Solar x-ray spectrometer (SOXS) mission – Low energy payload – First results

We present the first results from the ‘Low Energy Detector’ pay-load of ‘Solar X-ray Spectrometer (SOXS)’ mission, which was launched onboard GSAT-2 Indian spacecraft on 08 May 2003 by GSLV-D2 rocket to study the solar flares. The SOXS Low Energy Detector (SLD) payload was designed, developed and fabricated by Physical Research Laboratory (PRL) in collaboration with Space Application Centre (SAC), Ahmedabad and ISRO Satellite Centre (ISAC), Bangalore of the Indian Space Research Organization (ISRO). The SLD payload employs the state-of-the-art solid state detectors viz., Si PIN and Cadmium-Zinc-Telluride (CZT) devices that operate at near room temperature (-20°C). The dynamic energy range of Si PIN and CZT detectors are 4–25 keV and 4–56 keV respectively. The Si PIN provides sub-keV energy resolution while CZT reveals ∼1.7keV energy resolution throughout the dynamic range. The high sensitivity and sub-keV energy resolution of Si PIN detector allows the measuring of the intensity, peak energy and equivalent width of the Fe-line complex at approximately 6.7 keV as a function of time in all 8 M-class flares studied in this investigation. The peak energy (E _p) of Fe-line feature varies between 6.4 and 6.8 keV with increase in temperature from 9 to 34 MK. We found that the equivalent width (ω) of Fe-line feature increases exponentially with temperature up to 20 MK but later it increases very slowly up to 28 MK and then it remains uniform around 1.55 keV up to 34 MK. We compare our measurements ofw with calculations made earlier by various investigators and propose that these measurements may improve theoretical models. We interpret the variation of both E_pand ω with temperature as the changes in the ionization and recombination conditions in the plasma during the flare interval and as a consequence the contribution from different ionic emission lines also varies.     

The Fe-Line Feature in the X-Ray Spectrum of Solar Flares: First Results from the SOXS Mission

We present the first results from the low-energy detector payload of the solar X-ray spectrometer (SOXS) mission, which was launched onboard the GSAT-2 Indian spacecraft on May 08, 2003 by the GSLV-D2 rocket to study solar flares. The SOXS low-energy detector (SLD) payload was designed, developed, and fabricated by the Physical Research Laboratory (PRL) in collaboration with the Space Application Centre (SAC), Ahmedabad and the Indian Space Research Organization (ISRO) Satellite Centre (ISAC), Bangalore. The SLD payload employs state-of-the-art, solid-state detectors, viz., Si PIN and Cadmium-Zinc-Telluride (CZT) devices that operate at near room temperature (−20 ^°C). The energy ranges of the Si PIN and CZT detectors are 4 – 25 and 4 – 56 keV, respectively. The Si PIN provides sub-keV energy resolution, while the CZT provides ~1.7 keV energy resolution throughout the energy range. The high sensitivity and sub-keV energy resolution of the Si PIN detector allows measuring the intensity, peak energy, and the equivalent width of the Fe-line complex at approximately 6.7 keV, as a function of time in all ten M-class flares studied in this investigation. The peak energy (E _p) of the Fe-line feature varies between 6.4 and 6.7 keV with increase in temperature from 9 to 58 MK. We found that the equivalent width (w) of the Fe-line feature increases exponentially with temperature up to 30 MK and then increases very slowly up to 40 MK. It remains between 3.5 and 4 keV in the temperature range of 30 – 45 MK. We compare our measurements of w with calculations made earlier by various investigators and propose that these measurements may improve theoretical models. We interpret the variation of both E _p and w with temperature as being to the changes in the ionization and recombination conditions in the plasma during the flare, and as a consequence, the contribution from different ionic emission lines also varies.     

