An X-ray investigation of the NGC 346 field in the SMC (1) : the LBV HD 5980 and the NGC 346 cluster Y. Naz´e1,8, J.M. Hartwell2, I.R. Stevens2, M. F. Corcoran3, Y.-H. Chu4, G. Koenigsberger5, A.F.J. Moffat6, V.S. Niemela7 ABSTRACT We present results from a Chandra observation of the NGC 346 cluster. This cluster contains numerous massive stars and is responsible for the ionization of N66, the most luminous H ii region and the largest star formation region in the SMC. In this first paper, we will focus on the characteristics of the main objects of the field. The NGC 346 cluster itself shows only relatively unabs 34 −1 faint X-ray emission (with LX ∼ 1.5 × 10 erg s ), tightly correlated with the core of the cluster. In the field also lies HD 5980, a LBV star in a binary (or possibly a triple system) that is detected for the first time at X-ray energies. The star is X-ray bright, with an unabsorbed unabs 34 −1 luminosity of LX ∼ 1.7 × 10 erg s , but needs to be monitored further to investigate its X-ray variability over a complete 19 d orbital cycle. The high X-ray luminosity may be associated either with colliding winds in the binary system or with the 1994 eruption. HD 5980 is surrounded by a region of diffuse X-ray emission, which is a supernova remnant. While it may be only a chance alignment with HD 5980, such a spatial coincidence may indicate that the remnant is indeed related to this peculiar massive star. Subject headings: (galaxies:) Magellanic Clouds–X-rays: individual (NGC 346, HD 5980) 1. Introduction 1Institut d’Astrophysique et de G´eophysique, Universit´e de Li`ege, All´ee du 6 Aoˆut 17, Bat. B5c, B 4000 - Li`ege The Small Magellanic Cloud (SMC) is an irreg- (Belgium); [email protected] ular galaxy at a distance of 59 kpc (Mathewson, 2 School of Physics & Astronomy, University of Ford, & Visvanathan 1986) that forms a pair Birmingham, Edgbaston, Birmingham B15 2TT (UK); [email protected], [email protected] with the Large Magellanic Cloud (LMC). Both 3 are satellites of our own Galaxy. The interstellar arXiv:astro-ph/0208289v1 15 Aug 2002 Universities Space Research Association, High En- ergy Astrophysics Science Archive Research Center, God- extinction towards the Magellanic Clouds is low, dard Space Flight Center, Greenbelt, MD 20771; corco- allowing studies of the X-ray sources to be under- [email protected] taken with clarity. 4Astronomy Department, University of Illinois, 1002 W. Green Street, Urbana, IL 61801; [email protected] 5Instituto de Astronom´ıa, Universidad Nacional Previous X-ray observations of the SMC have Aut´onoma de Mexico, Apdo. Postal 70-264, 04510, Mexico been made with the Einstein Observatory, ASCA D.F. (Mexico); [email protected] and ROSAT. These observations included surveys 6 D´epartement de physique, Universit´e de Montreal, of the point source population (Kahabka et al. C.P. 6128, Succ. Centre-Ville, Montreal, QC, H3C 3J7 (Canada); moff[email protected] 1999; Schmidtke et al. 1999; Haberl et al. 2000; 7Facultad de Ciencias Astron´omicas y Geof´ısicas, Sasaki, Haberl, & Pietsch 2000; Yokogawa et al. Universidad Nacional de la Plata, Paseo del 2000) and in particular studies of the properties Bosque S/N, B19000FWA La Plata (Argentina); of the X-ray binary population (Haberl & Sasaki [email protected] 8 2000). The recent launch of Chandra, however, Research Fellow F.N.R.S. provides an opportunity to study the SMC with a far greater sensitivity and spatial resolution than 1 ever before. 2. The Observations and Data Analysis 2.1. X-ray Observations We have obtained a Chandra observation of the giant H ii region N66 (Henize 1956), the NGC 346 was observed with Chandra for the largest star formation region in the SMC, in XMEGA9 consortium on 2001 May 15–16 for order to study the young cluster NGC 346 and 100 ks (98.671 ks effective, ObsID = 1881, its interaction with the surrounding interstellar JD∼2452045.2d). The data were recorded by medium. This cluster contains numerous massive the ACIS-I0, I1, I2, I3, S2 and S3 CCD chips stars (Massey, Parker & Garmany 1989), of spec- maintained at a temperature of −120◦C. The data tral types as early as O2 (Walborn et al. 2002). were taken with a frame time of 3.241s in the faint The large number of massive stars is not the only mode. The exposure was centered on the cluster, feature of interest in this field. On the outskirts of with HD 5980 lying 1.9′ to the north-east of the NGC 346 lies the remarkable star HD 5980, which aimpoint on