ABSTRACT
We present the results of an analysis of hard X-ray observations of the C2.7 solar flare detected by the RT-2 experiment on board the Coronas–Photon satellite. We detect hard X-ray pulsations at periods of ∼12 s and ∼15 s. We find a marginal evidence for a decrease in period with time. We have augmented these results using the publicly available data from the RHESSI satellite. We present a spectral analysis and measure the spectral parameters.
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1. INTRODUCTION
Quasi-periodic pulsations (QPPs), a common feature of solar flare emission, have been observed for many years (Young et al. 1961) in all frequency bands ranging from radio to hard X-rays with periodicities varying from a few milliseconds to several seconds (Aschwanden 1987; Fleishman et al. 2002; Tan 2008). The long-period QPPs (periodicity >10 s) observed in the microwave emission of solar flares are also seen in hard X-rays (Kane et al. 1983; Nakajima et al. 1983; Asai et al. 2001; Nakariakov et al. 2003), and they could be resulting from some magnetohydrodynamic (MHD) oscillations in the source region or due to modulation of electron acceleration and injection mechanisms. QPPs are generally associated with the emission from flare-accelerated non-thermal electrons, because thermal parameters are not expected to show sudden changes and pulsations. QPPs can be observed in all the stages of a flare, prominently in hard X-rays, microwave, and white-light emissions.
MHD oscillations in the magnetic loop can cause modulation in the magnetic field strength and magnetic mirror ratio (Zaitsev & Stepanov 1982, 1989; Zimovets & Struminsky 2009) resulting in periodic variation toward the flare footpoints. Generally, most of the non-thermal hard X-rays are observed at the footpoints of the magnetic loops involved in the magnetic reconnection. This emission is produced by the accelerated non-thermal electrons from the reconnection region hitting the footpoints of the magnetic loops. The oscillations in the accelerated electrons in turn reflect as QPPs in the hard X-ray emission profile. Similarly, microwave emission produced by the interaction of the accelerated electrons with the magnetic field as a result of the gyrosynchrotron process also shows QPPs (like hard X-rays, as both the emissions are from the same population of electrons). Observationally, QPPs can be seen in the light curves of the solar flare emission in the respective wave bands. The profiles of photons or energy fluxes of hard X-rays associated with the non-thermal electrons show QPPs or damping oscillations.
Since the basic cause of QPPs has implications for particle acceleration mechanism, it is important to investigate QPPs at diverse source intensities. In this paper, we present the results obtained from the observation of the C2.7 solar flare detected on 2009 July 5 using the RT-2 experiment on board the Coronas–Photon satellite. Since this is the first result from this experiment, we describe in detail the methodology used in deriving the response matrix and spectral fitting. We augment our results by using the publicly available RHESSI data. We examine the spectral and temporal characteristics of the flare and investigate the implications for the electron acceleration mechanisms. In Section 2, a brief description of the RT-2 experiment is given. Observations and analysis results (RT-2 and RHESSI data) are given in Section 3, and finally in Section 4 a detailed discussion of the results is presented along with relevant conclusions.
2. RT-2 EXPERIMENT ONBOARD THE CORONAS–PHOTON SATELLITE
The RT-2 experiment (RT, Roentgen Telescope) is a part of the Coronas–Photon mission, launched on 2009 January 30 (Kotov et al. 2008; Nandi et al. 2009a). The primary objective of the mission is to make a detailed temporal and spectral study of hard X-ray and gamma-ray emission during solar flares. The satellite is in a near-polar (inclination 82
5), near-Earth (altitude 550 km) Sun-synchronous orbit. Though the large inclination gives low duty cycle of observation due to the increased background emission at high latitudes and South Atlantic Anomaly (SAA) regions, it facilitates Sun synchronization for long uninterrupted observations of the Sun.
