J/MNRAS/537/931 Chandra X-ray flares and dips (Dillmann+, 2025)
Representation learning for time-domain high-energy astrophysics:
Discovery of extragalactic fast X-ray transient XRT 200515.
Dillmann S., Martinez-Galarza J.R., Soria R., Di Stefano R., Kashyap V.L.
<Mon. Not. R. Astron. Soc. 537, 931 (2025)>
=2025MNRAS.537..931D 2025MNRAS.537..931D (SIMBAD/NED BibCode)
ADC_Keywords: X-ray sources ; Transient ; Photometry, infrared ;
Photometry, SDSS ; Optical
Keywords: methods: data analysis - software: machine learning -
stars: magnetars - stars: peculiar - X-rays: bursts -
(transients:) gamma-ray bursts
Abstract:
We present a novel representation learning method for downstream tasks
like anomaly detection, unsupervised classification, and similarity
searches in high- energy data sets. This enabled the discovery of a
new extragalactic fast X-ray transient (FXT) in Chandra archival data,
XRT 200515, a needle-in-the-haystack event and the first Chandra FXT
of its kind. Recent serendipitous discoveries in X-ray astronomy,
including FXTs from binary neutron star mergers and an extragalactic
planetary transit candidate, highlight the need for systematic
transient searches in X-ray archives. We introduce new event file
representations, E-t maps and E-t-dt cubes, that
effectively encode both temporal and spectral information, enabling
the seamless application of machine learning to variable-length event
file time series. Our unsupervised learning approach employs PCA or
sparse autoencoders to extract low-dimensional, informative features
from these data representations, followed by clustering in the
embedding space with DBSCAN. New transients are identified within
transient- dominant clusters or through nearest-neighbour searches
around known transients, producing a catalogue of 3559 candidates
(3447 flares and 112 dips). XRT 200515 exhibits unique temporal and
spectral variability, including an intense, hard <10s initial burst,
followed by spectral softening in an ∼800s oscillating tail. We
interpret XRT 200515 as either the first giant magnetar flare observed
at low X-ray energies or the first extragalactic Type I X-ray burst
from a faint, previously unknown low-mass X-ray binary in the LMC. Our
method extends to data sets from other observatories such as
XMM-Newton, Swift-XRT, eROSITA, Einstein Probe, and upcoming missions
like AXIS.
Description:
We tabulate the meta-data of X-ray flares and dips from the Chandra
Source Catalog (CSC) found as part of the paper.
Please get in touch with Steven Dillmann (stevendi(at)stanford.edu,
steven.dillmann(at)outlook.com) for further information or data (e.g.
the individual region eventfiles associated with each flaring/dipping
source, that can be used to plot the lightcurves).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
catalog.dat 1837 3559 Chandra X-ray flares and dips
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See also:
IX/70 : Chandra Source Catalog Release 2 (CSC 2.1) (Evans+, 2024)
II/349 : The Pan-STARRS release 1 (PS1) Survey - DR1 (Chambers+, 2016)
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
I/355 : Gaia DR3 Part 1. Main source (Gaia Collaboration, 2022)
Byte-by-byte Description of file: catalog.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 h RAh Right ascension (J2000)
3 A1 --- --- [:]
4- 5 I2 min RAm Right ascension (J2000)
6 A1 --- --- [:]
7- 11 F5.2 s RAs Right ascension (J2000)
13 A1 --- DE- Declination sign (J2000)
14- 15 I2 deg DEd Declination (J2000)
16 A1 --- --- [:]
17- 18 I2 arcmin DEm Declination (J2000)
19 A1 --- --- [:]
20- 24 F5.2 arcsec DEs Declination (J2000)
