arXiv:astro-ph/0702744v1 28 Feb 2007 omto n a efrigsasi h urn epoch current star the in of stars bursts 2006) forming occasional al. be through et may grow Rossa and 2005; may formation al. they et that (Walcher population so well older their as hidden present of a is Still, suggest Myr, masses 2006). 100 al. dynamical et large age, their (Rossa in component young stellar due luminous comparatively 2004) most the al. (B¨oker to than et luminous clusters part more globular much old are the they addition, and In more masses are scaled-up densities. 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Tremaine et Merritt clear (Graham 2000; & concentration al. light 1998; (Ferrarese central et al. dispersion Gebhardt et velocity 2000; Richstone & (Magorrian central (Kormendy mass 1995), luminosity early-type 2004), bulge in Rix their H¨aring holes & black and supermassive galaxies central of masses rpittpstuigL using typeset Preprint Journal Astrophysical to Submitted High-resolution the between measured been have correlations Tight 1 2 3 4 nvriyo lbm nHnsil,Dp.o hsc,Hunt Physics, of Dept. Huntsville, in Alabama of University nvriisSaeRsac soito,NS asalSp Marshall NASA Association, Research Space Universities avr-mtsna etrfrAtohsc,Cambridge, Astrophysics, for Center Harvard-Smithsonian pc cec eatet AAMrhl pc lgtCente Flight Space Marshall NASA Department, Science Space etro h erylt-yesia aayNC20.TeX-ray The 2403. NGC galaxy spiral levels, detection late-type below nearby from the of center fmlil lcbd opnnsadcrepnigpyial oac a to physically corresponding and components blackbody multiple of of observations between aatcncesi G 35 h rgteso h ula trclus star th nuclear to the then of binary, brightness X-ray The m low-mass a are 4395. o be NGC source curve to in the light believed nucleus X-ray of M33, galactic its properties in in X-ray X-8 evident The source is behavior observations. aperiodic individual nor pulsations No fcutr nlt-yesiasbtiseetv radius, effective its but spirals late-type in clusters of ujc headings: Subject niaigarltvl eae lse n o rbblt facentral a of probability low a and mass cluster a relaxed relatively a indicating ICVR FATASETXRYSUC NTECMATSTELLAR COMPACT THE IN SOURCE X-RAY TRANSIENT A OF DISCOVERY erpr h icvr fa -a orecicdn ihtenuc the with coincident source X-ray an of discovery the report We ∼ > 10 iooYukita Mihoko Hubble A 1. T 6 E . 5 tl mltajv 04/03/99 v. emulateapj style X INTRODUCTION M ⊙ aais vlto -as aais—Xry:binaries X-rays: — galaxies X-rays: — evolution galaxies: aais niiul(G 43 aais uli—glxe:sa clus star galaxies: — nuclei galaxies: — 2403) (NGC individual galaxies: mgs(aol ta.1998; al. et (Carollo images n naeof age an and < ots h -a pcrmi elfi ya crto ikmdlc model disk accretion an by well-fit is spectrum X-ray The months. 2 1 oga .Swartz A. Douglas , ∼ 10 35 ∼ . y siaigfo t bevdclr n brightness. and colors observed its from estimating Gyr 1.4 umte oAtohsclJournal Astrophysical to Submitted r s erg − 1 ∼ nte0 the in 75%, A USA MA, ABSTRACT vle L USA AL, sville, c lgtCne,V6,Hnsil,A,USA AL, Huntsville, VP62, Center, Flight ace G 2403 NGC 2 ,V6,Hnsil,A,USA AL, Huntsville, VP62, r, oet Soria Roberto , 1 . 5 − ru fglxe tadistance a at galaxies of group galax- that host holes holes. larger black black (intermediate-mass) the supermassive into seed of 2005) evolve the cores be the Dehnen may to & ies, migrate (King they candidates if sources potential and, X-ray be may ultraluminous (intermediate- they for host then holes, commonly & black they Layden mass) If 1993; Freeman ab- 2000). and (e.g. stripped Sarajendini galaxies tidally larger nuclei into dE globu- relic sorbed massive most be stars. the may forming of clusters some and lar that gas suggested accreting been whether has presently nor It there are originated they they al. not B¨oker if et or certain (e.g. not galaxies is it host 2002), their of centers dynamical levels. low at pcr n ih uv ftencersuc in X-ray source the nuclear present the We of curve 2004). light al. and et spectra Swartz (e.g., nucleus oreu20) edrv hsclpoete o this for properties physical recent central derive & from a We (Davidge cluster NSC lacks compact 2403 2002). luminous a NGC Courteau host does but 1991). bulge Freedman & Madore