Solar Flares and Coronal Mass Ejections: A Statistically Determined Flare Flux – CME Mass Correlation

In an effort to examine the relationship between flare flux and corresponding CME mass, we temporally and spatially correlate all X-ray flares and CMEs in the LASCO and GOES archives from 1996 to 2006. We cross-reference 6733 CMEs having well-measured masses against 12 050 X-ray flares having position information as determined from their optical counterparts. For a given flare, we search in time for CMEs which occur 10 – 80 minutes afterward, and we further require the flare and CME to occur within ± 45° in position angle on the solar disk. There are 826 CME/flare pairs which fit these criteria. Comparing the flare fluxes with CME masses of these paired events, we find CME mass increases with flare flux, following an approximately log-linear, broken relationship: in the limit of lower flare fluxes, log (CME mass)∝0.68×log (flare flux), and in the limit of higher flare fluxes, log (CME mass)∝0.33×log (flare flux). We show that this broken power-law, and in particular the flatter slope at higher flare fluxes, may be due to an observational bias against CMEs associated with the most energetic flares: halo CMEs. Correcting for this bias yields a single power-law relationship of the form log (CME mass)∝0.70×log (flare flux). This function describes the relationship between CME mass and flare flux over at least 3 dex in flare flux, from ≈ 10⁻⁷ – 10⁻⁴ W m⁻².     

Relative Timing and Spectra of Solar Flare Hard X-ray Sources

Hard X-ray lightcurves, spectrograms, images, and spectra of three medium-sized flares observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) are presented. Imaging spectroscopy of the 20 February 2002, 11:06 UT flare at 10′′ spatial resolution, comparable to the best previous hard X-ray imaging from Yohkoh, shows two footpoints with an ∼ 8 s delay of peak emission between footpoints. Subsequent imaging at le4′′ shows three sources consistent with two separate loops and simultaneous brightening in connected footpoints. Imaging for the simple two footpoint flare of 2 June 2002 also shows simultaneous footpoint brightening. The more complex 17 March 2002 flare shows at least four different sources during the main peak of the event, and it is difficult to clearly demonstrate simultaneous brightening of connected footpoints. Non-thermal power laws are observed down to ∼ 12–13 keV without flattening in all these events, indicating the energy content in energetic electrons may be significantly greater than previously estimated from assumed 25 keV low energy cutoff. Simultaneously brightening footpoints show similar spectra, at least in the three flares investigated. Double-power-law spectra with a relatively sharp break are often observed. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022469902940     

Energy-Dependent Timing of Thermal Emission in Solar Flares

We report solar flare plasma to be multi-thermal in nature based on the theoretical model and study of the energy-dependent timing of thermal emission in ten M-class flares. We employ high-resolution X-ray spectra observed by the Si detector of the “Solar X-ray Spectrometer” (SOXS). The SOXS onboard the Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May 2003. Firstly we model the spectral evolution of the X-ray line and continuum emission flux F(ε) from the flare by integrating a series of isothermal plasma flux. We find that the multi-temperature integrated flux F(ε) is a power-law function of ε with a spectral index (γ)≈−4.65. Next, based on spectral-temporal evolution of the flares we find that the emission in the energy range E=4 – 15 keV is dominated by temperatures of T=12 – 50 MK, while the multi-thermal power-law DEM index (δ) varies in the range of −4.4 and −5.7. The temporal evolution of the X-ray flux F(ε,t) assuming a multi-temperature plasma governed by thermal conduction cooling reveals that the temperature-dependent cooling time varies between 296 and 4640 s and the electron density (n _e) varies in the range of n _e=(1.77 – 29.3)×10¹⁰ cm⁻³. Employing temporal evolution technique in the current study as an alternative method for separating thermal from nonthermal components in the energy spectra, we measure the break-energy point, ranging between 14 and 21±1.0 keV.     

Solar X-ray Spectrometer (Soxs) Mission on Board GSAT2 Indian Spacecraft: The Low-Energy Payload

