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X-ray Flares (x-ray + flare)
Selected AbstractsA New X-Ray Flare from the Galactic Nucleus Detected with XMM-NewtonASTRONOMISCHE NACHRICHTEN, Issue S1 2003A. Goldwurm Abstract The compact radio source Sgr A*, believed to be the counterpart of the massive black hole at the Galactic nucleus, was observed to undergo rapid and intense flaring activity in X-rays with Chandra in October 2000. We report here the detection with XMM-Newton EPIC cameras of the early phase of a similar X-ray flare from this source, which occurred on 2001 September 4. The source 2,10 keV luminosity increased by a factor ,20 to reach a level of 4 1034 erg s,1 in a time interval of about 900 s, just before the end of the observation. The data indicate that the source spectrum was hard during the flare and can be described by simple power law of slope ,0.7. This XMM-Newton observation confirms the results obtained by Chandra, suggests that, in Sgr A* , rapid and intense X-ray flaring is not a rare event and therefore sets some constraints on the emission mechanism models proposed for this source. [source] The GRB early optical flashes from internal shocks: application to GRB 990123, GRB 041219a and GRB 060111bMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007D. M. Wei ABSTRACT With the successful launch of the Swift Gamma-ray Burst Explorer, it is widely expected that the prompt optical flashes like GRB 990123 would be easily detected. However, the observations show that for a number of Gamma-ray bursts (GRBs) no early optical flash has been detected, which indicates that the reverse shock emission must be suppressed. Here we explore the possibility that the optical flash may arise from the internal shock emission. For GRB 990123 and GRB 060111b, although their optical emission are not correlated with the gamma-ray emission, we propose here that their optical and gamma-ray emission may arise from different internal shocks (which can be formed by collision of different shells), and find that, under certain circumstances, the optical flashes of GRB 990123 and GRB 060111b can well be explained by the internal shock model. For GRB 041219a, the prompt optical emission was correlated with the gamma-ray emission, which can also be explained by the internal shock model if we assume the optical emission was the low-energy extension of the gamma-ray emission, and we find its redshift is about z, 0.2. As for GRB 050904, we have shown in previous paper that the optical flash was produced by synchrotron radiation and the X-ray flare was produced by the synchrotron,self-Compton (SSC) mechanism. Therefore we conclude that the early optical flashes of GRBs can usually arise from the internal shock emission. Meanwhile in our model since the shells producing the optical flashes would be easily disrupted by other shells, so we suggest that the bright optical flash should not be common in GRBs. In addition, we also discussed the SSC emission in the internal shock model, and find that for different values of parameters, there would be several kinds of high-energy emission (at ,100 keV, ,10 MeV or GeV) accompanying the optical flash. For a burst like GRB 990123, a GeV flare with fluence about 10,8 erg cm,2 s,1 is expected, which might be detected by the GLAST satellite. [source] The narrow-line quasar NAB 0205 + 024 observed with XMM,NewtonMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004L. C. Gallo ABSTRACT The XMM,Newton observation of the narrow-line quasar NAB 0205+024 reveals three striking differences since it was last observed in X-rays with ASCA. First, the 2,10 keV power law is notably steeper. Secondly, a hard X-ray flare is detected, very similar to that seen in I Zw 1. Thirdly, a strong and broad emission feature is detected with the bulk of its emission redward of 6.4 keV, and extending down to ,5 keV in the rest frame. The most likely explanation for the broad feature is neutral iron emission emitted from a narrow annulus of an accretion disc close to the black hole. The hard X-ray flare could be the mechanism that illuminates this region of the disc, allowing for the emission line to be detected. The combination of effects can be understood in terms of the ,thunder-cloud' model proposed by Merloni & Fabian. [source] A New X-Ray Flare from the Galactic Nucleus Detected with XMM-NewtonASTRONOMISCHE NACHRICHTEN, Issue S1 2003A. Goldwurm Abstract The compact radio source Sgr A*, believed to be the counterpart of the massive black hole at the Galactic nucleus, was observed to undergo rapid and intense flaring activity in X-rays with Chandra in October 2000. We report here the detection with XMM-Newton EPIC cameras of the early phase of a similar X-ray flare from this source, which occurred on 2001 September 4. The source 2,10 keV luminosity increased by a factor ,20 to reach a level of 4 1034 erg s,1 in a time interval of about 900 s, just before the end of the observation. The data indicate that the source spectrum was hard during the flare and can be described by simple power law of slope ,0.7. This XMM-Newton observation confirms the results obtained by Chandra, suggests that, in Sgr A* , rapid and intense X-ray flaring is not a rare event and therefore sets some constraints on the emission mechanism models proposed for this source. [source] Explosive reconnection in magnetarsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2003Maxim Lyutikov ABSTRACT The X-ray activity of anomalous X-ray pulsars and soft ,-ray repeaters may result from the heating of their magnetic corona by direct currents dissipated by magnetic reconnection. We investigate the possibility that X-ray flares and bursts observed from anomalous X-ray pulsars and soft ,-ray repeaters result from magnetospheric reconnection events initiated by development of the tearing mode in magnetically dominated relativistic plasma. We formulate equations of resistive force-free electrodynamics, discuss the relation of the latter to ideal electrodynamics, and give examples of both ideal and resistive equilibria. Resistive force-free current layers are unstable towards the development of small-scale current sheets where resistive effects become important. Thin current sheets are found to be unstable due to the development of the resistive force-free tearing mode. The growth rate of the tearing mode is intermediate between the short Alfvén time-scale ,A and a long resistive time-scale ,R: ,, 1/(,R,A)1/2, similar to the case of non-relativistic non-force-free plasma. We propose that growth of the tearing mode is related to the typical rise time of flares, ,10 ms. Finally, we discuss how reconnection may explain other magnetar phenomena and ways to test the model. [source] Did Swift measure gamma-ray burst prompt emission radii?