pia rpriso h S n h -a rprisof compact nearby properties in other X-ray to nuclei the compared are and source NSC transient the the of properties optical 2. about located source ultralumi- well-known X-ray the nous including from resolved sources easily X-ray is nearby transient nuclear archival The in observations. the NSC within source 2403 X-ray NGC transient bright a discovered have hntpclfrlt-yeglxe.Tepoete fthe of properties The galaxies. late-type for typical than esosrain n te rhvldt in data archival other and observations veys r e 8 h A()dglx G 43i ebro h M81 the of member a is 2403 NGC galaxy SAB(s)cd The the near very or at definition, by are, NSCs the While . ∼ e ad o7 to band, keV 0 2p,i eea ie agrta average than larger times several is pc, 12 § .W n h S sodradls compact less and older is NSC the find We 4. 3 n ly .Tennant F. Allyn and , r iia otoeo h nuclear the of those to similar ore ersa lse ttedynamical the at cluster star lear ter, uioiyo hssuc varies source this of luminosity matojc asof mass object ompact s ftelwlmnst active low-luminosity the of ose asv bet h lse has cluster The object. massive h hr ieclso the of timescales short the n × M Hubble 10 I 38 − ∼ r s erg Chandra 11 dacdCmr o Sur- for Camera Advanced . D a,i typical is mag, 8 − ULU OF NUCLEUS 1 3 = ntimescales on ′ otews fthe of west the to 6 es— ters 4 and . p (1 Mpc 2 onsisting ∼ > 5 XMM/Newton M ⊙ § . § ′ 1kpc; =1 .We 2. .The 3. 2

X-ray source are consistent with an X-ray binary contain- by Fraternali et al. (2002) from the Hi rotation curve. ing an ∼5 M⊙ compact object or larger accreting from a A model point spread function (PSF) of the Hub- low-mass companion. Further discussion is given in § 5 ble/ASC detector at the location of the source was con- structed using TinyTim (Krist & Hook 1997) assuming 2. THE NUCLEAR STAR CLUSTER IN NGC 2403 a spectral energy distribution equivalent to that of an The contrast between the nucleus of NGC 2403 and the A5V star (following B¨oker et al. 2004). We then fit an- underlying galaxy disk is clearly visible in optical and near- alytic models to the surface brightness distribution using IR images where it appears extended at high resolution the program ISHAPE (Larsen 1999) which convolves the (Figure 1). We used the method described in B¨oker et al. model PSF with these analytic profiles. ISHAPE6 allows (2004) to analyze the morphology of the cluster. for circular or elliptical models but cannot reproduce the Calibrated Hubble images acquired with the Advanced azimuthal asymmetries clearly seen in the Hubble images Camera for Surveys (ACS) operated in WFC mode were of NGC 2403. Therefore, we restricted our selection to cir- obtained from the Multimission Archive at STScI5. These cular Moffat and King model profiles. The standard power images were already cleaned of cosmic rays and bad pix- indices and concentration parameters for these models (see els and were corrected for geometric distortion; we per- Larsen 1999) provided poor fits to the data. A trial King formed no further processing of the data. The data were model applied to the azimuthally-averaged radial profile taken 2004-08-17 and include images through the F475W, suggested a Moffat model with power index of 1.35 would F606W, F658N, and F814W filters (dataset identifiers improve the model fit. This did give the best fit of all J90ZX1010 through 1040). our trials. Figure 2 displays this model, the data, and the fit residuals. Following B¨oker et al. (2004), we derive an effective radius of re = 11.7 ± 0.1 pc from the ISHAPE fitting. This is larger than the average, ∼3.5 pc, found by B¨oker et al. (2004) for a sample of 39 NSCs in late-type galaxies. We estimate the intrinsic, background-subtracted, I- 39 −1 2 band luminosity to be 1.1×10 erg s in a 2πre region centered on the NSC after correcting for an extinction of E(B − V )=0.2 mag (see below). The average surface −1 −2 brightness, Ie = 323 L⊙ erg s pc , is near the low lu- minosity range of the NSCs in Sd galaxies in the B¨oker et al. (2004) sample and near the high luminosity end of Milky Way globular clusters (cf. their Fig. 5). The ob- served absolute blue magnitude is MB = −9.3 mag (giving o an extinction-corrected MB = −10.2 mag, see below).

