A&A 528, A136 (2011) Astronomy DOI: 10.1051/0004-6361/201015866 & c ESO 2011 Astrophysics XMM-Newton observations of the superbubble in N 158 in the LMC M. Sasaki1, D. Breitschwerdt2, V. Baumgartner3, and F. Haberl4 1 Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: [email protected] 2 Department of Astronomy and Astrophysics, Berlin Institute of Technology, Hardenbergstr. 36, 10623 Berlin, Germany 3 Institut für Astronomie, Universität Wien, Türkenschanzstr. 17, 1180 Vienna, Austria 4 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany Received 5 October 2010 / Accepted 1 February 2011 ABSTRACT Aims. We study the diffuse X-ray emission observed in the field of view of the pulsar B 0540–69 in the Large Magellanic Cloud (LMC) by XMM-Newton. We wish to understand the nature of this soft diffuse emission, which coincides with the superbubble in the H ii region N 158, and improve our understanding of the evolution of superbubbles. Methods. We analyse the XMM-Newton spectra of the diffuse emission. Using the parameters obtained from the spectral fit, we per- form calculations of the evolution of the superbubble. The mass loss and energy input rates are based on the initial mass function (IMF) of the observed OB association inside the superbubble. Results. The analysis of the spectra shows that the soft X-ray emission arises from hot shocked gas surrounded by a thin shell of cooler, ionised gas. We show that the stellar winds alone cannot account for the energy inside the superbubble, but the energy release of 2−3 supernova explosions in the past ∼1 Myr provides a possible explanation. Conclusions. The combination of high sensitivity X-ray data, which permits a spectral analysis, and analytical models for superbub- bles can provide insight into the evolutionary state of interstellar bubbles, if the stellar content is known. Key words. shock waves – ISM: bubbles – evolution – HII regions – X-rays: ISM 1. Introduction N 158 (Henize 1956)isanHii region in these active re- gions of the LMC. It is elongated in the north-south direction Early observations in the radio and the optical have shown that and consists of a superbubble in the north and a more com- the interstellar medium (ISM) in the Milky Way mainly con- pact bright region in the south. It is known to host two OB as- sists of cool clouds (T ∼< 102 K) of neutral hydrogen embed- sociations LH 101 and LH 104 (Lucke & Hodge 1970). While ded in the warm (T 104 K) intercloud medium of partially LH 101 in the southern part of N 158 seems to power the very ionised hydrogen. Since the 1970s, observations in the ultra- bright region in Hα, LH 104 is found in the superbubble in the violet (UV) and X-rays have detected hot gas at coronal tem- northern part of N 158 and is dominated by B stars (Schild & peratures (T 105−6 K) in the ISM. The heat source of the Testor 1992), mainly consisting of a young population with an ISM are massive OB stars, which inject energy through their ra- age of 2−6Myr(Testor & Niemela 1998). Dunne et al. (2001) diation, stellar winds, and finally by supernova explosions. As analysed the ROSAT data and suggested that the X-ray emis- these processes are often correlated in space and time, super- sion seen at the position of N 158 is associated with the H ii re- bubbles with sizes of typically 100−1000 pc are created in the gion. N 158 is located near the X-ray bright pulsar (PSR) in the ISM. Therefore, supernova remnants (SNRs) and superbubbles LMC B 0540−69, which had been observed for calibration pur- are among the prime sources controlling the morphology and the poses by the X-ray Multi-Mirror Mission XMM-Newton (Jansen evolution of the ISM, and their observation is of key interest to et al. 2001; Aschenbach et al. 2000).Thefieldofviewofthe understanding the galactic matter cycle. However, they radiate European Photon Imaging Cameras (EPICs, Strüder et al. 2001; copiously in the soft X-rays below 2 keV, an energy range that is Turner et al. 2001) of these observations when performed in full difficult to study in the Milky Way because of absorption by the frame mode, covers the northern part of N 158 and allows us to Galactic disk. study the X-ray emission from the superbubble. The LMC, which is a dwarf irregular but appear to contain spiral structure, is one of the closest neighbours of our Galaxy. 