Soft Gamma-ray Repeaters Research Articles

Understanding AXPs and SGRs through the December 2004 SGR 1806–20's Hyperflare

On 27th December 2004 SGR 1806?20, one of the most active Soft ?-ray Repeaters (SGRs), displayed an extremely rare event, also known as giant flare, during which up to 1047 erg were released in the ~1?1000 keV range in less than 1s. Follow-up VLA observations after the giant flare largely increased the position accuracy of SGR 1806?20, allowing to perform sensitive searches at shorter wavelengths. In particular we carried out observations by using IR adaptive optics (NAOS-CONICA) mounted on VLT which provided images of unprecedented quality (FWHM better than 0.1''). We discovered the likely IR counterpart to SGR 1806?20 based on positional coincidence with the VLA uncertainty region and flux variability of a factor of about 2 correlated with that at higher energies. Among the several high energy datasets obtained during the 27th December 2004 event, we analyzed those taken from the Rossi?XTE which provided the highest ever statistics for a giant flare. We discovered rapid Quasi Periodic Oscillations (QPOs) in RXTE/PCA data of the pulsating tail of the 27th December 2004 giant flare. QPOs at ~92.5 Hz are detected in a 50 s interval starting 170 s after the onset of the giant flare. This is the first time that QPOs are unambiguously detected in the flux of a Soft Gamma-ray Repeater, or any other magnetar candidate. We interpret the highest QPOs in terms of the coupling of toroidal seismic modes with Alfv?n waves propagating along magnetospheric field lines.

The FirstXMM‐NewtonObservations of the Soft Gamma‐Ray Repeater SGR 1900+14

A ~50 ks XMM-Newton observation of SGR 1900+14 has been carried out in September 2005, after almost three years during which no bursts were detected from this soft gamma-ray repeater. The 0.8-10 keV spectrum was well fit by a power law plus blackbody model with photon index 1.9+/-0.1, temperature kT=0.47+/-0.02 keV and N_H = (2.12+/-0.08)x10^22 cm^-2, similar to previous observations of this source. The flux was ~5x10^{-12} erg/cm2/s, a factor 2 dimmer than the typical value and the smallest ever seen from SGR 1900+14. The long term fading of the persistent emission has been interrupted by the recent burst reactivation of the source. A target of opportunity XMM-Newton observation performed in April 2006 showed a flux ~15% higher. This variation was not accompanied by significant changes in the spectrum, pulsed fraction and light curve profile. We searched for emission and absorption lines in the spectra of the two observations, with negative results and setting tight upper limits of 50-200 eV (3 sigma), depending on the assumed line energy and width, on the equivalent width of lines in the 1-9 keV range.

Neutron star asteroseismology. Axial crust oscillations in the Cowling approximation

Recent observations of quasi-periodic oscillations in the aftermath of giant flares in soft gamma-ray repeaters suggest a close coupling between the seismic motion of the crust after a major quake and the modes of oscillations in a magnetar. In this paper, we consider the purely elastic modes of oscillation in the crust of a neutron star in the relativistic Cowling approximation (disregarding any magnetic field). We determine the axial crust modes for a large set of stellar models, using a state-of-the-art crust equation of state and a wide range of core masses and radii. We also devise useful approximate formulae for the mode-frequencies. We show that the relative crust thickness is well described by a function of the compactness of the star and a parameter describing the compressibility of the crust only. Considering the observational data for SGR 1900+14 and SGR 1806?20, we demonstrate how our results can be used to constrain the mass and radius of an oscillating neutron star.

Limits on the High-Energy Gamma and Neutrino Fluxes from the SGR 1806-20 Giant Flare of 27 December 2004 with the AMANDA-II Detector

On 27 December 2004, a giant gamma flare from the Soft Gamma-Ray Repeater 1806-20 saturated many satellite gamma-ray detectors, being the brightest transient event ever observed in the Galaxy. AMANDA-II was used to search for down-going muons indicative of high-energy gammas and/or neutrinos from this object. The data revealed no significant signal, so upper limits (at 90% C.L.) on the normalization constant were set: 0.05(0.5) TeV-1 m;{-2} s;{-1} for gamma=-1.47 (-2) in the gamma flux and 0.4(6.1) TeV-1 m;{-2} s;{-1} for gamma=-1.47 (-2) in the high-energy neutrino flux.

