A&A 420, 467–474 (2004) Astronomy DOI: 10.1051/0004-6361:20034126 & c ESO 2004 Astrophysics Jet emission in NGC 1052 at radio, optical, and X-ray frequencies M. Kadler1,J.Kerp2,E.Ros1, H. Falcke1,, R. W. Pogge3, and J. A. Zensus1 1 Max-Planck-Institut f¨ur Radioastronomie Bonn, Auf dem H¨ugel 69, 53121 Bonn, Germany 2 Radioastronomisches Institut, Universit¨at Bonn, Auf dem H¨ugel 71, 53121 Bonn, Germany 3 Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210-1173, USA Received 28 July 2003 / Accepted 19 February 2004 Abstract. We present a combined radio, optical, and X-ray study of the nearby LINER galaxy NGC 1052. Data from a short (2.3 ks) CHANDRA observation of NGC 1052 reveal the presence of various jet-related X-ray emitting regions, a bright compact core and unresolved knots in the jet structure as well as an extended emitting region inside the galaxy well aligned with the radio synchrotron jet-emission. The spectrum of the extended X-ray emission can best be fitted with a thermal model with kT = (0.4−0.5) keV, while the compact core exhibits a very flat spectrum, best approximated by an absorbed power- 22 −2 law with NH = (0.6−0.8) × 10 cm . We compare the radio structure to an optical “structure map” from a Hubble Space Telescope (HST) observation and find a good positional correlation between the radio jet and the optical emission cone. Bright, compact knots in the jet structure are visible in all three frequency bands whose spectrum is inconsistent with synchrotron emission. Key words. galaxies: individual: NGC 1052 – galaxies: individual: B 0238-084 – galaxies: active – galaxies: jets 1. Introduction X-ray emission on the same scales as accomplished by con- nected radio interferometers, e.g., MERLIN (Multi-Element 1 NGC 1052 is a nearby elliptical galaxy which harbors a low- Radio-Linked Interferometer Network). Disentangling the con- luminosity active galactic nucleus (LLAGN) in its very center tributions of compact (i.e., <1 arcsec) nuclear and extended = × 40 −1 (L1−100GHz 4.4 10 erg s ; Wrobel 1984). It hosts a two- (i.e., >1 arcsec) X-ray emitting regions to the total amount of sided radio jet emanating from the nucleus and reaching out to X-ray emission of NGC 1052 can serve as an important tool kiloparsec-scales which is, however, still fully enclosed within to study the interaction between the radio jet plasma and the the stellar body of the optical galaxy. In the optical, the spec- ambient interstellar medium. trum of NGC 1052 is characterized by strong forbidden lines from low-ionization states which has made NGC 1052 the pro- NGC 1052 has been observed by all major X-ray mis- totypical LINER (low-ionization nuclear emission line region; sions of the pre–CHANDRA era, like Einstein (Mc Dowell Heckman 1980) galaxy. As for LINERs in general, it has long 1994), ASCA and ROSAT (Weaver et al. 1999), and Beppo Sax been argued whether these low-ionization lines in NGC 1052 (Guainazzi & Antonelli 1999). For these X-ray missions, are excited by a central photo-ionizing source (e.g., Gabel NGC 1052 appeared as a point-like X-ray source. The X-ray et al. 2000) or if shock heating is the dominant mechanism spectrum of NGC 1052 is extremely flat, a finding that led to (e.g., Sugai & Malkan 2000). While there is overwhelming ev- the proposal of an advection dominated accretion flow (ADAF) idence for the presence of an active galactic nucleus (AGN) in as the origin of the observed X-ray emission (Guainazzi NGC 1052, the role of shocks in this galaxy is still unclear. The et al. 2000). To model the AGN X-ray spectrum above E 23 −2 improved angular resolution in X-rays offered by CHANDRA 2 keV absorbing column densities in excess of 10 cm makes it possible for the first time to image the distribution of have been discussed, supporting the idea of a high density obscuring torus, comparable to column densities found in other AGN (e.g., Malizia et al. 1997; Risaliti et al. 2002). Send offprint requests to:M.Kadler, e-mail: [email protected] Independent evidence for the existence of an obscuring torus at Present Adress: Radio Observatory, ASTRON, PO Box 2, 7990 the center of NGC 1052 is obtained from Very Long Baseline AA Dwingeloo, The Netherlands. Interferometry (VLBI) observations in the radio regime: on 1 D = 22.6 Mpc (assuming z = 0.0049, Knapp et al. 1978, and parsec-scales NGC 1052 exhibits a twin jet structure with a −1 −1 H0 = 65 km s Mpc ). At this distance 1 arcsec corresponds prominent emission gap between both jets (see e.g., Kadler to ∼110 pc. et al. 2003b). The inner part of the western jet shows a Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20034126 468 M. Kadler et al.: Jet emission in NGC 1052 at radio, optical, and X-ray frequencies strongly inverted radio spectrum, which was first discovered DEC by Kellermann et al. (1999) (see also Kameno et al. 2001). 