J/MNRAS/287/293ivo://CDS.VizieR/J/MNRAS/287/293doi:10.26093/cds/vizier.72870293CDSMaoz D.Ofek E.O.Shemi A.2024-06-25T20:17:43Z1998-02-08T13:48:27ZCDS support team

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

cds-question@unistra.frx-ray-sourcesultraviolet-astronomyphotometryMost of the sources detected in the extreme ultraviolet (EUV; 100-600{AA}) by the ROSAT/WFC and EUVE all-sky surveys have been identified with active late-type stars and hot white dwarfs that are near enough to the Earth to escape absorption by interstellar gas. However, about 15 per cent of EUV sources are as yet unidentified with any optical counterparts. We examine whether the unidentified EUV sources may consist of the same population of late-type stars and white dwarfs. We present B and R photometry of stars in the fields of seven of the unidentified EUV sources. We detect in the optical the entire main-sequence and white dwarf population out to the greatest distances where they could still avoid absorption. We use colour-magnitude diagrams to demonstrate that, in most of the fields, none of the observed stars has the colours and magnitudes of late-type dwarfs at distances less than 100pc. Similarly, none of the observed stars is a white dwarf within 500pc that is hot enough to be a EUV emitter. The unidentified EUV sources we study are not detected in X-rays, while cataclysmic variables, X-ray binaries, and active galactic nuclei generally are. We conclude that some of the EUV sources may be a new class of nearby objects, which are either very faint at optical bands or which mimic the colours and magnitudes of distant late-type stars or cool white dwarfs. One candidate for optically faint objects is isolated old neutron stars, slowly accreting interstellar matter. Such neutron stars are expected to be abundant in the Galaxy, and have not been unambiguously detected.1997MNRAS.287..293Mhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/287/293CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toJ/ApJS/93/569 : First EUVE source catalogue (Bowyer+, 1994)II/203 : Second Extreme Ultra-Violet Explorer Catalog (2EUVE, 1996)J/MNRAS/274/1165 : The 2RE Source Catalogue (Pye+ 1995)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/MNRAS/287/293https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/MNRAS/287/293http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/MNRAS/287/293https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/MNRAS/287/293https://vizier.iucaa.in/viz-bin/votable?-source=J/MNRAS/287/293http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/MNRAS/287/293GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/MNRAS/287/293/table1 (Unidentified EUV sources)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/MNRAS/287/293/table1?https://vizier.iucaa.in/viz-bin/conesearch/J/MNRAS/287/293/table1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/MNRAS/287/293/table1?GETtext/xml+votable180.050000true108.9583314.171670.0055555555555555566/4808 7398 7954 15391 22505 42862 48904https://cdsarc.cds.unistra.fr/viz-bin/moc/J/MNRAS/287/293?format=asciiX-rayUVdefaultJ/MNRAS/287/293/table1Unidentified EUV sourcesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintEUVEEUV source name (EUVE or RE)meta.id;meta.maincharBandEUV bandinstr.bandpasscharRAJ2000Right ascension (J2000)pos.eq.ra;meta.mainDEJ2000Declination signpos.eq.dec;meta.mainGLONGalactic longitudedegpos.galactic.lonintGLATGalactic latitudedegpos.galactic.latintDateObservation datetime.epochnullableBexpTime of B expositionstime.duration;obs.exposureintRexpTime of R expositionstime.duration;obs.exposureintJ/MNRAS/287/293/tab2-8BR photometry for unidentified EUV sourcesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintEUVEEUV source namemeta.id;meta.maincharStarStar number in the EUVE fieldmeta.idintSepSeparationarcsecpos.angDistancefloatoRAOffset in right ascension (1)arcsecpos.eq.ra;arith.difffloatoDEOffset in declination (1)arcsecpos.eq.dec;arith.difffloatBmagB magnitudemagphot.mag;em.opt.Bfloate_Bmagrms uncertainty on Bmagmagstat.errorfloatRmagR magnitudemagphot.mag;em.opt.Rfloate_Rmagrms uncertainty on Rmagmagstat.errorfloatB-RB-R colourmagphot.color;em.opt.B;em.opt.Rfloate_B-Rrms uncertainty on B-Rmagstat.errorfloatJ/MNRAS/287/293/table9X-ray visibility of EUV-detected CVse_EUVE200? rms uncertainty on EUVE200ct/ksstat.errorintnullableEUVE400? EUVE survey at 400{AA} countsct/ksphot.count;em.UVintnullablee_EUVE400? rms uncertainty on EUVE400ct/ksstat.errorintnullableEUVE600? EUVE survey at 600{AA} countsct/ksphot.count;em.UV.50-100nmintnullablee_EUVE600? rms uncertainty on EUVE600ct/ksstat.errorintnullableWFC100? ROSAT WFC survey at 100{AA} countsct/ksphot.count;em.X-rayintnullablee_WFC100? rms uncertainty on WFC100ct/ksstat.errorintnullableWFC150? ROSAT WFC survey at 150{AA} countsct/ksphot.count;em.X-rayintnullablee_WFC150? rms uncertainty on WFC150ct/ksstat.errorintnullableXRT? ROSAT XRT survey at 0.1-2.4keV countsct/ksphot.count;em.X-rayintnullablee_XRT? rms uncertainty on XRTct/ksstat.errorintnullablerecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameStar namemeta.id;meta.maincharTypeStellar typesrc.classcharEUVE100? EUVE survey at 100{AA} countsct/ksphot.count;em.UVfloatnullablen_EUVE100[*] *: EUVE observations with the Deep Survey Telescopemeta.notechare_EUVE100? rms uncertainty on EUVE100ct/ksstat.errorintnullableEUVE200? EUVE survey at 200{AA} countsct/ksphot.count;em.UVintnullable