Coronal X-ray Emission Research Articles

Simulating the environment around planet-hosting stars

We present the results of a detailed numerical simulation of the circumstellar environment around three exoplanet-hosting stars. A state-of-the-art global magnetohydrodynamic (MHD) model is considered, including Alfv\'en wave dissipation as a self-consistent coronal heating mechanism. This paper contains the description of the numerical set-up, evaluation procedure, and the simulated coronal structure of each system (HD 1237, HD 22049 and HD 147513). The simulations are driven by surface magnetic field maps, recovered with the observational technique of Zeeman Doppler Imaging (ZDI). A detailed comparison of the simulations is performed, where two different implementations of this mapping routine are used to generate the surface field distributions. Quantitative and qualitative descriptions of the coronae of these systems are presented, including synthetic high-energy emission maps in the Extreme Ultra-Violet (EUV) and Soft X-rays (SXR) ranges. Using the simulation results, we are able to recover similar trends as in previous observational studies, including the relation between the magnetic flux and the coronal X-ray emission. Furthermore, for HD 1237 we estimate the rotational modulation of the high-energy emission due to the various coronal features developed in the simulation. We obtain variations, during a single stellar rotation cycle, up to 15\% for the EUV and SXR ranges. The results presented here will be used, in a follow-up paper, to self-consistently simulate the stellar winds and inner astrospheres of these systems.

The influence of radiative core growth on coronal X-ray emission from pre-main-sequence stars

Pre-main sequence (PMS) stars of mass $\gtrsim0.35\,{\rm M}_\odot$ transition from hosting fully convective interiors to configurations with a radiative core and outer convective envelope during their gravitational contraction. This stellar structure change influences the external magnetic field topology and, as we demonstrate herein, affects the coronal X-ray emission as a stellar analog of the solar tachocline develops. We have combined archival X-ray, spectroscopic, and photometric data for $\sim$1000 PMS stars from five of the best studied star forming regions; the ONC, NGC 2264, IC 348, NGC 2362, and NGC 6530. Using a modern, PMS calibrated, spectral type-to-effective temperature and intrinsic colour scale, we deredden the photometry using colours appropriate for each spectral type, and determine the stellar mass, age, and internal structure consistently for the entire sample. We find that PMS stars on Henyey tracks have, on average, lower fractional X-ray luminosities ($L_{\rm X}/L_\ast$) than those on Hayashi tracks, where this effect is driven by changes in $L_{\rm X}$. X-ray emission decays faster with age for higher mass PMS stars. There is a strong correlation between $L_\ast$ and $L_{\rm X}$ for Hayashi track stars but no correlation for Henyey track stars. There is no correlation between $L_{\rm X}$ and radiative core mass or radius. However, the longer stars have spent with radiative cores, the less X-ray luminous they become. The decay of coronal X-ray emission from young early K to late G-type PMS stars, the progenitors of main sequence A-type stars, is consistent with the dearth of X-ray detections of the latter.

Relations Between He I λ10830 Absorption Strength and Stellar Activity Amongst Dwarf Stars

AbstractCorrelations are identified between the strength of the λ10830 He I triplet line and the following tracers of stellar activity amongst FGK dwarfs with colours of (B − V) > 0.47: coronal soft X-ray emission, emission in the λ1549 C IV and λ1335 C II lines originating from the transition region, and Ca II H and K emission from the chromosphere. No such correlations are present amongst dwarfs with spectral type earlier than F6. In addition, G and K dwarfs with strong triplet lines show evidence of excess flux in the GALEX FUV band compared to weak-triplet-line dwarfs. The X-ray spectra of late-F, G, and K dwarfs with He I triplets stronger than 160 mÅ have greater values of the ROSAT hardness ratio HR1 than are typical of weak-triplet dwarfs in the same range of spectral type. In other words, dwarfs later than F7V with strong He I triplet lines tend towards harder 0.1–2.0 keV X-ray spectra than weak-triplet dwarfs, although values of HR1 ~ −0.2 to +0.1 can still be encountered amongst a minority of weak-He-triplet stars. As regards, FGK main sequence stars the observational data on the λ10830 triplet line remains sparse. Progress could be made through spectroscopy of high resolution for samples of hundreds of stars, selected on the basis of having other measures of chromospheric and coronal activity available.

