J/MNRAS/537/931ivo://CDS.VizieR/J/MNRAS/537/931doi:10.26093/cds/vizier.75370931CDSDillmann S.Martinez-Galarza J.R.Soria R.Di Stefano R.Kashyap V.L.2025-03-07T20:14:51Z2025-02-24T13:17:32ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frx-ray-sourcestransient-sourcesinfrared-photometryvisible-astronomysloan-photometryWe 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.2025MNRAS.537..931Dhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/537/931CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toIX/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)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/MNRAS/537/931https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/MNRAS/537/931http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/MNRAS/537/931https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/MNRAS/537/931https://vizier.iucaa.in/viz-bin/votable?-source=J/MNRAS/537/931http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/MNRAS/537/931GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/MNRAS/537/931/catalog (Chandra X-ray flares and dips)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/MNRAS/537/931/catalog?https://vizier.iucaa.in/viz-bin/conesearch/J/MNRAS/537/931/catalog?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/MNRAS/537/931/catalog?GETtext/xml+votable180.050000true246.652125-51.94184720.0055555555555555565/7564 9504 10614-10615 6/87 349 393 399 463 474 764 862 932 1074-1075 1111 1199 1231 1285 1287 1293 1390 1405 1466 1539 1679 1703 1791 1848 1850 1852 1930 2052 2154-2155 2241 2250-2251 2374 2409 2411 2433 2436 2564 2569 2585 2623 2666 2687 2704 2708-2709 2737 2813 3011 3029 3042 3061 3063 3157 3256-3257 3278 3290 3307 3312 3384 3389 3438 3506-3507 3582 3649 3662 3681 3755 4095 4196 4285 4298 4410 4599 4653 4698 4795 4801 4808 4897 4965 5018 5046 5164 5283 5327 5350 5370 5437 5469 5617 5629-5630 5648 5731 5780 5793 5966 5995 6011 6073 6075 6292 6383 6399 6448 6502 6524 6536 6539 6585 6760-6761 6926 7273 7345 7552 7743 7921 8066 8074 8173 8195 8264-8265 8268 8282 8345 8440 8524 8555 8644 8675 8707 8733 8969 8972 9077 9145 9149 9181 9287 9436 9469 9498 9506 9666 9769 9997 9999 10153 10166 10307 10312 10364 10378 10464 10484 10513 10515-10517 10521 10543-10544 10662 10743 10841 10856 10877 10888 10905 10911 10927 10936 10979 11027 11044 11080 11252 11406 11425 11428 11434 11672-11673 11685 11824 12157 12181 12553 12693 12719 12943 12958 12970 12975 12983-12984 13050 13106 13209 13250 13256 13297 13476 13522 13564 13663 13707 13761 13855 13993 14029 14031 14056 14225 14227 14259 14317 14338 14410 14415 14441 14443 14527 14575 14600 14610 14619 14621-14622 14629 14636 14641 14644-14645 14649 14653 14655 14667 14670 14682 14689-14690 14692-14693 14696 14704 14712 14744 14858 15085 15106 15112 15135 15153 15172 15345 15351 15362 15370 15478-15479 15484 15511 15592-15593 15639 15662-15663 15679 15681 15683-15684 15752 16026 16056 16093 16189 16191 16197 16301 16312 16410 16441 16492 16653 16655 16703 16712-16714 16758 17061 17300 17492 17502 17662 17690 17699 17772 17819 17840 17899 18003 18176 18190 18398 18643 18784 18800 18935 19036 19042 19216 19282 19343 19456 19842 19881 20021 20136 20342 20481 20624 20657 21043 21433 21436 21438-21439 21456 21458 21609 21641 21643 21704 21916 21969 21987 22045 22093 22208 22243 22247 22249 22252 22254 22577 22581 22628 22751 22804-22805 22826 22828 22855 22873 22876 22895 22915 23027 23078 23183 23212 23306 23339 23392 23549 23637 23639 23705 23769 23798 23869 23952 23969 23995 24203 24325 24336 24345 24449 