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arXiv:0909.4048v1 [astro-ph.CO] 22 Sep 2009 fteAN n aevr o crto ae rrdaiee radiative or rates function accretion low very the have and nu of weak AGNs end the galactic a of faintest active shows the low-luminosity occupy (LLAGNs) normal These otherwise activity. of nuclear fraction large A Introduction 1. hnmnni h in litclNC19 sn h Space simila the a using quite 1399 unveiled NGC (2005) elliptical dimming giant al. a et the by O’Connell in followed 2. 4.5 phenomenon increase pe- of factor an factor years a showed a five by of (FOC) a luminosity Camera UV in Object the obtained Faint of nucleus the el- its with et the of Cappellari riod of 1995; images center al. The et the (Renzini 1999). in 4552 flare NGC UV liptical t an after of reports of discovery number in re serendipitous a been emission-line (HST) have nuclear There Telescope galaxies. low-ionization (LINER) some Space of Hubble nuclei the with observed variability see review). (SBH, a hole for black 2008, supermassive central the onto ciencies edo Send coe 9 2018 19, October srnm Astrophysics & Astronomy h lrvoe aea h etro h litclglx N galaxy elliptical the of center the at flare ultraviolet The .Cardullo A. n ftesgaue fteLAN steutailt(UV) ultraviolet the is LLAGNs the of signatures the of One rdcin ae nteglx eoiydispersion. velocity galaxy the on based predictions n 2 and n oioigo agt.TeSineDt rhv fteHu di the in of obtained Archive Data images UV Science The targets. of monitoring and 2 Vncertaset hc eeerirfudi te LLAG other the in of found time earlier scale were and which amplitude transients The nuclear months. UV 6 of period a in 1.6 1 lrvoe:glxe lc oephysics hole Black — galaxies Ultraviolet: activ variab words. AGN UV Key true and the LINERs that between idea link the missing the supports providing This source. variable UV Conclusions. Results. observ mass lines The emission 1995. the of January components and broad the 1994 measuring June between obtained images Methods. Aims. galaxies. (LINER) region variabili emission-line ultraviolet nuclear the is (LLAGNs) nuclei galactic tive Context. itiue nasphere. a in distributed litclrdoaayNC4278. NGC radiogalaxy elliptical ihteSaeTlsoeIaigSetorp SI)i the in (STIS) Spectrograph Imaging Telescope Space the with rpitoln eso:Otbr1,2018 19, October version: online Preprint ff rn eussto requests print NF sevtroAtooiod aoa ioodell’Oss vicolo Padova, di Astronomico Osservatorio INAF, iatmnod srnma nvri` iPdv,vicolo Padova, Universit`a di Astronomia, di Dipartimento hr sapesn edt cur etrsaitc bu U about statistics better acquire to need pressing a is There × h ulu fNC47 ot aeyrsle u togyv strongly but resolved barely a hosts 4278 NGC of nucleus The ag rcino tews omlglxe hw eknu weak a shows galaxies normal otherwise of fraction large A 10 1 h Vflxo h ula oreo G 28wsmaue ymea by measured was 4278 NGC of source nuclear the of flux UV The .M Corsini M. E. , 9 aais cie—Glxe:elpia n etclr cD lenticular, and elliptical Galaxies: — active Galaxies: M l h IE uliwt vial ut-pc Vobservat UV multi-epoch available with nuclei LINER the All ⊙ hsi nareetwt rvosfidnsbsdo di on based findings previous with agreement in is This . .Cardullo A. : ff aucitn.cardullo no. manuscript rn pcswt h ieFedPaeayCmr WP2 a (WFPC2) 2 Camera Planetary Field Wide the with epochs erent 1 .Beifiori A. , 1 .M Buson M. L. , eerhNote Research ywihwssrniiosydsoee ntecne fsom of center the in discovered serendipitously was which ty gion ABSTRACT elOsraoi ,I312Pdv,Italy Padova, I-35122 2, dell’Osservatorio clei di h STIS the in ed Ho ffi al. he beSaeTlsoewsqeidt n l h litclga elliptical the all find to queried was Telescope Space bble raoi ,I312Pdv,Italy Padova, I-35122 5, ervatorio s h aso h B a on ob nternebten7 between range the in be to found was SBH the of mass The Ns. Vflr nNC47 r eakbysmlrt hs ftebrig the of those to similar remarkably are 4278 NGC in flare UV 2 lt sarltvl omnpeoeo nglx etr,p centers, galaxy in phenomenon common relatively a is ility r - eino h H the of region aigadvraiiyi aaynce,bt ntrso th of terms in both nuclei, galaxy in