Discovery of a low-mass companion to the F7V star HD 984

We report the discovery of a low-mass companion to the nearby (d = 47 pc) F7V star HD 984. The companion is detected 0.19" away from its host star in the L' band with the Apodizing Phase Plate on NaCo/VLT and was recovered by L'-band non-coronagraphic imaging data taken a few days later. We confirm the companion is co-moving with the star with SINFONI integral field spectrograph H+K data. We present the first published data obtained with SINFONI in pupil-tracking mode. HD 984 has been argued to be a kinematic member of the 30 Myr-old Columba group, and its HR diagram position is not altogether inconsistent with being a ZAMS star of this age. By consolidating different age indicators, including isochronal age, coronal X-ray emission, and stellar rotation, we independently estimate a main sequence age of 115$\pm$85 Myr (95% CL) which does not rely on this kinematic association. The mass of directly imaged companions are usually inferred from theoretical evolutionary tracks, which are highly dependent on the age of the star. Based on the age extrema, we demonstrate that with our photometric data alone, the companion's mass is highly uncertain: between 33 and 96 M$_{\rm Jup}$ (0.03-0.09 M$_{\odot}$) using the COND evolutionary models. We compare the companion's SINFONI spectrum with field dwarf spectra to break this degeneracy. Based on the slope and shape of the spectrum in the H-band, we conclude that the companion is an M$6.0\pm0.5$ dwarf. The age of the system is not further constrained by the companion, as M dwarfs are poorly fit on low-mass evolutionary tracks. This discovery emphasizes the importance of obtaining a spectrum to spectral type companions around F-stars.

GENERALIZED INVESTIGATION OF THE ROTATION-ACTIVITY RELATION: FAVORING ROTATION PERIOD INSTEAD OF ROSSBY NUMBER

Magnetic activity in Sun-like and low-mass stars causes X-ray coronal emission, which is stronger for more rapidly rotating stars. This relation is often interpreted in terms of the Rossby number, i.e., the ratio of rotation period to convective overturn time. We reconsider this interpretation on the basis of the observed X-ray emission and rotation periods of 821 stars with masses below 1.4 Msun. A generalized analysis of the relation between X-ray luminosity normalized by bolometric luminosity, L_X/L_bol, and combinations of rotational period, P, and stellar radius, R, shows that the Rossby formulation does not provide the solution with minimal scatter. Instead, we find that the relation L_X/L_bol ~ P^{-2}R^{-4} optimally describes the non-saturated fraction of the stars. This relation is equivalent to L_X ~ P^{-2}, indicating that the rotation period alone determines the total X-ray emission. Since L_X is directly related to the magnetic flux at the stellar surface, this means that the surface flux is determined solely by the star's rotation and is independent of other stellar parameters. While a formulation in terms of a Rossby number would be consistent with these results if the convective overturn time scales exactly as L_bol^{-1/2}, our generalized approach emphasizes the need to test a broader range of mechanisms for dynamo action in cool stars.

Indications for an influence of hot Jupiters on the rotation and activity of their host stars

The magnetic activity of planet-hosting stars is an important factor to estimate the atmospheric stability of close-in exoplanets and the age of their host stars. It has long been speculated that close-in exoplanets can influence the stellar activity level. However, testing for tidal or magnetic interaction effects in samples of planet-hosting stars is difficult because stellar activity hinders exoplanet detection, so that stellar samples with detected exoplanets show a bias towards low activity for small exoplanets. We aim to test if exoplanets in close orbits influence the stellar rotation and magnetic activity of their host stars, and have developed a novel approach to test for such systematic activity enhancements. We use wide (several 100 AU) binary systems in which one of the stellar components is known to have an exoplanet, while the second stellar component does not have a detected planet and therefore acts as a negative control. We use the stellar coronal X-ray emission as an observational proxy for magnetic activity, and analyze observations performed with Chandra and XMM-Newton. We find that in two systems for which strong tidal interaction can be expected the planet-hosting primary displays a much higher magnetic activity level than the planet-free secondary. In three systems for which weaker tidal interaction can be expected the activity levels of both stellar components are in agreement. Our observations indicate that the presence of Hot Jupiters may inhibit the spin-down of host stars with thick outer convective layers. Possible causes for such an effect include a transfer of angular momentum from the planetary orbit to the stellar rotation through tidal interaction, or differences during the early evolution of the system, where the host star may decouple from the protoplanetary disk early due to a gap opened by the forming Hot Jupiter.