The first space-borne solar astronomy experiment of India, namely Solar X-ray Spectrometer (SOXS), was successfully launched on 08 May 2003 on board geostationary satellite GSAT-2 of India. The SOXS is composed of two independent payloads, viz. SOXS Low-Energy Detector (SLD) Payload and SOXS High-Energy Detector (SHD) Payload. The SOXS aims to study the full-disk integrated X-ray emission in the energy range from 4 keV to 10 MeV. In this paper we present the first report on the SLD instrumentation and its in-orbit performance. The SLD payload was designed and developed at the Physical Research Laboratory in collaboration with various centers of Indian Space Research Organisation (ISRO). The basic scientific aim of the SLD payload is to study solar flares in the energy range from 4 to 60 keV with high spectral and temporal resolution. To meet these requirements, the SLD payload employs state-of-the-art solid state detectors, the first time for a solar astronomy experiment, viz. Si PIN (4 –25 keV), and cadmium–zinc–telluride (4 –60 keV). With their superb high-energy resolution characteristics, SLD can observe iron and iron–nickel complex lines that are visible only during solar flares. In view of its 3.4 FOV, the detector package is mounted on a Sun Aspect System, for the first time, to get uninterrupted observations in a geostationary orbit. The SLD payload configuration, its in-flight operation, and the response of the detectors are presented. We also present the first observations of solar flares made by the SLD payload and briefly describe their temporal and spectral mode results.     

Balloon-Borne Hard X-Ray Spectrometer Using CdTe Detectors

Spectroscopic observation of solar flares in the hard X-ray energy range, particularly the 20 ∼ 100 keV region, is an invaluable tool for investigating the flare mechanism. This paper describes the design and performance of a balloon-borne hard X-ray spectrometer using CdTe detectors developed for solar flare observation. The instrument is a small balloon payload (gondola weight 70 kg) with sixteen 10×10×0.5 mm CdTe detectors, designed for a 1-day flight at 41 km altitude. It observes in an energy range of 20−120 keV and has an energy resolution of 3 keV at 60 keV. The second flight on 24 May 2002 succeeded in observing a class M1.1 flare.     

A Study of the Low Energy Cutoff of Nonthermal Electrons in RHESSI Flares

The flattening at the low energy end of the hard X-ray (HXR) photon spectrum of solar flares was generally thought to be due to a cutoff of nonthermal electrons in flares. However, some authors have suggested that inverse Compton scattering (i.e., the albedo effect) or certain other reaction of flare photons with the lower atmosphere can also lead to the flattening. This paper adopts the method of deriving the cutoff proposed by Gan et al. ^[12–14], and makes a statistical analysis on 100 flares observed by the satellite Ramaty High Energy Solar Spectroscopy Imager (RHESSI) in 2002–2005. We found that after the albedo correction, the HXR photon spectra of 18 flares can be fitted with single powerlaw spectra, and those of 80 flares, with double power-law spectra. Besides, 21 flares can be directly interpreted with a single power-law electron spectrum plus a low energy cutoff. The range of the low energy cutoff is 20–50 keV and the mean value is approximately 30 keV. Some other possible interpretations are also investigated.     

Spectral properties of the X-ray binary pulsar LMC X-4 during different Intensity states

We present spectral variations of the binary X-ray pulsar LMC X-4 observed with the RXTE/PCA during different phases of its 30.5 day long third period. Only out-of-eclipse data were used for this study. The 3–25 keV spectrum, modeled with high energy cut-off power-law and iron line emission is found to show strong dependence on the intensity state. Correlations between the Fe line emission flux and different parameters of the continuum are presented here.     

X-ray emission from solar flares

Solar X-ray Spectrometer (SOXS), the first space-borne solar astronomy experiment of India was designed to improve our current understanding of X-ray emission from the Sun in general and solar flares in particular. SOXS mission is composed of two solid state detectors, viz., Si and CZT semiconductors capable of observing the full disk Sun in X-ray energy range of 4–56 keV. The X-ray spectra of solar flares obtained by the Si detector in the 4–25 keV range show evidence of Fe and Fe/Ni line emission and multi-thermal plasma. The evolution of the break energy point that separates the thermal and non-thermal processes reveals increase with increasing flare plasma temperature. Small scale flare activities observed by both the detectors are found to be suitable to heat the active region corona; however their location appears to be in the transition region.     

