arXiv:0902.1652v1 [astro-ph.CO] 10 Feb 2009DRAFT VERSION FEBRUARY 10, 2009Preprint typeset using LATEX style emulateapj v. 10/09/06DEEP Chandra MONITORING OBSERVATIONS OF NGC 4278: CATALOG OF SOURCE PROPERTIESN. J. BRASSINGTON, G. FABBIANO, D.-W. KIM, A. ZEZASHarvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138S. ZEPF, A. KUNDUDepartment of Physics and Astronomy, Michigan State University, East Lansing, MI 48824-2320L. ANGELINILaboratory for X-Ray Astrophysics, NASA Goddard Space Flight Center, Greenbelt, MD 20771R. L. DAVIESSub-Department of Astrophysics, University of Oxford, Oxford OX1 3RH, UKJ. GALLAGHERDepartment of Astronomy, University of Wisconsin, Madison, WI 53706-1582V. KALOGERA, T. FRAGOSDepartment of Physics and Astronomy, Northwestern University, Evanston, IL 60208A. R. KINGTheoretical Astrophysics Group, University of Leicester, Leicester LE1 7RH, UKS. PELLEGRINIDipartimento di Astronomia, Universita di Bologna, Via Ranzani 1, 40127 Bologna, ItalyG. TRINCHIERIINAF-Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milan, ItalyDraft version February 10, 2009ABSTRACTWe present the properties of the discrete X-ray sources detected in our monitoring program of the globularcluster (GC) rich elliptical galaxy, NGC 4278, observed with Chandra ACIS-S in six separate pointings, result-ing in a co-added exposure of 458-ks. From this deep observation, 236 sources have been detected within theregion overlapped by all observations, 180 of which lie within the D25ellipse of the galaxy. These 236 sourcesrange in LXfrom 3.5×1036erg s−1(with 3σ upper limit ≤1×1037erg s−1) to ∼ 2 × 1040erg s−1, including thecentral nuclear source which has been classified as a LINER. From optical data, 39 X-ray sources have beendetermined to be coincident with a globular cluster, these sources tend to have high X-ray luminosity, withten of these sources exhibiting LX> 1× 1038erg s−1. From X-ray source photometry, it has been determinedthat the majority of the 236 point sources that have well constrained colors, have values that are consistentwith typical LMXB spectra, with 29 of these sources expected to be background objects from the logN −logSrelation. There are 103 sources in this population that exhibit long-term variability, indicating that they areaccreting compact objects. 3 of these sources have been identified as transient candidates, with a further 3 pos-sible transients. Spectral variations have also been identified in the majority of the source population, where adiverse range of variability has been identified, indicating that there are many different source classes locatedwithin this galaxy.Subject headings: galaxies: individual (NGC 4278) — X-rays: galaxies — X-ray: binaries1. INTRODUCTIONNGC 4278 is the second of two nearby, isolated, normal el-liptical galaxies for which we were awarded deep monitoringobservations with Chandra ACIS-S (Weisskopf et al. 2000).The purpose of these observations was to study the low-massX-ray binary (LMXB) population down to limiting luminosi-Electronic address: [email protected] of a few (0.3−8.0 keV) 1036erg s−1, well in the range ofthose LMXBs in the Galaxy and M31. LMXBs are the onlydirect fossil evidence of the formation and evolution of binarystars in the old stellar populations of early-type galaxies. Firstdiscovered in the Milky Way (see Giacconi 1974), these bina-ries are composed of a compact accretor, neutron star or blackhole, and a late-type stellar donor. The origin and evolutionof Galactic LMXBs has been the subject of much discussion,2 Brassington et al.centered on two main evolution paths (see Grindlay 1984; re-view by Verbunt & Van den Heuvel 1995): the evolution ofprimordialbinary systems in the stellar field, or formation andevolution in Globular Cluster.With the adventof Chandra, many LMXB populationshavebeen discovered in early-type galaxies (see review Fabbiano2006), and the same evolutionary themes (field or GC forma-tion and evolution) have again surfaced, supported and stim-ulated by a considerably larger and growing body of data.With our unique observations of NGC 3379 (Brassington etal. 2008) and NGC 4278 (this paper), we can now take a freshlook at these issues. Moreover, our observations have a timesampling that permits variability studies and the identificationof X-ray transients. Such data will extend the limited sam-ple of existing multi-epoch observations (albeit with higherlimiting luminosities; e.g. Irwin 2006; Sivakoff et al. 2008),where a variety of different variability behaviours of LMXBshave already been observed. Both multi-epoch observationsand low luminosity thresholds are importantaspects of the ob-servational characteristics of Galactic LMXBs and are neededfor constraining the evolution of these populations (e.g., Piro& Bildsten 2002, Bildsten & Deloye 2004).The first galaxy that was observedin this programwas NGC3379, in the nearby poor groupLeo(D=10.6 Mpc; Tonry et al.2001). This system was chosen for this study because it is arelatively isolated unperturbed ‘typical’ elliptical galaxy, withan old stellar population (age of 9.3 Gyr, Terlevich & Forbes2002) and a poor globular cluster system (SGC= 1.3± 0.7,Harris 1991; where SGC= No.GC×10(0.4(MV+15))). From the324-ks Chandra observation, reported in detail in Brassing-ton et al. (2008), 132 X-ray point sources were detected,98 of which were found to lie within the D25ellipse of thegalaxy. Through HST observations of the galaxy the globu-lar cluster system was identified in the central region and tenGC-LMXBs detected. These GC-LMXB associations werefound to have high luminosities, with three sources exhibitingLX≥ 1 × 1038erg s−1. Also from studying this population itwas determined that 48% of the detected LMXBs were foundto show some type of long-term variability, with 5 of thesesources being classified as transients and a further 3 of theseas possible transients.The characteristics of NGC 4278 are similar to those ofNGC 3379 in all but the GC population, where NGC 4278has a rich globular cluster system (SGC= 6.1 ± 1.5, Harris1991), making it an ideal galaxy to explore the evolution ofLMXB from primordial field binaries in a rich environment.Like NGC 3379, NGC 4278 is a nearby (D=16.1 Mpc; Tonryet al. 2001) relatively isolated, unperturbed ‘typical’ ellipti-cal galaxy with an old stellar population (age of 10.7 Gyr,Terlevich & Forbes 2002).Observationally, NGC