Small GOES flares with intense hard X-ray emission

In this paper we analyze soft and hard X-ray emission of the 2002 September 20 M1.8 GOES class solar flare observed by RHESSI and GOES satellites, where soft X-ray emission precedes the onset of the main bulk hard X-ray emission by ~5 min. This suggests that an additional heating mechanism may be at work at the early beginning of the flare. However RHESSI spectra indicate presence of the non-thermal electrons also before impulsive phase. So, we assumed that a dominant energy transport mechanism during rise phase of solar flares is electron beam-driven evaporation. We used non-thermal electron beams derived from RHESSI spectra as the heating source in a hydrodynamic model of the analyzed flare. We showed that energy delivered by non-thermal electron beams is sufficient to heat the flare loop to temperatures in which it emits soft X-ray closely following the GOES 1–8 Å light-curve.

We present an observational study on the impact of the dynamic evolution of kinking filaments on the production of hard X-ray (HXR) emission. The investigation of two kinking-filament events in this paper, occurring on 2003 June 12 and 2004 November 10, respectively, combined with our earlier study on the failed filament eruption of 2002 May 27, suggests that two distinct processes take place during the kink evolution, leading to HXR emission with different morphological connections to the overall magnetic configuration. The first phase of the evolution (Phase I) is characterized by compact HXR footpoint sources at the endpoints of the filament, and the second phase (Phase II) by a ribbon-like footpoint emission extending along the endpoints of the filament. The HXR emission in both the 2002 May 27 and 2004 November 10 events shows a transition from Phase I to Phase II. In the 2002 May 27 event, coronal emission was observed to be associated with EUV brightening sheaths aligned along two filament legs in Phase I, while in Phase II, it was located near the projected crossing point of the kink. The coronal emission in the 2004 November 10 event does not exhibit a clear morphological transition as in the 2002 May 27 event, probably due to the filament's relatively small size. The 2003 June 12 event mostly features a Phase I emission, with a compact footpoint emission located at one end of the filament, and an elongated coronal source oriented along the same filament leg. We propose the following scenarios to explain the different flare morphology: magnetic reconnection in Phase I occurs as a result of the interactions of the two writhing filament legs; reconnection in Phase II occurs at an X-type magnetic topology beneath the filament arch when the filament ascends and expands.

Noninductive plasma current start-up using 2nd harmonic electron cyclotron resonance heating (ECRH) with oblique radio frequency (RF) injection is demonstrated in a Q-shu University experiment with steady-state spherical tokamak. A strong transition was observed in the heating and plasma current ramp-up. The initial bulk electron heating regime exhibits T ebulk ∼ 140 eV and no hard x-ray (HXR) emission with a low I p of ∼15 kA; it abruptly transitions to a regime that exhibits a low T ebulk of ∼10 eV and a strong HXR emission with a high I p of ∼50 kA. This behavior is distinctly different from that observed in previous fundamental ECRH experiments. The mechanism of the heating and current drive transition are investigated considering wave power absorption and plasma power balance. The results indicate that the transition is caused by the favorable heating of tail electrons where the RF power absorption at the 2nd harmonic increases nearly linearly with T etail, while the power transfer from the tail electrons to the bulk electrons decreases with 1/T etail 0.5. This causes a rapid transition to a state with high T etail while reducing T ebulk towards colder ion temperature. The understanding of the transition mechanism helps to consider plasma current start-up using 2nd harmonic ECRH for tokamak reactors such as JT-60 SA and ITER.

Protostellar jets appear at all stages of star formation when the accretion process is still at work. Jets travel at velocities of hundreds of km/s, creating strong shocks when interacting with interstellar medium. Several cases of jets have been detected in X-rays, typically showing soft emission. For the first time, we report evidence of hard X-ray emission possibly related to non-thermal processes not explained by previous models of the post-shock emission predicted in the jet/ambient interaction scenario. HH 80 is located at the south head of the jet associated to the massive protostar IRAS 18162-2048. It shows soft and hard X-ray emission in regions that are spatially separated, with the soft X-ray emission region situated behind the region of hard X-ray emission. We propose a scenario for HH 80 where soft X-ray emission is associated to thermal processes from the interaction of the jet with denser ambient matter and the hard X-ray emission is produced by synchrotron radiation at the front shock.