Investigation of the transient X-ray burster IGR J17380-3749 discovered by the INTEGRAL observatory

Details of the discovery (in February 2004) and results of subsequent (in 2004–2009) INTEGRAL observations of the transient X-ray burster IGR J17380-3749 (IGR J17379-3747) are presented. Over the period of its observations, the INTEGRAL observatory recorded two hard X-ray flares and one type I X-ray burst from the source, which allowed the nature of IGR J17380-3749 to be determined. The burster radiation spectrum during the flares was hard—a power law with a photon index α = 1.8–2.0 or bremsstrahlung corresponding to a plasma with a temperature kT = 90–140 keV. The spectral shape at the flare peaks turned out to be the same, despite a more than twofold difference in flux (the peak flux recorded in the energy range 18–100 keV reached ∼20 mCrab). The upper limit on the flux from the source in its quiescent (off) state in the range of 18–40 keV was 0.15 mCrab (3σ).     

Energetic Particle Fluxes during the Bastille Day Solar Eruption

We report on our observations of solar energetic particle fluxes of p, He, C, O, Ne, Mg, Si, and Fe ions measured by the Energetic and Relativistic Nucleon and Electron (ERNE) experiment associated with the Bastille Day solar flare and coronal mass ejection (CME) on 14 July 2000. We observed two clear maxima of the Fe/O ratio at the energies 8.5–15 MeV nucl⁻¹. The first Fe/O maximum occurred ∼ 3 hours after the beginning of the particle event, and the second maximum ∼ 22 hours after the first one at the arrival of the shock associated with the Bastille Day eruption. We also observed a change in the energy spectrum of oxygen concurrent with a change in the direction of the interplanetary magnetic field at the start of the second enhancement of the Fe/O ratio. We propose an interpretation of the particle event where observed interplanetary particle fluxes are associated with two different particle sources near the Sun and in interplanetary space. We suggest that heavy ions observed during the first period of the Fe/O enhancement were released when a coronal shock reached a magnetic foot point connected to 1 AU. The second maximum of Fe/O occurred when spacecraft encountered Fe-rich material stored in magnetic field flux tubes early in the event and was possibly reaccelerated by the interplanetary shock.     

Simulation of photosphere and chromosphere of two powerful solar flares (October 28, 2003 and September 1, 1990)

The spectra of two powerful flares with approximately the same intensities in the optical region but with different spectral features and power in other regions are studied. One of them is the unique flare which occurred on October 28, 2003, importance X17.2/4B, ranking third in magnitude among the recorded flares. Another occurred on September 1, 1990, 3B importance. The flares vary in the Balmer decrement. The flare of October 28, 2003, has a ratio of I(H_β)/I(H_α) = 1.47. This is the largest value for solar flares ever observed. The flares also differ in magnitude of the D Na I lines emission: the emission of the flare of October 28, 2003, is substantially larger than that of the other flare. The chromosphere models of the flares are computed using the observed profiles of Balmer lines and D Na I lines. The satisfactory agreement of the calculated and observed profiles is obtained for the two-component models in which a hot component occupies 6% of the area. The hot component of the chromosphere model is characterized with the dense condensation available in the upper layers. For the flare of October 28, 2003, this condensation is located deeper and its substance concentration is greater than that for another flare. The H_α line intensity for the model hot component alone is approximately 30 and the continuous spectrum intensity is approximately 3% of the undisturbed level. The photosphere model is computed using the observed profiles of photosphere lines for the flare of October 28, 2003. It is found that very broad profiles of individual sigma-components of the Fe I λ 525.0 nm line may be only explained by the presence of magnetic fields having different directions. A great difference is detected between values of the magnetic field strength obtained in the splitting of sigma-components and those provided by simulation.     

Variability of the Fe K line relativistic component in a sample of Seyfert 1 galaxies

We present the analysis of X‐ray spectral variability made on a sample of 7 Seyfert 1 bright galaxies, using XMM‐Newton data. From the “XMM‐Newton Science Archive” we selected those bright Seyfert 1 showing one or more prominent flares in their 2–10 keV light curves. For each of them we extracted spectra in 3 different time intervals: before, during and after the flare. We fitted them with a simple power law and then shifted a narrow emission and absorption line template across the 2.5–10 keV data, in order to investigate the presence of line‐like features with a confidence level greater than 99%. Some highly significant features were detected in 3 out of 7 sources studied. In particular, the 3 sources, namely PG 1211+143, NGC 4051 and NGC 3783, showed the presence of a variable emission feature in the 4.5–5.8 keV band, characterized by an increase of its intensity after the flare peak. Because of the observed variability pattern, this feature seems to be ascribable to a reverbered redshifted relativistic component of the Fe K line. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Magnetic Structure and Generation of EUV Flare Ribbons