variability and flaring V ity. .DlaBont`a Dalla E. , ftecnrlsprasv lc oe(B)wsetmtdb estimated was (SBH) hole black supermassive central the of n pcsbfr 08 efud3 bet 1 IEs 2 LINERs, (12 di objects H in 37 1 filter and found galax transition same We 1 elliptical the 2008. Seyferts, 1991) with before obtained epochs al. et images ent Vaucouleurs (de UV having RC3 archive ies (WFPC2) the 2 We all Camera galaxies. Planetary for spiral Data Field mostly Science Wide observed HST the nu- and searched UV-variable the data queried the ACS nu already monitoring for the (1995) Archive of of al. terms et terms in Maoz in press clei. and both a targets statistics, is of better there ber acquire this, to to need Given ing centers. galaxy in phenomenon perce 50 to few from HST ranging amplitudes the which with galaxies, signifi- sample spirals, of the mostly a all means almost detected in by They variability UV (ACS). sources galax- cant Surveys LINER UV for 17 Camera nuclear of Advanced compact sample who a (2005), with re of al. research ies variability et of Maoz UV kind by the This approach monitored 4579 systematic 1996). NGC more al. spiral et a cluster Barth ceived Virgo 1995; the al. in et (Maoz detected fad was rapidly source a UV while (STIS), Spectrograph Imaging Telescope la ciiy n ftesgaue ftelow-luminosity the of signatures the of One activity. clear ral Vsuc.IsU uioiyicesdb atrof factor a by increased luminosity UV Its source. UV ariable hssget htteU aiblt sarltvl common relatively a is variability UV the that suggests This aais niiul G 28—Glxe:nce — nuclei Galaxies: — 4278 NGC individual: Galaxies: — ff rn supin bu h a itiuinadwt the with and distribution gas the about assumptions erent / 70 pcrmadasmn httegsi uniformly is gas the that assuming and spectrum G750M osadadtce ai oeaecaatrzdb a by characterized are core radio detected a and ions α msinln.Teedt eefudol o the for only found were data These line. emission so pruepooer nteWFPC2 the on photometry aperture of ns 1 .Morelli L. , dpsil ihncersetaobtained spectra nuclear with possibly nd 1 .Pizzella A. , II ulu,1 uecn galaxies, quiescent 17 nucleus, 1 low-ionization e n .Bertola F. and , C4278 GC aiswith laxies

number e c / F218W erhaps S 2018 ESO × htest 10 ac- y 7 were ff 1 ing nt. m- er- - - - 2 Cardullo et al.: UV flare in NGC 4278

P.I. A. Koratkar). Two exposures of 2200 s and 2300 s were ob- tained on 14 January 1995 (Prog. Id. 5381, P.I. A. Koratkar). The exposureswere taken with the telescope guiding in fine lock, which typically gave an rms tracking error of 0′′. 003. We focused our attention on the Planetary Camera (PC) chip where the nu- cleus of the galaxy was centered. This consists of 800 × 800 pixels of 0′′. 0455×0′′. 0455 each, yielding a field of view of about 36′′ × 36′′. The images were calibrated using the standard WFPC2 re- duction pipeline maintained by the Space Telescope Science Institute (STScI). Reduction steps including bias subtraction, dark current subtraction, and flat-fielding are described in detail Fig. 1. WFPC2/F218W images of the NGC 4278 nucleus of 2 in the WFPC2 instrument and data handbooks (Holtzman et al. June 1994 (left) and 14 January 1995 (right), plotted with the 1995b; Baggett et al. 2002; McMaster et al. 2008). Subsequent same grayscale. Each panel is 2′′. 5 on a side, north is up and reduction was completed using standard tasks in the STSDAS east at the left. The UV flux was measured within the aperture package of IRAF1. The bad pixels were corrected by means of a marked by the circle (r = 0′′. 