astro-legacy-archive/sas2-photon-events

SAS-2 photon events

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This dataset contains the archive-natural event product for SAS-2 pointing 17, including every source row, identifier, value, column shape, and auxiliary table. HEASARC states that the full corpus has one event file for each of the 28 satellite pointings; this dataset contains s2_0023n62p17.evt.Z.

How to use

python -m venv .venv && .venv/bin/pip install datasets huggingface_hub pyarrow

from datasets import load_dataset
dataset = load_dataset("astro-legacy-archive/sas2-photon-events", "s2_0023n62p17__EVENTS", split="train[:1]")
print(dataset.num_rows, dataset[0]["TIME"], dataset[0]["ENERGY"])

1 41754.188468113425 170

import json
from huggingface_hub import hf_hub_download
import pyarrow.parquet as pq
path = hf_hub_download("astro-legacy-archive/sas2-photon-events", "s2_0023n62p17__EVENTS.parquet", repo_type="dataset")
print(json.loads(pq.read_metadata(path).metadata[b"event_list"])["event_count"])

348

Dataset structure

The EVENTS table has 348 rows and these 29 source columns in source order:

ALLGTI has 130 rows with START 1D d and STOP 1D d. TSI has 15,057 rows and these 24 source columns in source order: START 1D d, STOP 1D d, RA_EXP 1E deg, DEC_EXP 1E deg, L_EXP 1E dec, B_EXP 1E deg, RADIUS 1E km, STATUS 1E, TEARTH00 1E s, TEARTH15 1E s, TEARTH20 1E s, TEARTH25 1E s, TEARTH30 1E s, TEARTH35 1E s, RSENSEXP 1E, ORBITEXP 1E, S3EXP 1E, EXP_POS1 1E, EXP_POS2 1E, EXP_POS3 1E, E_POS1 1E, E_POS2 1E, E_POS3 1E, and ZENITH 1E deg. The source's literal dec unit for TSI.L_EXP is retained.

The embedded event_list metadata is the machine-readable decoding route. It records every event field, source FITS header, source-column form and unit, the complete source-shaped ALLGTI and TSI arrays, provenance, rights, credit, and reconstruction instructions. No table is renamed, dropped, flattened, or normalised.

Provenance

The canonical source, checked 2026-08-30, is `s2_0023n62p17.evt.Z`, 1,312,605 bytes with SHA-256 7a21d9bee436cff25a6f2dc41c87b1ed6c541ea264f686535a70712b4b2ca74a. The Parquet artifact is 13,033,415 bytes with SHA-256 459798a2784c5c0a59b99c5d66a061af4875278bedee54fd162930b9c180e6b4.

The FITS is a HEASARC reconstruction: photon records came from the Event Encyclopedia, exposures from the original Orbit Attitude Live Time tapes, and HEASARC combined them into the present three-extension event layout. It is not a native spacecraft FITS product. See the archive description, SAS-2 archive account, and SAS-2/COS-B database-reconstruction account.

Considerations for use

The released calibration was not present in the original SAS-2 data; HEASARC reconstructed it from the literature, including Fichtel et al. (1975).

One event (source row 164) has ENERGY=-999 MeV, outside the source TLMIN/TLMAX=1/5000, with no TNULL. It is a measured special value, not a physical photon energy. The artifact preserves it verbatim so consumers can identify it explicitly; it must not be interpreted as energy.

The archive's analysis bands are 35--100, 100--1000, and 35--1000 MeV; the mission's nominal instrumental range is 20 MeV--1 GeV, while valid-looking values in this file reach 1,079 MeV. The response changed as spark-chamber gas deteriorated: sensitivity declined nearly linearly and that decline was incorporated into the archive calibration, but HEASARC says only rough instrumental calibration is possible. Its available redistribution matrix covers input channels only from 20 to 500 MeV, with the final 200--500 MeV channel integral above 200 MeV. Consequently, neither the 1,079 MeV event value nor the archive's 1,000 MeV map boundary implies resolved response support at those energies.

RA and DEC are FK4 B1950 coordinates. X and Y are HEASARC-generated TAN pseudo-image sky pixels, while DETX and DETY are corrected detector coordinates. X (-29..545) and Y (-1..535) exceed the header's nominal 1..512 limits and are preserved, never clipped.

TIME is absolute MJD with TIMEREF=LOCAL and CLOCKAPP=T; 130 ALLGTI intervals run from MJD 41754.14194444445 to 41760.830830833336 and sum to 5.209249722 days. The 348 distinct event times span 41754.188468113425 to 41760.731864305555 d. The source header records TIMESYS=MJD, RADECSYS=FK4, EQUINOX=1950.0, OBJECT=N/A, OBS_ID=17, TELESCOP=SAS-2, and INSTRUME=SAS-2.

OGIP/94-003 identifies FLAG only as an encoded bit flag, FOVCLASS as a 0--5 field-of-view class, and SPKDAT as 12 integers of spark-chamber data; it does not provide a complete bit/code interpretation. Finally, 342 of 348 Event Encyclopedia NEVENT identifiers are negative source values, spanning -1930219..1698759; they are documented anomalies, not normalised identifiers.

Licensing and credit

No explicit data licence was found. HEASARC states:

HEASARC materials are all available freely for your use.

HEASARC requests the acknowledgement below.

This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC.

Citation

Fichtel, C. E. et al. 1975, "High-Energy Gamma-Ray Results from the Second Small Astronomy Satellite", The Astrophysical Journal 198, 163--182, DOI 10.1086/153590.

Errata and requests

Report corrections or dataset requests in the dataset's Community tab.