a low intensity of the IBL and a ratio of 10 times more energy in hadrons than nonthermal electrons. Fits are also made to a TeV flare observed in 2001 from Mrk 421 which require Doppler factors ≳30 and jet powers ≳1045 ergs s−1.

Multiwavelength observations of a giant flare on CN Leonis

Multiwavelength observations of a giant flare on CN Leonis

Keynote address: Outstanding problems in solar physics

Celebrating the diamond jubilee of the Physics Research Laboratory (PRL) in Ahmedabad, India, we look back over the last six decades in solar physics and contemplate on the ten outstanding problems (or research foci) in solar physics: 1. The solar neutrino problem 2. Structure of the solar interior (helioseismology) 3. The solar magnetic field (dynamo, solar cycle, corona) 4. Hydrodynamics of coronal loops 5. MHD oscillations and waves (coronal seismology) 6. The coronal heating problem 7. Self-organized criticality (from nanoflares to giant flares) 8. Magnetic reconnection processes 9. Particle acceleration processes 10. Coronal mass ejections and coronal dimming

A downward revision to the distance of the 1806−20 cluster and associated magnetar from Gemini Near-Infrared Spectroscopy

Abstract We present H- and K-band spectroscopy of OB and Wolf–Rayet (WR) members of the Milky Way cluster 1806−20 (G10.0–0.3) to obtain a revised cluster distance, of relevance to the 2004 giant flare from the (soft gamma repeater) SGR 1806−20 magnetar. From GNIRS (Gemini Near-Infrared Spectrograph) spectroscopy obtained with Gemini South, four candidate OB stars are confirmed as late O/early B supergiants, while we support previous mid-WN and late WC classifications for two WR stars. Based upon an absolute Ks-band magnitude calibration for B supergiants and WR stars, and near-infrared (IR) photometry from NIRI (Near-Infrared Imager) at Gemini North plus archival VLT/ISAAC (Very Large Telescope/Infrared Spectrometer And Array Camera) data sets, we obtain a cluster distance modulus of 14.7 ± 0.35 mag. The known stellar content of the 1806−20 cluster suggests an age of 3–5 Myr, from which theoretical isochrone fits infer a distance modulus of 14.7 ± 0.7 mag. Together, our results favour a distance modulus of 14.7 ± 0.4 mag (8.7+1.8−1.5 kpc) to the 1806−20 cluster, which is significantly lower than the nominal 15 kpc distance to the magnetar. For our preferred distance, the peak luminosity of the 2004 December giant flare is reduced by a factor of 3 to 7 × 1046 erg s−1, such that the contamination of BATSE (Burst And Transient Source Experiment) short gamma-ray bursts (GRBs) from giant flares of extragalactic magnetars is reduced to a few per cent. We infer a magnetar progenitor mass of ∼48+20−8 M⊙, in close agreement with that obtained recently for the magnetar in Westerlund 1.

An infrared ring around the magnetar SGR 1900+14

Magnetars are a special class of slowly rotating (period approximately 5-12 s) neutron stars with extremely strong magnetic fields (>10(14 )G)--at least an order of magnitude larger than those of the 'normal' radio pulsars. The potential evolutionary links and differences between these two types of object are still unknown; recent studies, however, have provided circumstantial evidence connecting magnetars with very massive progenitor stars. Here we report the discovery of an infrared elliptical ring or shell surrounding the magnetar SGR 1900+14. The appearance and energetics of the ring are difficult to interpret within the framework of the progenitor's stellar mass loss or the subsequent evolution of the supernova remnant. We suggest instead that a dust-free cavity was produced in the magnetar environment by the giant flare emitted by the source in August 1998. Considering the total energy released in the flare, the theoretical dust-destruction radius matches well with the observed dimensions of the ring. We conclude that SGR 1900+14 is unambiguously associated with a cluster of massive stars, thereby solidifying the link between magnetars and massive stars.

A Morphological Approach to the Pulsed Emission from Soft Gamma Repeaters

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 starquake is trapped in the magnetosphere, and photons are emitted tangentially 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 suggest that the pulse profile evolution of SGR 1806–20 during the 2004 December 27 giant flare is due to a lateral drift of the emitting region in the magnetosphere.