Extreme Ultraviolet Late-Phase Flares: Before and During the Solar Dynamics Observatory Mission

The SDO EUV observations have revealed interesting characteristics of warm coronal emissions, which peak soon after the hot coronal X-ray emissions peak during a flare and then sometimes peak for a second time hours after the X-ray flare peak. This flare type, with two warm coronal emission peaks but only one X-ray peak, has been named the EUV late phase. These flares have the distinct properties of i) having a complex magnetic field structure with two initial sets of coronal loops, with one upper set overlaying a lower set, ii) having an eruptive flare initiated in the lower set and disturbing both loop sets, iii) having the hot coronal emissions emitted only from the lower set, and iv) having the first peak of the warm coronal emissions associated with the lower set and its second peak emitted from the upper set much later. The disturbance of the coronal loops by the eruption is at about the same time, but the relaxation and cooling down of the heated coronal loops during the post-flare reconnections have different time scales with the longer, upper loops being significantly delayed from the lower loops. The difference in these cooling time scales is related to the difference between the two peak times of the warm coronal emission and is also apparent in the decay profile of the X-ray emissions having two distinct decays, with the first decay slope being steeper and the delayed decay slope being smaller during the time of the warm coronal emission second peak. The frequency and relationship of the EUV late-phase decay times between the Fe XVI two flare peaks and X-ray decay slopes are examined using three years of SDO/EVE data, and the X-ray dual-decay character is then exploited to estimate the frequency of EUV late-phase flares during the past four solar cycles. This study indicates that the frequency of EUV late-phase flares peaks before and after each solar cycle minimum.

MAGNETIC INHIBITION OF CONVECTION AND THE FUNDAMENTAL PROPERTIES OF LOW-MASS STARS. I. STARS WITH A RADIATIVE CORE

Magnetic fields are hypothesized to inflate the radii of low-mass stars---defined as less massive than 0.8$M_\odot$---in detached eclipsing binaries (DEBs). We investigate this hypothesis using the recently introduced magnetic Dartmouth stellar evolution code. In particular, we focus on stars thought to have a radiative core and convective outer envelope by studying in detail three individual DEBs: UV Psc, YY Gem, and CU Cnc. The results suggest that the stabilization of thermal convection by a magnetic field is a plausible explanation for the observed model-radius discrepancies. However, surface magnetic field strengths required by the models are significantly stronger than those estimated from the observed coronal X-ray emission. Agreement between model predicted surface magnetic field strengths and those inferred from X-ray observations can be found by assuming that the magnetic field sources its energy from convection. This approach makes the transport of heat by convection less efficient and is akin to reduced convective mixing length methods used in other studies. Predictions for the metallicity and magnetic field strengths of the aforementioned systems are reported. We also develop an expression relating a reduction in the convective mixing length to a magnetic field strength in units of the equipartition value. Our results are compared with those from previous investigations to incorporate magnetic fields to explain the low-mass DEB radius inflation. Finally, we explore how the effects of magnetic fields might affect mass determinations using asteroseismic data and the implication of magnetic fields on exoplanet studies.