0.2−1.0 keV The cm-wavelength spectral index in this central region is −8:14 larger than 2.5, exceeding the theoretical limit for synchrotron self-absorption. Combined studies of the core region of NGC 1052 in the radio and X-ray regime are of essential importance for con- straining the physical properties of the parsec-scale radio jet −8:15 and the obscuring torus as well as to determine the nature of the nuclear X-ray emission. In this paper we present a com- bined radio, optical, and X-ray study of the jet-related emission in NGC 1052 on arcsecond scales. In particular, we focus on the soft X-ray excess in the source-spectrum below E = 2keV. −8:16 This soft component was identified first by Weaver et al. (1999) based on ROSAT PSPC data. CHANDRAs superior angular res- olution makes it possible to present evidence that this soft ex- cess emission is associated with the well known radio jet. In Sect. 2 we present the CHANDRA, MERLIN, and 2h41m09s 2h41m06s 2h41m03s 2h41m00s HST data as well as their reduction. In Sect. 3 we discuss the RA arcsecond-scale morphology of NGC 1052 in the radio, opti- Fig. 1. Raw CHANDRA image of NGC 1052 showing the spatial dis- cal, and X-ray regime and the correlations between the dif- tribution of photon events in the energy range (0.2−1.0) keV. ferent wave bands. In Sect. 4 we derive models for the nu- clear and extended X-ray emission and Sect. 5 summarizes our conclusions. the energy range (0.2−1) keV. Even from the small number of counts during this snapshot observation the distribution of soft ff 2. Observations and data reduction X-rays clearly di ers from a point spread function. Using the program which is part of the software we also 2.1. CHANDRA data produced an adaptively smoothed map of the field of interest (see Fig. 2) for which we reduced the angular resolution of the CHANDRA observed NGC 1052 on August 29/30, 20002. data from 0.5 arcsec to 4 arcsec and set a minimum significance During the 2342 s observation, the Advanced CCD Imaging threshold to 3σ. Spectrometer (ACIS) Chip S3 was in the focus of the High Resolution Mirror Assembly (HRMA). The ACIS-S3 detec- tor offers high angular resolution as well as information on 2.2. MERLIN data the X-ray source spectrum because of its intrinsic energy res- A MERLIN observation of NGC 1052 at 1.4 GHz was per- olution and has a higher sensitivity in the soft X-ray energy formed on November 22, 1995. The data from this experi- regime below E < 1 keV then the front-side illuminated detec- ment have been obtained from the public archive3 and ana- tors of CHANDRA. lyzed applying standard methods using the program . The nucleus of NGC 1052 is the brightest X-ray source − Two different maps of the brightness distribution of NGC 1052 within the field of interest. The count rate of 0.12 cts s 1 is suf- at 1.4 GHz were produced. First, a strong (u,v)-taper was used ficiently high to affect the measured AGN X-ray spectrum by to map the extended emission resulting in a restoring beam the pile-up effect. The pile-up effect changes the shape of the of (1.5 × 1.1)arcsec at a position angle (PA) of −31◦. Second, a measured X-ray spectrum as well as the measured count rate, pure naturally weighted image was produced yielding a restor- because during a single read-out period of the chip multiple ing beam of (0.5 × 0.3) arcsec at a PA of 27◦. X-ray photons may be detected within a single pixel. Due to insufficient time resolution their combined signal is registered as a single photon event. The presented CHANDRA observa- 2.3. HST data tion was performed using the standard timed exposure mode. Depending on the detailed shape of the AGN X-ray spectrum For this paper, we re-examined the archival HST data up to 27% of the available data can be affected by the pile-up of NGC 1052 previously published by Pogge et al. (2000). effect. In principle, this introduces a bias which results in spec- The archival data are three narrow-band F658N filter im- tral hardening and mimics lower count rates.