[vsnet-grb-info 23865] LIGO/Virgo S190930t: Swift discovery of Swift J224718-581442 (AT2019sbk): a fading UV transient with an X-ray counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25964
SUBJECT: LIGO/Virgo S190930t: Swift discovery of Swift J224718-581442 (AT2019sbk): a fading UV transient with an X-ray counterpart
DATE:    19/10/07 00:23:29 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.kholodenko at gmail.com>

A . Tohuvavohu (U. Toronto), N. P. M. Kuin (UCL-MSSL), S. R. Oates
(Uni. of Warwick),  A. A. Breeveld (MSSL-UCL),
F. E. Marshall (NASA/GSFC), P. Brown (TAMU),C. Gronwall (PSU),
M. J. Page (UCL-MSSL), M. De Pasquale (Istanbul U.),
M. H. Siegel (PSU), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB),
S. D. Barthelmy (NASA/GSFC), A. P. Beardmore (U. Leicester), D. N.
Burrows (PSU),
S. Campana (INAF-OAB), S. B. Cenko (NASA/GSFC), G. Cusumano (INAF-IASF PA),
V. D'Elia(ASDC), P. A. Evans (U. Leicester), P. Giommi (ASI),
D. Hartmann (Clemson U.), J. A. Kennea (PSU), N. J. Klingler (PSU),
H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC), D. B. Malesani (DTU Space),
A. Melandri (INAF-OAB),  J. A. Nousek (PSU), P. T. O'Brien (U. Leicester),
J. P. Osborne (U. Leicester), C. Pagani (U. Leicester), K. L. Page
(U.Leicester),
D.M. Palmer (LANL),  M. Perri (ASDC), J. L. Racusin (NASA/GSFC),
T. Sakamoto (AGU),  B. Sbarufatti (INAF-OAB/PSU), G. Tagliaferri (INAF-OAB),
and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift team:

We report the discovery of Swift J224718-581442 a  transient found
in the UVOT and XRT search results of the LVC event
S190930t (LIGO/Virgo Collaboration GCN Circ. 25876).

On 2019-10-01 07:19:41 (GW T0+17 hours) UT Swift UVOT took
 a 75 s exposure in the u band with observation ID 07021863001.
An uncatalogued source was found.
The new source is not listed in the USNO-B1,  Gaia DR1, GSC2.3 or 2MASS
catalogues and is not listed as a minor planet.

The UVOT position is

    RA (J2000)  22:47:18.100= 341.825417
   Dec (J2000) -58:14:50.80  = -58.247444

The magnitude using the UVOT photometric system  (Breeveld et al. 2011, AIP
Conf. Proc. 1358, 373) was u = 18.76 +/- 0.21 mag (Vega).

On 2019-10-04 05:01:0 UT Swift UVOT took another 78 s exposure in the u band
with observation ID 07400055001. The source is found to have faded by
 > 1 mag, to u = 19.84  (Vega).

A faint uncatalogued X-ray source is found by XRT 1.9 arcseconds away,
consistent with the location of the UVOT source.

The  X-ray source is located at RA, Dec 341.82803, -58.2451 which
is equivalent to:
   RA(J2000)  = 22h 47m 18.73s
   Dec(J2000) = -58d 14′ 42.3″

The count rate of this source is measured in XRT at 0.013 (±0.004) ct s-1.

This source is located on the outskirts of the galaxy 2MASX J22471856-5814422,
with a redshift of z=0.054 (D~240 Mpc), ~ 4 sigma above the mean GW
distance along this line of sight (mean = 94 Mpc, sigma = 34 Mpc).

We encourage spectroscopic followup of this transient.
Further Swift UVOT and XRT observations are planned.

This transient has been reported to the TNS as AT2019sbk.

This circular is an official product of the Swift team.