RT-2 consists of an ensemble of the low-energy gamma-ray (or hard X-ray) detectors sensitive in the energy range of 15–150 keV and also has an extended detection capability up to 1000 keV. It consists of three instruments called RT-2/S, RT-2/G, and RT-2/CZT. RT-2/S and RT-2/G detector assemblies have an identical configuration of NaI(Tl)/CsI(Na) scintillators in phoswich combination. Both the detector assemblies sit behind respective mechanical slat collimators surrounded by uniform shields of tantalum material having different viewing angles of 4° × 4° (RT-2/S) and 6° × 6° (RT-2/G). The collimation is effective up to about 150 keV, and above this energy these detectors act as omni-directional low-energy gamma-ray detectors with sensitivity up to ∼1000 keV. The low-background high-sensitivity range for RT-2/S is 15–150 keV whereas the use of an aluminum filter in RT-2/G sets the lower energy threshold at 25 keV. The RT-2/CZT consists of three CZT detector modules (OMS40G256) and one CMOS detector (RadEye-1) arranged in a 2 × 2 array. The CZT–CMOS detector assembly is mounted behind a collimator with two different types of coding devices, namely, a coded aperture mask and a Fresnel zone plate, surrounded by a uniform shield of tantalum material and has a viewing angle ranging from 6' to 6°. The RT-2/CZT payload is the only imaging device in the Coronas–Photon mission to image the solar flares in hard X-rays in the energy range of 20–150 keV.
During the "GOOD" regions (that is, outside the high-background regions of Polar Caps and SAA), the RT-2/S and RT-2/G generally operate in the solar quiet mode (SQM) when count rates in eight channels (for each detector) are stored every second. The spectral data are stored every 100 s. The low-energy spectra are stored separately for NaI(Tl) (15–150 keV) and NaI(Cs) (25–215 keV) detectors based on the pulse shape along with the high-energy spectrum in the energy range of 215–1000 keV (see Debnath et al. 2009 for details). The onboard software automatically stores the data in finer time resolution (0.1 s count rates and 10 s spectra) during the solar flare mode (SFM), when the count rates exceed a pre-determined limit. RT-2/CZT operates only in the SQM when 1 s count rates, 100 s spectra, and images are stored. The test and evaluation results of this payload are described in Nandi et al. (2009b), Debnath et al. (2009), Kotoch et al. (2009), Sarkar et al. (2009), and Sreekumar et al. (2009).
3. OBSERVATION AND ANALYSIS
The flare of class C2.7 occurred near the center of the disk (S27W12) in the NOAA active region 11024 on 2009 July 5, which peaked at 07:12 UT. From the X-ray light curves derived from RT-2, GOES, and RHESSI observations, it can be concluded that this flare is compact and impulsive in nature. During the solar flare, the RT-2/S and RT-2/G were in the SQM. The count rates were too low to trigger the SFM. Due to some operational constraints, the low-energy threshold of CZT detectors was kept at ∼40 keV and hence RT-2/CZT did not detect this flare.
The observed count rates of the solar flare from the RT-2/S detector are shown in Figure 1 for low energies (20–35 keV), high energies (35–59 keV), along with the count rates above 215 keV, which represents the high-energy particle background rates. The bin size is 1 s and the time is given in UT seconds on 2009 July 5. The satellite was at high latitudes at the beginning of the observations, and the background rates slowly stabilized when the satellite approached the low-background equatorial region. Since the detectors use the phoswich technique, the changing background has negligible impact on the <35 keV light curves. For example, the count rate during the first 200 s of observation (in this energy band) is 36.6 ± 0.4 s−1 and it is 36.4 ± 0.4 s−1 toward the end of observation. During this time the background rate decreased from 576 ± 2 s−1 to 235 ± 1 s−1. There is, however, some decreasing trend in the >35 keV count rates. Solar flare starting from 25,800 s UT (07:10) is seen clearly in both the energy channels.
Figure 1. RT-2/S light curve of the solar flare in 20–35 keV (top panel) and 35–59 keV (middle panel) with a bin size of 1 s. The background rate (above 215 keV) is shown in the bottom panel. The time is in UT seconds on 2009 July 5.
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Standard image High-resolution imageThe RT-2/G detector has an aluminum window to block X-rays below ∼25 keV and hence it samples the high-energy photons. The light curves of the solar flare as measured by the RT-2/S and RT-2/G detectors are shown in two channels each in Figure 2. The bin size is 1 s and T0 is UT 07:08:50 on 2009 July 5. QPPs are clearly seen in the light curve. We define the rising phase of the flare as between 125 and 225 s (07:10:55–07:12:35) and the falling phase as between 225 s and 325 s (07:12:35–07:14:15). These regions are demarcated by vertical dashed lines in the top panel of Figure 2. They also correspond to the availability of the spectral data in the SQM.
