HEL1OS on Aditya-L1 Mission: Operations, Data Processing and Monitoring of Sun in Hard X-rays

Introduction to the show: ident: Astrophysics. The day in astrophysics: observations, models and what the data forces.

Vera: I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "HEL1OS on Aditya-L1 Mission".

Jocelyn: Here's the gist of the paper. HEL1OS (High Energy L1 Orbiting X-ray Spectrometer) is the hard X-ray spectrometer on-board Aditya-L1 solar mission, monitoring the Sun continuously in the hard X-ray band of 8 keV to 150 keV The instrument always operates in the event mode, and the down-linked data is processed in an automated pipeline to generate Science-ready products comprising mainly of solar spectra in 8 keV to 150 keV energy band This paper provides an overview of the HEL1OS operational aspects during the crucial commissioning and performance verification phases (in section 2.) Instrument Configuration and Commissioning The HEL1OS payload is designed to monitor Sun in the hard X-ray energy band of 8 keV to 150 keV by employing two different kinds of detectors The wide energy band of 8 keV to 150 keV is achieved by employing a pair of Cadmium Telluride (CdTe) single pixel detectors operating over 8 keV – 70 keV, and a pair of Cadmium Zinc Telluride (CZT) 256 pixel detectors operating over 20 keV – 150 keV After the launch of the Aditya-L1 spacecraft on September 2, 2023, HEL1OS was commissioned on October 27, 2023 while Aditya-L1 was in the cruise phase towards the L1 point During this performance verification phase the on-board logics tested and tuned include: hot-pixel & bunched-pixel logics of the CZT detectors, and, the digital pulse processing (DPP) parameters of the CdTe detectors The total geometric areas of the CdTe and CZT detector pairs are 0.5 cm2 and 32 cm2, respectively Data Pipeline Architecture The architecture of the automated pipeline operating between the Indian Space Science Data Centre (ISSDC) and the Payload Operation Centre (POC) is described in section 3 This whole processing chain of Level-0 generation till dissemination of the Level-1 data products happens in an automated fashion (see Figure 2) The data transfer between the data centre at ISSDC and the POC system is via an interface PC using the secure copy protocol (SCP) over secure shell (SSH) and File Transfer Protocol (FTP) server The automation is achieved by a few concurrent cron jobs and daemons Data Processing Stages The Level-0 to Level-1 data processing chain at the POC system involves several stages depicted in Figure 3 The first stage of ‘data quality check’ involves checking the page sync codes and the CRC codes for in each 2k page of the ‘HEL1OS data’ In the HK Data handling stage, metadata read from the page envelope includes recorded temperature values of each of the four detectors, status-bits of different onboard logics, and count rates of each detector Event data handling involves saving information for every photon recorded as and when the electronics system registers them Time correlation scheme computes the UTC corresponding to each page start time by using the DH-OBT and TCT entries made available Scientific Results and Analysis HEL1OS provides a continuous view of the Sun in hard X-rays from the vantage point of Aditya-L1 mission’s orbit about the Lagrangian Point L1 The two X-ray payloads on Aditya-L1 mission itself, SoLEXS (Sankarasubramanian et al.

Vera: 2025) and HEL1OS (Nandi et al.

Jocelyn: 2025) provide a continuous and complementary view of the Sun spanning a broad energy range of ∼ 2.8 keV to 150 keV The peak detection method applied on the light curves in Figure 5 involved dynamic thresholding The peak times of the flare emission differ systematically across energy bands, with hard X-ray emission generally peaking earlier than softer X-ray emission Multi-energy observations from HEL1OS, GOES, and SoLEXS are vital for diagnosing flare energy release, particle acceleration, and plasma heating Long Term Monitoring Capabilities HEL1OS provides a continuous view of the Sun in hard X-rays from the vantage point of Aditya-L1 mission’s orbit about the Lagrangian Point L1 Plotted in Figure 7 are the long term light curves of the CdTe and CZT data, from 01 November, 2023 till 31 October, 2025 The combined data is filtered for observed background level as mentioned earlier Over the past two years the transitioning of the current solar cycle 25 from its maxima towards minima is also evident from the gradual decrease in the frequency and intensity of the flaring activities Data Dissemination and Products The Level-1 data products generated at the POC are transferred to ISSDC data server for dissemination The Level-1 data product bundles transferred to the ISSDC data server are ingested and made available through the PRADAN portal The science-ready Level-1 products include the Light curves and Spectra, both as OGIP compliant FITS data files The URL of the portal is https://pradan1.issdc.gov.in/al1/, and the data is accessible to all the registered users The Level-1 data product bundles transferred to the ISSDC data server are ingested and made available through the PRADAN portal Spectral Analysis Techniques It is possible to undertake the spectral analysis on the HEL1OS data with OSPEX, X-ray Spectral Fitting Package (XSPEC) or Sherpa tools XSPEC allows for combined spectral fitting using data from multiple instruments or detectors The fits were performed using XSPEC models apec+powerlaw along with a constant factor to handle the cross-calibration difference between the two detectors A systematic error between 2% and 3% has been included for the overall model fit for both detectors The feature observed in the residuals of all tied to the training domain, not just a general physical law?

