Solar Low Energy X-ray Spectrometer on board Aditya-L1: Ground Calibration and In-flight Performance

TL;DR

SoLEXS on Aditya-L1 delivers continuous Sun-as-a-star soft X-ray spectroscopy in the $2-22\ \mathrm{keV}$ band with $\sim 170\ \mathrm{eV}$ resolution at $5.9\ \mathrm{keV}$ and a 1 s cadence, spanning the full solar activity range with two aperture SDDs to avoid saturation. The paper presents comprehensive ground and in-flight calibrations, including energy-channel and resolution calibrations, SRF/ARF modeling via HYPERMET and Beer-Lambert-based area calculations, thermo-vacuum validation, and deadtime/pile-up corrections, along with cross-calibration against GOES-XRS and Chandrayaan-2/XSM. It demonstrates robust onboard calibration using a $^{55}$Fe source, confirms radiometric accuracy to within $\sim$10-15% across typical conditions, and shows the capability to extract coronal plasma parameters through forward-modeling of flare spectra. The results enable precise monitoring of flare evolution and coronal heating processes, establishing SoLEXS as a key instrument for coronal physics at the Sun-Earth L1 point.

Abstract

The Solar Low-Energy X-ray Spectrometer (SoLEXS) on board India's Aditya-L1 mission was launched on 2 September 2023 and commenced solar observations on 13 December 2023 following successful aperture cover deployment. Operating from the Sun-Earth L1 Lagrange point, SoLEXS has been providing continuous Sun-as-a-star soft X-ray spectroscopy across 2-22 keV with 170 eV resolution at 5.9 keV and 1-second temporal cadence since 6 January 2024. The instrument employs two Silicon Drift Detectors with aperture areas of 7.1 mm$^2$ and 0.1 mm$^2$ to accommodate the full dynamic range of solar activity from A-class to X-class flares. This paper presents comprehensive ground and on board calibration procedures that establish SoLEXS's quantitative spectroscopic capabilities. Ground calibration encompassed energy-channel relationships, spectral resolution characterization, instrument response functions, and collimator angular response measurements, with thermo-vacuum testing validating performance stability across operational temperature ranges. On board calibration utilizing an internal $^{55}$Fe source demonstrated preserved post-launch spectral resolution (164.9-171.2 eV), while cross-calibration with GOES-XRS and Chandrayaan-2/XSM confirmed radiometric accuracy and flux agreement. The instrument's 100% observational duty cycle at L1 enables unprecedented continuous monitoring of solar flare evolution across all intensity classes, providing calibrated data for advancing coronal heating mechanisms, flare energetics, and flare-coronal mass ejection relationship studies through soft X-ray spectroscopy.

Figures (24)

• Figure 1: Schematic of SoLEXS instrument, (a) A front view of the integrated SoLEXS package, showing the Detector Module (top) and the Electronics Module (bottom), which are thermally isolated by GFRP spacers. Aperture cover mechanism (grey) is visible on the outside of the detector module. (b) An internal front view of the package with the detector module's top cover removed, revealing the two SDDs (yellow) mounted on the PCB (green). The imaginary FOVs are shown originating from the two apertures on the front plate: the blue FOV for the large-aperture SDD1 and the red FOV for the small-aperture SDD2. (c) A close-up back view of the detector module with the top cover removed, showing the internal calibration source encapsulated in its Aluminium holder (grey) and the Copper thermal flanges (brown).
• Figure 2: DPP demonstration for non-pileup (left panel) and pile-up (right panel) events. Each panel shows the input exponential pulse ($\tau$ = 3.2µs) and the corresponding triangular-shaped outputs from the spectral channel (2µs peaking time) and the timing channel (.35µs peaking time, scaled $\times$4.0 for visibility). In pile-up scenario, the faster timing channel resolves individual events that appear as single distorted pulses in the spectral channel.
• Figure 3: Sample XRF spectrum of JSC-1A lunar simulant with salt obtained using SDD1, demonstrating the instrument's 340.0-channel binning scheme. Channels up to 168.0 (left region) represent single-channel binning, while channels 169340 (right, shaded region) show paired-channel binning. The absence of signal above channel 280.0 results from the X-ray gun operating voltage of 20kV, which limits the maximum photon energy available.
• Figure 4: XRF spectrum of JSC-1A lunar simulant with salt obtained using SDD1 (a) and Mu metal obtained using SDD2 (b), demonstrating the energy-channel calibration methodology. The spectrum exhibits characteristic fluorescence lines which serve as reference energies for establishing energy-channel relationship. The absence of signal above $\approx$19 in SDD1 spectrum (a) results from the X-ray gun operating voltage of 20kV, which limits the maximum photon energy available. The SDD2 spectrum (b) below 5 keV is significantly attenuated due to absorption by the Mu metal target.
• Figure 5: Energy-channel calibration plots for SDD1 (a) and SDD2 (b) displaying the linear relationship between incident photon energy and recorded PHA channel. The plots demonstrate the instrument's energy binning scheme, with single-channel binning up to 168.0 channel, followed by paired-channel binning that doubles the energy width for channels 169340, as indicated by the change in slope after the 168th channel.