X-rays from magnetic massive OB stars

AbstractThe magnetic activity of solar-type and low-mass stars is a well known source of coronal X-ray emission. At the other end of the main sequence, X-rays emission is instead associated with the powerful, radiatively driven winds of massive stars. Indeed, the intrinsically unstable line-driving mechanism of OB star winds gives rise to shock-heated, soft emission (~0.5 keV) distributed throughout the wind. Recently, the latest generation of spectropolarimetric instrumentation has uncovered a population of massive OB-stars hosting strong, organized magnetic fields. The magnetic characteristics of these stars are similar to the apparently fossil magnetic fields of the chemically peculiar ApBp stars. Magnetic channeling of these OB stars' strong winds leads to the formation of large-scale shock-heated magnetospheres, which can modify UV resonance lines, create complex distributions of cooled Halpha emitting material, and radiate hard (~2-5 keV) X-rays. This presentation summarizes our coordinated observational and modelling efforts to characterize the manifestation of these magnetospheres in the X-ray domain, providing an important contrast between the emission originating in shocks associated with the large-scale fossil fields of massive stars, and the X-rays associated with the activity of complex, dynamo-generated fields in lower-mass stars.

CONSTRAINING A MODEL OF TURBULENT CORONAL HEATING FOR AU MICROSCOPII WITH X-RAY, RADIO, AND MILLIMETER OBSERVATIONS

Many low-mass pre-main-sequence stars exhibit strong magnetic activity and coronal X-ray emission. Even after the primordial accretion disk has been cleared out, the star's high-energy radiation continues to affect the formation and evolution of dust, planetesimals, and large planets. Young stars with debris disks are thus ideal environments for studying the earliest stages of non-accretion-driven coronae. In this paper we simulate the corona of AU Mic, a nearby active M dwarf with an edge-on debris disk. We apply a self-consistent model of coronal loop heating that was derived from numerical simulations of solar field-line tangling and magnetohydrodynamic turbulence. We also synthesize the modeled star's X-ray luminosity and thermal radio/millimeter continuum emission. A realistic set of parameter choices for AU Mic produces simulated observations that agree with all existing measurements and upper limits. This coronal model thus represents an alternative explanation for a recently discovered ALMA central emission peak that was suggested to be the result of an inner "asteroid belt" within 3 AU of the star. However, it is also possible that the central 1.3 mm peak is caused by a combination of active coronal emission and a bright inner source of dusty debris. Additional observations of this source's spatial extent and spectral energy distribution at millimeter and radio wavelengths will better constrain the relative contributions of the proposed mechanisms.

Study of X-pinch dynamics using a low current (25 kA) and slower current (400 ns) pulse

We report on experiments examining the dynamics of low current (25 kA), long rise-time (400 ns) X-pinches. The experiments used two- and four-wire 5 μm tungsten X-pinches. We discuss coronal plasma dynamics, X-ray emission, cross-point evolution, and axial column dynamics. We show that the pinch dynamics are akin to those in X-pinches driven by 0.1–1 MA and at rise-rates from 0.25 to 10 kA/ns. We observe soft X-rays (∼1–2 keV range) emitted from the cross point in both two-wire and four-wire X-pinches at about peak current. These results show that compact current generators consisting of 2–4 small capacitors in a simple capacitive discharge circuit can produce good quality pinches for applications in Thomson X-ray scattering and backlighting among others.

THE EFFECT OF LIMITED SPATIAL RESOLUTION OF STELLAR SURFACE MAGNETIC FIELD MAPS ON MAGNETOHYDRODYNAMIC WIND AND CORONAL X-RAY EMISSION MODELS

We study the influence of the spatial resolution on scales of $5\deg$ and smaller of solar surface magnetic field maps on global magnetohydrodynamic solar wind models, and on a model of coronal heating and X-ray emission. We compare the solutions driven by a low-resolution Wilcox Solar Observatory magnetic map, the same map with spatial resolution artificially increased by a refinement algorithm, and a high-resolution Solar and Heliospheric Observatory Michelson Doppler Imager map. We find that both the wind structure and the X-ray morphology are affected by the fine-scale surface magnetic structure. Moreover, the X-ray morphology is dominated by the closed loop structure between mixed polarities on smaller scales and shows significant changes between high and low resolution maps. We conclude that three-dimensional modeling of coronal X-ray emission has greater surface magnetic field spatial resolution requirements than wind modeling, and can be unreliable unless the dominant mixed polarity magnetic flux is properly resolved.