2. Data Its proximity with a distance of 48 kpc (Macri et al. 2006)and modest extinction along the line of sight (average Galactic fore- The pulsar B 0540−69 in the LMC is a Crab-like pulsar with 21 −2 ground NH = 1.6 × 10 cm ) make it the ideal laboratory for a pulsar wind nebula (PWN), which has been spatially re- exploring the global structure of the ISM in a galaxy. The well- solved and studied with the Chandra X-ray Observatory (Petre known and well-studied extended emission region in the LMC et al. 2007). To study the diffuse emission in the vicinity of is the 30 Doradus region and the region south of it, which har- B 0540−69, we chose observations for which the EPICs were op- bour star-formation sites, superbubbles, and SNRs. ROSAT data erated in full frame mode. The observation IDs are 0117510201, of the superbubbles in the LMC have been studied in detail by, 0117730501, and 0125120101. The observations were all car- e.g., Chu et al. (1995)andDunne et al. (2001). ried out using the medium filter. Starting from the observational Article published by EDP Sciences A136, page 1 of 7 A&A 528, A136 (2011) -69:00:00.0 05:00.0 10:00.0 15:00.0 20:00.0 Dec 25:00.0 Fig. 1. XMM-Newton EPIC mosaic image of 30:00.0 the PSR B 0540−69 and its surroundings in true colour presentation (red: 0.3−0.8 keV, green: 0.8−1.5 keV, blue: 1.5−2.3 keV). The bright − ∼ 35:00.0 X-ray emission from PSR B 0540 69 at RA = 05h40m,Dec= –69◦20, other point sources, and the out-of-time events have been removed 40:00.0 from the data. The arc-shaped features in the south are caused by stray light from the bright X-ray source LMC X-1. The position of the 44:00.0 43:00.0 42:00.0 41:00.0 5:40:00.0 39:00.0 38:00.0 37:00.0 36:00.0 superbubble in the H ii region N 158 is shown RA with a dashed line. Table 1. XMM-Newton data used for the analysis. relatively bright extended region in the south of the PSR coin- cides well with a superbubble in the H ii region N 158 (Henize Obs. ID EPIC Start Date Effective Exposure 1956), which contains the OB association LH 104 (Lucke & [ks] Hodge 1970). To study the spectral properties of the diffuse emission, we selected two regions: region 1, which covers the 01175102 PN 2000-02-11 8.3 brighter spot in the east of the PSR, and a region that covers the 01175102 MOS1,2 3.5 ii 01177305 PN 2000-02-17 8.3 superbubble in the H region N 158. The regions are shown in 01177305 MOS1,2 9.8 the left panel in Fig. 2. The PSR and the PWN around it have an 01251201 PN 2000-05-26 29. extent of about 1 . They were completely removed from the data. 01251201 MOS1,2 27. The soft, extended emission east to the PSR is not directly con- nected to the PWN and has, as we see in Sect. 2.2.4, a perfectly Notes. All the analysed data were obtained in full frame mode using the thermal spectrum. We therefore assume that it is not related to medium filter. the PWN. data files (ODFs), the data are processed with the XMM- 2.2. EPIC spectra Newton Science Analysis System (XMMSAS) version 10.0.0. For EPIC PN, only single and double pattern events are used, For the spectral analysis of an extended diffuse emission the con- whereas for the MOS1 and 2, singles to quadruples are selected. tribution of the background is significant. Since the emission fills The exposure times that we obtain after filtering out the time a large part of the detector, we are unable to extract a local back- intervals with background flares are listed in Table 1. ground close to the source emission. We note that as the effective area of the mirrors depends on the off-axis angle, photons are subject to vignetting while particles are not. The high-energy 2.1. EPIC image particles that interact with the material surrounding the detec- After filtering out the background flares, we created a mosaic tor, however, produce fluorescence, which varies with position image out of the full frame mode data of EPIC PN, MOS1, and on the detector, especially for the PN detector. In addition, the MOS2 for all three observations (Fig. 1). To enhance the not-so- spectral response depends on the position on the detector. A de- bright diffuse emission, we filtered out all point sources found tailed description of the XMM-Newton background is given by using a source detection routine as well as the so-called out- Read & Ponman (2003)andCarter & Read (2007), and a com- ff of-time events of EPIC PN.