Fig. 1.— 3-color combined Hubble/ACS image of the 20′′×20′′ re- gion centered on the nuclear star cluster in NGC 2403. Colors blue, Fig. 2.— ISHAPE (Larsen 1999) model fit to the I-band sur- green, and red correspond to the filters F475W, F606W and F814W, face brightness distribution of the nuclear star cluster in NGC 2403. respectively. The image has been rotated for display purposes such From left to right: Model, data, and fit residuals. Grayscale of that north is up and east is to the left. model and data cover the same range. The model is a Moffat profile with a power index of 1.35 (see text). The 200′′×200′′ Hubble/ACS field contained both SN 2004dj (the target of the original investigation) and the Lacking an optical spectrum of the NGC 2403 nucleus nucleus of NGC 2403. Although the location of SN 2004dj we cannot accurately constrain the mass and age of the is known to high precision (Beswick et al. 2005) and the cluster. However, we can obtain estimates of the cluster ACS resolution is ∼0.′′1 (with 0.′′05 pixel−1 sampling), age from the observed colors and the cluster mass from o the supernova is saturated in these images and this is the MB using the stellar synthesis models of, e.g., Leitherer et dominant contribution to the uncertainty in the position of al. (1999). The observed colors are (using the conversions the nucleus: Our best estimated position for the nucleus is from Hubble/ACS to Johnson-Cousins given by Sirianni R.A.=7h36m50.s070, Decl.= +65◦36′3.′′54 (J2000.0) with et al. 2005) B − V = 1.0 mag, V − I = 1.2 mag, and an uncertainty of ∼0.′′1 radius based on a circular Gaus- J − K = 0.6 mag (the latter from Davidge & Courteau sian model fit to the location of the supernova in the 2002). These colors can be self-consistently reproduced by F606W filter image. The position of the nucleus is consis- a single starburst episode aged ∼1.4 Gyr viewed through tent with the kinematic center of the galaxy determined an extinction of E(B − V )=0.2 mag. They cannot be 5http://archive.stsci.edu/ 6ISHAPE documentation is available from http://www.astro.uu.nl/∼larsen/baolab/ 3 reproduced by models with continuous star formation nor observations with the Einstein, ASCA, and ROSAT Ob- can the metallicity of the cluster be radically different from servatories show no evidence for a nuclear X-ray source the solar value. The implied intrinsic colors are consistent although the quality of the data is poor compared to the with those of G2V to K0V stars. As shown in § 3.2, the recent Chandra and XMM/Newton observations. extinction estimate is consistent with the hydrogen col- We obtained level 1 event lists from the Chandra data umn density towards the NSC X-ray source according to archive7 and reprocessed them using the Chandra Inter- the best-fitting X-ray spectral model. active Analysis of Observations (CIAO) version 3.3.0.1 Rossa et al. (2006) show there is a strong correlation tool acis process events and the calibration database between the observed B − V color of NSCs and their (CALDB) version 3.2.1. Reprocessing removed pixel ran- luminosity-weighted cluster age, τL. Using their linear domization and applied CTI and time-dependent-gain cor- 9.8±0.4 correlation coefficients gives τL = 10 yr. The age- rections. We then filtered the data of events with non- mass correlation (Rossa et al. 2006) then gives log M = ASCA grades and bad status bits as well as hot pixels and 8.3±1.8 10 M⊙ for the total mass of the NSC. These values columns. are much higher than expected for a late-type spiral galaxy The European Photon Imaging Camera (EPIC) event like NGC 2403 based on the (weaker) correlations Rossa lists from the XMM/Newton observations were obtained et al. (2006) find between cluster mass or cluster age and from the HEASARC data archive8. We processed the galaxy morphological type. event lists using the current calibration files with the rou- There is no correction for extinction in the work of Rossa tines in SAS 6.5.0. et al. (2006). One possibility is that the NSC in NGC 2403 We checked for periods of high background that could af- is more reddened than typical. Figure 1 shows the clus- fect our spectra and timing analysis; excluding some short ter is, in fact, at the edge of a dark region however our intervals from the XMM/Newton datasets. The final Good best-fitting extinction is only E(B − V )=0.2 mag. Using Time Intervals for data used in our analysis are listed in the implied intrinsic B − V = 0.8 mag color reduces the Table 1. 