A&A 616, A36 (2018) Astronomy https://doi.org/10.1051/0004-6361/201833086 & c ESO 2018 Astrophysics EXTraS discovery of a peculiar flaring X-ray source in the Galactic globular cluster NGC 6540 Sandro Mereghetti1, Andrea De Luca1,4, David Salvetti1, Andrea Belfiore1, Martino Marelli1,3, Adamantia Paizis1, Michela Rigoselli1,2, Ruben Salvaterra1, Lara Sidoli1, and Andrea Tiengo3,4,1 1 INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via E. Bassini 15, 20133 Milano, Italy e-mail: [email protected] 2 Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italy 3 IUSS, Piazza della Vittoria 15, 27100 Pavia, Italy 4 INFN, Sezione di Pavia, Via A. Bassi 6, 27100 Pavia, Italy Received 23 March 2018 / Accepted 1 June 2018 ABSTRACT We report the discovery of a flaring X-ray source 700 from the center of the globular cluster NGC 6540 obtained during the EXTraS project devoted to a systematic search for variability in archival data of the XMM–Newton satellite. The source had a quiescent X-ray luminosity on the order of ∼1032 erg s−1 in the 0.5–10 keV range (for a distance of NGC 6540 of 4 kpc) and showed a flare lasting about 300 s. During the flare, the X-ray luminosity increased by more than a factor 40, with a total emitted energy of ∼1036 erg. These properties, as well as Hubble Space Telescope photometry of the possible optical counterparts, suggest the identification with a chromospherically active binary in the cluster. However, the flare luminosity is significantly higher than what is commonly observed in stellar flares of such a short duration, leaving open the possibility of other interpretations. Key words. globular clusters: individual: NGC 6540 – stars: flare – X-rays: binaries – X-rays: bursts 1. Introduction 2. Data analysis and results Globular clusters host a large number of X-ray sources. The lu- 2.1. XMM–Newton 36 −1 minous sources (LX > 10 erg s ) were soon recognized as low mass X-ray binaries (LMXB) containing neutron stars, owing to NGC 6540 was observed with XMM–Newton for about 9 ks on the detection of type I X-ray bursts in many of these sources 2005 September 21, starting at 03:09 UT. Our results are based and to their overall properties similar to those of LMXBs in the on data obtained with the three CCD cameras of the EPIC in- field (Grindlay et al. 1984). The nature of the much larger popu- strument. During this observation, all of the cameras used the lation of sources with lower X-ray luminosity has been more dif- medium thickness optical filter and were operated in full imag- ficult to understand. It is now clear that low luminosity sources ing mode, resulting in a time resolution of 73 ms for the pn (<1035 erg s−1) in globular clusters comprise a mix of different camera (Strüder et al. 2001) and 2.6 s for the two MOS cameras classes, including compact objects powered by accretion and/or (Turner et al. 2001). 0 × 0 rotation (transient LMXBs in quiescence, cataclysmic variables, The left panel of Fig.1 shows a 8 8 region of the and millisecond pulsars), as well as nondegenerate stars with en- 0.2–12 keV image obtained by summing the data of the three hanced X-ray emission related to chromospheric activity (see, EPIC cameras. This image, integrated over the whole observa- e.g., Heinke 2010). tion, shows the presence of four sources. The variable source, at h m s − ◦ 0 00 We report the discovery of a transient X-ray source with pe- coordinates RA = 18 6 8.9 , Dec = 27 45 53 , is indicated culiar variability properties in the globular cluster NGC 6540. by the arrow. Its light curve is shown in Fig.2, where a flare This result was obtained in the course of EXTraS1, which is lasting about 300 s is clearly visible. an EU/FP7 project devoted to a systematic variability study This source is also present in the 3XMM Catalog of XMM– of the X-ray sources in the XMM–Newton public archive Newton serendipitous sources (Rosen et al. 2016) and is called (De Luca et al. 2016). 3XMM J180608.9–274553. In the following we refer to this NGC 6540 is located in the Galactic bulge and distances be- source with the abbreviated name J1806–27. tween 3 and 5.3 kpc have been derived by various authors (see The middle panel of Fig.1 shows the same sky region accu- Rossi et al. 2015, and references therein). In the following we mulated during the short time interval (300 s) corresponding to give all the distance-dependent quantities normalized to an as- the flare. J1806–27 is detected with high significance in this im- sumed value d = d=4 kpc. age, which is too short to reveal the other sources, but it is also 4 visible when the flare time interval is excluded, as shown in the right panel of Fig.1. Considering the limited count statistics, we extracted the source spectra during the flare using a maximum likelihood 1 http://www.extras-fp7.eu/ (ML) method, as described in Rigoselli & Mereghetti(2018). Article published by EDP Sciences A36, page 1 of5 2A&A S. Mereghetti 616, A36 et (2018) al.: 0 0 Fig.Fig. 1. 