Hard Flares Research Articles

Chandra Resolves the Double FU Orionis System RNO 1B/1C in X-Rays

Abstract We present new Chandra X-ray observations of the close pair of young stars RNO 1B and 1C (6″ separation) located in the L1287 cloud. RNO 1B erupted in 1978–1990 and is classified as an FU Orionis star (FUor). RNO 1C also shows most of the properties of an FUor but no eruption has been seen yet. Only a few dozen FUors are known and the presence of two such objects with a small angular separation is rare, suggesting a common origin. Both stars were faintly detected by Chandra and we summarize their X-ray properties within the framework of other previously detected FUors. We also report other X-ray detections in L1287 including the deeply embedded young star RNO 1G, the jet-like radio source VLA 3, and an enigmatic hard flaring source with no Two Micron All Sky Survey counterpart that was only detected in the second of the two Chandra exposures.

Wind, jet, hybrid corona and hard X-ray flares: multiwavelength evolution of GRO J1655−40 during the 2005 outburst rise

We have investigated the complex multiwavelength evolution of GRO J1655-40 during the rise of its 2005 outburst. We detected two hard X-ray flares, the first one during the transition from the soft state to the ultra-soft state, and the second one in the ultra-soft state. The first X-ray flare coincided with an optically thin radio flare. We also observed a hint of increased radio emission during the second X-ray flare. To explain the hard flares without invoking a secondary emission component, we fit the entire data set with the eqpair model. This single, hybrid Comptonization model sufficiently fits the data even during the hard X-ray flares if we allow reflection fractions greater than unity. In this case, the hard X-ray flares correspond to a Comptonizing corona dominated by non-thermal electrons. The fits also require absorption features in the soft and ultra-soft state which are likely due to a wind. In this work we show that the wind and the optically thin radio flare co-exist. Finally, we have also investigated the radio to optical spectral energy distribution, tracking the radio spectral evolution through the quenching of the compact jet and rise of the optically thin flare, and interpreted all data using state transition models.

Wavelets, Intermittency and Solar Flare Hard X-rays 2. LIM Analysis of High Time Resolution BATSE Data

We apply local intermittency measure (LIM) analysis to 64 ms time resolved BATSE measurements of hard X-rays from several solar flares. LIM analysis yields information on the character of the processes giving rise to fluctuations in light curves, e.g. avalanche vs. cascade pictures. We present LIM scaleograms for several flares and summarise scaleogram morphological features. More detailed descriptions are given of the behaviour in the flare of 13 January 1992 (the well-known ‘Masuda flare’) and an interesting event on 22 November 1998. All flares display episodes of intermittency across a range of scales. LIM scaleogram features may be consistent with both ‘top-down’ and ‘bottom-up’ scenarios for the development of small-scale structure, sometimes within the same flare. Thus neither class of model captures the totality of the events in a flare.

Wavelets, Intermittency and Solar Flare Hard X-rays 1. Local Intermittency Measure in Cascade and Avalanche Scenarios

We introduce the local intermittency measure (LIM) as a tool for the investigation of solar flare hard X-ray light curves. Constructed from wavelet amplitudes, the LIM allows us to investigate the extent to which rapid fluctuations reveal an underlying, scale-invariant process, and to identify those episodes during a flare which definitely represent ‘intermittent’ behaviour (e.g. involvement of new spatial structures). We carry out two sets of simple simulations intended to provide generic examples of ‘top-down’ and ‘bottom-up’ scenarios for the development of flares, and show how LIM may discriminate between them.

Very hard gamma-ray emission from a flare of Mrk 501

We investigate the peculiar properties of a large TeV gamma-ray flare from Mrk~501 detected during the 2009 multiwavelength campaign. We identify the counterpart of the flare in the Fermi/LAT telescope data and study its spectral and timing characteristics. A strong order-of-magnitude increase of the very-high-energy gamma-ray flux during the flare was not accompanied by an increase in the X-ray flux, so that the flare was one of the "orphan"-type TeV flares observed in BL Lacs. The flare lasted about 1 month at energies above 10 GeV. The flaring source spectrum in the 10-200 GeV range was very hard, with a photon index 1.1+/-0.2, harder than that observed in any other blazar in the gamma-ray band. No simultaneous flaring activity was detected below 10 GeV. Different variability properties of the emission below and above 10 GeV indicate the existence of two separate components in the spectrum. We investigate possible explanations of the very hard flaring component. We consider, among others, the possibility that the flare is produced by an electromagnetic cascade initiated by very-high-energy gamma-rays in the intergalactic medium. Within such an interpretation, peculiar spectral and temporal characteristics of the flare could be explained if the magnetic field in the intergalactic medium is of the order of 1e-17 - 1e-16 G.

