Research Note

INTEGRAL and Swift/XRT observations of IGR J19405-3016

¹ Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Beijing 100049, PR China e-mail: szhang@mail.ihep.ac.cn
² ICREA & Institut de Ciències de l'Espai (IEEC-CSIC), Campus UAB, Facultat de Ciències, Torre C5-parell, 2a planta, 08193 Barcelona, Spain
³ Theoretical Physics Center for Science Facilities (TPCSF), CAS, PR China
⁴ Center for Astrophysics,Tsinghua University, Beijing 100084, PR China

Received: 9 June 2009
Accepted: 21 July 2009

Abstract

Aims. IGR J19405-3016 is reported in the 3rd IBIS catalog as one of its lowest significance sources (~4.6σ under an exposure of about 371 ks). This leads to a caveat in multi-wavelength studies, although the source was identified in the optical as a Seyfert 1. The currently available INTEGRAL data on the source have increased to an exposure time of ~1376 ks, which stimulates us to investigate the reality of this source again by using all the available data from INTEGRAL and Swift/XRT.

Methods. We analyzed all available observations carried out by the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) on IGR J19405-3016. The data were processed by using the latest version OSA 7.0. In addition, we analyzed all the available Swift/XRT data on this source.

Results. We find that IGR J19405-3016 has a detection significance of ~9.4σ in the 20–60 keV band during the observational period between March 2003 and March 2008. This confirms a real source detection reported previously. The source position and error location are therefore updated. The source is found to be constant over years at the hard X-rays. We analyzed the Swift/XRT observations on IGR J19405-3016 as well, and find that the spectrum can be fitted with a simple power law model. Over the three XRT observations, the source flux varied by up to 39% from the average, and the spectrum is generally soft. The combined XRT/ISGRI spectrum is well fitted with a simple power law model (photon index ) with a column density fixed at  atoms/cm². Such a photon index is consistent with the mean value 1.98 (dispersion 0.27) obtained from Swift/BAT AGN samples at 14–195 keV. The spectral slope of IGR J19405-3016 is softer than the average spectral slope found elsewhere. A similar discrepancy is found with other results regarding Seyfert 1 AGNs. A possible explanation for this simple spectral description may be that the low level of the column density allows for the “true” spectrum to appear at soft X-rays as well.