Another Shell-Type Supernova Remnant: RX J0852.0–4622 ("Vela Junior")

March 2005

Rosat image of the Vela supernova remnant and its surroundings, for X-ray energies above 1.3 keV, showing the big Vela remnant, the small Puppis-A supernova remnant in the upper right, and the in this image newly discovered remnant RX J0852.0-4622 in the lower left (Aschenbach, 1998)

The supernova remnant RX J1713.7–3946 (SOM 2005-01) was the first remnant where the shell structure was detected in VHE gamma rays, demonstrating that the supernova shock wave accelerates particles. The detailed interpretation is complicated by the fact that there is significant uncertainty in a number of crucial parameters, such as the distance to the remnant and the characteristics of the ambient interstellar medium. It is therefore important to enlarge the ensemble of supernova remnants studied in VHE gamma rays, which allows one to average over such parameters and to extract the key features. “Vela Junior”, or RX J0852.0–4622 is supernova remnant discovered in 1998 in ROSAT X-ray images (Aschenbach 1998, Aschenbach et al. 1999). The remnant is only a faint radio emitter (Duncan & Green, 2000) and was not identified in earlier radio surveys. Age and distance are estimated to 680 years and 200 pc (Aschenbach et al. 1999). It has been argued that three nearby supernova explosions might be responsible for spikes in the nitrate abundance found in South Pole ice cores (Rood et al, 1979); with an age around 700 years, the Vela Junior supernova might be responsible for a fourth, previously unidentified spike (Burgess and Zuber, 2000) (Fig. 1). The CANGAROO instrument reported the detection at the 6 sigma level of VHE gamma rays from the north-western part of Vela Junior, based on about 100 h of observations (Katagiri et al., 2005).

**Fig. 1:** Nitrate abundance in South Pole ice cores. The two spikes at ~50 m correspond to the dates around 1600, the spikes at ~85 m and ~100 m to 1300 and 1150, respectively. The increases in Nitrate abundance have been associated with Supernovae observed in 1604 (Kepler’s supernova), 1572 (Tycho’s supernova), and in 1181. The 700 year old Vela Junior supernova might have caused the fourth spike at ~85 m. See Rood et al. and Burgess and Zuber.

With the H.E.S.S. telescopes, a clear signature of Vela Junior in VHE gamma rays was detected in only 3.2 h of exposure, with a significance of 12 sigma (Fig. 2). In the H.E.S.S. image, a shell-like morphology is clearly visible, coincident with the X-ray morphology of the remnant. The radius of the shell is almost 2 degrees, with a peak of VHE emission in the north-western (upper-left) section of the remnant. The total flux from the remnant is at the same level as the flux from the Crab Nebula, which makes it one of the strongest galactic sources of VHE gamma rays. The close resemblance between the X-ray image and the gamma-ray image is demonstrated by a correlation coefficient of 0.7, obtained by dividing the image into bins of 0.4 by 0.4 degrees. X-ray images show a compact X-ray source at the center of the remnant (AX J0851.9–4617.4, Slane et al. 2001), which could be a neutron star created in the explosion; in the H.E.S.S. image, no significant gamma-ray excess from this object is detected.

**Fig. 2:** Count map of VHE gamma rays detected with H.E.S.S. from the direction of RX J0852.0-4622, after background subtraction (preliminary). Data are smoothed with a Gaussian of width 0.1 degr. The contour lines represent the X-ray intensity measured by ASCA (Slane et al.).

The differential gamma-ray spectrum obtained with the H.E.S.S. telescopes is shown in Fig. 3, it follows a power law with a spectral index of 2.1, in good agreement with the spectra predicted for shock-wave acceleration of nuclei in the supernova shock wave. In such a scenario, the VHE gamma rays result from interactions of accelerated nuclei with the ambient interstellar medium. Assuming a local gas density of 1/cm³, the total energy in accelerated nuclei is estimated to 10⁴⁹ ergs, which would imply a conversion efficiency of a few percent between kinetic energy released in the explosion and energy of accelerated cosmic rays. With its large size and high gamma-ray flux, Vela Junior is obviously a good target for more detailed studies of the morphology of VHE gamma ray emission, and it is planned to record larger data sets for this source.

**Fig. 3:** Spectrum of gamma rays, integrated over the whole remnant (preliminary). The solid line shows a power law fit with a spectral index of 2.1.

References:

Detection of TeV gamma-ray emission from the shell-type supernova remnant RX J0852.0-4622 with H.E.S.S, H.E.S.S. collaboration, F. Aharonian et al., Astron. Astrophys. 437 (2005) L7-L10