Mira: Precisely.

Title and authors: Vera: Moving on to the summary of the paper, they describe the operational aspects during those crucial commissioning and performance verification phases. It gives us a snapshot of how the instrument performed when it first came online.

Jocelyn: It’s a detailed look at Figure one which plots the lightcurve of combined CdTe data for October twenty-seven two thousand twenty-three showing C2 point 1 class flare and C1 point 1 flare detections during that initial configuration before DPP tuning <ref:2609.01307#pg3>.

Subrahmanyan: And seeing those specific class detections like C2 point 1 and C1 point 1 helps us understand the energy spectrum the instrument was sensitive to at that time, which is a direct constraint on the solar atmosphere models we use for hard X-rays <ref:2609.01307#pg3>.

Vera: It also compares this combined CdTe data against GOES XRSB and XRSA fluxes for that same time interval, allowing them to see how HEL1OS measured compared to existing sensors.

Jocelyn: And they note that the Geostationary Operational Environmental Satellite GOES X-ray sensor was at B8 class level during that initial measurement period, which sets a baseline for comparison against the instrument’s performance <ref:2609.01307#pg3>.

Subrahmanyan: That B8 level flux gives us a concrete benchmark to see if HEL1OS was capturing events above that noise floor as expected for its design.

Vera: They also mention the specific energy bands they were looking at, like one hundred fifty-five to one hundred twenty-four kiloelectron volts in the XRSB channel and zero point five to four kiloelectron volts in the XRSA channel <ref:2609.01307#pg3>.

Jocelyn: That energy range is what makes this instrument so specific; it’s not just looking at any X-ray, it’s targeting these particular hard X-ray windows where we expect certain physical processes to dominate.

The paper's summary: Subrahmanyan: Now, the paper discusses improvements they suggest for the operational setup. They point out that they have to tune those DPP parameters because the instrument always functions in its final set configuration and operates in event-mode.

Vera: So, one improvement is related to making sure that tuning process itself is robust so it doesn't introduce too much noise or systematic error into the resulting measurements.

Jocelyn: And they detail how they manage the pile-up rejection within the CdTe detectors, which is important because when you have a lot of photons coming in quickly, pile-up can obscure real events <ref:2609.01307#pg2>.

Subrahmanyan: If they can improve that pile-up rejection, then it means their ability to resolve fainter signals during intense solar activity will be much better.

Vera: And the paper suggests that by using these improved methods, they could potentially reduce the cross-calibration difference between detectors A systematic error between two percent and three percent has been included for the overall model fit for both detectors <ref:2609.01307#pg1>.

Jocelyn: That reduction in systematic error is significant because it means when you combine data from different instruments, our final results will be more trustworthy.

Subrahmanyan: If that error is reduced, it directly impacts the ability of theorists to use these observational constraints to build better models of solar flares and particle acceleration.

The paper's improvements: Vera: So wrapping up, the paper shows that while commissioning is complex and requires tuning, the automated pipeline successfully processes data from HEL1OS into accessible products quickly.

Jocelyn: The main implication for us is seeing a validated path for deploying this instrument on Aditya-L1 and understanding how to leverage those automated products immediately for solar monitoring.

Subrahmanyan: For cosmology and structure formation, the specific spectral information they are gathering from those hard X-ray bands is what connects back to the larger picture of how energy is transported in the solar corona.

Vera: And that connection is made clearer by showing how their operational performance directly constrains our understanding of those processes.

Jocelyn: We’re looking forward to seeing more data from this mission and figuring out if these operational results hold up when they get into steady-state science mode.

Conclusion: Vera: So we’ve walked through how they built HEL1OS on Aditya-L1, covering everything from operations to data processing and monitoring of the sun in hard X-rays.

Jocelyn: The main implication for us is seeing a validated path for deploying this instrument on Aditya-L1 and understanding how to leverage those automated products immediately for solar monitoring.

Subrahmanyan: For cosmology and structure formation, the specific spectral information they are gathering from those hard X-ray bands is what connects back to the larger picture of how energy is transported in the solar corona.

Vera: And that connection is made clearer by showing how their operational performance directly constrains our understanding of those processes.

Jocelyn: We’re looking forward to seeing more data from this mission and figuring out if these operational results hold up when they get into steady-state science mode.

Subrahmanyan: I just hope the pipeline stays robust enough to handle the expected variability over the long term without introducing unexpected artifacts.

Vera: That robustness is definitely a key focus for them in their monitoring phase.

Jocelyn: It’s about getting that data clean and consistent so we can trust the scientific conclusions drawn from it.

Subrahmanyan: And once that consistency is achieved, those constraints on coronal physics will be very powerful tools for theorists.