28). linear one-dimensional interpolation using the data quality files and the WFIXUP task. The two 1995 images were aligned and combined using IMSHIFT and knowledge of the offset shifts. and 4 objects with unknownnuclear activity; Ho et al. 1997, and Cosmic ray events and residual hot pixels were removed using NASA/IPAC Extragalactic Database) with at least one F218W the LACOS IMA procedure (van Dokkum 2001). The cosmic or F300W image. NGC 4278 was the only galaxy with multi- ray removal and bad pixel correction were checked by inspec- epoch WFPC2/F218W images, which was not studied before. tion of the residual image between the cleaned and the original A nuclear spectrum obtained within a subarcsecond aperture by frame to ensure that the nuclear region was not affected by light STIS in the Hα region was also available in HST Science Data loss. The residual cosmic rays and bad pixels in the PC were cor- Archive. In this paper, we present and discuss the results about rected by manually editing the resulting image with IMEDIT. −1 the UV variability of its nucleus including the estimate of the The sky level (∼ 1 count pixel ) was determined from appar- mass of its SBH based on STIS spectroscopy. ently empty regions in the Wide Field chips and subtracted from the PC frame after appropriate scaling. The flux calibration was performed by adopting the 2. NGC 4278 Vega magnitude system (Whitmore 1995) and by taking ′ ′ into account for the time dependence of the UV response NGC 4278 is a large (4.1 × 3.8, RC3) and bright (BT = 11.09, (Mc Master & Whitmore 2002). In fact, the presence of con- RC3) . It is classified E1–2 and hosts a LINER taminants within WFPC2 causes a gradual build-up of mate- nucleus (L1.9, Ho et al. 1997). Its total absolute magnitude rial on the cold CCD faceplate of the camera, resulting in a is M0 = −20.03 (RC3), adopting a distance of 16.1 Mpc BT decrease in the UV throughput. The contaminants are evapo- (Tonry et al. 2001). NGC 4278 is member of the Coma I cloud rated by periodically heating the camera in order to restore the (Forbes 1996). It was one of the first elliptical galaxies in instrumental throughput to its nominal value. The contamina- which neutral hydrogenwas detected (Gallagher et al. 1977) and tion rate is remarkably constant during each decontamination used to infer the dark mater content at large radii (Bertola et al. cycle, and can be accurately modeled by a simple linear de- 1993). It is distributed in an inclined outer ring (Raimond et al. cline following the decontaminations. The observed fluxes were 1981), which is possibly associated to the inner disk of ion- corrected by assuming a decline in the F218W/PC normalized ized gas (Goudfrooijet al. 1994; Sarzi et al. 2006). The north- count rate of (4.78±0.28)×10−3 per day since decontamination. west side of the galaxy is heavily obscured by large-scale dust This was derived during the decontamination cycles performed patches and filaments which seem to spiral down into the nucleus between April 1994 and June 1995 (Mc Master & Whitmore (Carollo et al. 1997). 2002), which nicely bracket the observations of NGC 4278. The optical and radio properties of the nucleus have been Evidence of the presence of a nuclear source was found investigated in detail (see Giroletti et al. 2005, and references in the two final images of NGC 4278. It is barely resolved therein). The radio data reveal two symmetric steep-spectrum (FWHM = 0′′. 07) when compared to the WFPC2/F218W point jets on sub scale. They emerge from a flat-spectrum core spread function (PSF, FWHM = 0′′. 06) derived with the TINY and are responsible for the bulk of the emission at radio to op- TIM package (Krist & Hook 1999). The total flux of the nu- tical frequencies in a similar way to that seen in more power- clear source was estimated as the flux in the circular aperture ful radio loud AGNs. However, the total radio luminosity of of radius of 0′′. 28 (Fig. 1). The background level was deter- NGC 4278 is at least 2 orders of magnitude less than those mined as the median of the flux within the annulus of 0′′. 37– objects (Condon et al 1998). This makes NGC 4278 a LLAGN 0′′. 51 centered on the source. The correction for the finite aper- (Giroletti et al. 2005). ture radius has been done by multiplying counts by 1.18 ± 0.01, based on the encircled-energy value of a point source tabulated in Holtzman et al. (1995a). The errors were calculated taking in 3. Observations, data reduction, and analysis account the Poisson and CCD readout noises, charge transfer ef- 3.1. Nuclear ultraviolet variability ficiency, correction for contamination, and correction for finite aperture. The UV flux of the central source of NGC 4278 in the The multi-epoch images obtained with the WFPC2 and the F218W filter were retrieved from the HST Science Data Archive. 1 Imaging Reduction and Analysis Facilities is distributed by A 1800 s exposure was taken on 2 June 1994 (Prog. Id. 5380, National Optical Astronomy Observatories (NOAO). Cardullo et al.: UV flare in NGC 4278 3