9.3±0.3 estimated age to τL = 10 yr and the cluster mass 7.6±1.7 to M = 10 M⊙. This age is consistent with the age estimated above from the colors using the starburst mod- 3.1. X-ray Light Curve els of Leitherer et al. (1999). A mass estimate can also be A bright, point-like X-ray source was detected in sev- deduced from the age and intrinsic blue magnitude using eral of the observations at the location of the nuclear star these starburst models. The mass estimated in this way is cluster. We used the common source, SN 2004dj, to match 6.4 M ∼ 10 M⊙ depending weakly on starburst metallicity the location of the X-ray source and the NSC. The X-ray and IMF. source is well within the >1′′ optical extent of the clus- The age estimated here is much older than the ∼100 Myr ter. Figure 3 shows the 0.5 – 8.0 keV light curve of the age deduced by Davidge & Courteau (2002) for individual source, designated CXOU J073650.0+653603, constructed AGB stars in the vicinity of the nucleus. (Many of these from the average flux during each individual observation stars are visible as blue sources in the composite Hubble based on spectral fits (§ 3.2). For observations in which image of Figure 1.) Davidge & Courteau (2002) point out the source was not detected, a 2σ flux upper limit was that the J − K color of the cluster is somewhat bluer than calculated by scaling from the fitted measurements by the these surroundings suggesting an age gradient. According background-subtracted count rate in an appropriate source to Leitherer et al. (1999), the J − K color for a starburst region. The corresponding observed X-ray luminosities, becomes bluer with age (after an early reddening phase) assuming a distance of 3.2 Mpc, are tabulated in Table 1. implying that the NSC in NGC 2403 is older than its sur- The X-ray source was undetected in five observations and roundings. This is consistent with our calculations. orders of magnitude above (Chandra) detection thresholds Although the bulk of the star formation in the cluster in three observations which qualifies it as a transient source appears to have occurred some ∼1.4 Gyr ago, there may (e.g. van Paradijs & McClintock 1995). The shortest mea- be a low level of current star formation present (and, likely, sured interval between a detection and a non-detection is an underlying older population of stars as well). For star- 21 days (observations number 5 and 6 of Table 1) but the forming clusters, the current star formation rate can be XMM/Newton upper limit is not particularly compelling. estimated from the Hα luminosity. We have constructed The time between the Chandra non-detection observation a continuum-subtracted Hα image from the Hubble/ACS number 4 and the detection observation 6 is 41 days. images and find no net Hα emission; the 2σ upper limit We also inspected the light curve from observations in ′′ to the Hα luminosity within a 1. 5 radius circle about the which the source was detected for variability during the − NSC is 3×1035 erg s 1. Thus there is no evidence for observation. These X-ray light curves, binned into 1 ks in- current star formation in the NSC. tervals, displayed no conspicuous variability and were for- mally consistent with a constant flux model. Searches for short-term aperiodic variability using a power-spectrum 3. THE TRANSIENT X-RAY SOURCE IN THE NUCLEUS OF NGC 2403 analysis found no excess power above the Poisson noise. 2 Searches for coherent pulsations, using the Zn statistics The nuclear region of NGC 2403 was observed with (Buccheri et al. 1983), detected nothing significant. Chandra ACIS-S four times over a 3.7 year interval and three times with XMM/Newton within this same time in- terval. Table 1 provides a log of these observations. Earlier 7http://cxc.harvard.edu/cda/ 8http://heasarc.gsfc.nasa.gov/db-perl/W3Browse/w3browse.pl 4 0.01 −3 10 normalized counts/sec/keV −4 10 residuals