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2006M. Lyutikov ABSTRACT The Swift X-Ray Telescope often observes a rapidly decaying X-ray emission stretching to as long as t, 103 s after a conventional prompt phase. This component is most likely due to a prompt emission viewed at large observer angles , > 1/,, where ,, 0.1 is a typical viewing angle of the jet and ,, 100 is the Lorentz factor of the flow during the prompt phase. This can be used to estimate the prompt emission radii, rem, 2t c/,2, 6 × 1015 cm. These radii are much larger than is assumed within the framework of a fireball model. Such large emission radii can be reconciled with a fast variability, on time-scales as short as milliseconds, if the emission is beamed in the bulk outflow frame, e.g. because of a random relativistic motion of ,fundamental emitters'. This may also offer a possible explanation for X-ray flares observed during early afterglows. [source] Limits on the Short Term Variability of Sagittarius A* in the Near-InfraredASTRONOMISCHE NACHRICHTEN, Issue S1 2003S. D. Hornstein Abstract The detection of X-ray flares by the Chandra X-ray Observatory and XMM-Newton has raised the possibility of enhanced emission over a broad range of wavelengths from Sagittarius A*, the suspected 2.6 × 106 M, black hole at the Galactic center, during a flaring event. We have, therefore, reconstructed 3,4 hr data sets from 2 ,m speckle and adaptive optics images (,core = 50,100 mas) obtained with the W. M. Keck 10 m telescopes between 1995 and 2002. The results for 25 of these observations were reported by Hornstein et al. (2002) and an additional 11 observations are presented here. In the 36 separate observations, no evidence of any significant excess emission associated with Sgr A* was detected. The lowest of our detection limits gives an observed limit for the quiescent state of Sgr A* of 0.09 ± 0.005 mJy, or, equivalently, a dereddened value of 2.0 ± 0.1 mJy. Under the assumption that there are random 3 hr flares producing both enhanced X-ray and near-infrared emission, our highest limit constrains the variable state of Sgr A* to ,0.8 mJy (observed) or 19 mJy (dereddened). These results suggest that the early model favored by Markoff et al. (2002), in which the flare is produced through local heating of relativistic particles surrounding Sgr A* (e.g., a sudden magnetic reconnection event), is unlikely because it predicts peak 2 ,m emission of ,300 mJy, well above our detection limit. [source] A Relativistic Disk in Sagittarius A*ASTRONOMISCHE NACHRICHTEN, Issue S1 2003Siming Liu The detection of a mm/Sub-mm "bump" in Sgr A*'s radio spectrum suggests that at least a portion of its overall emission is produced within a compact accretion disk. This inference is strengthened by observations of strong linear polarization (at the 10 percent level) within this bump. No linear polarization has been detected yet at other wavelengths. Given that radiation from this source is produced on progressively smaller spatial scales with increasing frequency, the mm/Sub-mm bump apparently arises within a mere handful of Schwarzschild radii of the black hole. We have found that a small (10-Schwarzschild-radii) magnetized accretion disk can not only account for the spectral bump via thermal synchrotron processes, but that it can also reproduce the corresponding polarimetric results. In addition, the quiescent X-ray emission appears to be associated with synchrotron self-Comptonization, while X-ray flares detected from Sgr A* may be induced by a sudden enhancement of accretion through this disk. The hardening of the flare-state X-ray spectrum appears to favor thermal bremsstrahlung as the dominant X-ray emission mechanism during the transient event. This picture predicts correlations among the mm, IR, and X-ray flux densities, that appear to be consistent with recent multi-wavelength observations. Further evidence for such a disk in Sgr A* is provided by its radio variability. Recent monitoring of Sgr A* at cm and mm wavelengths suggests that a spectral break is manifested at 3 mm during cm/Sub-mm flares. The flat cm spectrum, combined with a weak X-ray flux in the quiescent state, rules out models in which the radio emission is produced by thermal synchrotron process in a bounded plasma. One possibility is that nonthermal particles may be produced when the large scale quasi-spherical inflow circularizes and settles down into the small accretion disk. Dissipation of kinetic energy associated with radial motion may lead to particle acceleration in shocks or via magnetic reconnection. On the other hand, the identification of a 106-day cycle in Sgr A*'s radio variability may signal a precession of the disk around a spinning black hole. The disk's characteristics imply rigid-body rotation, so the long precession period is indicative of a small black-hole spin with a spin parameter a/M around 0.1. It is interesting to note that such a small value of a/M would be favored if the nonthermal portion of Sgr A*'s spectrum is powered by a Blandford- Znajek type of process; in this situation, the observed luminosity would correspond to an outer disk radius of about 30 Schwarzschild radii. This disk structure is consistent with earlier hydrodynamical and recent MHD simulations and is implied by Sgr A*'s mm/Sub-mm spectral and polarimetric characteristics. For the disk to precess with such a long (106-day) period, the angular momentum flux flowing through it must be sufficiently small that any modulation of the total angular momentum is mostly due to its coupling with the black-hole spin. This requires that the torque exerted on the inner boundary of the disk via magnetic stresses is close to the angular momentum accretion rate associated with the infalling gas. Significant heating at the inner edge of the disk then leaves the gas marginally bounded near the black hole. A strong wind from the central region may ensue and produce a scaled down version of relativistic (possibly magnetized) jets in AGNs. [source] | ||||||||||