Swift/XRT follow-up of two unidentified INTEGRAL sources
ATel #3185; R. Landi, A. Malizia (INAF/IASF Bologna), A. Bazzano, M. Fiocchi (INAF/IASF Rome), A. J. Bird (Univ. Southampton), N. Gehrels (NASA/GSFC)
on 23 Feb 2011; 14:57 UT
Credential Certification: Raffaella Landi (firstname.lastname@example.org)
Referred to by ATel #: 3186
We report the results of X-ray follow-up observations performed with Swift/XRT of two unidentified
INTEGRAL sources listed in the 4th IBIS Survey Catalogue (Bird et al. 2010, ApJS, 186, 1).
There are two sources detected by XRT above 3 sigma c.l. and located within the 90% IBIS positional
Source #1, detected at 6.1 sigma c.l. (0.3-10 keV), is located at R.A.(J2000) = 06h 52m
31.35s and Dec.(J2000) = +53d 31m 26.2s (4.7 arcsec uncertainty). This position is compatible with
that of the ROSAT Faint source 1RXS J065230.8+533130. This X-ray source has an optical counterpart
in QORG J065231.2+533130, a QSO candidate having B and R magnitudes of 18.3 and 17.7, respectively
(Million QSO catalogue in HEASARC); it is also detected in the 2MASS catalogue with magnitudes J ~17, H ~16 and
K ~15. The X-ray data are described by a power law with photon index 1.8 and a 2-10 keV flux of 1.7
x 10-13 erg cm-2 s-1.
Source #2 is detected at 3.7 sigma c.l. (0.3-10 keV) and its position is at R.A.(J2000) = 06h 52m
07.1s and Dec.(J2000) = +53d 30m 28.8s (5.5 arcsec uncertainty). It coincides with a USNO-B1.0
object (USNO-B1.0 1435-0176077) having magnitudes R ~20 and B ~21. The source has no counterpart
in the 2MASS catalogue.
From X-ray data we can only infer a 2-10 keV flux of 6.7 x 10-14 erg cm-2
s-1 assuming a power law with photon index 1.8.
Source #1, the brighter of the two and also the only one detected above 3 keV (at 2.7 sigma), is
the most likely association.
Within the IBIS error circle XRT does not detect any X-ray source above 3 sigma c.l.. However, two
objects are found outside, of which one is at border of the 90% error circle (#1) and one is inside
the 99% uncertainty region (#2).
Source #1, detected at 3.3 sigma c.l. (0.3-10 keV), is located at R.A.(J2000) = 18h 37m
and Dec.(J2000) = +26d 32m 31.2s (6 arcsec uncertainty). The XRT position is compatible with that of
a ROSAT Faint source (1RXS J183728.8+263225). The XRT detection coincides with a 2MASS object having
magnitudes J ~11, H ~10 and K ~10, also detected in optical with B ~14 and R ~13. The low X-ray
statistics provide only a flux estimate, which in the 2-10 keV band is 1.3 x 10-13 erg
cm-2 s-1 (Gamma = 1.8).
Source #2 is instead detected at 10.2 sigma c.l.(0.3-10 keV) and its position is at R.A.(J2000) =
18h 37m 28.36s and Dec.(J2000) = +26d 29m 44.4s (4.5 arcsec uncertainty). The XRT position is
compatible with an XMM Slew source XMMSL1 J183728.2+262942 (flux 1.2 x 10-12 erg
cm-2 s-1 in the 0.2-2
keV energy band); this source has been associated to the star HD 336589 of spectral type F8D. The X-ray
spectroscopy indicates that the source is very soft with no counts above 2 keV and a 0.2-2 keV
flux of 1.3 x 10-12 erg cm-2 s-1 (Gamma = 1.8). Comparison between
XRT observations indicates source variability by a factor of 4.6 in around one month.
Despite being the brightest detection, source #2 is a highly unlikely counterpart of the IBIS object
because of its spectral softness, location and optical spectrum, thus leaving source #1 as a viable