Mon.Not.R.Astron.Soc.000,1–7(2011) Printed7January2011 (MNLATEXstylefilev2.2) Gamma-ray variability of radio-loud narrow-line Seyfert 1 galaxies G. Calderone1⋆, L. Foschini2, G. Ghisellini2, M. Colpi1, L. Maraschi3, F. Tavecchio2, R. Decarli4, G. Tagliaferri2 1Universita`di Milano - Bicocca, Dip. di Fisica G. Occhialini, Piazza della Scienza 3, I-20126 Milano, Italy 1 2INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy 1 3INAF Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milano, Italy 0 4Max-Planck-Institut fu¨r Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany 2 n a J Accepted 2011January06.Received2011January06;inoriginalform2010November16 6 ABSTRACT ] E The recent detection of γ-ray emission from four radio-loud narrow-line Seyfert 1 H galaxiessuggeststhattheenginedrivingtheAGNactivityoftheseobjectssharesome similaritieswiththatofblazars,namelythepresenceofaγ-rayemitting,variable,jetof . h plasmacloselyalignedtothelineofsight.Inthisworkweanalyzetheγ-raylightcurves p ofthefourradio-loudnarrow-lineSeyfert1galaxiesforwhichhigh-energygamma-ray o- emission has been discovered by Fermi/LAT, in order to study their variability. We r findsignificantflux variabilityinallthe sources.Thisallowsus to exclude astarburst t s originoftheγ-rayphotonsandconfirmsthepresenceofarelativisticjet.Furthermore a we estimate the minimum e-folding variability timescale (3 – 30 days) and infer an [ upper limit for the size of the emitting region (0.2 – 2 pc, assuming a relativistic 1 Doppler factor δ =10 and a jet aperture of θ =0.1 rad). v Keywords: galaxies:jets–galaxies:Seyfert–galaxies:individual:PMNJ0948+0022 5 - 1H 0323+342- PKS 1502+036- PKS 2004-447– gamma-rays:observations. 6 2 1 . 1 0 1 INTRODUCTION (z=0.585, Williams et al. 2002) is one of the strongest ra- 1 dio source among the RL-NLS1; for its fast radio variabil- 1 Narrow-line Seyfert 1 (NLS1) galaxies are a class of AGN ity, source compactness, inverted radio spectrum and high : characterized by a rather narrow width of the Hβ emission iv line(FWHM <∼ 2000km/s),andbythepresenceofastrong bRrLig-NhtLnSe1ssftoermwpheircahtutrhee,PprMesNenJc0e94o8f+a0j0e2t2hiassobneeeonfhthyepofitrhst- X FeII bump,a soft X-ray excess and fluxratio [OIII]/Hβ<3 esized (Zhou et al. 2003; Doi et al. 2006). The detection of (Osterbrock & Pogge 1985; Pogge 2000). These sources are r γ-ray emission definitively confirms the presence of a jet a usually radio-quiet, although, in a few cases, they show and allows to build a complete SED which closely resem- a radio-loudness parameter R (ratio between the 5 GHz ble that of a typical blazar, with two broad peaks in the andopticalBfluxdensities,Kellermann et al.1989)greater far IR and in the γ-ray range respectively. By modeling than 1000 (Komossa et al. 2006; Yuanet al. 2008). Such the SED with the synchrotron and inverse-Compton model sources, dubbed radio-loud narrow-line Seyfert 1 galaxies (Ghisellini & Tavecchio 2009) it is possible to roughly esti- (RL-NLS1),werethoughttobeinactiveinγ-rays,although mateimportantparameterssuchastheblackholemass,the severalauthorsspeculatedtheoccurrenceofsimilaritieswith Eddington ratio, and the power carried by the jet. The re- blazars (Zhou et al. 2003; Komossa et al. 2006; Zhou et al. sultingvalues, although model-dependent,are usually com- 2007; Yuanet al. 2008; Foschini et al. 2009; Gu & Chen patiblewithestimatesfoundindependentlyinotherstudies. 