−0.02 0 0.02 0.04 0.5 1 2 5 channel energy (keV)

Fig. 4.— XMM/Newton spectrum of CXOU J073650.0+653603 with the best fit disk-blackbody model (upper panel) and fit resid- uals. Fig. 3.— Lightcurve of the X-ray source in the nucleus of 4. COMPARISON TO OTHER NUCLEAR STAR CLUSTERS NGC 2403. Time is measured since the first Chandra observation on 2001-04-17. Upper limits for non-detections are represented with As mentioned in the Introduction, some 75% of late- arrows. Crosses denote Chandra observations. Triangles denote XMM/Newton observations. Error bars denote two-sided errors. type spirals host NSCs. In this section we briefly compare Table 1 provides the numerical values shown here. NGC 2403 to two well-studied objects with very differ- ent interpretations for their X-ray emission mechanisms: 3.2. X-ray Spectrum NGC 598 (M33), another Scd galaxy with a NSC and a Models were fit to observed spectra of the source. Ta- bright point-like nuclear X-ray source, is thought to con- ble 2 lists the models attempted, the resulting best-fit tain a luminous X-ray binary; and NGC 4395, the optically model parameters with 90% confidence extremes for a sin- least-luminous broad-line AGN known, is thought to con- gle interesting parameter, and the fit statistic for each of tain a “proper” nuclear black hole at the low end of their 5 the three observations in which the source was detected. mass distribution (M ∼ 10 M⊙). CXOU J073650.0+653603 was most luminous during the 4.1. M33 X8 XMM/Newton observation of 2003 September. The spec- trum from this observation is shown in Figure 4. Following NGC 598 is a disk-dominated late type spiral galaxy as the procedures outlined in Page et al. (2003), the aver- is NGC 2403. Its NSC is as luminous (MB = −10.2, Ko- age XMM/Newton spectrum from the three detectors was rmendy & McClure 1993) as that of NGC 2403 but much modeled in XSPEC using the χ2 fit statistic. Absorbed more compact (FWHM <1 pc, Gordon et al. 1999). Anal- diskbb and powerlaw models were applied. Adding ad- ysis of the cluster optical spectrum shows it formed from ditional model components did not significantly improve two major starburst episodes; one ∼1 Gyr ago and the the fit statistics according to the F-test. We used an un- other occurring only 40−70 Myr in the past (O’Connell binned spectrum with C-statistic in XSPEC in fitting the 1983; Gordon et al. 1999; Long et al. 2002). The best es- low-count Chandra observations. Only single-component timated upper limit for the mass of any central compact models were applied to the Chandra spectra. It was neces- object in the cluster is MBH∼1500 M⊙ based on modeling sary to fix the multiplicative absorption model component the narrow-slit Hubble spectrum (Gebhardt et al. 2001). 20 −2 at the Galactic value nH =4 × 10 cm in fits to some The nucleus contains a bright unresolved X-ray source, of the Chandra data to prevent convergence to an unphys- designated X-8 (Trinchieri et al. 1988), with a luminosity ically low value. as high as ∼2×1039 erg s−1 in the 0.5−10 keV band (Fos- The best-fitting model in all cases is the diskbb model chini et al. 2004). It has long been debated whether this (Makishima et al. 1986, 2000) representing a spectrum source could be a weak AGN. The optical spectrum shows dominated by a geometrically-thin accretion disk which no evidence of AGN activity although a strong component extends to the innermost stable orbit often referred to as would be expected if the measured X-ray flux is extrapo- a high/soft state (Remillard & McClintock 2006). The lated using the known X-ray to optical flux relationship for model fit parameters are proportional to the inner disk AGNs (Long et al. 2002). The current consensus is that temperature and radius or, alternatively, can be expressed X-8 is a luminous X-ray binary in the NSC of NGC 598 in terms of the mass of the central object. These parame- rather than a weak AGN (e.g., La