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Swift/XRT follow-up observation of IGR J18179-1621

ATel #3950; J. Li, S. Zhang, Y. P. Chen (IHEP, China), M. Tuerler (ISDC, Univ. of Geneva, Switzerland), J. Chenevez (DTU Space, Denmark); E. Bozzo, C. Ferrigno, A. Tramacere (ISDC, U. of Geneva, Switzerland); I. Caballero, J. Rodriguez (CEA Saclay, France); M. Cadolle-Bel, C. Sanchez-Fernandez (ESA/ESAC, Spain); M. del Santo, A. Tarana, M. Fiocchi(INAF/IASF-Roma, Italy), P. R. den Hartog (Stanford, USA), I. Kreykenbohm, M. Kuehnel (Dr. Karl Remeis Obs. and ECAP, Germany), A. Paizis (INAF/IASF-Milano, Italy), G. Puehlhofer (IAA-Tuebingen, Germany), K. Watanabe (FGCU, USA), G. Weidenspointner (MPE and MPI HLL, Germany)
on 2 Mar 2012; 13:29 UT
Credential Certification: Jian Li (jianli@ihep.ac.cn)

Subjects: X-ray, Gamma Ray, Binary, Black Hole, Neutron Star, Transient, Pulsar

Referred to by ATel #: 3988

Following the discovery of IGR J18179-1621 (Tuerler et al. ATel #3947) on 2012-02-29, we requested two Swift ToO, which were executed on 2012-02-29 from 20:10:02 to 22:00:01 UT and on 2012-03-01 15:25:55 from to 2012-03-01 15:59:48 UT for a total exposure of ~3.9 ks. These two observations were carried out in Photon Counting mode and the sky image is piled up. We have corrected the pile up effect in our analysis. Using the online Swift tool we find an enhanced position of

RA = 274.4675 (18h 17m 52.20s),
Dec = -16.3589 (-16d 21' 31.9'') (J2000),

with a 90% confidence level error of 2.2 arcsec, consistent with the position measured by INTEGRAL/JEM-X (ATel #3947)(We have used the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue, see http://www.swift.ac.uk/user_objects and Evans et al. 2009, MNRAS, 397, 1177).

The two XRT spectra can be well fitted (reduced chi^2=0.72 for 47 dof and reduced chi^2=0.98 for 49 dof ) with an absorbed power-law model. The spectral parameters we derived in the 1-10 keV energy band are (model and flux uncertainties are 90% c. l.): In 2012-02-29, N_H = 11.0+/-2.7 e22 atoms/cm2 and photon index = 0.4+/-0.4. The 1-10 keV unabsorbed flux is 4.5+/-0.5 e-10 erg/cm2/s. In 2012-03-01, N_H = 15.1+/-3.6 e22 atoms/cm2 and photon index = 0.7+/-0.4. The 1-10 keV unabsorbed flux is 4.2+/-0.8 e-10 erg/cm2/s.

We derive from Swift/XRT a flux, between 3-10 keV, of 21.5+/-1.8 mCrab on 2012-02-29 and 22.0+/-2.2 mCrab on 2012-03-01, consistent with what was derived from JEM-X observation (ATel #3947).This may mean that IGR J18179-1621 now is still in a relatively bright phase.

No significant variation of the source spectral parameters are revealed in the XRT data.

We notice that the absorption is much larger than the Galactic column density (1.2e22 atoms/cm2 as estimated at the source position by using the tool available at the HEASARC website, Kalberla et al. 2005), and hence indicates an additional absorption intrinsic to the source.

In ATel #3947 absorption line was claimed in joint ISGRI/JEMX energy spectrum, suggestive of having a high mass X-ray binary pulsar. We confirm the detection of a possible period at ~11.8 seconds by using a Lomb-Scargle method on all available Swift/XRT data corrected for pile-up. This is consistent with the period reported by J. P. Halpern (ATEL #3949). The estimated pulsed fraction is ~22%.

An infrared counterpart is found within the XRT error box (at 0.33"): 2MASS J18175218-1621316 with coordinates RA =274.467438 dec=-16.358791 and an uncertainty of about 0.1". This source has a well measured magnitude only in the Ks band: Ks=11.14.