1FGL J1018.6-5856: a New Gamma-ray Binary
ATel #3221; R. H.D. Corbet (UMBC/NASA GSFC), C. C. Cheung (NRC, resident at NRL), M. Kerr (Stanford), R. Dubois (SLAC), D. Donato (UMCP/NASA GSFC), G. A. Caliandro (IEEC-CSIC), on behalf of the LAT collaboration, M. J. Coe (Southampton), P. G. Edwards (CSIRO), M. D. Filipovic (UWS), J. L. Payne (UWS), J. Stevens (CSIRO)
on 15 Mar 2011; 17:14 UT
Credential Certification: Robin Corbet (corbet@umbc.edu)
Subjects: Radio, Optical, X-ray, Gamma Ray, >GeV, Binary, Black Hole, Neutron Star, Star, Variables
Referred to by ATel #: 3228
Fermi LAT observations of the gamma-ray source 1FGL
J1018.6-5856 (Abdo et al., 2010, ApJS, 188, 405) obtained between MJD 54682 and 55627
in the energy range 100 MeV to 200 GeV show the presence of periodic modulation with a period of
16.58 ± 0.04 days. In the power spectrum of the light curve the modulation at the 16.6 day
period has a strength 23 times the mean power level, and the false alarm probability,
considering periods longer than 0.5 days, is ~10-7.
Modulation at this period is not seen in LAT observations of any
other source, and so it appears to be intrinsic to 1FGL J1018.6-5856 and not a
systematic effect. The epoch of maximum gamma-ray flux is MJD 55403.3 ± 0.4.
Swift XRT observations of the vicinity of 1FGL J1018.6-5856 reveal
an X-ray source located at R.A. = 10h 18m 55.54s, decl. = -58° 56' 45.9"
(J2000, 90% confidence radius = 4.5"), consistent with the location
of the gamma-ray source. A number of Swift XRT observations obtained
between September 2009 and February 2011 show
this source to be highly variable with 0.3 to 10 keV count rates ranging
from approximately 0.01 to 0.05 counts/s. The highest count rates obtained
so far are from times close to the epoch of maximum gamma-ray flux.
The DSS2 image of this region shows a star of magnitude B2 = 13.1 ±
0.3 within the XRT error circle at R.A. = 10h 18m 55.6s, decl. = -58° 56'
46.2" (J2000). A spectrum of this star obtained with the SAAO 1.9m telescope
indicates a spectral type of approximately O6V((f)), very similar to that of the gamma-ray binary LS 5039.
Three radio observations of this location with the Australia Telescope Compact
Array in February 2011 at frequencies of 5.5 and 9 GHz show a variable
source with intensities that range from 2 to 5 mJy/beam
at 5.5 GHz and from 2.5 to 5.5 mJy/beam at 9 GHz.
We conclude that 1FGL J1018.6-5856 is new member of the
rare gamma-ray binary class of objects, and that it shares several
properties with LS 5039. Continued observations in other wavebands
are encouraged.
For this source the Fermi LAT contact person is Robin Corbet (corbet@umbc.edu).
The Fermi LAT is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.