The `ribbons' of two-ribbon flares show complicated patterns reflecting the linkages of coronal magnetic field lines through the lower solar atmosphere. We describe the morphology of the EUV ribbons of the July 14, 2000 flare, as seen in SOHO, TRACE, and Yohkoh data, from this point of view. A successful co-alignment of the TRACE, SOHO/MDI and Yohkoh/HXT data has allowed us to locate the EUV ribbon positions on the underlying field to within ∼ 2′′, and thus to investigate the relationship between the ribbons and the field, and also the sites of electron precipitation. We have also made a determination of the longitudinal magnetic flux involved in the flare reconnection event, an important parameter in flare energetic considerations. There are several respects in which the observations differ from what would be expected in the commonly-adopted models for flares. Firstly, the flare ribbons differ in fine structure from the (line-of-sight) magnetic field patterns underlying them, apparently propagating through regions of very weak and probably mixed polarity. Secondly, the ribbons split or bifurcate. Thirdly, the amount of line-of-sight flux passed over by the ribbons in the negative and positive fields is not equal. Fourthly, the strongest hard X-ray sources are observed to originate in stronger field regions. Based on a comparison between HXT and EUV time-profiles we suggest that emission in the EUV ribbons is caused by electron bombardment of the lower atmosphere, supporting the hypothesis that flare ribbons map out the chromospheric footpoints of magnetic field lines newly linked by reconnection. We describe the interpretation of our observations within the standard model, and the implications for the distribution of magnetic fields in this active region.     

Flare Plasma Cooling from 30 MK down to 1 MK modeled from Yohkoh,GOES, and TRACE observations during the Bastille Day Event (14 July 2000)

We present an analysis of the evolution of the thermal flare plasma during the 14 July 2000, 10 UT, Bastille Day flare event, using spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and TRACE. The spatial structure of this double-ribbon flare consists of a curved arcade with some 100 post-flare loops which brighten up in a sequential manner from highly-sheared low-lying to less-sheared higher-lying bipolar loops. We reconstruct an instrument-combined, average differential emission measure distribution dEM(T)/dT that ranges from T=1 MK to 40 MK and peaks at T ₀=10.9 MK. We find that the time profiles of the different instrument fluxes peak sequentially over 7 minutes with decreasing temperatures from T≈30 MK to 1 MK, indicating the systematic cooling of the flare plasma. From these temperature-dependent relative peak times t _peak(T) we reconstruct the average plasma cooling function T(t) for loops observed near the flare peak time, and find that their temperature decrease is initially controlled by conductive cooling during the first 188 s, T(t)∼[1+(t/τ_cond)]^−2/7, and then by radiative cooling during the next 592 s, T(t)∼[1−(t/τ_rad)]^3/5. From the radiative cooling phase we infer an average electron density of n _e=4.2×10¹¹ cm⁻³, which implies a filling factor near 100% for the brightest observed 23 loops with diameters of ∼1.8 Mm that appear simultaneously over the flare peak time and are fully resolved with TRACE. We reproduce the time delays and fluxes of the observed time profiles near the flare peak self-consistently with a forward-fitting method of a fully analytical model. The total integrated thermal energy of this flare amounts to E _thermal=2.6×10³¹ erg. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014257826116     

The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)