Parola et al. 2003). The ters are listed in Table 2. They suggest a compact object mass of the compact object must be ∼10 M⊙ or more un- 2 38 1/2 mass, M ∼ 2.3/To (Ldisk,bol/10 ) where To ∼ Tin is less it is radiating above the Eddington limit in the X-ray the disk color temperature, of about 5 M⊙ and that the band. peak observed luminosity is therefore about the Edding- The source X-ray flux has remained at about its current ton value. The mass could be up to about a factor-of-two level since discovery although it does vary by about a fac- larger depending on viewing angle, black hole spin, and tor of two on short timescales. This is in contrast with hardening factor. many low-mass XRBs which are soft X-ray transients that 5 vary by factors of 100 or more on timescales of weeks or et al. 2006; Wehner & Harris 2006), where the CMO is a months (as does the nuclear source in NGC 2403). It has supermassive BH in ellipticals and early-type spirals and a been suggested that M33 X-8 is similar to GRS 1915+105 NSC in late-type spirals. The CMO mass is ∼1/500 of the (e.g., Long et al. 2002; Dubus & Rutledge 2002) in that spheroidal mass (bulge mass in spirals). Qualitatively, this both are steady sources with comparable X-ray luminosi- suggests that the initial galaxy assembly processes led to ties and spectral states. Both are also radio sources the rapid collapse of a nuclear gas component into a BH in (Dubus & Rutledge 2002). GRS 1915+105 is a low-mass the more massive galaxies, while a star cluster was formed X-ray binary with a 14±4 M⊙ compact object accret- in galaxies with a shallower gravitational potential, where ing from a low mass, evolved (K or M giant) companion the central accumulation of gas occurred more slowly. (Greiner, Cuby, & McCaughrean 2001). However, many questions remain open. Some relatively massive ellipticals do have NSCs (Rossa et al. 2006; al- 4.2. NGC 4395 though they have not been found in galaxies brighter than NGC 4395 is the optically least-luminous AGN known. MB ∼ −20 mag; Ferrarese et al. 2006). The range in which NSCs and SMBHs overlap may span several orders It is a SA(s)m dwarf galaxy with a distinct though weak 6 type 1 Seyfert spectrum (Filippenko & Sargent 1989). The of magnitude: a SMBH with MBH ∼ 3 × 10 M⊙ is the CMO in the center of the Milky Way, and a SMBH with nuclear brightness is only MB ∼−11 mag which makes it 5 a mass ∼ 10 M⊙ is present in NGC 4395, but NSCs as only twice as bright as the NSC in NGC 2403. Filippenko 8 & Ho (2003) estimate the mass of its compact object to massive as a few 10 M⊙ have been found in some elliptical 5 galaxies (Rossa et al. 2006). It is not known whether the be less than ∼10 M⊙. Its X-ray spectrum is a moder- ately absorbed, N =(1.2−2.3)×1022 cm−2, hard power same galaxy can harbor both an SMBH and a NSC, or if H they are mutually exclusive. It is also not yet clear which < ∼ law (Γ ∼1.5, Shih et al. 2003; Γ 0.6, Moran et al. 2005). of the two classes of objects defines the more fundamen- The absorption-corrected 2−10 keV luminosity is nearly 40 −1 tal correlation with the galactic bulge; it was suggested 10 erg s (Iwasawa et al. 2000; Moran et al. 2005). It (Rossa et al. 2006) that NSCs in large galaxies may be a is highly variable in X-rays (Moran et al. 1999, 2005; Shih factor of 3 more massive than SMBHs, for a given bulge et al. 2003) exhibiting factor of 10 changes in X-ray flux mass. Furthermore, it is not known whether a linear rela- in less than 2 ks. tion between bulge and CMO masses may extend all the The low power law index, moderate absorption, and way down to bulgeless Scd galaxies (implying the existence rapid flux variability of the AGN in NGC 4395 is unlike the 3 5 of CMOs with masses as low as 10 − 10 M⊙), or if the behavior of the nuclear source in NGC 2403 in all respects. < 6 relation breaks down for CMO masses ∼10 M⊙. 