2010). The important discovery of γ-ray emission from the In the case of PMN J0948+0022 the black hole mass turns RL-NLS1 source PMN J0948+0022 (Foschini et al. 2010b; out to be ∼1.5 × 108 M⊙ (compatible with the range of Abdoet al.2009a)confirmedthesesimilarities,i.e.thepres- values found by Zhou et al. 2003 using the empirical rela- ence of a jet closely aligned to the line of sight as a source tion between the FWHM of emission lines and the contin- of Compton up-scattered γ-ray photons. PMN J0948+0022 uum luminosity), and the Eddington ratio which is ∼40%. A γ-ray variability of a factor 2.2 rules out the possibility that the γ-ray emission is due to a starburst contribu- ⋆ E-mail:giorgio.calderone@mib.infn.it tion (Abdoet al. 2009b). Shortly after this discovery, three (cid:13)c 2011RAS 2 G. Calderone et al. more RL-NLS1 had been observed in γ-rays (Abdoet al. Whiletheγ-rayvariabilityofPMNJ0948+0022hasal- 2009c): 1H 0323+342, PKS 1502+036 and PKS 2004-447, readybeenstudiedinAbdoet al.(2009b)and,withahigher thusconfirmingthatatleastsomeRL-NLS1areγ-rayemit- level of significance, during the outburst occurred in July ting AGN. The SED of these sources has also been mod- 2010 (Foschini et al. 2010c), it has never been studied for eledwiththesynchrotronandinverse-Comptonmodel,pro- the remaining three sources. Aim of this paper is to ana- viding black hole mass estimates in agreement with previ- lyzetheFermi/LATlightcurvesoftheafore-mentionedRL- ous studies (see Abdoet al. 2009c, and references therein). NLS1galaxiesinordertoputtheγ-rayvariabilityonafirm 1H 0323+342 (z=0.06, Marcha et al. 1996) is considered to basis, and to find the minimum γ-ray variability timescale. be a composite nucleus of a NLS1 (for its optical spec- Throughoutthepaper,weassumeaΛCDMcosmologywith trum properties typical of NLS1) and a blazar (for its flat H0 = 71 km s−1 Mpc−1, Ωm = 0.27, ΩΛ = 0.73. radio spectrum, core compactness and X-rays variability, Zhou et al. 2007). It is the only γ-ray detected RL-NLS1 for which we have an HST/WFPC2 optical image of the 2 DATA ANALYSIS host galaxy. The host shows a ring-like structure of 15.6 kpc in diameter and the entire galaxy looks like a one- The data analysis has been performed following the stan- armed spiral (Zhou et al. 2007). Another study, based on dardproceduredescribedintheFermi/LATdocumentation. NOTdata,suggeststhat1H0323+342 maybetheremnant The Science Tools software version is 9.15.2 and the IRF is of a galaxy merger (Antónet al. 2008). The most striking P6_V3_DIFFUSE.Dataspantheperiodfrom2008august4to feature of this source is the high luminosity of the accre- 2010 october 08 (∼26 months). For each RL-NLS1all class tion disc which, according to SED modeling, is approxi- 3 (diffuse) events coming from a zenith angle < 105◦ were mately 90% of Eddington value (Abdoet al. 2009c). PKS extracted within an acceptance cone of radius 10◦ around 1502+036 (z=0.41, Snellen et al. 2002) is the second most thecatalog sourceposition. Theunbinnedlikelihood analy- powerful source in terms of Eddington ratio (80 per cent). sis has been performed modeling the spectrum of each RL- It carries a jet power, inferred from SED modeling, sligthly NLS1and of all nearby(<10◦) sources presentin theLAT lower than PMN J0948+002 (Abdoet al. 2009c). Finally, 1-yearpointsourcecatalog(Abdoet al.2010b)withapower PKS 2004-447 (z=0.24, Drinkwateret al. 1997) is an un- lawintherange0.1–100GeV.Wecomputedtheintegrated usualRL-NLS1.Itisastrongradioemitter,withthehighest fluxforeachsourceanalyzingalldataovertheentireperiod radio-loudness parameter (R>6000) among the four γ-ray of26 months.Theextension of