Figure 2. RT-2/S light curve in the 20–35 keV channel (top panel) and the 35–59 keV channel (second from top) shown along with the RT-2/G light curves (third and fourth panels). The bin size is 1 s and T0 is UT 07:08:50 on 2009 July 5. The vertical dashed lines in panel (a) demarcate the rising (07:10:55–07:12:35) and the falling (07:12:35–07:14:15) phase of the flare.
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Standard image High-resolution imageThe light curves obtained from the GOES 10 and RHESSI satellites are shown in Figure 3 and they show that the flare is an impulsive one, where the rising phase takes approximately 4 minutes from the onset to peak flux. The GOES light curves in two channels (1–8 Å: 1.6–12.4 keV and 0.5–4 Å: 3.1–24.8 keV) are shown in the top panel of the figure with a bin size of 1 minute. The RHESSI light curves in three energy bands (3–6 keV, 6–12 keV, and 12–25 keV) are shown in the bottom panel of the figure with a bin size of 4 s.
Figure 3. Top panel: GOES light curves in the bands 1–8 Å (1.6–12.4 keV) and 0.5–4 Å (3.1–24.8 keV) with a time resolution of 1 minute. Bottom panel: RHESSI light curves in the energy bands 6–12 keV, 3–6 keV, and 12–25 keV (from top). The bin size is 4 s. T0 is UT 07:08:50 on 2009 July 5.
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Standard image High-resolution image3.1. Timing Analysis
We follow the method given in Fleishman et al. (2008) to find the modulation power and the periodicities. If C(t) is the count rate at time t, the normalized modulation is
where 〈C(t)〉 is the running average taken over a number of bins, 20 s in our case. The modulation power over a period of time is the average of S2(t), and the square root of modulation power is the modulation amplitude (see Fleishman et al. 2008).
In Figure 4, we have plotted the normalized modulation along with the count rates, with a bin size of 1 s. The top panel shows the count rates in the RT-2/S 20–35 keV range and the successive panels downward show the normalized modulation for RT-2/S 20–35 keV, 35–59 keV, and similarly for RT-2/G, respectively. To estimate the errors in the modulation power, we have calculated the modulation power in the light curves outside the flares, in batches of 100 s, and the rms variation in them is deemed as the error in the measured values. The background-subtracted modulation powers, for 4 s integration time, are given in Table 1, for the rising and the falling phase of the flare, respectively. The modulation amplitudes are also calculated for the RHESSI data. The flare shows the highest modulation of 13.5% in the low-energy RT-2/S band (20–35 keV) and it is ∼5%–8% in low energies (from RHESSI data) as well as above 35 keV.
Figure 4. RT-2/S low-energy light curve (top panel) is shown with the normalized modulation (see the text) in the four energy bands shown in Figure 2.
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Standard image High-resolution imageTable 1. Modulation Amplitude of the 2009 July 5 Flare (in percent)
| Data | Rising Phase | Falling Phase |
|---|---|---|
| RT-2/S 20–35 keV | 13.5 ± 0.4 | 5.2 ± 0.4 |
| 35–59 keV | 4.6 ± 2.8 | <5.6 |
| RT-2/G 25–35 keV | 6.6 ± 0.4 | <0.8 |
| RHESSI 3–6 keV | 7.6 ± 0.4 | 2.4 ± 0.4 |
| 6–12 keV | 6.2 ± 0.4 | 1.5 ± 0.4 |
| 12–25 keV | 5.5 ± 0.3 | <0.5 |
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To investigate the values of periodicities and their variation, we have followed the method used in Fleishman et al. (2008) and calculated the Fourier transform of the normalized modulation derived with 1 s time resolution. We have shown the Fourier transform of the normalized modulation in Figure 5 for the RT-2/S 20–35 keV total light curve (top panel) and the rising phase of the flare (second panel from top). The third and fourth panels show similar power spectra for the RT-2/G light curves. To quantify the errors in periods and the significance level of the period determination, we represent the amplitudes of the Fourier components with a normalization as suggested by Horne & Baliunas (1986). This method has the added advantage of quantifying the false alarm probability, which is the probability of getting a power higher than the observed peak, by random distribution. The highest peaks in the power spectrum, along with the confidence levels (false alarm probability), are shown in Table 2. To increase the signal-to-noise ratio, we have added all counts (two channels of RT-2/S and the lowest channel of RT-2/G) and have given the corresponding peaks in Table 2. It can be seen from the table (also see Figure 5) that two prominent peaks are seen in the periodogram corresponding to the periods of ∼12 s and ∼15 s, respectively. Both periods are seen very significantly in the full data set, whereas the 15 s periodicity is more significant in the rising phase.
