HESS J1503-582 - a "Forbidden Velocity Wing"?

November 2008

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The HI 21 cm line traces the distribution of neutral hydrogen in the Galaxy; from the intensity and Doppler shift of the line the amount of hydrogen gas and its velocity along the line of sight can be determined. Due to the differential rotation of the Galaxy, the mean velocity and velocity range vary with Galactic longitude, showing that regions of the Galaxy seen around longitude 80-160 degr. move towards us, whereas the regions around 20-60 degr. tend to move away, with a wide dispersion. The distribution of velocities shows, however, also compact regions protruding from the band, indicative of gas masses with large velocity dispersion, termed Forbidden Velocity Winds (FVW) (from Kang & Koo 2007).

Forbidden velocity wings (FVW, see top image) obviously represent objects involving gas masses with high velocity dispersion, such as supernova remnants or strong stellar winds. However, only a small fraction of all FVWs as identified by Kang & Koo (2007) coincide with known supernova remnants (SNRs) (Fig. 1). Previously unknown, old SNRs in the radiative phase could be the most likely candidates, as in the case of the discovery of the SNR associated with FVW 190.2+1.1 (Koo et al. 2006).

HESS J1503-582 (Fig. 2) is a 6-sigma source candidate revealed in HESS data taken during the Galactic Plane Survey and in observations of the nearby object MSH 15-52. The region was first targeted in 2004 and was later observed regularly until 2007. HESS J1503-582 shows up as an extended gamma-ray excess at longitude 319.7 degr. and latitude 0.3 degr., with an rms size of about 0.3 degr. Two potential counterparts were found in catalogs: the X-ray source AX J1504.6-5824, which is, however, catalogued as a Cataclysmic Variable,  and the very old pulsar PSR J1502-5828, with an age of 0.3 Myr. Pulsar wind nebulae indeed represent a significant fraction of all Galactic H.E.S.S. sources. The spin-down energy loss of this particular pulsar is, however, one to two orders of magnitude too small to plausibly account for the gamma-ray source. Interestingly, the forbidden velocity wing FVW 319.8+0.3, marked with the highest detection rank by Kang & Koo (2007), is spatially coincident with HESS J1503-582, as shown in Fig. 3. This FVW appears in the HI line image integrated between gas velocities of  -123 to -98 km/s, two velocities which are not permitted by the canonical Galactic rotation curve along this line of sight. The exact nature of the source, however, remains unclear. Old supernova remnants with ages of some 10 kyr are not expected to accelerate multi-TeV particles any longer, mainly because of the very low shock speed. On the other hand, the joint activity of stellar winds and supernova explosions from massive stars in nearby and powerful OB associations could produce fast-moving neutral hydrogen gas detectable at the sensitivity level of the current HI surveys, and serve to accelerate particles and create gamma rays, as seen in the source HESS J1023–575 co-located with the Westerlund 2 cluster.

Reference: "On the nature of HESS J1503-582 revealed by the H.E.S.S. experiment: Coincidence with a FVW?", H.E.S.S. collaboration, M. Renaud et al., to appear in Proc. of the 4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy.

Fig. 1: Distribution of FVWs in (ℓ,b) plane. The color of each symbol indicates the mean velocity of the HI emitting gas and corresponds to the velocity in the color bar on the top in km/s. Each symbol indicates a related object, and its occupied area indicates the integrated antenna temperature as shown in the bottom. The areas filled with solid black show gaps between data, and the dotted line shows the boundary of the search in latitude. (From Kang & Koo 2007)
Fig. 2: Gamma-ray image of HESS J1503-582. Its peak significance reaches 6 sigma. The black cross and circle denote the uncertainty of the source centroid and its intrinsic size, respectively. The Galactic Plane is shown as the dashed line. The bright source in the lower left corner is MSH 15-52.
Fig. 3: SGPS (Parkes) image of the HI line emission centered on FVW 319.8+0.3, velocity-integrated between -123 and -98 km/s . H.E.S.S. significance contours are shown as 4 and 5 sigma levels. Note that the image is in Galactic coordinates, rotated compared to Fig. 2.