Radio Properties of the Hard X-ray transient IGR J18175-1530

ATel #1289; C. C. Cheung (NASA GSFC)
on 19 Nov 2007; 20:09 UT
Credential Certification: Teddy Cheung (ccheung@milkyway.gsfc.nasa.gov)

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In response to the discovery of the new hard X-ray transient IGR J18175-1530 and its possible association with the radio source GPSR5 15.308+0.226 (hereafter GPS15.3) by Paizis et al. (ATEL #1248), radio images of the field from MAGPIS and NVSS were examined.

The 1.4 GHz MAGPIS image (5.4" x 6.2" resolution; Helfand et al. 2006 AJ 131, 2525) shows GPS15.3 to be a discrete 30 mJy (18 mJy peak) source, whereas the lower resolution (45") 1.4 GHz NVSS map shows a tail of emission extending to the northeast with a 7 mJy peak that is slightly offset (to the east) of GPS15.3. It is therefore evident that the GPS15.3 has varied by 10-20 mJy between the time of the NVSS (June 1996) and MAGPIS observations (accumulated between Apr 2003 - Mar 2004). The radio tail extends in the direction of the extended (~15') galactic diffuse source 15.4350+0.1600 (Helfand et al. 2006).

It is also apparent that the radio source is persistent on many year timescales. From other available MAGPIS data, GPS15.3 is a discrete source with integrated flux densities of 10 mJy at 4.9 GHz (9" x 4" beam with 6 mJy peak, June 1989; White et al. 2005 AJ 130 586) and 301 mJy at 325 MHz (24" x 18" beam with 235 mJy peak, Aug 2002). Relative to the non-contemporaneous MAGPIS 1.4 GHz detection, the radio spectrum appears steep with power-law slopes of -1.6 (325 MHz to 1.4 GHz) and -0.9 (1.4 to 4.9 GHz). This points to a non-thermal origin for the radio emission, but the uncertainty in the spectral slopes due to variability and resolution effects precludes a more definitive interpretation for IGR J18175-1530 (e.g., the more negative value points to a pulsar origin, but the less negative one is typical of a microquasar or background AGN). Simultaneous multi-band radio measurements to determine a reliable spectrum would enlighten the situation while higher resolution imaging would reveal any structure and will aid in identifying the optical/infrared counterpart.