Non-thermal recombination – a neglected source of flare hard X-rays and fast electron diagnostic

Context. Flare Hard X-rays (HXRs) from non-thermal electrons are commonly treated as solely bremsstrahlung (free-free = f-f), recombination (free-bound = f-b) being neglected. This assumption is shown to be substantially in error, especially in hot sources, mainly due to recombination onto Fe ions. Aims. We analyse the effects on HXR spectra J(� ) and electron diagnostics by including non-thermal recombination onto heavy elements in our model. Methods. Using Kramers hydrogenic cross sections with effective Z = Zeff, we calculate f-f and f-b spectra for power-law electron spectra within both thin and thick target limits and for Maxwellians with summation over all important ions. Results. We find that non-thermal electron recombination, especially onto Fe, must, in general, be included with f-f for reliable spectral interpretation, when the HXR source is hot, such as occulted loops containing high ions of Fe (f-b cross-section ∝Z 4 ). The f-b contribution is greatest when the electron spectral index δ is large and any low energy cut-off Ec is small, because the electron flux

Regularized Reconstruction of the Differential Emission Measure from Solar Flare Hard X-Ray Spectra

We address the problem of how to test whether an observed solar hard X-ray bremsstrahlung spectrum (I(∊)) is consistent with a purely thermal (locally Maxwellian) distribution of source electrons, and, if so, how to reconstruct the corresponding differential emission measure (ξ(T)). Unlike previous analysis based on the Kramers and Bethe-Heitler approximations to the bremsstrahlung cross-section, here we use an exact (solid-angle-averaged) cross-section. We show that the problem of determining ξ(T) from measurements of I(∊) invOlves two successive inverse problems: the first, to recover the mean source-electron flux spectrum (\(\overline{F}\)(E)) from I(∊) and the second, to recover ξ(T) from \(\overline{F}\)(E). We discuss the highly pathological numerical properties of this second problem within the framework of the regularization theory for linear inverse problems. In particular, we show that an iterative scheme with a positivity constraint is effective in recovering δ-like forms of ξ(T) while first-order Tikhonov regularization with boundary conditions works well in the case of power-law-like forms. Therefore, we introduce a restoration approach whereby the low-energy part of \(\overline{F}\) (E), dominated by the thermal component, is inverted by using the iterative algorithm with positivity, while the high-energy part, dominated by the power-law component, is inverted by using first-order regularization. This approach is first tested by using simulated \(\overline{F}\)(E) derived from a priori known forms of ξ(T) and then applied to hard X-ray spectral data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI).

Tracing a Z-track in the M 31 X-ray binary RX J0042.6+4115

Four XMM-Newton observations of the core of M 31, spaced at 6 month intervals, show that the brightest point X-ray source, RX J0042.6+4115, has a 0.4-10 keV luminosity of5 10 38 erg s 1 , and exhibits significant variability in intensity and X-ray spectrum over a time scale of100 s including hard flares; such behaviour is only observed in Z-sources and transient black hole binaries in our Galaxy. The lightcurves, X-ray spectra and hardness-intensity data from the four XMM-Newton observations all strongly suggest that it is a Z-source, bringing the total number of known Z-sources to nine.

Hard X-Ray Flares Preceding Soft X-Ray Outbursts in Aquila X-1: A Link between Neutron Star and Black Hole State Transitions

We have analyzed Rossi X-Ray Timing Explorer data of the neutron star transient Aquila X-1 obtained during its outbursts in 1999 May/June and 2000 September/October. 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 3 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 sharp spectral transition occurred with spectral pivoting in the range 8-12 keV. Our timing analysis shows that during the hard flare, the power spectra were composed mainly of band-limited noise and a ~1-20 Hz quasi-periodic oscillation (QPO), which correlate in frequency. Immediately after the hard flare, the power spectra turned into power-law noise. The spectral and timing properties during and after the hard flares are very similar to those in black hole transients during the early rise of an outburst. We suggest that these hard flares and spectral transitions in Aql X-1 are of the same origin as those observed in black hole transients. This leads to the association of the 1-20 Hz QPOs and band-limited noise in Aql X-1 with those in black hole transients. We discuss the impact of this discovery on our understanding of soft X-ray transient outbursts, state transitions, and variability in X-ray binaries.