ACIS-I3. Our faintest sources have underwent a Luminous Blue Variable (LBV)-type fluxes of about 2 × 1032 erg s−1 (see paper II), eruption in 1994. This massive binary (or triple?) assuming a distance of 59 kpc for the SMC. system has been monitored and analysed for its varying spectral and photometric properties in We excluded bad pixels from the analysis, us- visible and UV wavelengths since the early 80’s ing the customary bad-pixel file provided by the (Koenigsberger et al. 2000; Sterken & Breysacher Chandra X-ray Center (CXC) for this particular 1997, and references therein). Previous X-ray ob- observation. We have searched the data for back- servations of NGC 346 (Inoue, Koyama, & Tanaka ground flares, which are known to affect Chandra 1983; Haberl et al. 2000) detected a bright ex- data, by examining the lightcurve of the total tended source around HD5980 which has been count rate, but no flares were found. Event Pulse attributed to a supernova remnant (SNR); how- Invariant (PI) values and photon energies were ever, the crude instrumental resolution prohibited determined using the FITS Embedded File (FEF) unambiguous detections of a point source associ- acisD2000-01-29fef piN0001.fits. ated with HD5980. In addition to HD5980, the young massive stars of NGC 346 and their inter- For long exposures, removing the afterglow acting winds are likely to produce X-ray emission, events can adversely affect the science analysis but have not been detected previously. The sen- (underestimation of the fluxes, alteration of the sitive, high-resolution Chandra observation of the 10 spectra and so on) . We thus computed a new NGC 346 field thus provides an excellent oppor- level 2 events file by filtering the level 1 file and tunity to study a variety of phenomena involving keeping the events with ASCA grades of 0, 2, 3, emission at X-ray energies. 4 and 6, but without applying a status=0 filter. Throughout this paper, we will use this new file In this paper, we will describe in § 2 the ob- for all scientific analysis. servations used in this study, then discuss the available data on NGC346, HD5980, and the ex- Further analysis was performed using the CIAO tended emission in § 3, 4, and 5, respectively. v2.1.2 software provided by the CXC and also Finally, conclusions are given in § 6. The second with the FTOOLS tasks. The spectra were anal- part of this work (Naz´eet al., paper II) will de- ysed and fitted within XSPEC v11.0.1 (Arnaud scribe the properties of the other sources detected 1996). In Fig. 1, we show a 3 color image of the in the field. Chandra data of the NGC 346 cluster, HD 5980 and its surroundings. We will discuss the features of this image further in later sections. 9http://lheawww.gsfc.nasa.gov/users/corcoran/xmega/xmega.html 10see caveat on http://asc.harvard.edu/ciao/caveats/acis cray.html 2 We have investigated problems due to Charge NGC 346 cluster has not been detected in X- Transfer Inefficiency (CTI). To remove these CTI rays until this Chandra observation. Even though effects, we used the algorithm of Townsley et al. NGC 346 lies near the joint of the four ACIS-I (2000). Checking several sources on different chips CCD chips and part of the cluster is lost in the and/or at different positions on each CCD, we con- gaps between CCDs, the X-ray emission from the cluded that the impact of the CTI was small and core of NGC 346 is unambiguously detected in the that it did not change significantly the spectral corner of the ACIS-I3 chip. The X-ray emission parameters found by using non-corrected data. appears extended with multiple peaks correspond- When a difference was apparently found, we dis- ing to the bright blue stars MPG 396, 417, 435, covered that the best fits found on corrected and 470, and 476(Fig. 3). As only ∼300 counts are de- non-corrected data gave similar χ2. As the work tected and the X-ray peaks are not well-resolved, of Townsley et al. (2000)is not yet an official we will analyze only the overall emission from the CXC product11, we chose to present here only the cluster. results of the non-corrected data. The total count rate of this emission in the 0.3 − 10.0 keV band is 3.04 ± 0.36 × 10−3 cnts s−1. 2.2. HST WFPC2 Images The spectrum of the NGC346 cluster was ex- ′′ ′′ Two HST WFPC2 observations of the N66 tracted from a square aperture of ∼45 ×45 with / NGC 346 region in Hα are available from the a carefully chosen background region, as close as STScI archives. They were taken on 1998 Sept.