5. DISCUSSION From its optical brightness and colors, we estimate a 6 stellar mass of ∼2×10 M⊙ for the NSC in NGC 2403, We have presented a new study of the NSC in comparable to the mass of the SMBH in the Milky Way, NGC 2403. We discovered that it contains a transient even though NGC 2403 is bulgeless. Although our X-ray X-ray source; its X-ray luminosity varies from ∼7×1038 study suggests that the nuclear X-ray source is not from < 34 −1 to ∼5×10 erg s over a few months. The question is a SMBH, this does not rule out the presence of a SMBH whether this accreting source is the galaxy’s nuclear black inside the NSC: it may simply be quiescent. In that case, hole or, instead, a lower-mass black hole formed in the star it would be among the faintest SMBHs, with a luminos- cluster from stellar processes. ity <10−10 L . In a work currently in preparation, we Its luminosity is consistent both with a stellar-mass ∼ Edd shall use an X-ray survey of galactic nuclei to determine black hole (BH) and with a supermassive black hole whether galaxies with NSCs may also have signs of AGN (SMBH) emitting a few orders of magnitude below its activity. Eddington limit (as are the large majority of SMBHs in Even if we accept that a late-type galaxy such as the Local Universe). Its X-ray variability pattern, over at NGC 2403 formed a NSC but no SMBH, there is a strong least 4 orders of magnitude over a few months to years, possibility that the NSC contains stellar-mass BHs formed is more consistent with typical X-ray binaries. Its X-ray via ordinary stellar processes, or perhaps an intermediate- spectrum when the source is bright is exactly what we ex- mass BH (IMBH) formed via runaway core collapse and pect from a BH of mass ∼5−10 M⊙ in the thermal dom- 38 −1 stellar coalescence (if the NSC was sufficiently compact). inant state (LX ∼ 8 × 10 erg s ∼ LEdd, Tin ∼ 1 keV, 1/2 Then, those BHs could have grown over a Hubble time, Rin(cos θ) ∼ 50 km). SMBHs may have comparable X- via accretion during large-scale gas inflows towards the ray luminosities but tend to have power-law spectra in the galactic center, via coalescence of stellar mass BHs inside X-ray band, because their characteristic disk temperatures the NSC, or via orbital decay and coalescence of accreted < are ∼0.1 keV. We conclude that the X-ray source in the satellite galaxies or primordial halo remnants. Processes NSC of NGC 2403 is more likely to be an ordinary X-ray involving BH mergers may lead to the displacement or binary, similar to the nuclear source in the NSC of M33. escape of the merged BH due to gravitational radiation X-ray studies of nuclear sources associated with NSCs recoil, and to the formation of a lower-density stellar core provide an additional tool to understand and quantify the (Merritt et al. 2004). It is possible that some late-type relation between the formation of galactic bulges and the galaxies have a three-tier hierarchical relation between a presence and properties of a central “compact massive ob- bulge, a NSC, and an IMBH inside the NSC. Theoreti- ject” (CMO). Ideally, one should look for correlations that cal arguments have suggested that if an IMBH forms via extend unbroken from large ellipticals to small late-type core collapse in the core of a massive star cluster, its mass disk galaxies. One such fundamental correlation seems to should be ∼0.2% of the host cluster mass, coincidentally hold between the bulge mass and the CMO mass (Ferrarese 6 similar to the mass ratio between the bulge and its CMO. atively massive BH is the role of feedback. More powerful In principle, the effective radius of a NSC may provide feedback is expected from a nuclear IMBH or SMBH, lead- clues on its dynamical processes. In NGC 2403, the NSC ing to gas expulsions, perhaps large-scale outflows, and is older and less compact (re = 11.7 pc) than most other cyclic episodes of star formations. In this sense, the dis- NSCs in late-type spirals (typical re ∼ 3.5 pc). If the clus- tribution of stellar populations in a NSC provides a fossil ter was formed with such a large re, it would not have been record of past galactic activity, and may be used to re- compact enough to form an IMBH via core collapse (Porte- construct episodes of galactic mergers or phases of nuclear gies Zwart et al. 2004). Alternatively, one may speculate activity. However, in