View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Servicio de Difusión de la Creación Intelectual Mon. Not. R. Astron. Soc. 346, 704–718 (2003) XMM–Newton X-ray observations of the Carina nebula ? J. F. Albacete Colombo,1 M. M´endez2 and N. I. Morrell1†‡§ 1Facultad de Ciencias Astronomicas´ y Geof´ısicas, Universidad Nacional de La Plata Paseo del Bosque S/N, 1900 La Plata, Argentina 2SRON, National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands Accepted 2003 August 27. Received 2003 May 28; in original form 2002 December 3 ABSTRACT We use new XMM–Newton observations to perform a detailed X-ray analysis of the Carina nebula region in the 0.3–12 keV energy range. Our source detection yields 80 discrete X- ray sources, from which about 20 per cent seem not to have optical counterparts. To get an idea of the energy spectrum of these sources, we construct an X-ray colour–colour diagram using the energy bands 0.3–2, 2–4.5 and 4.5–12 keV. We analyse the spectra of the most intense X-ray sources associated with early-type stars, including the luminous blue variable η Carina and WR25. We show that the X-ray emission from these sources is well fitted by multitemperature model spectra. We detect surprisingly intense X-ray emission at energies above 4 keV for some of the observed early-type stars, especially from CPD-59 2629 (Tr16- 22) which presents particularly hard X-ray emission. We detect intense soft X-ray emission, below <2 keV, in HDE 303311, which presents an X-ray excess of about 100 times higher than has been observed in other O5V stars. We use these data to construct the Lx/Lbol relation for the 0.3–12 and 3.0–12 keV energy ranges, for all the observed O-type stars, plus η Carina and WR25. Most of the bright stars seem to agree with low metallicity spectral models. The / ≈ 7.5 × Lx Lbol ratio for O-type stars in the 0.3–12 keV range is well fitted by a constant 6.04.8 −7 −7 10 , in fair agreement with the canonical expression Lx/Lbol ∼ 2 × 10 formerly estimated for the 0.3–2.4 keV energy band. In contrast, the Lx/Lbol relation for the 3.0–12 keV range presents a strong deviation from the canonical relation, with a high dispersion of about four orders of magnitude. We also detect intrinsic X-ray time variability in seven sources, over the time-scale of about 50 h covered by the observations. This includes an X-ray flare of about 2-h duration detected in DETWC Tr16 J104429.2−594143, a source probably not physically associated with the Carina nebula. We discuss the different underlying physical mechanisms that can be responsible for the X-ray emission from early-type stars. Key words: H II regions – ISM: individual: NGC 3372 – X-rays: ISM – X-rays: stars. 32 −1 28 −1 1 INTRODUCTION 10 erg s for early O stars, to 10 erg s for late B stars, and broadly following the relation log(Lx/Lbol) ≈−7or−8.5, respec- The Carina nebula region (NGC 3372) is one of the most interest- tively (Chlebowski, Harnden & Sciortino 1989). ing massive star-forming regions in the Galaxy. It harbours the open The first X-ray observation of the Carina nebula made by the clusters Trumpler 14 and Trumpler 16 which contain dozens of mas- Einstein observatory in the 0.2–3.5 keV energy range (Seward et al. sive O-type stars, three Wolf–Rayet stars, and the still controversial 1979), and later ROSAT Position Sensitive Proportional Counter luminous blue variable (LBV) η Carina. (PSPC) and High Resolution Imager (HRI) observations in the band Surveys made with the Einstein observatory two decades ago 0.2–2.4 keV (Corcoran et al. 1995), allowed the detection of large- showed that essentially all OB stars are X-ray emitters in the 0.2– scale diffuse emission in the Carina nebula. Most of this emission 7 3 keV band (Seward et al. 1979), with luminosities ranging from comes from hot gas at a temperature of about 10 K and ne ∼ 0.1– 1cm−3. The energy to heat this gas probably comes from the strong stellar winds of several early-type stars embedded in the nebula, ? Fellow of CIC, Prov. de Buenos Aires, Argentina. instead of resolved X-ray sources (Seward & Chlebowski 1982). †E-mail: [email protected] Subsequently, in the context of the international X-Mega cam- ‡Member of Carrera del Investigador Cient´ıfico, CONICET, Argentina. paign (Corcoran et al. 1999), the Carina nebula has been observed §Visiting Scientist, Carnegie Institution of Washington, Las Campanas Ob- with ROSAT between 