A&A 495, L9–L12 (2009) Astronomy DOI: 10.1051/0004-6361/200811479 & c ESO 2009 Astrophysics Letter to the Editor A candidate tidal disruption event in the Galaxy cluster Abell 3571 N. Cappelluti1,2, M. Ajello3,4, P. Rebusco5, S. Komossa1, A. Bongiorno1,C.Clemens1,M.Salvato6,P.Esquej7,1, T. Aldcroft8, J. Greiner1, and H. Quintana9 1 Max-Planck-Institute für Extraterrestrische Physik, Postfach 1312, 85741, Garching bei München, Germany e-mail: [email protected] 2 University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA 3 SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA 4 KIPAC, 2575 Sand Hill Road, Menlo Park, CA 94025, USA 5 Kavli Institute for Astrophysics and Space Research, MIT, Cambridge, MA 02139, USA 6 California Institute of Technology, 105-24 Robinson, 1200 East California Boulevard, Pasadena, CA 91125, USA 7 Dept. of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK 8 Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA 9 Department of Astronomy and Astrophysics, Pontificia Universidad Catolica de Chile. Casilla 306, Santiago 22, Santiago, Chile Received 5 December 2008 / Accepted 18 January 2009 ABSTRACT Context. Tidal disruption events are possible sources of temporary nuclear activity in galactic nuclei and can be considered as good indicators of the existence of supermassive black holes in the centers of galaxies. Aims. A new X-ray source has been detected serendipitously with ROSAT in a PSPC pointed observation of the galaxy cluster A3571. Given the strong flux decay of the object in subsequent detections, the tidal disruption scenario is investigated as a possible explanation of the event. Methods. We followed the evolution of the X-ray transient with ROSAT, XMM-Newton and Chandra for a total period of ∼13 years. We also obtained 7-band optical/NIR photometry with GROND at the ESO/MPI 2.2 m telescope. Results. We report a very large decay of the X-ray flux of the ROSAT source identified with the galaxy LEDA 095953, a member of −1 the cluster Abell 3571. We measured a maximum 0.3−2.4 keV luminosity Log (LX) = 42.8 erg s . The high state of the source lasted −2 at least 150 ks; afterwards LX declined as ∼t . The spectrum of the brightest epoch is consistent with a black body with temperature kT ∼ 0.12 keV. Conclusions. The total energy released by this event in 10 yr was estimated to be Δ E > 2 × 1050 erg. We interpret this event as a tidal 7 disruption of a solar type star by the central supermassive black hole (i.e. ∼10 M) of the galaxy. Key words. X-rays: galaxies – galaxies: clusters: individual: Abell 3571 – galaxies: nuclei – X-rays: bursts – galaxies: active 1. Introduction In the case of rotating BHs, the tidal disruption also can oc- cur for higher BH masses if the star approaches from a favor- There is growing evidence that most galaxies with a bulge host able direction (Beloborodov et al. 1992). However atmosphere a supermassive black hole (BH) in their center (Kormendy & stripping of giant stars is possible also for larger BH masses. Richstone 1995; Magorrian et al. 1998). The most recent the- When assuming a black body radiating at the Eddington lumi- oretical and observational developments in the field of galaxy nosity, the temperature of the emitting region calculated at the evolution suggest that during their life, galaxies experience one last stable orbit (3 RS,whereRS is the Schwarzschild radius) 6−7 or more phases where the BH is active and powered by accre- is ∼30−300 eV for a 10 M black hole. After the disrup- ff tion, the mass accretion rate, and therefore the bolometric lu- tion. One of the e ects of this phenomenon are the so called ac- − tive galactic nuclei or quasars where accretion occurs on time minosity, may decay approximately as t 5/3 (Rees 1990; Evans scales longer than 106−7 yr. It is widely accepted that during & Kochanek 1989), even though deviations from this law are ex- this period the BH is fed by an accretion disk. On the other pected (see e.g. Lodato et al. 2008). 