EIGENFREQUENCIES OF NODELESS ELASTIC VIBRATIONS LOCKED IN THE CRUST OF QUAKING NEUTRON STAR

The Newtonian solid-mechanical theory of non-compressional spheroidal and torsional nodeless elastic vibrations in the homogenous crust model of a quaking neutron star is developed and applied to the modal classification of the quasi-periodic oscillations (QPOs) of X-ray luminosity in the aftermath of giant flares in SGR 1806-20 and SGR 1900+14. Particular attention is given to the low-frequency QPOs in the data for SGR 1806-20 whose physical origin has been called into question. Our calculations suggest that unspecified QPOs are due to nodeless dipole torsional and dipole spheroidal elastic shear vibrations.

On the nature of the short-duration GRB 050906 ★

We present deep optical and infrared observations of the short duration GRB 050906. Although no X-ray or optical/IR afterglow was discovered to deep limits, the error circle of the GRB (as derived from the Swift BAT) is unusual in containing the relatively local starburst galaxy IC328. This makes GRB 050906 a candidate burst from a soft-gamma repeater, similar to the giant flare from SGR 1806-20. The probability of chance alignment of a given BAT position with such a galaxy is small ( � < 1%), although the size of the error circle (2.6 arcminute radius) is such that a higher z origin can’t be ruled out. Indeed, the error circle also includes a moderately rich galaxy cluster at z = 0.43, which is a plausible location for the burst given the apparent preference that short GRBs have for regions of high mass density. No residual optical or infrared emission has been observed, either in the form of an afterglow or later time emission from any associated supernova-like event. We discuss the constraints these limits place on the progenitor of GRB 050906 based on the expected optical signatures from both SGRs and merging compact object systems.

ASTROPHYSICAL ASPECTS IN THE STUDIES OF SOLAR COSMIC RAYS

This review paper comprises main concepts, available observational data and recent theoretical results related to astrophysical aspects of particle acceleration at/near the Sun and extreme capacities of the solar accelerator(s). We summarize underground and ground-based observations of solar cosmic rays (SCR) accumulated since 1942, direct spacecraft measurements of solar energetic particles (SEP) near the Earth's orbit, indirect information on the SCR variations in the past, and other relevant astrophysical, solar and geophysical data. The list of the problems under discussion includes: upper limit spectrum (ULS) for solar cosmic rays; maximum energy (rigidity), Em(Rm), of particles accelerated at/near the Sun; production of the flare neutrinos; energetics of SCR and solar flares; production of flare neutrons and gamma rays; charge states and elemental abundances of accelerated solar ions; coronal mass ejections (CME's) and extended coronal structures in acceleration models; magnetic reconnection in acceleration scenarios; size (frequency) distributions of solar proton events (SPE) and stellar flares; occurrence probability of giant flares; archaeology of solar cosmic rays. The discussion allows us to outline a series of interesting conceptual and physical associations of SCR generation with the high-energy processes at other stars. The most reliable estimates of various parameters are given in each of research fields mentioned above; a set of promising lines of future studies is highlighted. A great importance of SCR data for resolving some general astrophysical problems is emphasized.

Anomalous X-ray emission in GRB 060904B: a nickel line?

Context. The detection of an extra component in GRB 060904B X-ray spectra in addition to the standard single power-law behaviour has recently been reported in the literature. This component can be fit with di fferent models; in particular the addition of a spectral line provides the best representation. Aims. In this paper we investigate the physical properties that th e surrounding medium must have in order to produce a spectral feature that can explain the detected emission. Methods. We analyse and discuss how and if the detected spectral excess fits in di fferent theoretical models developed to explain the nature of line emission during the afterglow phase of Gamma-Ray Bursts (GRBs). Trasmission and reflection models hav e been considered. Results. Given the high value (≫ 1) of the Thomson optical depth, the emission is likely to arise in a reflection scenario. Within reflection models, the external reflection geometry fails to predict the observed luminosity. On the contrary, the detected feature can be explained in a funnel scenario with typical opening angleθ∼ 5 ◦ , Nickel mass∼ 0.1 M⊙ and T = 10 6 K. Forθ∼ 20 ◦ , assuming the reprocessing material to be the SN shell, the detected emission implies a Nickel mass∼ 0.4 M⊙ at T ∼ 10 7 K and a metallicity∼ 10 times the solar value. If the giant X-ray flare that dominates the early XRT light curve is identified with the ionizing sour ce, the SN expansion began∼ 3000 s before the GRB event .