We present the results from sensitive, multi-epoch NuSTAR observations of the late-type star-forming galaxy M83 (d = 4.6 Mpc). This is the first investigation to spatially resolve the hard ( E > 10 keV) X-ray emission of this galaxy. The nuclear region and ∼20 off-nuclear point sources, including a previously discovered ultraluminous X-ray source, are detected in our NuSTAR observations. The X-ray hardnesses and luminosities of the majority of the point sources are consistent with hard X-ray sources resolved in the starburst galaxy NGC 253. We infer that the hard X-ray emission is most likely dominated by intermediate accretion state black hole binaries and neutron star low-mass X-ray binaries (Z-sources). We construct the X-ray binary luminosity function (XLF) in the NuSTAR band for an extragalactic environment for the first time. The M83 XLF has a steeper XLF than the X-ray binary XLF in NGC 253, which is consistent with previous measurements by Chandra at softer X-ray energies. The NuSTAR integrated galaxy spectrum of M83 drops quickly above 10 keV, which is also seen in the starburst galaxies NGC 253, NGC 3310, and NGC 3256. The NuSTAR observations constrain any active galactic nucleus (AGN) to be either highly obscured or to have an extremely low luminosity of ≲1038 erg s−1 (10–30 keV), implying that it is emitting at a very low Eddington ratio. An X-ray point source that is consistent with the location of the nuclear star cluster with an X-ray luminosity of a few times 1038 erg s−1 may be a low-luminosity AGN but is more consistent with being an X-ray binary.

view Abstract Citations (29) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Beam-driven Chromospheric Evaporation Model of Solar Flares: A Model Not Supported by Observations from Nonimpulsive Large Flares Feldman, U. Abstract Most large solar flares exhibit hard X-ray emission which is usually impulsive, as well as thermal soft X-ray emission, which is gradual. The beam-driven chromospheric evaporation model of solar flares was proposed to explain the origin of the soft X-ray emitting flare plasma. A careful evaluation of the issue under discussion reveals contradictions between predictions from the theoretical chromospheric evaporation model and actual observations from a set of large X- and M-type flares. It is shown that although the soft X-ray and hard X-ray emissions are a result of the same flare, one is not a result of the other. Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/169414 Bibcode: 1990ApJ...364..322F Keywords: Chromosphere; Solar Flares; Solar X-Rays; Coronal Loops; Emission Spectra; Light Curve; Solar Physics; SUN: CHROMOSPHERE; SUN: FLARES; SUN: X-RAYS full text sources ADS |

We use 13 yr of Swift/BAT observations to probe the nature and origin of the hard X-ray (14–195 KeV) emission in Centaurus A. Since the beginning of the Swift operation in 2004, significant X-ray variability in the 14–195 KeV band has been detected, with mild changes in the source spectrum. Spectral variations became more eminent after 2013, following a softer-when-brighter trend. Using the power spectral density (PSD) method, we find that the observed hard X-ray photon flux variations are consistent with a red-noise process of slope, −1.3, with no evidence for a break in the PSD. We find a significant correlation between the hard X-ray and 230 GHz radio flux variations, with no time delay longer than 30 days. The temporal and spectral analysis confirms that the X-ray emission generated by the accretion in the ADAF model is sub-dominant as compared with the emission arising from that produced by the inner regions of the radio jet.

The gas puff Z-pinch is an intense pulsed plasma X-ray source energized by a capacitor bank. Typically 5-10% of the electrical energy stored in the bank is converted into X-ray radiations with wavelengths less than 100 A. Such devices have applications in X-ray microscopy, lithography, spectroscopy and X-ray optics engineering. Here we present the results of a parmetric study of the X-ray emission from a small gas puff Z-pinch which has 4kJ of stored electrical energy. The quantities measured are the soft X-ray emission in the wavelength range 4 to 85 A and the hard X-ray emission with wavelength less than 2.5 A. These quantities are found to depend on the gas used and the mass of gas imploded. Conditions exist that maximise the soft X-ray emission from the pinch. It is also shown that the hard X-ray emission can be reduced significantly by changing the anode material and by varying the mass of the gas imploded.

