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 (firstname.lastname@example.org)
Subjects: Radio, X-ray, Gamma Ray, A Comment, AGN, Binary, Black Hole, Neutron Star, Transient, Variables, Pulsar
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.