60 NGC 4278 60 NGC 4278 ) −2 40 40 cm −1 Å −1 20 20 erg s

−17 0 0 (10 λ F

−20 −20

6500 6550 6600 6650 6700 6720 6740 6760 6780 λ(Å) λ(Å) Fig. 2. UV light curve in WFPC2/F218W band for the nucleus Fig. 3. Continuum-subtracted central spectrum of NGC 4278 in of NGC 4278. Points correspond to fluxes measured on 2 June the Hα+[N II] (left panel)and [S II] (right panel) regions.In each 1994 and 14 January 1995, respectively. panel, the red solid line shows the overall line blend, whereas the green dashed-dotted lines and blue dotted lines show the adopted narrow and broad Gaussian components, respectively. Shown are WFPC2/F218W passband increased from (6.94 ± 0.46) × 10−16 also the fit residuals, offset for better visibility. to (10.83 ± 0.25) × 10−16 erg cm−2 s−1 Å−1 from 2 June 1994 to 14 January 1995 (Fig. 2). of NGC 4278 with the fitted emission lines. It was extracted from 3.2. Estimate of the mass of the central black hole the three central rows of Hα STIS/G750M spectrum centered on the continuum peak. It thus consists of the central emission The long-slit spectrum of the nucleus of NGC 4278 obtained convolved with the STIS spatial PSF and sampled over a nearly with STIS in the region of the [N II] λλ6548, 6583, Hα, and square aperture of 0′′. 15 × 0′′. 2 (corresponding to 12 × 16 pc2). [S II] λλ6716, 6731 emission lines (Prog. Id. 7403, P.I. A. Filippenko) was retrieved from the HST Science Data Archive. The mass M• of the central SBH can be estimated from the virial theorem by assuming that the gas is uniformly distributed The G750M grating was used at the secondary tilt λ = 6581 Å c within a sphere of radius R and moves around the SBH with a covering the wavelength range 6295–6867 Å. The spectrum was mean velocity measured from the width of the broad component ′′. × ′′ taken with the 0 2 52 slit placed across the galaxy nucleus of the Hα line. with a position angle of 87◦.9. The total exposure time was 3128 The lower limit on the size of the broad-line emitting region s. The dispersion is 0.55 Å pixel−1. The instrumental resolution −1 is given by the upper limit to the gas density as set by the forbid- is 1.6 Å (FWHM) correspondingto σinst ≈ 32km s atHα. The den lines and Hα luminosity. Following Osterbrock (1989) spatial scale of the 1024 × 1024 SITe CCD is 0′′. 05 pixel−1. The spectrum was reduced using IRAF and the STIS re- 1 3 duction pipeline maintained by the STScI. The basic reduction  3LHα  steps including overscan subtraction, bias subtraction, dark sub- Rmin =   (1) 4π fn2αeff hν  traction, and flatfield correction are described in detail in the e Hα Hα STIS instrument and data handbooks (Kim Quijano et al. 2007; Dressel et al. 2007). The cosmic ray events and hot pixels were where LHα is the total luminosity of the Hα broad component, removed using the task LACOS SPEC by van Dokkum (2001). f is the volume filling factor, ne is the electron density of the eff The residual bad pixels were corrected by means of a linear one- gas, αHα is the recombination coefficient for Hα, h is the Planck dimensional interpolation using the data quality files. The wave- constant, and νHα is the frequency of the Hα line. The ratio of 5 −3 length and flux calibration as well as geometrical correction for the [S II] lines sets ne ≈ 10 cm , whereas the spherical and two-dimensional distortion were performed following the stan- uniform distribution of the gas gives a filling factor equal to 1.It 38 −1 dard STIS reduction pipeline and applying the X2D task. This is LHα = 4.13×10 erg s from the measuredflux (1.34±0.10)× −14 −1 −2 task corrected the wavelength scale to the heliocentric frame as 10 erg s cm and assumed distance. It results Rmin = 0.1 −1 well. pc. In this region σV = 1224 ± 40 km s , which