Detection of transient high frequency optical oscillations of the flare star YZ CMin

AbstractIn this paper we present the results of the analysis of the B ‐light curve for the flares of the red dwarf YZ CMin (dM4.5e), which were observed on February of 2002, with the help of the 30‐inch Cassegrain telescope of the Stephanion Observatory, Greece. Discrete Fourier Transform analysis and the use of the Brownian Walk noise enable us to estimate the proper random noise and detect possible weak transient optical oscillations. Our results indicate that: (1) Transient high frequency oscillations occur during the flare event and during the quiet‐star phase as well. (2) The observed frequencies range between 0.0083 Hz (period 2 min) and 0.3 Hz (period 3 s) is not rigorously bounded. The phenomenon is most pronounced during the flare state. (3) During the flare state: (a) Oscillations with period 2 to 1.5 min, 60 s, 11 s, 7.5 s, and 4 s appear around the maximum light state and persist during the whole flare state, (b) from the flare maximum phase on, a progressive increase of oscillations with periods 30 s, 20 s down to 4.0 s is markedly indicated, and (c) at the end of the flare only the oscillation of the pre‐flare state do remain. Our observations are consistent with the phenomenology of impulsively exited oscillations on a coronal magnetic loop and subsequent chromospheric heating by electronic flux at the foot of the loop or/and by soft X‐ray coronal emission. Our observation give evidence that more than one impulsive events may occur in the course of an observed flare (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An Observational Study on Coronal Hard X-ray Sources in Flares

Observational studies on solar flares with footpoints partially occulted by the solar limb provide an important method for diagnostics of coronal hard X-ray emissions. The statistics of hard X-ray sources in 71 such flares observed by RHESSI (Reuven Ramaty High-Energy Solar Spectroscopic Imager) show that the two kinds of hard X-ray sources proposed in previous studies (i.e., the sources with respectively a smaller and larger spatial separations between the thermal and non-thermal sources of coronal hard X-ray emissions) have no evident difference in the aspects of their photon spectra, images, light curves, GOES durations, etc. The area of the radiation region, the flare's total thermal energy and GOES duration are well correlated with the distance of separation. These results support some unified models of solar flares proposed in recent years, and indicate that the Masuda flare is only a kind of special event, which does not possess the general features of coronal hard X-ray emissions

Coronal activity cycles in nearby G and K stars

We use X-ray observations of the nearby binaries 61 Cyg A/B (K5V and K7V) and Alpha Cen A/B (G2V and K1V) to study the long-term evolution of magnetic activity in weakly to moderately active G + K dwarfs over nearly a decade. Specifically we search for X-ray activity cycles and related coronal changes and compare them to the solar behavior. For 61 Cyg A we find a regular coronal activity cycle analog to its 7.3 yr chromospheric cycle. The X-ray brightness variations are with a factor of three significantly lower than on the Sun, yet the changes of coronal properties resemble the solar behavior with larger variations occurring in the respective hotter plasma components. 61 Cyg B does not show a clear cyclic coronal trend so far, but the X-ray data matches the more irregular chromospheric cycle. Both Alpha Cen stars exhibit significant long-term X-ray variability. Alpha Cen A shows indications for cyclic variability of an order of magnitude with a period of about 12-15 years; the Alpha Cen B data suggests an X-ray cycle with an amplitude of about six to eight and a period of 8-9 years. The sample stars exhibit X-ray luminosities ranging between Lx < 1x10^26 - 3x10^27 erg s^-1 in the 0.2-2.0 keV band and have coronae dominated by cool plasma with variable average temperatures of around 1.0-2.5 MK. We find that coronal activity cycles are apparently a common phenomenon in older, slowly rotating G and K stars. The spectral changes of the coronal X-ray emission over the cycles are solar-like in all studied targets.

