The HI absorption distance of HESS J1943+213 favours its extragalactic nature

arXiv:1112.2655v3 [astro-ph.GA] 26 Jan 2012 Astronomy & Astrophysics manuscript no. ms c⃝ESO 2018 November 10, 2018 The HI absorption distance of HESS J1943+213 favours its extragalactic nature (Research Note) D.A. Leahy1 and W.W. Tian2 1 Department of Physics & Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada; 2 National Astronomical Observatories, CAS, Beijing 100012, China. tww@bao.ac.cn Received 2011/Accepted 25th, Jan. 2012 ABSTRACT Context. The H.E.S.S. collaboration (Abramowski et al. 2011) dicovered a new TeV point-like source HESS J1943+213 in the Galactic plane and suggested three possible low-energy-band counterparts: a γ-ray binary, a pulsar wind nebula (PWN), or a BL Lacertae object. Aims. We measure the distance to the radio counterpart G57.76-1.29 of HESS J1943+213. Methods. We analyze Very Large Array observations to obtain a reliable HI absorption spectrum. Results. The resulting distance limit is ≥16 kpc. This distance strongly supports that HESS J1943+213 is an extragalactic source, consistent with the preferred counterpart of the HESS collaboration Conclusions. . Key words. Radio continuum: general – Radio lines: ISM – ISM: supernova remnants 1. Introduction Very-high-energy (VHE) γ-ray observations shed a light on the question of origin and acceleration of cosmic rays (Caprioli 2011). A multi-wavelength approach helps to solve the ques- tion, especially by seeking VHE sources’ counterparts in lower energy bands. More than 110 VHE γ-ray sources have been detected by ground-based high energy telescopes recently (http://www.mppmu.mpg.de/∼rwagner/sources/). About 30 of them have no counterparts identifed so far. The identification of counterparts plays a key role in understanding the nature of the VHE sources and in distinguishing different emission mech- anisms for TeV γ-rays (Halpern & Gotthelf 2010, Misanovic et al. 2011, Tian et al. 2008). HESS J1943+213 was recently discovered by the H.E.S.S. collaboration (Abramowski et al. 2011) and was suggested to have three possible low-energy-band counterparts: a γ-ray bi- nary, a pulsar wind nebula (PWN), or a BL Lacertae object. HESS J1943+213 has a compact radio counterpart G57.76-1.29 which has been observed in the VGPS (also a NVSS source, NVSS J194356+211826, see Condon et al. 1998). We analyze 1.4 GHz continuum and HI-line observations of G57.76-1.29 from Very Large Array (VLA) Galactic Plane Survey (VGPS, Stil et al. 2006). Based on our well-tested meth- ods to build HI absorption spectrum against a continuum back- ground radio source, we extract an HI absorption spectrum to- wards G57.76-1.29/HESS J1943+213. We report our results in this research note. Fig. 1. 1.4-GHz VLA image of HESS J1943+213 (contours 7, 15, 20 K) from the VGPS data. 2. Results and discussion Fig. 1 shows the 1420 MHz continuum map in a 0.1◦field con- taining G57.76-1.29. The VGPS data has an angular resolution of 1′ and an rms noise inthe HI Line of 2 K (Stil et al. 2006). It is a good database to study Galactic supernova remnants (SNRs) which usually extend from a few arcmin to ∼1 degree in size. We have previously used the data to study some interesting SNRs (see a review paper by Leahy & Tian 2010) and have also devel- Send offprint requests to: tww@bao.ac.cn 1 Leahy & Tian: HESS J1943+213 is an Extragalactic Source (RN) 0 20 40 60 80 TB (K) G57.76-1.29 background -100 -50 0 50 100 Velocity (km/s) 0 0.5 1 exp(-Tau) absorption spectrum Fig. 2. Top panel: HI emission spectrum towards J1943+2118. Bottom panel: HI absorption spectrum. The standard deviation of exp(-Tau) is 0.14, taken from the velocity range of 60 to 100 km/s (the mean value is 1.04) Fig. 3. The HI channel map at -67 km s−1 with 1420 MHz con- tinuum contours (the same as in Figure 1). oped methods to estimate kinemtic distances to them by analyz- ing 21cm continuum, HI-line and CO-line data (Tian, Leahy & Wang 2007; Leahy & Tian 2008). Fig. 2 displays the HI emission spectra of source and back- ground regions (top panel) and the absorption spectrum (bot- tom panel) towards G57.76-1.29. The background region was taken as directly surrounding the source region. Pixels were as- signed to source or background regions depending on whether they have 1420 MHz continuum brightness above or below a specified threshold, respectively. We used the program mean- lev in the DRAO (Dominion Radio Astrophysical Observatory) Export Software Package to calculate the source and background spectra. Then we used our methods, which take into account the background continuum radiation, to calculate optical depth in the form exp(−τ). The resulting HI absorption spectrum shows absorption at all Galactic velocities. This includes absorption at positive velocities, from the interstellar medium interior to the solar circle, up to the tangient point velocity. Also we see ab- sorption for negative velocities above ∼-67 km/s, which corre- sponds approximately to the outer edge of the Galaxy. The low- est velocity abs

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