RXTESpectral Observations of the 1996–1997 Outburst of the Microquasar GRO J1655−40

Excellent coverage of the entire 16 month 1996-1997 outburst cycle of GRO J1655-40 was provided by the Rossi X-Ray Timing Explorer (RXTE). We present a full spectral analysis of these data, which includes 52 Proportional Counter Array spectra from 2.5 to 20 keV and High Energy X-Ray Timing Experiment spectra above 20 keV. We also include a nearly continuous All-Sky Monitor light curve with several intensity measurements per day. The data are interpreted in the context of the multicolor blackbody disk/power-law model. The source exhibits two principal outburst states which we associate with the very high and the high/soft states. During the very high state, the spectrum is often dominated by a power-law component with photon index (Γ) ~ 2.3-2.7. The source exhibits intense hard flares on timescales of hours to days which are correlated with changes in both the fitted temperature and radius of the inner accretion disk. During the high/soft state, the spectrum is dominated by the soft thermal emission from the accretion disk with spectral parameters that suggest approximately constant inner disk radius and temperature. The power-law component is relatively weak with Γ ~ 2-3. During the last few observations, the source undergoes a transition to the low/hard state. We find that a tight relationship exists between the observed inner radius of the disk and the flux in the power-law component. During intense hard flares, the inner disk radius is observed to decrease by as much as a factor of 3 on a timescale of days. The apparent decrease of the inner disk radius observed during the flares may be due to the failure of the multicolor disk model caused by a steepening of the radial temperature profile in the disk coupled with increased spectral hardening and not physical changes of the inner disk radius. The distortion of the inner disk spectrum by the power-law flares indicates that the physical mechanism responsible for producing the power-law emission is linked to the inner disk region. Assuming that our spectral model is valid during periods of weak power-law emission, our most likely value for the inner disk radius implies a* < 0.7. Such a low value for the black hole angular momentum is inconsistent with the relativistic frame dragging and the "diskoseismic" models as interpretations for the 300 Hz X-ray QPO seen during some of these RXTE observations.

Production of Energy‐dependent Time Delays in Impulsive Solar Flare Hard X‐Ray Emission by Short‐Duration Spectral Index Variations

Cross-correlation techniques have been used recently to study the relative timing of solar flare hard X-ray emission at different energies. These studies find that for the majority of the impulsive flares observed with BATSE there is a systematic time delay of a few tens of milliseconds between low (≈ 50 keV) and higher energy emission (≈ 100 keV). These time delays have been interpreted as energy-dependent time-of-flight differences for electron propagation from the corona, where they are accelerated, to the chromosphere, where the bulk of the hard X-rays are emitted. We show in this paper that cross-correlation methods fail if the spectral index of the flare is not constant. BATSE channel ratios typically display variations of factors of 2 to 5 over time intervals as short as a few seconds. Using simulated and observed data, we demonstrate that cross-correlating energy channels with identical timing characteristics, but with variations in the amplitudes of one or a small number of relatively strong emission spikes, produces asymmetric time delays of either sign. The reported time delays are therefore largely due to spectral index variations and are not signatures of time-of-flight effects.

Deconvolution of Directly Precipitating and Trap‐Precipitating Electrons in Solar Flare Hard X‐Rays. I. Method and Tests

We develop and test a numerical code that provides a self-consistent deconvolution of energy-dependent hard X-ray (HXR) time profiles I(e, t) into two HXR-producing electron components, i.e., directly precipitating and trap-precipitating electrons. These two HXR components can be physically distinguished because their energy-dependent time delays have an opposite sign. The deconvolution is based on the following model assumptions: (1) nonthermal electrons are injected from the acceleration site into coronal flare loops by an injection function f(E, α, t) that consists of synchronized pulses in energy E and pitch angle α, (2) electrons with initially small pitch angles (α ≤ α0) precipitate directly to the HXR emission site, (3) electrons with initially large pitch angles (α ≥ α0) are temporarily trapped and precipitate after the collisional deflection time, and (4) nonthermal electrons lose their energy by Coulomb collisions and emit thick-target HXR bremsstrahlung in a high-density (fully collisional) site (near or inside the chromosphere). The numerical deconvolution provides a self-consistent determination of three physical parameters: (1) the electron time-of-flight distance lTOF between the acceleration/injection site and the HXR emission site, (2) the electron density ne in the trap region, and (3) the fraction of HXR-emitting electrons that precipitate directly, qprec, which relates to the loss cone angle by qprec(α0) = (1 - cos α0) for isotropic pitch angle distributions. This yields the magnetic mirror ratio R = Bloss/Binj = 1/sin2 (α0) between the injection and loss cone site. With this method, we measure for the first time magnetic field ratios in coronal loops by means of HXR data. Based on this ratio, together with the knowledge of the photospheric field at the footpoint, a direct measurement of the magnetic field in the coronal acceleration region can be obtained. We simulate energy-dependent HXR data I(e, t) with typical solar flare parameters (lTOF = 15,000 km, ne = 1011 cm-3, qprec = 0.5) and test the accuracy of the inversion code. We perform the inversion in 30 different simulations over the entire physically plausible parameter space and demonstrate that a satisfactory inversion of all three physical parameters lTOF, ne, and qprec is achieved in a density range of ne = 1010-1012 cm-3 for precipitation ratios of qprec = 0.1-0.9 and for signal-to-noise ratios of 100 (requiring HXR count rates of 104 counts s-1). Applications of this inversion method to solar flare observations in hard X-rays (CGRO/BATSE, Yohkoh/Hard X-Ray Telescope) and microwaves (Nobeyama) will be presented in subsequent papers.