The coronal X-ray-age relation and its implications for the evaporation of exoplanets

We study the relationship between coronal X-ray emission and stellar age for late-type stars, and the variation of this relationship with spectral type. We select 717 stars from 13 open clusters and find that the ratio of X-ray to bolometric luminosity during the saturated phase of coronal emission decreases from 10^-3.1 for late K-dwarfs to 10^-4.3 for early F-type stars (across the range 0.29<(B-V)_0<1.41). Our determined saturation timescales vary between 10^7.6 and 10^8.3 years, though with no clear trend across the whole FGK range. We apply our X-ray emission - age relations to the investigation of the evaporation history of 121 known transiting exoplanets using a simple energy-limited model of evaporation and taking into consideration Roche lobe effects and different heating/evaporation efficiencies. We confirm that a linear cut-off of the planet distribution in the M^2/R^3 versus a^-2 plane is an expected result of population modification by evaporation and that the known transiting exoplanets display such a cut-off. We find that for an evaporation efficiency of 25% we expect around 1 in 10 of the known transiting exoplanets to have lost > 5% of their mass since formation. In addition we provide estimates of the minimum formation mass for which a planet could be expected to survive for 4 Gyrs for a range of stellar and planetary parameters. We emphasise the importance of the earliest periods of a planet's life for its evaporation history with 75% expected to occur within the first Gyr. This raises the possibility of using evaporation histories to distinguish between different migration scenarios. For planets with available spin-orbit angles no difference is found between the distributions of planets with misaligned orbits and those with aligned orbits. This suggests that misalignment occurs early in the life of the planetary system, though more data is needed.

Simulated performance of a single pixel PIN spectrometer SCXM equipped with a concentrator optics in Solar coronal X-ray observations

In this paper we present simulated solar coronal X-ray observations to verify the sensitivity of a new hypothetical instrument design. These simulations are folded through this X-ray spectrometer having a moderate size circular field of view ( FoV) of 1.6°. This SCXM (Solar Coronal X-ray Mapper) is designed to compose of a single pixel silicon PIN detector equipped with a single reflection double frustum X-ray optics. A moderate FoV would enable a morphological study of the expanded X-ray emission from the solar corona during a high activity of the Sun. The main scientific task of SCXM would be the mapping of the coronal X-ray emission, i.e. to resolve the radial distribution of the X-ray surface brightness around the Sun. These kinds of off-limb observations would help to interpret the coronal plasma diagnostics as a function of the elongation angle. Direct solar full disc observations could be also performed with SCXM. In this work we have applied real solar coronal X-ray data obtained by the SMART-1 XSM (X-ray Solar Monitor) [3] to simulate on-solar observations at different flux levels to derive full disc sensitivity and performance of SCXM. A challenging attempt for SCXM would also be to distinguish the X-ray spectrum of the decaying axions around the Sun. These axions are assumed to be created as side products of fusion reactions in the core of the Sun. These axions are predicted to be gravitationally trapped to orbit the Sun forming a halo-like X-ray emitting object. No signature of an axion X-ray emission around the Sun has been observed to this day. This simple X-ray spectrometer with an optical concentrator would be an inexpensive instrument with low mass and telemetry budgets compared with more accurate X-ray instruments of imaging capability. Hence SCXM would be an advanced choice as an auxiliary instrument for solar coronal X-ray observations.

The corona and companion of CoRoT-2a. Insights from X-rays and optical spectroscopy

CoRoT-2 is one of the most unusual planetary systems known to date. Its host star is exceptionally active, showing a pronounced, regular pattern of optical variability caused by magnetic activity. The transiting hot Jupiter, CoRoT-2b, shows one of the largest known radius anomalies. We analyze the properties and activity of CoRoT-2A in the optical and X-ray regime by means of a high-quality UVES spectrum and a 15 ks Chandra exposure both obtained during planetary transits. The UVES data are analyzed using various complementary methods of high-resolution stellar spectroscopy. We characterize the photosphere of the host star by deriving accurate stellar parameters such as effective temperature, surface gravity, and abundances. Signatures of stellar activity, Li abundance, and interstellar absorption are investigated to provide constraints on the age and distance of CoRoT-2. Furthermore, our UVES data confirm the presence of a late-type stellar companion to CoRoT-2A that is gravitationally bound to the system. The Chandra data provide a clear detection of coronal X-ray emission from CoRoT-2A, for which we obtain an X-ray luminosity of 1.9e29 erg/s. The potential stellar companion remains undetected in X-rays. Our results indicate that the distance to the CoRoT-2 system is approximately 270 pc, and the most likely age lies between 100 and 300 Ma. Our X-ray observations show that the planet is immersed in an intense field of high-energy radiation. Surprisingly, CoRoT-2A's likely coeval stellar companion, which we find to be of late-K spectral type, remains X-ray dark. Yet, as a potential third body in the system, the companion could account for CoRoT-2b's slightly eccentric orbit.