the case of a relatively low-mass NSC that the NSC was originally more compact and has evolved such as the one in NGC 2403, we find that normal stellar to such a large radius because of the presence of one or winds and SNe can provide enough energy to expel cool in- more BHs in its core (Ebisuzaki et al. 1991; Milosavljevi´c terstellar gas from the cluster and quench star formation, et al. 2002; Graham 2004). Other physical processes not without the need of AGN feedback. related to BH coalescence may also cause a star cluster to Interestingly, NGC 2403 is one of those rare galaxies expand. For example, studies of clusters in the LMC show with gas column densities below the critical value needed clusters are born compact and evolve to a range of re as to sustain activity but has a normal star formation rate they age (Elson et al. 1989; Mackey & Gilmore 2003). But (Martin & Kennicutt 2001). Perhaps a strong outflow studies also show this evolution is not due to differences from the central region has helped trigger star formation in IMF (deGrijs et al. 2002), time-varying external tidal at larger radii. Outflow may also account for some of the fields, nor differences in the number of hard primordial bi- anomalous Hi discovered in the halo of NGC 2403 (Frater- naries present (Wilkinson et al. 2003). If no massive BH nalli et al. (2002, 2004). These points will be addressed in formed, the cluster could still evolve to large re. For the a future paper (Yukita et al., in preparation). LMC clusters (none of which are thought to host a mas- The time interval since the last star formation episode sive BH!), re evolution is likely due to differences in star is longer for the NSC in NGC 2403 than is typical for clus- formation efficiencies with lower efficiencies tending to ex- ters in late-type spirals. Davidge & Courteau (2002) also pel gas before a dense stellar core (with a population of noted the lack of star formation activity more recent than massive stars) can form and leading eventually to cluster ∼100 Myr in the inner disk of NGC 2403 and suggested it expansion (Goodwin 1997; Vine & Bonnell 2003). If evo- may be due to expulsion of potential star-forming material lution is through this mechanism, then we would expect a from the central regions. Indeed, active star formation is smooth distribution of cluster re values that included the ongoing in several giant Hii regions (Drissen et al. 1999; value re = 11.7 pc found here for NGC 2403. This does Yukita et al., in preparation) surrounding the nucleus of not appear to be the case (cf. Fig. 4 of B¨oker et al. 2004) NGC 2403 but the NSC itself contains perhaps the oldest although the B¨oker et al. sample is not large. collection of stars in the region. Another difference between NSCs with or without a rel-

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TABLE 1 X-ray Observations of NGC 2403 and its Nuclear Source Luminosity

No. Mission/Instrument Date/Identifier LX(0.5-8.0 keV) texp (1037 erg s−1) (ks) 1 Chandra/ACIS-S 2001-04-17/2014 < 0.049 36.1 2 XMM/Newton/MOS+PN 2003-04-30/0150651101 < 2.8 8.0 +11.3 3 XMM/Newton/MOS+PN 2003-09-11/0150651201 69.8−8.2 7.5 4 Chandra/ACIS-S 2004-08-23/4628 < 0.005 47.1 5 XMM/Newton/MOS+PN 2004-09-12/0164560901 < 1.0 60.0 +0.1 6 Chandra/ACIS-S 2004-10-03/4629 2.9−2.5 45.1 +0.9 7 Chandra/ACIS-S 2004-12-22/4630 18.7−7.0 50.6

TABLE 2 X-Ray Spectral Fit Parameters Observation 3 Observation 6 Observation 7 Fitted energy range (keV) 0.3 - 10.0 0.5 - 2.0 0.5 - 3.5 Disk Blackbody Model 21 −2 a +0.2 +1.0 +0.7 nH (10 cm ) 0.9−0.2 0.4−0.0 0.4−0.0 +0.09 +0.06 +0.10 Tin (keV) 1.11−0.09 0.46−0.12 0.81−0.13 b +7.9 +75.5 +20.4 Rin (km) 47.0−6.6 60.1−15.1 45.7−9.1 M/M⊙ 5.4 7.2 5.4 −7 −1 M˙ (10 M⊙ yr ) 1.24 0.05 0.30 37 −1 Ldisk,bol (10 erg s ) 85.4 4.52 23.7 χ2/DOFc 73.8/97 115.6/101 215.7/204 Power Law Model 21 −2 +0.4 +2.1 +1.1 nH (10 cm ) 3.0−0.4 2.2−1.8 2.1−1.0 +0.13 +1.24 +0.42 Γ 2.22−0.13 3.05−1.15 2.28−0.40 χ2/DOFc 91.7/97 116.5/101 218.2/204 aFixed at Galactic value for Observations 6 & 7 bAssumes cos θ ∼ 1 cC-statistic/Number of data bins for Observations 6 & 7