Figure 5. Power spectra of the RT-2/S low-energy light curves for the rising (top panel) and the falling phase (second panel from the top). The next two panels show the corresponding power spectra of the RT-2/G low-energy light curves.
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Standard image High-resolution imageTable 2. QPP Periods in Seconds (with False Alarm Probability)
| Data | Full Flare | Rising Phase | Falling Phase |
|---|---|---|---|
| RT-2/S 20–35 keV | 11.6 ± 0.1 (7 × 10−4) | 15.3 ± 0.1 (3 × 10−4) | 12.2 ± 0.2 (2 × 10−2) |
| (2nd peak) | ⋅⋅⋅ | 11.9 ± 0.2 (2 × 10−2) | ⋅⋅⋅ |
| RT-2/G 25–35 keV | 15.5 ± 0.1 (6 × 10−4) | 12.1 ± 0.2 (3 × 10−3) | 15.6 ± 0.3 (0.23) |
| (2nd peak) | ⋅⋅⋅ | 15.5 ± 0.3 (0.1) | ⋅⋅⋅ |
| Full data | 11.62 ± 0.6 (2 × 10−4) | 15.5 ± 0.2 (5 × 10−4) | 12.2 ± 0.2 (2 × 10−2) |
| (2nd peak) | 15.7 ± 0.1 (3 × 10−3) | 11.9 ± 0.2 (5 × 10−3) | ⋅⋅⋅ |
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To investigate any possible period changes during the flare, we have used a running window of 60 s duration and measured the periodicities in the full data set. The variation of the period with time (starting from the flare onset) is shown in Figure 6. There is an indication of the value of period decreasing with time. A straight line fit to the data gives a value of period derivative as −0.06 ± 0.03 s s−1 for the 15 s periodicity and −0.02 ± 0.01 s s−1 for the 12 s periodicity.
Figure 6. Variation of the period of QPPs with time from the start of the flare (07:10:55 UT on 2009 July 5).
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Standard image High-resolution image3.2. Spectral Analysis
We have generated the appropriate response matrices of the RT-2/S and RT-2/G detectors. The background line at 56.9 keV (due to I121 decay) is used for channel to energy conversion. The values of energy resolution function measured during the ground calibration and the effective areas from the known geometrical properties of the detectors are used for response matrix generation. The background spectrum obtained away from the solar flare is used. The XSPEC tool of the ftools package is used for spectral fitting. The deconvolved spectrum is shown in Figure 7 for RT-2/S (filled circles) and RT-2/G (open circles). The spectrum is very steep and it is best fit by a simple bremsstrahlung function of energy 3.43 ± 0.30 keV. This model is shown as a dashed line in the figure.
Figure 7. Deconvolved spectra from RT-2/S (filled circles) and RT-2/G (open circles) along with a simple bremsstrahlung spectrum (dashed line).
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Standard image High-resolution imageWe have also generated the spatially integrated, background-subtracted spectra from RHESSI observations for the rising and the falling phase of the flare. The count spectral files were created using the standard RHESSI software of Solar SoftWare (SSW). The data were accumulated over 30 s with 97 energy bands from 3 to 100 keV using all front detector segments excluding 2 and 7 (for their lower energy resolution and high-threshold energies, respectively). The full spectrum response matrix was used to calibrate the data. Then, the RHESSI OSPEX package is used for spectral fitting of the count spectra.
The background spectra were accumulated for the non-flare period of (07:06:28–07:06:58) 30 s in all the energy levels. The count spectra were fitted with a two-component model consisting of an optically thin thermal bremsstrahlung radiation function parameterized by the plasma temperature kT and the emission measure (EM) and a thick target bremsstrahlung characterized by the electron flux and the power-law index (Γ) of the electron distribution function below the break energy. The best fit parameters, temperature kT of the isothermal emitting plasma, and its EM are derived by these fitted spectra and are given in Table 3. The deconvolved photon spectrum is shown in Figure 8. It can be seen that the 20–30 keV RHESSI spectrum agrees quite closely with the RT-2 data.
