{
  "eventId": "LIGO/Virgo G298048",
  "submitter": "S. J. Smartt at Queens U Belfast  <s.smartt@qub.ac.uk>",
  "body": "J. Lyman (Univ. of Warwick), D. Homan (Univ. of Edinburgh), K. Maguire\n(QUB), M. T. Botticella (INAF-Capodimonte), M. Fraser (UCD), C.\nInserra (Southampton), E. Kankare (QUB), S. J. Smartt (QUB), K. W.\nSmith (QUB), M. Sullivan (Southampton), S. Valenti (UC Davis), O.\nYaron (Weizmann), I. Manulis (Weizmann), D. Young (QUB), T.-W. Chen\n(MPE), S. Campana (INAF-Brera), S. Benetti, L. Tomasella\n(INAF-Padova), G. Leloudas (DARK, Copenhagen), Z. Cano (IAA, Granada).\n\nUnder the extended Public extened ESO Spectroscopic Survey for\nTransient Objects (ePESSTO; see Smartt et al. 2015, A&A, 579, 40\nhttp://www.pessto.org), we report spectra of the optical/NIR candiate\ncounterpart of the gravitational wave source G298048 in NGC4993.\n\nThis object was discovered by Coulter et al. (GCN 21529, named SSS17a)\nand further detected by Allam et al. (GCN 21530), Yang et al. (GCN\n21531), Melandri et al. (GCN 21532), Nicholl et al. (GCN21541), Tanvir\n& Levan (GCN21544), Chambers et al. (GCN 21553), Yoshida et al. (GCN\n21549).\n\nThe observations were performed on the ESO New Technology Telescope at\nLa Silla with the EFOSC2 instrument in spectroscopic mode starting\nfrom 2018-08-18 at 23:20 UT, using the Gr11 and Gr16 which provide\ncoverage 3345-9995 at a resolution between 260-396 (v = 1150 - 756\nkm/s) for a 1.5\" slit, which was used in roughly 1.5-2.0\" seeing. \n\nQuick look reductions (not including fringe and flat correction)\nindicate the spectrum is highly unusual for a nearby transient. \n\nWe have dereddening the spectrum with A_v = 0.34, for Milky \nWay correction. \n\nThe continuum is featureless, similar to the report of the first\nMagellan spectrum in Drout et al. (GCN 21547). But it peaks at 5500\nand falls off rapidly in the blue. This is consistent with the Swift\ncolours reported and the fading U-band and UV flux. The red part\n(beyond 5000Angs) can be fit with a black-body temperature of around\n6000K or a power law.  But neither are consistent with the turn over\nin the blue.  There are no strong features in the blue that indicate\nline absorption or could be identified with known transitions.  There\nare no Balmer lines, no Ca II H&K, or Si II absorption seen in\nsupernova-like transients. \n\nWe can certainly conclude this is not a young supernova of any type in\nthis host, nor is it consistent with being a background supernova of \nany standard type. The UV dimming and maintaining of the red and NIR\nflux appears broadly consistent with many flavours of kilonova models. \n\nFurther observations with ePESSTO will continue over the next 4\nnights, combining EFOSC2 and SOFI observations. During the 5 years of\nPESSTO spectroscopic surveys, this transient stands out as exceptional\nin its colours, evolution and spectra (supporting the ideas in Foley\nGCN 21557).",
  "editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>",
  "bibcode": "2017GCN.21582....1L",
  "createdOn": 1503107186000,
  "subject": "LIGO/VIRGO G298048:  ePESSTO optical spectra of the candidate optical/NIR  counterpart of the gravitational wave G298048 in NGC4993.",
  "editedOn": 1777052863929,
  "email": "s.smartt@qub.ac.uk",
  "circularId": 21582,
  "version": 2
}