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On the nature of the INTEGRAL sources IGR J16181-5407 and IGR J17157-5449

ATel #4233; R. Landi, L. Bassani, N. Masetti (INAF - IASF Bologna), A. Bazzano, A. Tarana (INAF - IAPS Rome), A. J. Bird (University of Southampton)
on 2 Jul 2012; 15:28 UT
Credential Certification: Raffaella Landi (landi@iasfbo.inaf.it)

Subjects: X-ray, Gamma Ray, Request for Observations

With respect to the recent INTEGRAL/IBIS 9-year Galactic Hard X-ray Survey (Krivonos et al. 2012, arXiv:1205.3941), we use archival Swift/XRT observations in conjunction with multi-wavelength information to discuss the counterparts for two newly discovered INTEGRAL sources: IGR J16181-5407 and IGR J17157-5449.

IGR J16181-5407

XRT pointed at the region of this IBIS source for a total exposure of ~1.4 ks. Within the 90% IBIS error circle, XRT detects one X-ray source at 4 sigma confidence level (c.l.) both in the 0.1-3 keV energy range and above 3 keV, implying a hard source. This object is located at RA(J2000) = 16h 18m 08.02s and Dec(J2000) = -54d 06m 09.8s (6 arcsec uncertainty at 90% c.l.). Within the XRT error circle, there are three USNO-B1.0 sources, USNO-B1.0 0358-0591958, USNO-B1.0 0358-0591923 and USNO-B1.0 0358-0591979; all three have a counterpart in the 2MASS catalogue.

From the X-ray data we can only infer an observed 2-10 keV flux of ~4 x 10-13 erg cm-2 s-1, adopting a power law (NH(Gal) = 5.75 x 1021 cm-2, Gamma = 1.8).

IGR J17157-5449

This source also appears in the 54-month Palermo BAT Catalogue (Cusumano et al., 2010, A&A, 524A, 64) as 2PBC J1715.2-5448 and is identified in the Krivonos et al. (2012) survey as the star HD 155573. We use archival Swift/XRT data to investigate its nature further. XRT pointed at the region of IGR J17157-5449 for a total exposure of ~1.5 ks. There is only one X-ray excess within the IBIS/BAT error circles located at RA(J2000) = 17h 15m 37.39s and Dec(J2000) = -54d 50m 04.5s (4.3 arcsec uncertainty, at 90% c.l.). It is detected at 9.6 and 4.5 sigma c.l. in the 0.3-10 keV energy range and above 3 keV, respectively.

The XRT spectrum is well fitted with a power law having (NH(Gal) = 1.25 x 1021 cm-2) a photon index Gamma ~1.5 and an observed 2-10 keV flux of ~3.3 x 10-12 erg cm-2 s-1.

As shown in the XRT image (available at the link reported at the bottom of this ATel), this X-ray source position is compatible with that of the bright ROSAT source 1RXS J171535.6-545015 also listed in Krivonos et al. (2012) catalogue (smaller green circle); it falls also within the positional uncertainty of an Einstein Slew source 1ES 1711-54.7 (bigger red circle). In the light of the XRT findings, the association between the star HD 155573 (black cross in the image) and the IBIS source, proposed by Krivonos et al. (2012), is ruled out. Indeed, although the star is located within the IBIS and BAT error circles, it is not detected by XRT and it lies far away from both the XRT and the ROSAT source detection.

Furthermore, a closer scrutiny of the XRT image indicates that the X-ray excess is composed of two distinct objects (#1 and #2 in the image), both of which are still observed above 3 keV. By inspection of the XRT image, we provide a tentative estimation of the position of each source (6 arcsec uncertainty at 90% c.l. assumed).

Source #1 is located at RA(J2000) = 17h 15m 37.11s and Dec(J2000) = -54d 50m 10.7s. Its position is compatible with two USNO-B1.0 objects: USNO-B1.0 0351-0716548 with R ~15.6 and USNO-B1.0 0351-0716550 with R ~14.5-15.0.

Source #2 is located at RA(J2000) = 17h 15m 37.83s and Dec(J2000) = -54d 50m 00.2s and has no counterpart in publicly available databases.

Source #1 and #2 have comparable X-ray fluxes, which suggest that both objects could contribute to the high energy emission seen by IBIS and BAT.

We acknowledge the use of public data from the Swift data archive.

Swift/XRT image of the region surrounding IGR J17157-5449