Return of the hard solar flares' 154 day periodicity in solar cycle 22 and evidence for a phase relationship of this periodicity between cycles 21 and 22

Return of the hard solar flares' 154 day periodicity in solar cycle 22 and evidence for a phase relationship of this periodicity between cycles 21 and 22

Analysis of Prognoz - 9 Solar Flare Hard X-ray Data-support for the Non-thermal thick target Model

Solar flare hard X-ray data obtained by Prognoz-9 spacecraft (Abrosimov et al 1988) in the energy range 10-200 keV are analysed. In examples of events which we consider here, high energy X-ray pulses appear earlier than low energy ones, which is contrary to many other events where the low energy X-ray peak emission takes place earlier. The variation of the spectral index was dynamical.

A long-duration balloon payload for hard X-ray and gamma-ray observations of the sun

We describe a balloon payload designed to study the processes of energy release, particle acceleration, and heating of the active corona, in hard X-ray microflares and normal flares. An array of liquid nitrogen-cooled germanium detectors together with large area phoswich scintillation detectors provide the highest sensitivity (∼500 cm2) and energy resolution (≤0.7 keV) ever achieved for solar hard X-ray (∼15–600 keV) measurements. These detectors were flown in February 1987 from Australia on a long duration RAdiation COntrolled balloON (RACOON) flight (LDBF) which provided 12 days of observations before cutdown in Brazil. The payload includes solar cells for power, pointing and navigation sensors, a microprocessor controlled data system with VCR tape storage, and transmitters for GOES and ARGOS spacecraft. This successful flight illustrates the potential of LDBF's for solar flare studies.

Fast transients - A search in X-rays for short flares, bursts, and related phenomena

view Abstract Citations (20) References (69) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Fast Transients: A Search in X-Rays for Short Flares, Bursts, and Related Phenomena Connors, A. ; Serlemitsos, P. J. ; Swank, J. H. Abstract The HEAO 1 A-2 database on fast high energy X-ray transients was examined for any discernible regularities. The data were taken over the interval 1977-79 with six collimated multiwire, multilayer, proportional counters that had a sensitivity sufficient for detecting events lasting 1-5 sec at energies as low as 8-120 keV. The entire sky was surveyed completely three times in the observational period. Best-fit position, error box corners, mean transient flux and quiescent flux data are provided for all six of the type 3 events that were found. All the sources were within the Galaxy. The duration of the events ranged from 60-2000 sec. The limited number of events observed leads to estimates of 10,000-200,000 events per year. It is suggested that all the events originate from hard flares occurring at a rate of 20,000/yr on dMe/dKe stars. Publication: The Astrophysical Journal Pub Date: April 1986 DOI: 10.1086/164124 Bibcode: 1986ApJ...303..769C Keywords: Bursts; Stellar Flares; Stellar Radiation; X Ray Sources; Bremsstrahlung; Gamma Ray Bursts; Heao 1; Searching; Stellar Spectra; Transient Response; Astrophysics; STARS: FLARE; X-RAYS: BURSTS full text sources ADS | data products SIMBAD (8)

Hard solar flare X-ray Bursts on 8 December 1970

Hard X-ray bursts have been observed from two 1B flares located in the same sunspot region and separated in time by about 70 min on 8 December 1970. The bursts are composed by many ‘flashes’ of 2 to 20 seconds duration. Power spectrum analysis reveals no strong periodicities although a significant peak appears at 7.5 s. The characteristic times of 2 to 20 s do not seem likely to be electron transit times since the flare size is so small. These times are evidently connected with the acceleration of the electrons. Fitting Takakura's model of the radio noise region to the observations we find this region to be small but the density of electrons above 100 keV is high, on the order of 107 cm-3.

High-energy X rays from the solar flare of July 7, 1966

Highest differential energy range of X rays during July 1966 solar flare suggests nonthermal bremsstrahlung origin of hard flare X rays