corresponds 2 7 The central wavelengths, FWHMs, and intensities of the to a lower limit on the SBH mass of M•≥ RminσV /G ≃ 7 × 10 all the observed emission lines were measured following M⊙. Beifiori et al. (2009). The broad and narrow components of the The upper limit on the size of the broad-line emitting re- emission lines were fitted with multiple Gaussians, while de- gion Rmax can be estimated from the intrinsic emissivity dis- scribing the stellar continuum with a low-order polynomial.A tribution of the gaseous sphere. An intrinsically Gaussian flux flux ratio of 1:2.96 was assumed for the [N II] doublet, as dic- profile centered on the stellar nucleus was assumed. It has a tated by atomic physics (e.g., Osterbrock 1989), both the [N II] FWHM = 0′′. 19 (15 pc) when accounting for the STIS PSF and [S II] doublets were assumed to share a common line cen- (FWHM = 0′′. 09). The choice of a Gaussian parametriza- troid and width. A broad component was needed to describe tion is also conservative, since cuspier functions would have ′′ the Hα emission out to 0. 20 from the center. The forbidden led us to estimate smaller M•. It results Rmax = 5.9 pc and ′′ −1 [N II]and[S II] lines required a broad component from −0. 05 to σV = 1127 ± 30 kms which translates into an upper limit ′′ 2 9 +0. 1. Fig. 3 shows the continuum-subtracted nuclear spectrum on the SBH mass of M•≤ RmaxσV /G ≃ 2 × 10 M⊙. 4 Cardullo et al.: UV flare in NGC 4278

4. Conclusions observed by HST in different epochs in the UV and which was not studied before. This fact does not allow us to derive any firm UV variability with amplitudes ranging from few to 50 percent statistical conclusionabout the frequencyof UV flares at the cen- over a timescale of a decade was detected in most of the LINER ter of elliptical galaxies. Nevertheless, it has to be remarked that nuclei observed more than once (Maoz et al. 2005) suggesting out of 37 galaxies the only object with multi-epoch UV obser- a possible link between UV flares and SBH-related activity in vations turned out to be variable. This is a further suggestion LLAGNs. To acquire better statistics, both in terms of the num- that this phenomenon has to be quite common. Additional imag- ber of targets and their monitoring we queried the HST Science ing with the Wide Field Camera 3 recently installed on HST to Archive for all the elliptical galaxies with available UV images monitor the UV variablity in a statistically significant sample of obtained with the WFPC2 in different epochs. quiescent and active nuclei and STIS spectra to measure their Multi-epoch images were found only for NGC 4278, a SBHs are highly desirable to gain insights on this subject in the nearby radiogalaxy known to host a LLAGN (Giroletti et al. next future. 2005). It is characterized by a barely resolved nuclear source, which increased its UV luminosity by a factor of 1.6 in a pe- Acknowledgements. AC acknowledges the Space Telescope Science Institute for hospility while this work was in progress. We thank Massimo Stiavelli and riod of 6 months. The amplitude and scale time of the varia- Michele Cappellari for useful discussions and their helpful comments. This tion are similar to those of the UV-brightest nuclear transients, work was made possible through grants CPDA068415 and CPDA089220 by which were earlier discovered in NGC 4552 (Renzini et al. Padua University. This research has made use of the Lyon-Meudon Extragalactic 1995; Cappellariet al. 1999), NGC 4579 (Maoz et al. 1995; Database (LEDA) and NASA/IPAC Extragalactic Database (NED). Barth et al. 1996), and NGC 1399 (O’Connell et al. 2005). These serendipituous findings support the idea the UV vari- References ability is a common event at the center of galaxies where SBHs reside. Some alternatives to the AGN interpretation were ex- Baggett, S., et al. 2002, WFPC2 Data Handbook, Version 4.0 (STScI, Baltimore) Barth, A. J., Reichert, G. A., Filippenko, A. V., Ho, L. C., Shields, J. C., plored to explain the UV variability. Maoz et al. (2005) pointed Mushotzky, R. F., & Puchnarewicz, E. 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