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Astron. Astrophys. 354, L57-L61 (2000)

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1. Introduction

Supernova remnants (SNRs) are currently believed to be a major source of galactic cosmic rays (GCRs) from the arguments of energetics, shock acceleration mechanisms (Blandford & Eichler 1987, Jones & Ellison 1991), and the elemental abundances in the source of GCR (Yanagita et al. 1990, Yanagita & Nomoto 1999). EGRET observations suggest that the acceleration sites of GCRs at GeV energies are SNRs (Esposito et al. 1996). However, direct evidence for the SN origin of GCRs at TeV energies is scarce (e.g. Koyama et al. 1995, Allen et al. 1997, Buckley et al. 1998). Arguably the best evidence for the existence of relativistic electrons with energies around 100 TeV is the CANGAROO observation of TeV gamma-rays from the northeast rim of SN1006, which coincides with the region of maximum flux in the 2-10 keV band of the ASCA data (Tanimori et al. 1998b). This TeV gamma-ray emission was explained as arising from 2.7 K Cosmic Microwave Background Radiation (CMBR) photons being Inverse Compton (IC) up-scattered by electrons with energies up to [FORMULA] 100 TeV and allowed, together with the observation of non-thermal radio and X-ray emission, the estimation of the physical parameters of the remnant, such as the magnetic field strength (Pohl 1996, Mastichiadis 1996, Mastichiadis & de Jager 1996, Yoshida & Yanagita 1997, Naito et al. 1999).

The shell type SNR RX J1713.7-3946 was discovered in the ROSAT All-Sky Survey (Pfeffermann & Aschenbach 1996). The remnant has a slightly elliptical shape with a maximum extent of [FORMULA]. The 0.1-2.4 keV X-ray flux from the whole remnant is [FORMULA] 4.4 [FORMULA] 10-10 erg cm-2 s-1 ranking it among the brightest galactic supernova remnants. Subsequent observations of this remnant by the ASCA Galactic Plane Survey revealed strong non-thermal hard X-ray emission from the northwest (NW) rim of the remnant that is three times brighter than that from SN1006 (Koyama et al. 1997). The non-thermal emission from the NW rim dominates the X-ray emission from RX J1713.7-3946, and the SNR X-ray emission as a whole is dominated by non-thermal emission (Slane et al. 1999, Tomida 1999). It is notable that the observed emission region of hard X-rays extends over an area [FORMULA] in diameter. Slane et al. (1999) carried out 843 MHz radio observations using the Molonglo Observatory Synthesis Telescope, and discovered faint emission which extends along most of the SNR perimeter, with the most distinct emission from the region bright in X-rays. Slane et al. (1999) suggest the distance to RX J1713.7-3946 is about 6 kpc based upon the observation of CO emission from molecular clouds which are likely to be associated with the remnant.

The dominance of non-thermal emission from the shell is reminiscent of SN1006. Koyama et al. (1997) proposed from the global similarity of the new remnant to SN1006 in its shell type morphology, the non-thermal nature of the X-ray emission, and apparent lack of central engine like a pulsar, that RX J1713.7-3946 is the second example, after SN1006, of synchrotron X-ray radiation from a shell type SNR. These findings from X-ray observations would suggest that TeV gamma-ray emission could be expected, as observed in SN1006, from regions in the remnant extended over an area larger than the point spread function of a typical imaging telescope ([FORMULA].2).

Both SN1006 and RX J1713.7-3946 show notably lower radio flux densities and relatively lower matter densities in their ambient space when compared with those for the other shell type SNRs (Green 1998) for which the Whipple group (Buckley et al. 1998) and CANGAROO group (Rowell et al. 1999) have reported upper limits to the TeV gamma-ray emission. These characteristics might be related to the reason why TeV gamma-rays have been detected only for SN1006 and not from other shell type SNRs: the lower radio flux may indicate a weaker magnetic field which may result in a higher electron energies due to reduced synchrotron losses. In addition, the lower matter density would suppress the production of [FORMULA] decay gamma-rays. An observation of TeV gamma-rays from RX J1713.7-3946 would provide not only further direct evidence for the existence of very high energy electrons accelerated in the remnant but also other important information on some physical parameters such as the strength of the magnetic field which are relevant to the particle acceleration phenomena occurring in the remnant, and would also help clarify the reason why TeV gamma-rays have until now been detected only from SN1006.

With the above motivation, we have observed RX J1713.7-3946 with the CANGAROO imaging TeV gamma-ray telescope in 1998. Here we report the result of these observations.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000