The Short‐Hard GRB 051103: Observations and Implications for Its Nature

The bright short-hard GRB 051103 was triangulated by the inter-planetary network and found to occur in the direction of the nearby M81/M82 galaxy group. Given its possible local-Universe nature, we searched for an afterglow associated with this burst. We observed the entire 3-sigma error quadrilateral using the Palomar 60-inch robotic telescope and the Very Large Array (VLA) about three days after the burst. We used the optical and radio observations to constrain the flux of any afterglow related to this burst, and to show that this burst is not associated with a typical supernova out to z~0.15. Our optical and radio observations, along with the Konus/Wind gamma-ray energy and light curve are consistent with this burst being a giant flare of a Soft Gamma-ray Repeater (SGR) within the M81 galaxy group. Furthermore, we find a star forming region associated with M81 within the error quadrilateral of this burst which supports the SGR hypothesis. If confirmed, this will be the first case of a soft gamma-ray repeater outside the local group.

Optical and Infrared Emission from the Anomalous X-Ray Pulsars and Soft Gamma-Ray Repeaters

We show that the irradiated accretion disk model can account for all the optical and infrared observations of the anomalous X-ray pulsars in the persistent state. Model fits do not constrain the outer disk radii, while placing an upper limit to the inner disk radii, and thus to the strength of the dipole component of the stellar magnetic field. While magnetar fields (B_* > 10^{14} G) in higher multipoles are compatible with the irradiated disk model, magnetic dipole components of magnetar strength are not consistent with optical data.

Five years of SGR 1900+14 observations withBeppoSAX

We present a systematic analysis of all the BeppoSAX data of the soft gamma-ray repeater SGR 1900+14: these observations allowed us to study the long term properties of the source quiescent emission. In the observation carried out before the 1998 giant flare the spectrum in the 0.8-10 keV energy range was harder and there was evidence for a 20-150 keV emission, possibly associated with SGR 1900+14. This possible hard tail, if compared with the recent INTEGRAL detection of SGR 1900+14, has a harder spectrum (power-law photon index ~1.6 versus ~3) and a 20-100 keV flux ~4 times larger. In the last BeppoSAX observation (April 2002), while the source was entering the long quiescent period that lasted until 2006, the 2-10 keV flux was ~25% below the historical level. We also studied in detail the spectral evolution during the 2001 flare afterglow. This was characterized by a softening that can be interpreted in terms of a cooling blackbody-like component.

Magnetic Field Decay and Period Evolution of Anomalous X-Ray Pulsarsin the Context of Quark Stars

We discuss a model wherein soft gamma-ray repeaters (SGRs), anomalous X-ray pulsars (AXPs), and radio-quiet isolated neutron stars (RQINSs) are all compact objects exhibiting superconductivity, namely, color-flavor-locked quark stars. In particular, we calculate the magnetic field decay due to the expulsion of spin-induced vortices from the star's superfluid-superconducting interior, and the resultant spin-down rate. We find that for initial parameters characteristic of AXPs/SGRs (1013 G < B < 1014 G, 3 s < P < 12 s), the magnetic field strengths and periods remain unchanged within a factor of 2 for timescales on the order of 5 × 105 to 5 × 107 yr given a quark star of radius 10 km. Within these timescales, we show that the observed period clustering in RQINSs can be explained by compactness, and we calculate how the magnetic field and period evolve in a manner concurrent with RQINS observations.

Supernova remnant energetics and magnetars: no evidence in favour of millisecond proto-neutron stars

Abstract It is generally accepted that anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are magnetars, i.e. neutron stars with extremely high surface magnetic fields (B &amp;gt; 1014 G). The origin of these high magnetic fields is uncertain, but a popular hypothesis is that magnetars are born with an initial spin period not much exceeding the convective overturn time (∼3 ms), which results in a powerful dynamo action, amplifying the seed magnetic field to ≳1015 G. Part of this rotation energy is then expected to power the supernova through rapid magnetic braking. It is therefore possible that magnetar creation is accompanied by supernovae that are an order of magnitude more energetic than normal supernovae, provided their initial spin period is ∼1 ms. However, we list here evidence that the explosion energies of these supernova remnants associated with AXPs and SGRs - Kes 73 (AXP 1E 1841-045), CTB 109 (AXP 1E2259+586) and N49 (SGR 0526-66) - are close to the canonical supernova explosion energy of 1051 erg, suggesting an initial spin period of P0≳ 5 ms. We therefore do not find evidence that magnetars are formed from rapidly rotating proto-neutron stars, allowing for the possibility that they descend from stellar progenitors with high magnetic field cores, and discuss the merits of both formation scenarios. In an appendix we describe the analysis of XMM-Newton observations of Kes 73 and N49 used to derive the explosion energies for these remnants.