The Galactic plane at faint X-ray fluxes - I. Properties and characteristics of the X-ray source population

We investigate the serendipitous X-ray source population revealed in XMM-Newton observations targeted in the Galactic Plane within the region 315<l<45 and |b|<2.5 deg. Our study focuses on a sample of 2204 X-ray sources at intermediate to faint fluxes, which were detected in a total of 116 XMM fields and are listed in the 2XMMi catalogue. We characterise each source as spectrally soft or hard on the basis of whether the bulk of the recorded counts have energies below or above 2 keV and find that the sample divides roughly equally (56%:44%) into these soft and hard categories. The X-ray spectral form underlying the soft sources may be represented as either a power-law continuum with Gamma~2.5 or a thermal spectrum with kT~0.5 keV, with N_H ranging from 10^{20-22} cm^{-2}. For the hard sources, a significantly harder continuum form is likely, i.e., Gamma~1 with N_H=10^{22-24} cm^{-2}. For ~50% of the hard sources, the inferred column density is commensurate with the total Galactic line-of-sight value; many of these sources will be located at significant distances across the Galaxy implying a hard band luminosity L_X>10^{32} erg/s, whereas some will be extragalactic interlopers. >90% of the soft sources have potential NIR (2MASS and/or UKIDSS) counterparts inside their error circles, consistent with the dominant soft X-ray source population being relatively nearby coronally-active stars. These stellar counterparts are generally brighter than J=16, a brightness cutoff which corresponds to the saturation of the X-ray coronal emission at L_X=10^{-3} L_{bol}. In contrast, the success rate in finding likely IR counterparts to the hard X-ray sample is no more than ~15% down to J=16 and ~25% down to J=20, set against a rapidly rising chance coincidence rate. The make-up of the hard X-ray source population, in terms of the known classes of accreting and non-accreting systems, remains uncertain.

MEASUREMENTS OF THE CORONAL ACCELERATION REGION OF A SOLAR FLARE

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph (NoRH) are used to investigate coronal hard X-ray and microwave emissions in the partially disk-occulted solar flare of 2007 December 31. The STEREO mission provides EUV images of the flare site at different viewing angles, establishing a two-ribbon flare geometry and occultation heights of the RHESSI and NoRH observations of {approx}16 Mm and {approx}25 Mm, respectively. Despite the occultation, intense hard X-ray emission up to {approx}80 keV occurs during the impulsive phase from a coronal source that is also seen in microwaves. The hard X-ray and microwave source during the impulsive phase is located {approx}6 Mm above thermal flare loops seen later at the soft X-ray peak time, similar in location to the above-the-loop-top source in the Masuda flare. A single non-thermal electron population with a power-law distribution (with spectral index of {approx}3.7 from {approx}16 keV up to the MeV range) radiating in both bremsstrahlung and gyrosynchrotron emission can explain the observed hard X-ray and microwave spectrum, respectively. This clearly establishes the non-thermal nature of the above-the-loop-top source. The large hard X-ray intensity requires a very large number (>5 x 10{sup 35} above 16 keV for themore » derived upper limit of the ambient density of {approx}8 x 10{sup 9} cm{sup -3}) of suprathermal electrons to be present in this above-the-loop-top source. This is of the same order of magnitude as the number of ambient thermal electrons. We show that collisional losses of these accelerated electrons would heat all ambient electrons to superhot temperatures (tens of keV) within seconds. Hence, the standard scenario, with hard X-rays produced by a beam comprising the tail of a dominant thermal core plasma, does not work. Instead, all electrons in the above-the-loop-top source seem to be accelerated, suggesting that the above-the-loop-top source is itself the electron acceleration region.« less