1997 December and 1998 January, for a total servatory, Chile. exposure time of about 47 ks in the soft and medium energy bands C 2003 RAS XMM–Newton X-ray observations of the Carina nebula 705 Table 1. Observing log. The observing revolutions 115 and 116 are identified by the numbers 0112580601 and 0112580701. PFF and PPSF refer to Prime-Full-Frame and Prime-Partial-Small-Frame observing modes, respectively. EPIC Revolution Start time Exposure Blocking Observing camera no yr/m/d at h:m:s (UTC) time (s) filter mode MOS-1 115 2000/07/26 at 05:07:47 33598 Thick PFF MOS-2 115 2000/07/26 at 06:00:45 30497 Thick PPSF PN 115 2000/07/26 at 05:48:51 31409 Thick PFF MOS-1 116 2000/07/27 at 23:57:54 10867 Thick PFF MOS-2 116 2000/07/28 at 00:50:51 7998 Thick PPSF PN 116 2000/07/28 at 00:38:57 9216 Thick FF (0.2–2.4 keV). Most of the detected X-ray point-like sources were reducing uncertainties in parameters derived from the spectral fits. associated with massive early-type stars, which was the starting Spectra were analysed with XSPEC version 11.2 (Arnaud 1996). point of a coordinated optical and X-ray survey of several O-type stars in the region (Albacete Colombo et al. 2001; Morrell et al. 2001; Rauw et al. 2001; Albacete Colombo et al. 2002). 3 X-RAY IMAGES Recently, the Carina nebula region has been observed with the The X-ray image obtained by XMM–Newton gives us the possibility XMM–Newton X-ray telescope. The large effective area of the in- of obtaining the deepest X-ray source detection ever carried out in struments on board XMM–Newton provides the deepest observa- the 0.3–12 keV energy range, in the Carina nebula. In Fig. 1 we tions, at moderate spectral and spatial resolution, of the Carina neb- show a region of ≈30 arcmin around η Carina, comprising the open ula obtained to date, over the unprecedented broad energy band of clusters Trumpler 14 and Trumpler 16, as observed with the PN and 0.3–12 keV. We used these data to make the deepest X-ray source MOS-1 (full frame, exposure corrected image). detection, identification and analysis ever carried out in this region, Because of the strong noise affecting the PN and MOS-1 detectors which we present in this paper. In order to better understand the below 0.3 keV,and the uncertain calibration of the EPIC instruments underlying mechanisms and stellar environment responsible for the at low energies, we ignored all energy channels below 0.3 keV. It / large differences observed between individual values of Lx Lbol, should also be noted that the sensitivity of XMM–Newton above we reconstruct this relation for many O-type stars detected as X-ray 12 keV drops to less than 25 per cent of that at 10 keV, and so sources in the region. we decided to limit the high-energy range to a maximum value of 12 keV. To correct for differences in exposure and vignetting of different 2 X-RAY OBSERVATIONS parts of the images we divided each image by an exposure map using 2 The Carina nebula field, centred at the position of η Carina (α2000 = the EEXPMAP routine. We did this in three different energy bands, h m s ◦ 0 00 10 45 03.4,δ2000 =−59 41 03.1), was observed with the XMM– the soft (0.3–2 keV), medium (2–4.5 keV) and hard (4.5–12 keV) Newton satellite in two separate revolutions (115 and 116) between bands. We selected the bands such that the instrument effective areas 2000 July 26 and 28. The observations were not affected by high are approximately equal in each band. flaring background fluxes, giving a good time interval (≈44 ks) of The colour image in Fig. 1 is the combination of the soft (red), useful exposure time (Table 1). The observatory consists of three medium (green) and hard (blue) energy bands. This image is useful X-ray telescopes, as described by Jansen et al. (2001). Data were to obtain a rough estimate of the hardness of the X-ray radiation of acquired with the European Photon Imaging Camera (EPIC), which each detected source, which in turn provides an idea of the physical provides simultaneous CCD imaging spectroscopy from the PN mechanism responsible for the X-ray emission. The fact that some camera (Struder¨ et al. 2001) and two identical MOS cameras, MOS- sources are brighter in the soft bands, whereas others are brighter 1 and MOS-2 (Turner et al. 2001)). Full-frame mode and the thick in the hard bands, suggested to us the possibility of carrying out blocking filter were used in order to reject optical light.