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.

Rotational periods and other parameters of magnetars

The rotational periods P, period derivatives dP/dt, and magnetic fields B in the region where the emission of anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) is generated are calculated using a model that associates the emission of these objects with the existence of drift waves at the periphery of the magnetosphere of a neutron star. The values obtained for these parameters are P = 11−737 ms, dP/dt = 3.7 × 10−16−5.5 × 10−12, and log B (G) = 2.63−6.25. We find a dependence between the X-ray luminosity of AXPs and SGRs, Lx, and the rate at which they lose rotational energy, dE/dt, which is similar to the Lx(dE/dt) dependence for radio pulsars with detected X-ray emission. Within the errors, AXPs/SGRs and radio pulsars with short periods (P < 0.1 s) display the same slopes for their log(dP/dt)-log P relations and for the dependence of the efficiency of their transformation of rotational energy into radiation on their periods. A dipole model is used to calculate the surface magnetic fields of the neutron stars in AXPs and SGRs, which turn out to be, on average, comparable to the surface fields of normal radio pulsars (〈log Bs (G)〉 = 11.90).

XMM-Newton observations of the Soft Gamma Ray Repeater SGR 1627-41 in a low luminosity state

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 similar to 9 x 10(-14) erg cm(-2) s(-1) ( 2-10 keV). For a distance of 11 kpc, this corresponds to a luminosity of similar to 3 x 10(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 quiescent level. The spectrum is equally well fit by a steep power law ( photon index similar to 3.2) or by a blackbody with temperature kT similar to 0.8 keV. We also report on the INTEGRAL transient IGR J16358-4726 that lies at similar to 10' from SGR 1627-41. It was detected only in September 2004 with a luminosity of similar to 4 x 10(33) erg s(-1) ( for d = 7 kpc), while in February 2004 it was at least a factor 10 fainter.

Magnetars: structure and evolution from p-star models

P-stars are compact stars made of up-and-down quarks in β -equilibrium with electrons in a chromomagnetic condensate. We discuss p-stars endowed with super strong dipolar magnetic field that, following consolidated tradition in literature, are referred to as magnetars. We show that soft gamma-ray repeaters and anomalous X-ray pulsars can be understood within our theory. We find a well-defined criterion to distinguish rotation-powered pulsars from magnetic-powered pulsars. We show that glitches, which in our magnetars are triggered by magnetic dissipative effects in the inner core, explain both the quiescent emission and bursts in soft gamma-ray repeaters and anomalous X-ray pulsars. We account for the braking glitch from SGR 1900+14 and the normal glitch from AXP 1E 2259+586 following a giant burst. We discuss and explain the observed anti correlation between hardness ratio and intensity. Within our magnetar theory we are able to quantitatively account for light curves for both gamma-ray repeaters and anomalous X-ray pulsars. In particular we explain the puzzling light curve after the June 18, 2002 giant burst from AXP 1E 2259+586.

A Possible Mechanism for the Origin of Ultrastrong MagneticField of Magnetars

Growing observations reveal that soft gamma-ray repeaters and anomalousx-ray pulsars are magnetars. Their magneticfields may achieve 1014−1015 G. We explore the origin of thesuperstrong magnetic field by considering the magnetization of the3P2 superfluid neutrons inside neutron stars (NSs). By solvingthe Tolman–Oppenheimer–Volkov equations together with theequation of state adopted by Elgarøy et al. [Phys. Rev. Lett.77 (1996) 1428] in the calculation of the neutron pairing gap, wespecifically calculate the NS internal structure, the permissible regionfor 3P2 superfluid neutrons inside the NS, and the total magneticmoment contributed by the orderly arranged neutron vortexes. Theresult shows that the induced magnetic field may cover a widerange, which is consistent with the magnetic field predicted bythe standard magnetic dipole radiation for pulsar spindown.

