Research note An unlikely radio halo in the low X-ray luminosity galaxy cluster RXCJ1S14.9-1S23 - - S. Giacintucci 1, D. Dallacasa2,3, T. Venturi2, G. Brunetti2, R. Cassano2, M. Markevitch4, R. M. Athreya5 1 Department of Astronomy, University of Maryland, College Park, MD, 20742-2421, USA 2 INAF- Istituto di Radioastronomia, via Gobetti 101,1-40129, Bologna, Italy 3 Department of Astronomy, University of Bologna, via Ranzani 1, 1-40127 Bologna, Italy 4 Astrophysics Science Division, Laboratory for High Energy Astrophysics, Code 662, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA 5 Indian Institute of Science Education and Research, Central Tower, Sai Trinity Building, Sutarwadi Road, Pashan, Pune 411021, India Received 00 -00 -0000; accepted 00 -00 -0000 ABSTRACT Aims, We report the discovery of a giant radio halo in the galaxy cluster RXCJlSI4,9-1S23 at z=0.22 with a relatively low X-ray luminosity, LX[O.1-2.4kevl ~ 7 X 1044 erg S-I. Methods. This faint, diffuse radio source is detected with the Giant Metrewave Radio Telescope at 327 MHz. The source is barely detected at 1.4 GHz in a NVSS pointing that we have reanalyzed. Results. The integrated radio spectrum of the halo is quite steep, with a slope a 1.6 between 327 MHz and 1.4 GHz. While giant radio halos are common in more X-ray luminous cluster mergers, there is a less than 10% probability to detect a halo in systems with Lx <f: 8 x 1044 erg S-I. The detection of a new giant halo in this borderline luminosity regime can be particularly useful for discriminating between the competing theories for the origin of ultrarelativistic electrons in clusters. Furthermore, if our steep radio spectral index is confirmed by future deeper radio observations, this cluster would provide another example of the very rare, new class of ultra-steep spectrum radio halos, predicted by the model in which the cluster cosmic ray electrons are produced by turbulent reacceleration. Key words. radiation mechanism: non-thermal-galaxies: clusters: general-galaxies: clusters: individual: RXCJlS14.9-1S23 1. Introduction more relaxed (e.g., Buote 2001, Cassano et al. 20lO, and refer ences therein). Few exceptions are known, where a radio halo Hot, X-ray emitting gas is the dominant constituent of the intra is not present in a merging system, typically with relatively low cluster medium (ICM) in galaxy clusters. The ICM is also per X-ray luminosity (Lx::8 x lO44 erg S-I; e.g., Cassano et al. 20lO, meated by magnetic fields and ultra-relativistic particles, whose Russell et al. 2011). energy densities and dynamical effects are still uncertain. The The halo-merger connection suggests that the energy nec presence of these non-thermal components is evidenced by gi essary to generate radio halos - through acceleration of parti ant, faint synchrotron radio halos, detected in the central ~ Mpc cles and amplification of magnetic fields - is provided by clus regions of a number of massive clusters (e.g., Ferrari et al. 2008, ter mergers. Although the origin of radio halos is still debated Cassano 2009, Venturi 2011 for reviews). Unlike radio galaxies, (e.g., Brunetti et al. 2008, Pfrommer et al. 2008, Donnert et al. these diffuse, cluster-scale radio sources lack any optical identi 2010, Keshet and Loeb 2010, Brown and Rudnick 2011, leltema fication and are associated directly with the ICM, in good spa and Profumo 2011), current observations (e.g., Cassano 2009, tial coincidence with the distribution of the hot, X-ray emitting and references therein) appear to favour models where the gi gas. They are produced by electrons with Lorentz factor y~1 000 ant halos are caused by merger-driven turbulence that reacceler spinning in large-scale flG magnetic fields. Their radio spectra ates relativistic particles (reacceleration model; Petrosian 2001, are steep, with spectral indices a > 1 (we adopt S v cc v-a, where Brunetti et al. 2001). In line with present data, these models S v is the flux density at the frequency v). predict that halos are more probably found in massive clusters Observations show that radio halos are not common in and become quite rare in systems with mass:: 1015 Mo (i.e .. galaxy clusters. Large halos are found in only ~ 1/3 of the Lx::7 8 x 1044 erg ) at intermediate redshift (2 ~ 0.2 -:-O.S; most massive and X-ray luminous clusters (e.g., Giovannini et Cassano et al. 2008). al. 1999, Kempner & Sarazin 2001, Venturi et al. 2008, Cassano Here, we report the discovery of a giant radio halo in et al. 2008), and become even rarer in less massive systems. The RXC11S14.9-1S23, a galaxy cluster at z=0.22 with a relatively rest of the clusters seems to form a distinct population of radio low X-ray luminousity, = 7.2 X 1044 erg S-1 quiet systems (Brunetti et al. 2007, 2009). (Bohringer et al. 2004). Clusters with and without a giant halo appear segregated in We adopt the ACDM cosmology with Ho=70 km S-1 Mpc-I, terms of their dynamical state: halos are located exclusively in Qm 0.3 and QA = 0.7. At the redshift ofRXC 11S14.9-1S23, merging systems, while clusters without radio halos are typically this gives a scale of I" 3.S9 kpc.