Figure 8. RHESSI photon energy spectra during the rising phase of the flare (07:11:40–07:12:30). The two-component thin and thick target bremsstrahlung model is also shown.
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Table 3. Spectral Parameters Derived from the RHESSI Data for the 2009 July 5 Flare
| Parameter | Rising Phase | Falling Phase |
|---|---|---|
| Time period | 7:11:40–07:12:30 | 07:12:35–07:14:15 |
| Model: thin target | ||
| EM (1049 cm−3) | 1.26 | 0.77 |
| kT (keV) | 0.79 | 0.80 |
| Model: thick target | ||
| Electron flux (1035 s−1) | 18.4 | 11.3 |
| Γ | 8.4 | 8.6 |
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4. DISCUSSION AND CONCLUSIONS
Jakimiec & Tomczak (2010) have investigated QPPs in about 50 flares using Yohkov and BATSE hard X-ray data and have derived a correlation between the QPP periods (ranging from 10 s to 150 s) and sizes of loop-top sources. From the RHESSI data we derive a size of the X-ray emitting region in the 6–12 keV region of 7'' (5 Mm), corresponding to the 50% level of the peak emission, for the 2009 July 5 flare. This size agrees with the correlation derived by Jakimiec & Tomczak (2010).
Desai et al. (1987) detected fast oscillations in several solar hard X-ray flares and observed MHD signature of the loop dynamics. Jakimiec & Tomczak (2010) conclude that the hard X-ray oscillations are confined to the loop-top sources and the observations are described with a model of oscillating magnetic traps. Fleishman et al. (2008) have made a detailed analysis of the 2003 June 15 solar flare (GOES X1.3 class) and detected hard X-ray (based on RHESSI data) and microwave oscillations with periods ranging from 10 s to 20 s. They, however, conclude that QPPs are associated with quasi-periodic acceleration and injection of electrons. The possible detection of a decreasing trend in the periodicity can put further constraints on the MHD models.
Several flare observations as well as numerical simulation studies have been reported on the periodic and quasi-periodic oscillations of flare intensity in the radio and X-ray energy bands. Such oscillations show the typical size of reconnection site, configuration of loops formed during the reconnection, and plasmoid or coronal mass ejection (CME) launched above the reconnection X-point.
As shown in Figure 2, the rising phase of the flare shows the nominal exponential increase. However, as the flare attains the peak level, the intensity goes through moderate quasi-periodic oscillations, which are more prominent in the 20–35 keV energy band. Moreover, the modulation index (i.e., the degree of quasi-periodic oscillation) is higher at the low-energy band (i.e., ∼13.5% in the 20–35 keV band) than in the high-energy band (∼7% above 35 keV bands). It shows the production of copious amounts of electrons over a limited range of energies. The flare profile observed at 15.4 GHz correlates with the rising phase of the flare, but the oscillations are not clearly seen, which may be a limitation imposed by the sensitivity of the measurement.
The white-light images from Large Angle and Spectromeric Coronagraph Experiment associated with this flare show a rather slow moving CME (i.e., speed in the range of 50–150 km s−1). This is consistent with our finding of the production of particles in a limited energy range. A comparison of profiles shown in Figure 2 with the RHESSI spectrum reveals a gradual steepening of the spectrum from the flare rising phase to the start of the decay phase, although the average intensity of the flare remained nearly at the same level in this period. Thus, most of the accelerated electrons have been generated and injected from the reconnection site.
This is the brightest solar flare detected by the RT-2 experiment in the first 10 months of operation. Several other solar flares, particularly during the eruptions that have taken place from 2009 October 22 and November 2, are also recorded. A detailed investigation into faint flares during this solar minimum is going on and a flare list would be published separately. From 2009 December onward, the satellite has not been responding to communication, though attempts are on to revive the system.
This work was made possible in part from a grant from Indian Space Research Organization (ISRO). The whole-hearted support from G. Madhavan Nair, Ex-chairman, ISRO, who initiated the RT-2 project, is gratefully acknowledged. Significant contributions from several organizations for the realization of the RT-2 payload are gratefully acknowledged. We are also grateful to the RHESSI and GOES spacecraft teams for the respective data.