26- 35 A10 --- CatId Catalog ID, NNNNN_NNNN
37- 58 A22 --- CSCName Name of the source in the CSC,
2CXO JHHMMSS.s+DDMMSS
60 A1 --- TransType [F/D] Transient (F:flare, D:dip)
62- 78 A17 --- CSCcoord J2000 coordinates in the CSC,
JHHMMSS.s+DDMMSS
80- 99 F20.16 deg RACdeg Right ascension of the source in the CSC
101- 120 F20.16 deg DECdeg Declination of the source in the CSC
122- 126 I5 --- CSCobsId Observation ID in the CSC
128- 131 I4 --- CSCregId Region ID in the CSC
133- 152 F20.16 --- CSCsigni Highest flux significance
154- 175 E22.17 --- CSCFluxap-b ? Broad band flux in the CSC
177- 195 F19.16 --- CSChardhm ? Hard-to-medium hardness ratio
197- 215 F19.16 --- CSChard-ms ? Medium-to-soft hardness ratio
217- 235 E19.16 --- CSChard-hs ? Hard-to-soft hardness ratio
237- 255 E19.15 --- CSCvarProb-b ? Variability probability in broad band
257- 273 F17.15 --- CSCvarProb-h ? Variability probability in soft band
275- 292 F18.16 --- CSCvarProb-m ? Variability probability in medium band
294- 310 F17.15 --- CSCvarProb-s ? Variability probability in hard band
312- 315 F4.1 --- CSCvarIndx-b ? Variability index in broad band
317- 335 A19 "datime" CSCgti-obs Start time of observation
337- 355 A19 "datime" CSCgti-end End time of observation
357- 375 F19.16 arcmin CSCtheta Off-axis angle of the source
377- 381 I5 ct CSCctapb Source region counts
383- 387 I5 ct CSCctapbkgb Background region counts
389 A1 --- CSCtype [P/X] Source type (P:point, X:ext.)
391- 395 A5 --- CSC2.1Flag True: CSC2.1, False: CSC2.0
397- 414 I18 --- PSid ?=0 PanSTARRS-1 unique object ID
416- 432 F17.1 --- PSintId ? PanSTARRS-1 internal object ID
434- 443 F10.8 arcsec sepCSC-PS ? Distance CSC-PS source
445- 461 F17.12 mag PSgmag ?=- PanSTARRS-1 g magnitude
463- 479 F17.12 mag PSrmag ?=- PanSTARRS-1 r magnitude
481- 497 F17.12 mag PSimag ?=- PanSTARRS-1 i magnitude
499- 515 F17.12 mag PSzmag ?=- PanSTARRS-1 z magnitude
517- 535 I19 --- GaiaId ?=0 Gaia unique object ID
537- 549 E13.7 arcsec sepCSC-Gaia ? Distance CSC-GAIA source
551- 565 F15.12 mag Gmag ? Gaia G magnitude
567- 579 F13.8 km/s RV ? Gaia radial velocity
581- 599 F19.16 mas plx ? Gaia parallax
601- 611 F11.8 mag Bp-Rp ? Gaia BP-RP magnitude
613- 622 F10.7 mag GMAG ? Gaia absolute magnitude
624- 640 I17 --- LSId ?=0 Legacy Survey DR10 ID
642- 648 F7.1 --- LSobjectId ? Legacy Survey DR10 object ID
650- 661 E12.8 arcsec sepCSC-LS ? Distance CSC-LS source
663- 671 F9.6 mag LSgmag ? Legacy Survey DR10 g magnitude
673- 681 F9.6 mag LSrmag ? Legacy Survey DR10 r magnitude
683- 691 F9.6 mag LSzmag ? Legacy Survey DR10 z magnitude
693- 708 A16 --- 2MASSId 2MASS name
710- 720 F11.9 arcsec sepCSC-2M ? Distance CSC-2MASS source
722- 727 F6.3 mag Jmag ? 2MASS J magnitude
729- 734 F6.3 mag Hmag ? 2MASS H magnitude
736- 741 F6.3 mag Kmag ? 2MASS K magnitude
743- 760 F18.16 arcsec sepCSC-Sp ? Distance CSC-SPEC source
762- 767 A6 --- Spclass SDSS spectroscopic clas
769- 787 A19 --- Spsubclass SDSS spectroscopic subclass
789- 802 E14.11 --- Spz ? SDSS spectroscopic redshift
804- 817 E14.11 --- e_Spz ? SDSS spectroscopic redshift error
819- 827 F9.4 0.1nm Spwavemin ? SDSS spectroscopic minimum wavelength
829- 837 F9.3 0.1nm Spwavemax ? SDSS spectroscopic maximum wavelength
839- 841 F3.1 --- Spzwarning ? SDSS spectroscopic redshift warning
843- 852 E10.7 --- Spsn ? SDSS spectroscopy median SN
854- 872 I19 --- SDSSId ? SDSS object ID
874- 881 F8.6 --- sepCSC-SDSS ? Distance CSC-SDSS source
883 A1 --- SDSSType [S/G] SDSS type (S:star, G:galaxy)
885-1718 A834 --- SIM-ids SIMBAD object ID
1720-1741 A22 --- SIM-otype SIMBAD object type
1743-1763 A21 --- SIM-sptype SIMBAD spectral type
1765-1771 F7.4 mas SIM-plx ? SIMBAD parallax
1773-1780 F8.3 mas/yr SIM-pmRA ? SIMBAD proper motion in RA
1782-1789 F8.3 mas/yr SIM-pmDE ? SIMBAD proper motion in DEC
1791-1808 E18.14 km/s SIM-RV ? SIMBAD radial velocity
1810-1818 E9.6 --- SIM-z ? SIMBAD redshift
1820-1837 F18.16 arcsec sepCSC-SIM ? Distance CSC-SIMBAD source
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Acknowledgements:
Steven Dillmann, stevendi(at)stanford.edu
(End) Patricia Vannier [CDS] 17-Feb-2025