Soft gamma repeaters activity in time

AbstractIn 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 (∝t5/2), 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 1044–1046 erg as the total energy stored in a magnetar's magnetic field is not enough to support thousands of bursts similar to the prototype 1979 March 5 flare. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Nonrelativistic and Relativistic Treatments for Propagation of Torsional Resonant Alfven Waves in Strongly Magnetized Neutron Stars

We study the propagation and transmission of torsional Alfven waves in magnetars. First, we consider a simple two-layer model composed of an outer crust and vaccum magnetosphere. In this model, we derive the classical formulation for the transmission coefficient of Alfven waves using an MHD approximation. We then include a displacement current in Maxwell’s equations, which can be shown to become important in the magnetically relativistic region of the crust, B 4πρc. The effects of strong magnetic fields of magnetars are specifically quantified and discussed. Second, we consider a three-layer model in order to investigate the propagation properties of Alfven waves excited in the deep crust. This model is adopted for the inner crust, outer crust, and vacuum magnetosphere. It is shown that the outer crust plays a significant role as a resonant cavity if a certain condition holds. Some nonrelativistic resonance of Alfven modes with low frequencies obtained in this work is probably associated with the observed QPOs from soft gamma-ray repeaters. The estimated energy flux carried by relativistic Alfven waves is in good agreement with the observed values, E = 10–10 erg s−1, of giant flares.

Interacting coronae of two T Tauri stars: first observational evidence for solar-like helmet streamers

Context. The young binary system V773 Tau A exhibits a persistent radio flaring activity that gradually increases from a level of a few mJy at apoastron to more than 100 mJy at periastron. Interbinary collisions between very large (>15 R∗) magnetic structures anchored on the two rotating stars of the system have been proposed to be the origin of these periodic radio flares. Magnetic structures extended over tens of stellar radii, that can also account for the observed fast decay of the radio flares, seem to correspond to the typical solar semi-open quite extended magnetic configurations called helmet streamers. Aims. We aim to find direct observational evidence for the postulated, solar-like, coronal topologies. Methods. We performed seven-consecutive-day VLBI observations at 8.4 GHz using an array consisting of the VLBA and the 100-m Effelsberg telescope. V773 Tau A was phase-referenced to QSO B0400+258. Results. Two distinctive structures appear in the radio images here presented. They happen to be associated with the primary and secondary stars of the V773 Tau A system. In one image (Fig. 2B) the two features are extended up to 18 R∗ each and are nearly parallel revealing the presence of two interacting helmet streamers. One image (Fig. 2E) taken a few hours after a flare monitored by the 100-m Effelsberg telescope shows one elongated fading structure substantially rotated with respect to those seen in the B run. The same decay scenario is seen in Fig. 2G for the helmet streamer associated with the other star. Conclusions. This is the very first direct evidence revealing that even if the flare origin is magnetic reconnection due to interbinary collision, both stars independently emit in the radio range with structures of their own. These structures are helmet streamers, observed for the first time in stars other than the Sun. The complete extent of each helmet streamer above the stellar surface is about 24 R∗ which implies that they can practically interact throughout the whole orbit, even rather close to apoastron where the stellar separation is 52 R∗. However, the radio flares become stronger when the stars approach. Around periastron the stellar separation is only 30 R∗, nearly covered by a single streamer: the two streamers overlap producing the observed giant flares.

SGR 1806-20 about two years after the giant flare:Suzaku,XMM-Newton andINTEGRAL observations

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 also remained quite active after the giant flare, allowing us to study several short bursts observed by Suzaku in the 1–100 keV range. We discuss the broad-band spectral properties of SGR 1806-20, covering both persistent and bursting emission, in the context of the magnetar model, and consider its recent theoretical developments.

Radio emission of the magnetar SGR 1806-20: Evolution of the magnetic field in the region of the radio afterglow

The results of radio observations of the afterglow produced by a giant gamma-ray flare from the magnetar SGR 1806-20 on December 27, 2004 are reported. The observations were carried out on the 32-m radio telescope of the Zelenchuk Observatory of the Institute of Applied Astronomy, Russian Academy of Sciences, at a wavelength of 3.5 cm. The observations confirm the enhanced radio brightness of the nebula around the magnetar in the period from 25 to 31 days after the flare. A comparison of the Zelenchuk observations with other data have enabled us to estimate the magnetic-field intensity in the radio-afterglow region using a model of synchrotron radiation with self-absorption in a relativistic plasma. The kinetic energy of the blast wave produced by the giant explosion of the magnetar is estimated.