24451 24491 24510 24522 24580 24582 24735 24740 24758-24759 24824 24949 24951 25083 25344 25413 25451 25669 25673 25768 25946 25990 26002 26189 26208 26238 26277 26292 26303 26316 26469 26594 26785 26803 26809 26913 26956 26990 27105 27223 27245 27247 27256 27258 27440 27515 27537 27714 27731 27773 27872 27947 27962 28033 28035 28086 28141 28167 28350 28370 28448 28516 28535 28574 28631 28717 28721 28723 28735 28768 28770 28778 28798 28846 28860 28862 28864 28868-28869 28890-28891 28894 28897 28899 28905 28912-28913 28915 28918-28919 28970 29070 29091 29094 29097 29100-29101 29113-29116 29118 29162-29163 29166 29184 29231 29259 29261 29267 29289 29460-29461 29463 29505-29506 29511 29523 29532 29632 29728 29730 30140 30199 30219 30222 30243-30245 30262-30263 30268-30269 30319 30330 30341 30343 30356 30366 30378 30416 30423 30505-30507 30594-30595 30604 30616 30618 30634 30694 30709 30719 30771 30946 31332-31333 31509 31552 31781 31817 32083 32104 32209 32500 32960 33047 33053 33094-33095 33097 33116 33124 33228 33297 33321 33646 33905 33980 33986 34049 34052 34069 34104 34159 34181 34273 34326 34471 34532 34534 34640-34641 34668 34793 35003 35174 35388 35390 35514 35536 35545 35565 35576 35720 35770 36131 36427 36466 36956-36957 37170 37294 37333 37341 37343 37361 37364 37388 37475 37710 37721 37841 37848 37850 37863 37865 37871-37872 37884 37901 37904 37912 37915 37921 37923-37926 37928-37929 37933 37937 37959 38020 38024 38026 38465 38551 38821 38868 38926 38973 39087 39235 39241 39318 39432 39784 39835 39907 39991 40124 40321 40323 40564 40612 40809 40952 41062 41304 41327 41417 41576 41586 41591 41594 41598-41599 41643 41654-41655 41663 41671 41674 41677 41680 41683 41685 41743 41758 41761 41780 41802-41803 41809 41812 41814-41815 41852 41937 42076 42163 42168-42169 42171-42172 42190 42206-42207 42212-42213 42215 42224-42226 42234 42370-42371 42373 42384 42390 42394 42396 42398 42419 42434 42437-42439 42441 42446-42447 42450 42486-42487 42502 42611 42666 42789 42792 42841 42892 42952 42955 42958 42981 42991 43009 43017 43019 43033 43036 43227 43390 43412 43544 43585 43631 43846 43874 43881 43956-43957 44094 44226 44284 44301 44555 44563 44712 44921 45564 45748 45823 45881 45984 46005 46108 46174 46388 46402 46424 46474 46491 46549 46571 47027 47058 47125 47205 47217 47777 47786 47828 47866 47868 48035 48071 48076 48105 48108 48423 48476 48500-48501 491251.1166583743361574e-19 3.916500758235022e-19https://cdsarc.cds.unistra.fr/viz-bin/moc/J/MNRAS/537/931?format=asciiX-rayOpticalInfrareddefaultJ/MNRAS/537/931/catalogChandra X-ray flares and dipsrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right ascension (J2000)pos.eq.ra;meta.mainHmag? 2MASS H magnitudemagphot.mag;em.IR.HfloatnullableKmag? 2MASS K magnitudemagphot.mag;em.IR.KfloatnullablesepCSC-Sp? Distance CSC-SPEC sourcearcsecpos.angDistancedoublenullableSpclassSDSS spectroscopic classrc.spTypecharSpsubclassSDSS spectroscopic subclasssrc.spTypecharSpz? SDSS spectroscopic redshiftsrc.redshift;pos.heliocentricdoublenullablee_Spz? SDSS spectroscopic redshift errorstat.errordoublenullableSpwavemin? SDSS spectroscopic minimum wavelength0.1nmem.wldoublenullableSpwavemax? SDSS spectroscopic maximum wavelength0.1nmem.wldoublenullableSpzwarning? SDSS spectroscopic redshift warningmeta.codefloatnullableSimbadNameSimbad column added by the CDSmeta.idcharnullableDEJ2000Declination (J2000)pos.eq.dec;meta.mainTransType[F/D] Transient (F:flare, D:dip)meta.codecharCSCcoordJ2000 coordinates in the CSC, JHHMMSS.s+DDMMSSmeta.idcharRACdegRight ascension of