Using RHESSI and some auxiliary observations we examine possible connections between spatial and temporal morphology of the sources of non-thermal hard X-ray (HXR) emission which revealed minute quasi-periodic pulsations (QPPs) during the two-ribbon flares on 2003 May 29 and 2005 January 19. Microwave emission also reveals the same quasi-periodicity. The sources of non-thermal HXR emission are situated mainly inside the footpoints of the flare arcade loops observed by the TRACE and SOHO instruments. At least one of the sources moves systematically both during the QPP-phase and after it in each flare that allows to examine the sources velocities and the energy release rate via the process of magnetic reconnection. The sources move predominantly parallel to the magnetic inversion line or the appropriate flare ribbon during the QPP-phase whereas the movement slightly changes to more perpendicular regime after the QPPs. Each QPP is emitted from its own position. It is also seen that the velocity and the energy release rate don't correlate well with the flux of the HXR emission calculated from the sources. The sources of microwaves and thermal HXRs are situated near the apex of the loop arcade and are not stationary either. Almost all QPPs and some spikes of HXR emission during the post-QPP-phase reveal the soft-hard-soft spectral behavior indicating separate acts of electrons acceleration and injection, rather than modulation of emission flux by some kinds of magnetohydrodynamic (MHD) oscillations of coronal loops. In all likelihood, the flare scenarios based on the successively firing arcade loops are more preferable to interpret the observations, although we can not conclude exactly what mechanism forces these loops to flare up.

\n We present a multi-wavelength analysis of a gradual radio flare\non June 27, 1993 which showed emission at millimeter waves long after\nthe soft X-ray flux had peaked. The radio flare located at S12 E75\nwas associated with a GOES class M3.6 flare that lasted for more than\none hour and hard X-ray emission during the rising phase of the \nsoft X-ray/radio emission. The maximum radio flux\ndensity at 35 GHz was 60 sfu, but the calculated thermal\nbremsstrahlung flux from the GOES soft X-rays was less than half of\nthat. The possible explanations for this prolonged millimeter wave\nemission could be accelerated high-energy electrons gyrating along the\nfield-lines (nonthermal gyrosynchrotron emission) or thermal\nbremsstrahlung from evaporating chromospheric warm and dense plasma\n(cool enough to go undetected by GOES), or a mixture of these. Our\nmodel calculations show that even an inhomogeneous source containing\nboth kinds of particles would not be able to produce such a spectral\nshape. A second source with extremely high electron densities\n(>1016 m-3), large source dimensions (>1015 m2),\nand very low temperatures (<106 K) must be assumed to explain the\nobserved radio spectra.\n\n\n

We present joint XMM–Newton and NuSTAR observations of the ‘bare’ narrow-line Seyfert 1 Ton S180 (z = 0.062), carried out in 2016 and providing the first hard X-ray view of this luminous galaxy. We find that the 0.4–30 keV band cannot be self-consistently reproduced by relativistic reflection models, which fail to account simultaneously for the soft and hard X-ray emission. The smooth soft excess prefers extreme blurring parameters, confirmed by the nearly featureless nature of the Reflection Grating Spectrometer (RGS) spectrum, while the moderately broad Fe K line and the modest hard excess above 10 keV appear to arise in a milder gravity regime. By allowing a different origin of the soft excess, the broad-band X-ray spectrum and overall spectral energy distribution (SED) are well explained by a combination of (a) direct thermal emission from the accretion disc, dominating from the optical to the far/extreme UV; (b) Comptonization of seed disc photons by a warm (kTe ∼ 0.3 keV) and optically thick (τ ∼ 10) corona, mostly contributing to the soft X-rays; (c) Comptonization by a standard hot ($kT_{\rm \mathrm{ e}}\gtrsim 100$ keV) and optically thin (τ < 0.5) corona, responsible for the primary X-ray continuum; and (d) reflection from the mid/outer part of the disc. The two coronae are suggested to be rather compact, with $R_{\rm hot}\lesssim R_{\rm warm}\lesssim 10\, r_{\rm g}$. Our SED analysis implies that Ton S180 accretes at super-Eddington rates. This is a key condition for the launch of a wind, marginal (i.e. 3.1σ significance) evidence of which is indeed found in the RGS spectrum.