RHESSI is the sixth in the NASA line of Small Explorer (SMEX) missions and the first managed in the Principal Investigator mode, where the PI is responsible for all aspects of the mission except the launch vehicle. RHESSI is designed to investigate particle acceleration and energy release in solar flares, through imaging and spectroscopy of hard X-ray/gamma-ray continua emitted by energetic electrons, and of gamma-ray lines produced by energetic ions. The single instrument consists of an imager, made up of nine bi-grid rotating modulation collimators (RMCs), in front of a spectrometer with nine cryogenically-cooled germanium detectors (GeDs), one behind each RMC. It provides the first high-resolution hard X-ray imaging spectroscopy, the first high-resolution gamma-ray line spectroscopy, and the first imaging above 100 keV including the first imaging of gamma-ray lines. The spatial resolution is as fine as ∼ 2.3 arc sec with a full-Sun (≳ 1°) field of view, and the spectral resolution is ∼ 1–10 keV FWHM over the energy range from soft X-rays (3 keV) to gamma-rays (17 MeV). An automated shutter system allows a wide dynamic range (>10⁷) of flare intensities to be handled without instrument saturation. Data for every photon is stored in a solid-state memory and telemetered to the ground, thus allowing for versatile data analysis keyed to specific science objectives. The spin-stabilized (∼ 15 rpm) spacecraft is Sun-pointing to within ∼ 0.2° and operates autonomously. RHESSI was launched on 5 February 2002, into a nearly circular, 38° inclination, 600-km altitude orbit and began observations a week later. The mission is operated from Berkeley using a dedicated 11-m antenna for telemetry reception and command uplinks. All data and analysis software are made freely and immediately available to the scientific community. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022428818870     

Investigation of X-Ray Flares with Long Rising Phases

We have used Yohkoh and GOES X-ray observations to investigate flares with a long rising phase. We have found that a characteristic feature of such flares is a long time interval, Δ t ≥ 20 min, between the temperature maximum and the maximum of the emission measure. We have carried out detailed analysis for 10 limb flares of this type. Time variation of the heating function, E_H(t), has been determined for their loop-top X-ray kernels. The time variation of E_H(t), together with the temperature–density diagnostic diagrams, have been used to explain the large value of the time interval, Δ t. The main point is that for these flares the heating function E_H(t) decreases so slowly after the temperature maximum, that for the long time, Δ t, the energy flux reaching flare foot points is sufficient to maintain significant chromospheric evaporation. Investigation of the flare evolution in the temperature–density diagnostic diagrams allowed us to work out a new method of determination of the density for flare kernels. This method can be applied to all the kernels for which their altitudes can be estimated. The advantage of this method is that for the density determination it is not necessary to assume what is the extension of the emitting plasma along the line of sight.     

An Invariable Point in the Energy Spectra of Non-Thermal Electrons of Solar Flares

The power-law energy spectra of non-thermal electrons for each 1.024 second have been drawn together during the flare. For some flares, it is discovered that the energy spectra taken at different times present a roughly fixed crossing point, at which point the flux of non-thermal electrons keeps the same during the flare. The possible significance of this phenomenon is discussed. We conjecture that there may be a flux saturation at the low-energy cutoff in the process of electron acceleration.     

Hard X-ray Microflares down to 3 keV

The excellent sensitivity, spectral and spatial resolution, and energy coverage down to 3 keV provided by the Reuven Ramaty High-Energy Solar Spectroscopic Imager mission (RHESSI) allows for the first time the detailed study of the locations and the spectra of solar microflares down to 3 keV. During a one-hour quiet interval (GOES soft X-ray level around B6) on 2 May, 1:40–2:40 UT, at least 7 microflares occurred with the largest peaking at A6 GOES level. The microflares are found to come from 4 different active regions including one behind the west limb. At 7′′ resolution, some events show elongated sources, while others are unresolved point sources. In the impulsive phase of the microflares, the spectra can generally be fitted better with a thermal model plus power law above ∼ 6–7 keV than with a thermal only. The decay phase sometimes can be fitted with a thermal only, but in some events, power-law emission is detected late in the event indicating particle acceleration after the thermal peak of the event. The behind-the-limb microflare shows thermal emissions only, suggesting that the non-thermal power law emission originates lower, in footpoints that are occulted. The power-law fits extend to below 7 keV with exponents between −5 and −8, and imply a total non-thermal electron energy content between 10²⁶–10²⁷ erg. Except for the fact that the power-law indices are steeper than what is generally found in regular flares, the investigated microflares show characteristics similar to large flares. Since the total energy in non-thermal electrons is very sensitive to the value of the power law and the energy cutoff, these observations will give us better estimates of the total energy input into the corona. (Note that color versions of figures are on the accompanying CD-ROM.) Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022404512780