the source in the CSCdegpos.eq.raDECdegDeclination of the source in the CSCdegpos.eq.decCSCobsIdObservation ID in the CSCmeta.id;obsintCSCregIdRegion ID in the CSCmeta.id;obs.fieldintCSCsigniHighest flux significancestat.fit.goodnessdoubleCSCFluxap-b? Broad band flux in the CSCphot.flux;em.X-raydoublenullableCSChardhm? Hard-to-medium hardness ratiophot.flux;arith.ratiodoublenullableCSChard-ms? Medium-to-soft hardness ratiophot.flux;arith.ratiodoublenullableCSChard-hs? Hard-to-soft hardness ratiophot.flux;arith.ratiodoublenullableCSCvarProb-b? Variability probability in broad bandstat.fit.goodnessdoublenullableCSCvarProb-h? Variability probability in soft bandstat.fit.goodnessdoublenullableCSCvarProb-m? Variability probability in medium bandstat.fit.goodnessdoublenullableCSCvarProb-s? Variability probability in hard bandstat.fit.goodnessdoublenullableCSCvarIndx-b? Variability index in broad bandmeta.code;src.var.indexfloatnullableCSCgti-obsStart time of observationstime.epochnullableCSCgti-endEnd time of observationstime.epochnullableCSCthetaOff-axis angle of the sourcearcminpos.posAngdoubleCSCctapbSource region countsctphot.count;em.X-rayintCSCctapbkgbBackground region countsctinstr.background;obs.fieldintCSCtype[P/X] Source type (P:point, X:ext.)meta.codecharCSC2.1FlagTrue: CSC2.1, False: CSC2.0meta.codecharPSid? PanSTARRS-1 unique object IDmeta.idnullablePSintId? PanSTARRS-1 internal object IDmeta.iddoublenullablesepCSC-PS? Distance CSC-PS sourcearcsecpos.angDistancedoublenullablePSgmag? PanSTARRS-1 g magnitudemagphot.mag;em.opt.BdoublenullablePSrmag? PanSTARRS-1 r magnitudemagphot.mag;em.opt.RdoublenullablePSimag? PanSTARRS-1 i magnitudemagphot.mag;em.opt.IdoublenullablePSzmag? PanSTARRS-1 z magnitudemagphot.mag;em.opt.IdoublenullableGaiaId? Gaia unique object IDmeta.idnullablesepCSC-Gaia? Distance CSC-GAIA sourcearcsecpos.angDistancedoublenullableGmag? Gaia G magnitudemagphot.mag;em.optdoublenullableRV? Gaia radial velocitykm/sphys.veloc;pos.heliocentricdoublenullableplx? Gaia parallaxmaspos.parallax.trigdoublenullableBp-Rp? Gaia BP-RP magnitudemagphot.color;em.opt.B;em.opt.RdoublenullableGMAG? Gaia absolute magnitudemagphys.magAbsdoublenullableLSId? Legacy Survey DR10 IDmeta.idnullableLSobjectId? Legacy Survey DR10 object IDmeta.idfloatnullablesepCSC-LS? Distance CSC-LS sourcearcsecpos.angDistancedoublenullableLSgmag? Legacy Survey DR10 g magnitudemagphot.mag;em.opt.BdoublenullableLSrmag? Legacy Survey DR10 r magnitudemagphot.mag;em.opt.RdoublenullableLSzmag? Legacy Survey DR10 z magnitudemagphot.mag;em.opt.Idoublenullable2MASSId2MASS namemeta.idcharsepCSC-2M? Distance CSC-2MASS sourcearcsecpos.angDistancedoublenullableJmag? 2MASS J magnitudemagphot.mag;em.IR.JfloatnullableSpsn? SDSS spectroscopy median SNstat.snrdoublenullableSDSSId? SDSS object IDmeta.idnullablesepCSC-SDSS? Distance CSC-SDSS sourcepos.angDistancedoublenullableSDSSType[S/G] SDSS type (S:star, G:galaxy)meta.codecharSIM-otypeSIMBAD object typesrc.classcharSIM-sptypeSIMBAD spectral typesrc.spTypecharSIM-plx? SIMBAD parallaxmaspos.parallax.trigfloatnullableSIM-pmRA? SIMBAD proper motion in RAmas/yrpos.pm;pos.eq.radoublenullableSIM-pmDE? SIMBAD proper motion in DECmas/yrpos.pm;pos.eq.decdoublenullableSIM-RV? SIMBAD radial velocitykm/sphys.veloc;pos.heliocentricdoublenullableSIM-z? SIMBAD redshiftsrc.redshift;pos.heliocentricdoublenullablesepCSC-SIM? Distance CSC-SIMBAD sourcearcsecpos.angDistancedoublenullableCatIdCatalog ID, NNNNN_NNNNmeta.id;meta.tablecharCSCNameName of the source in the CSC, 2CXO JHHMMSS.s+DDMMSSmeta.id;meta.mainchar