1:X-ray X-ray images images of a of 8 a× 880 ×field80 field toward toward NGC 6540NGC (north 6540 to (north the top, to the east top, to the east left). to theThe left). images The are images obtained are by obtained summing by the summing data of the three EPIC cameras in the 0.2–12 keV energy range. Left panel: whole observation with an exposure time of about 9 ks. Center panel: a 300 s time intervalthe data centered of the on three the flare. EPICRight cameras panel: in whole the observation 0.2-12 keV with energy the above range. time Left interval panel: excluded. whole Theobservation source indicated with an by exposure the arrow time is clearly of variable;about 9 the ks. majority Center panel:of its counts a 300 are s time detected interval during centered the short on time the flare. interval Right containing panel: the whole flare, observation but the source with is thealso above visible time when interval the time intervalexcluded. containing The source the flare indicated is excluded. by the arrow is clearly variable; the majority of its counts are detected during the short time interval containing the flare, but the source is also visible when the time interval containing the flare is excluded. 2.2. Chandra The middle panel of Fig. 1 shows the same sky region ac- NGCAcceptable 6540 fitswas were observed also obtained with the withChandra other singleACIS component instrument cumulated during the short time interval (300 s) corresponding (modelsGarmire (blackbody, et al. 2003) thermal for 5.1 ksbremsstrahlung, on 2008 November thermal 1. plasma The tar- to the flare. J1806–27 is detected with high significance in this getemission, was imaged and multi-blackbody on the backside-illuminated disk emission); Chipthe best-fit S3 of pa- the image, which is too short to reveal the other sources, but it is ACIS-Srameters array. are summarized A source detection, in Table in 1. the 0.3–8 keV energy range, also visible when the flare time interval is excluded, as shown revealedWith severalthe ML sources method in it thewas cluster also possible region, to including extract the one in the right panel of Fig. 1. positionallyspectra of J1806–27 coincident during with J1806–27 the quiescent and with emission, a count i.e., rate from con- sistentthe whole with observation that observed excluding for its the quiescent time interval level of with theXMM– flare. Newton. This yielded exposure times of 6.5 ks in the pn and 8.8 ks in the We improved the Chandra astrometry by cross-correlating MOS (100 and 85 source counts, respectively). To avoid con- X-ray sources with the 2MASS catalog (also used as00 a ref- erencetamination for from HST a astrometry, nearby persistent see next source section). (at only We 18 usedfrom 25 ChandraJ1806–27,sources see Figure located 1), within we included 4 arcmin it in of the the ML aimpoint, model where and Fig. 2. X-ray light curve (PN+MOS) of the transient source J1806–27 theextracted sharper simultaneously point spread the function spectra yields of the a two better sources. localization In the with a bin time of 30 s. Start time is set at the beginning of the pn obser- accuracy.spectral fits We of found J1806–27, five matches we fixed within the absorption a correlation at the distance values of vation (MJD = 53634.14811 – all times are in Barycentric Dynamical 0.5derived00 – no from more the matches spectrum were of the found flare up with to distances the corresponding larger than Time). The inset shows a zoom on the flare. Error bars are computed 1model.00. In view Wefound of the 2MASSa quiescent source source density flux of slightly∼8:3 × smaller10−3 sources than according to Gehrels(1986). The flare is well described by a Gaussian per10−13 squareerg cm arcsec,−2 s−1 theseand, five although matches the have uncertainties a chance occurrenceare large, centered at MJD = 53634:18937 ± 0:00014 and with σ = 54 ± 11 s. −7 probabilitythere is some of evidence∼8 × 10 that– these the spectrum are very likely during the the actual quiescent X-ray counterpartsperiod is softer of than the 2MASS that of the sources.