We have studied the energetics of two impulsive solar flares of X-ray class X1.7 by assuming the electrons accelerated in several episodes of energy release to be the main source of plasma heating and reached conclusions about their morphology. The time profiles of the flare plasma temperature, emission measure, and their derivatives, and the intensity of nonthermal X-ray emission are compared; images of the X-ray sources and magnetograms of the flare region at key instants of time have been constructed. Based on a spectral analysis of the hard X-ray emission from RHESSI data and GOES observations of the soft X-ray emission, we have estimated the spatially integrated kinetic power of nonthermal electrons and the change in flare-plasma internal energy by taking into account the heat losses through thermal conduction and radiation and determined the parameters needed for thermal balance. We have established that the electrons accelerated at the beginning of the events with a relatively soft spectrum directly heat up the coronal part of the flare loops, with the increase in emission measure and hard X-ray emission from the chromosphere being negligible. The succeeding episodes of electron acceleration with a harder spectrum have virtually no effect on the temperature rise, but they lead to an increase in emission measure and hard X-ray emission from the footpoints of the flare loops.

view Abstract Citations (52) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The discovery of low-energy X-ray emission from U Geminorum. Mason, K. O. ; Lampton, M. ; Charles, P. ; Bowyer, S. Abstract Strong X-ray emission in the 0.15-0.5-keV band has been detected from U Gem during optical outburst. The peak flux observed in the 0.15-0.5-keV range is 3.2 x 10 to the -10th erg/sq cm per sec (about 3 x 10 to the 32nd erg/sec at 100 pc). No soft X-ray emission was detected during the quiescent optical phase at a level which is at least 100 times less than the peak outburst flux. During outburst, the soft X-ray source appears to be highly variable on a time scale of a few hours, but this variability is consistent with orbital dependence for the limited data sample available. The origin of the soft X-ray emission is discussed. Publication: The Astrophysical Journal Pub Date: December 1978 DOI: 10.1086/182847 Bibcode: 1978ApJ...226L.129M Keywords: Binary Stars; Novae; Satellite Observation; White Dwarf Stars; X Ray Sources; Heao 1; Spectral Energy Distribution; Ultraviolet Astronomy; Astrophysics; Dwarf Novae:X Rays; U Geminorum Stars:X Rays full text sources ADS | data products SIMBAD (2)

Initial results of hard X-ray spectroscopy by LaBr[formula omitted](Ce) detector for runaway electron study in Thailand Tokamak-1

Context. Hard X-ray (HXR) sources are frequently observed near the top of solar flare loops, which are also bright in soft X-ray (SXR) and extreme ultraviolet (EUV) wavebands. We revisit a recent scenario proposed by Fang et al. (2016) to trigger loop-top turbulence in flaring loops, which can help explain variations seen in SXR and EUV brightenings and potentially impact and induce HXR emission. It is conjectured that evaporation flows from flare-impacted chromospheric footpoints interact with each other near the loop top and produce turbulence via the Kelvin–Helmholtz instability (KHI). Aims. By performing a rigorous parameter survey, in which we vary the duration, total amount, and asymmetry of the energy deposition at both footpoints, we assess the relevance of the KHI in triggering and sustaining loop-top turbulence. We synthesize SXR and EUV emission and discuss the possibility of HXR emission through bremsstrahlung or inverse Compton processes, which scatter SXR photons to HXR photons via the inverse Compton mechanism. Methods. We performed 2.5D numerical simulations in which the magnetohydrodynamic model incorporates a realistic photosphere to coronal stratification, parametrized heating, radiative losses, and field-aligned anisotropic thermal conduction. We focus on the trigger of the KHI and the resulting turbulence, as well as identify various oscillatory patterns that appear in the evolutions. Results. We find that a M2.2-class related amount of energy should be deposited in less than four minutes to trigger a KHI interaction. Slower deposition, or lesser energy (< 0.33 × 1029 ergs) rather leads to mere loop-top compression sites bounded by shocks, without KHI development. Asymmetry in the footpoint deposition determines whether the KHI turbulent zone gets produced away from the apex, and asymmetric cases can show a slow-mode mediated, periodic displacement of the turbulent zone. Our reference simulation further demonstrates a clear 25 s periodicity in the declining phase of the SXR light curve, wherein compressional effects dominate. Conclusions. When turbulence is produced in the loop apex, an index of −5/3 can be found in the spectra of velocity and magnetic field fluctuations. Typical values for M-class flares routinely show KHI development. The synthesized SXR light curve shows a clear periodic signal related to the sloshing motion of the vortex pattern created by the KHI.