Magnetars as persistent hard X-ray sources: INTEGRAL discovery of a hard tail in SGR 1900+14

Using 2.5 Ms of data obtained by the INTEGRAL satellite in 2003-2004, we discovered persistent hard X-ray emission from the soft gamma-ray repeater SGR 1900+14. Its 20-100 keV spectrum is well described by a steep power law with photon index r = 3.1 ± 0.5 and flux 1.5 x 10 -11 erg cm -2 s -1 . Contrary to SGR 1806-20, the only other soft gamma-ray repeater for which persistent emission above 20 keV was reported, SGR 1900+14 has been detected in the hard X-ray range while it was in a quiescent state (the last bursts from this source were observed in 2002). By comparing the broad band spectra (1-100 keV) of all the magnetars detected by INTEGRAL (the two SGRs and three anomalous X-ray pulsars) we find evidence for a different spectral behaviour of these two classes of sources.

Radiation Spectra and Polarization in Magnetar Bursts

We present Monte Carlo simulations of radiative transfer in magnetar atmospheres. We include the effects of vacuum polarization, electron and proton scattering, and free-free absorption. Simulations are performed for the atmosphere model with the magnetic field perpendicular and also tilted with respect to the neutron star surface, and we show that the average spectrum does not strongly depend on the orientation of the magnetic field. We investigate the region of the parameter space where the vacuum absorption-like feature appears in the spectrum and we analyze the shape of the proton cyclotron line. Our results indicate that the existence of the vacuum polarization feature should be a general attribute of soft gamma-ray repeaters burst spectra, provided that the energy release takes place at the sufficiently dense region, and the atmosphere scaleheight is large enough. We discuss the existence of such a feature in recent observational data on these sources.

Two years of INTEGRAL monitoring of the soft gamma-ray repeater SGR 1806-20: from quiescence to frenzy

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 an energy release of 3 x 1E42 erg. We present a detailed analysis of it and discuss our results in the framework of the magnetar model.

Shear Waves and Giant-Flare Oscillations from Soft Gamma-Ray Repeaters

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 (B ~ 1014-1015 G). The lowest radial-order mode has a redshifted frequency of ≈28 Hz[l(l + 1)/6]1/2, with the prefactor depending on the NS's mass and radius and its crust's depth and composition. This mode is independent of the magnetic field for B 4 × 1015 G, a limit much greater than the inferred dipole magnetic fields for these objects. Although this is a good fit to the observed oscillations, only rather loose constraints can be made for the NSs' properties because all that can be fit is this prefactor (a single parameter). Modes with shorter radial wavelengths are more sensitive to the magnetic field starting at B ~ 2 × 1014 G and have higher frequencies (~600-2000 Hz). The discovery of these modes, coupled with the oscillations observed thus far, would provide a powerful probe to the NS crustal structure.

Three‐Dimensional Simulations of the Reorganization of a Quark Star’s Magnetic Field as Induced by the Meissner Effect

In a previous paper we presented a new model for soft gamma-ray repeaters (SGRs), based on the onset of color superconductivity in quark stars. In this model, a single burst results from the reorganization of the exterior magnetic field following the formation of vortices that confine the internal magnetic field (the Meissner effect). Here we present full three-dimensional simulations of the evolution of the inclined exterior magnetic field immediately following vortex formation. The simulations capture the violent reconnection events in the entangled surface magnetic field as it evolves into a smooth, more stable configuration that consists of a dipole field aligned with the star's rotation axis. The total magnetic energy dissipated in this process is found to be of the order of 1044 ergs, and if it is emitted as synchrotron radiation, it peaks typically at 280 keV. The intensity decays with time in a way characteristic of SGR giant flares with a tail lasting from a few to a few hundred times the rotation period of the star, depending on the initial inclination between the rotation and dipole axes. One of the obvious consequences of our model's final state (aligned rotator) is the suppression of persistent pulsed radio emission in SGRs and anomalous X-ray pulsars (AXPs) following their bursting era. We compare our model to observations and highlight our predictions.

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.