7 candidate tidal disruption hand, during the non-active phase of the galaxy life the BHs events have been detected in the X-ray by the ROSAT all-sky can be briefly powered by stars tidally disrupted during close survey and in the XMM-Newton slew survey (see e.g. Komossa encounters with the BHs (e.g. Frank & Rees 1976). Recent stud- & Bade 1999; Greiner et al. 2000; Grupe et al. 1999; Komossa ies (see e.g. Wang & Merritt 2004; Esquej et al. 2008; Donley et al. 2004; Esquej et al. 2007). Candidate tidal disruption have et al. 2002) pointed out that these events happen with a rate been observed also in the Optical/UV (e.g. Renzini et al. 1995; of ∼10−4−10−5 galaxy−1 year−1. These phenomena can gener- Gezari et al. 2008). The X-ray events show strong X-ray vari- ate strong flares in the UV-X-ray band as the result of the on- ability (i.e. more than a factor of 100), 0.3−2.4 keV luminosi- / ties typical of AGN and quasars (i.e. >1042 erg s−1) declining as set of an accretion disk. The X-ray UV luminosity of the galaxy − is therefore expected to grow to values close to those of AGNs ∼t 5/3, ultrasoft (i.e. ∼0.1 keV) X-ray spectra and the absence of (i.e. >1042 erg s−1). If the star is Sun-like, tidal disruption can Seyfert activity in ground-based optical spectra (see Komossa 8 only occur if the mass of the BHs does not exceed ∼10 M 2002, for a review). In this letter we report the serendipitous since, in this case the tidal radius would typically fall within detection of a very large decay in the X-ray flux of the galaxy the Schwarzschild radius and the star is swallowed as a whole. LEDA 095953 at z = 0.0366 (Quintana & de Souza 1993)in Article published by EDP Sciences L10 N. Cappelluti et al.: A candidate tidal disruption event in the Galaxy cluster Abell 3571 5’ 30" Fig. 1. Left panel:theROSAT-PSPC 0.3−2.4 keV colour coded image of the field of A3571. The X-ray transient is indicated with the arrow. Right panel: GROND image in the KS-band of the region of the X-ray transient, the red circle represents the ROSAT-PSPC error-box and the red ellipsoid is the Chandra confidence region. Fig. 2. The ROSAT-PSPC unfolded spectrum of the source TDXFJ134730.3-325451 (crosses) and the best fit blackbody model Table 1. The log of the X-ray and optical/NIR observation of the field (continuous line). of Abell 3571 used for this work. Instrument Date Band Exposure significance >10σ with coordinates of the centroid 13h47m29.8s dd-mm-yyyy s ROSAT-PSPC 12-08-1992 0.3–2.4 keV 5800 −32◦55 00 (J2000) with an error box of ∼25. Considering that ROSAT-HRI 05-08-1994 0.3–2.4 keV 19 200 the source is embedded in the X-ray emission of the galaxy clus- XMM-Newton-EPIC-PN 29-07-2002 0.3–2.4 keV 24 200 ter Abell 3571, the photometric properties have been evaluated Chandra-ACIS-S 31-07-2003 0.3–2.4 keV 33 900 / using aperture photometry instead of using the estimate of the ESO MPI-GROND 12-08-2008 g, r , i , z 458 detection software. We extracted the source counts in a circular ESO/MPI-GROND 12-08-2008 J, H, K 480 region with a radius corresponding to the 90% Encircled Energy Radius (EER) around the centroid estimated by the maximum likelyhood fit. Background counts have been estimated in a ring the field of view of the galaxy cluster Abell 3571 using ROSAT, around the source with an inner radius 1.5 times the 90% EER XMM-Newton and Chandra archival X-ray data and GROND / and outer radius 2 times the 90% ERR. As a result we obtained optical NIR follow-up. 305 ± 27 net source counts corresponding to a count-rate of (5.2 ± 0.4) × 10−2 s−1. PSF fitting estimated a flux ∼30% lower as a result of a likely overestimate of the background. In the same 2. Observations and data processing regions we also extracted source and background lightcurves and During the soft X-ray follow-up of the Swift-BAT, hard X-ray spectra. All the epochs but one have flux consistent with a mean − − selected sample of galaxy clusters (Ajello et al. 2008), we count-rate of ∼5.2 × 10 2 s 1. Note that the observation was split serendipitously discovered a bright source in a ROSAT-PSPC in two parts separated by ∼150 ks. This indicates that no strong image of Abell 3571 (see Fig. 1) that apparently disappeared variability is present on such a time scale. However in one time − − in subsequent observations with XMM-Newton and Chandra. bin the count-rate reached (8.3 ± 1.0) × 10 2 s 1. This flux is The field of Abell 3571 has been observed in X-rays in pointed ∼3σ above the mean and could indicate a small flare during the mode with the ROSAT-PSPC/HRI detectors, XMM-Newton event. EPIC, Chandra ACIS-S and during the ROSAT all-sky survey. The unfolded spectrum of the source is shown in Fig.