theregion of interest to15◦ detected RL-NLS1. PKS 2004-447 has an unusually weak around the catalog position results in non-significant varia- Fe II complex compared to other NLS1 and a rather steep tions of the flux, number of counts and TS (Test Statistic, (although variable) radio spectral index compared to the Mattox et al. 1996), as expected since the PSF of LAT is other RL-NLS1. Oshlack et al. (2001) suggests that it may at most 5◦ wide at 100 MeV. We also performed the same be well classified as a low-luminosity compact steep spec- analysisusingalternativespectralmodels.Theuseofabro- trum (CSS) radio quasar, also considering its low inferred kenpowerlawinsteadofasimplepowerlawmodelyieldsto blackholemass(∼106M⊙)(Abdoet al.2009c).Spectralfit a slight increase in the values of TS for PMN J0948+0022 of the SED may require a thermal Comptonization compo- (from 993 to1194) and1H 0323+342 (from 86 to115), and nent(Gallo et al.2006)oranexternalComptoncomponent a decrease for PKS 1502+036 and PKS 2004-447. With a (Abdoet al. 2009c). log-parabola, the values of TS remain essentially the same The detection of γ-rays from the four mentioned RL- as for the case of a power law for PMN J0948+0022 (from NLS1allows toidentifyanewclass ofγ-rayemittingAGN. 993 to 999) and PKS 2004-447 (from 97.4 to 98.5), while Theradioproperties(namelytemporalvariability,flatspec- they decrease for 1H 0323+342 and PKS 1502+036. In the trum and high brightness temperature) together with the following analysis we will therefore use thepower law mod- γ-ray detection suggests the presence of a relativistic jet els. Then we extracted light curves with time binning of 15 closely aligned to the line of sight. Low power, mildly rela- days(Fig.1;thetimebinningforPKS2004-447 is30days) tivistic and poorly collimated radio jets have already been using a TS threshold of 10 (TS>10, roughly equivalent to observedin afew spiralgalaxies hostingSeyfert nuclei(e.g. 3σ, Mattox et al. 1996) and performed a chi-squared test Keel et al. 2006, and references therein), but in the case of against the null hypotesis of constant flux. If the detection PMNJ0948+0022 andPKS1502+036thepowercarriedby wasnotsignificant(TS<10)wecomputedanupperlimitto the jet, as inferred from SED modeling, is in the range of theflux by varyingthesource fluxvalue (obtained through quasars, while in 1H 0323+342 and PKS 2004-447 is in the maximization of likelihood) until TS reaches a value of 4 rangeofBLLacobjects(Abdoet al.2009c).Suchpowerful (Abdoet al. 2010b). The resulting fluxes (denoted with ar- jets are observed only in blazars hosted in elliptical galax- rows in the figure) corresponds to 2σ upper limits. When ies (Marscher 2009) with black hole masses in the range TS<1 we didn’t compute the upper limit since it would be 108 - 109 M⊙. By contrast black hole masses in γ-ray de- overestimated. tected RL-NLS1 (106 - 108 M⊙) are up to three orders of Wefurtherproceededontheanalysisofthelightcurves magnitude smaller than for blazars, thus suggesting that in order to compute the minimum e-folding timescale for these sources emit at high Eddington ratios. Furthermore, each source. We extracted light curves with different time itseemsthattheMBH-σ∗ scalingrelation doesnotapplyto binnings starting from 30 days. When the detection is sig- NLS1,mainlyduetotheirsmallFWHMofpermittedlines. nificant (TS>10) we re-run the analysis halving the time Decarli et al. (2008) showed that a reconciliation with the bin interval, down to a minimum of approximately 6 hours scalingrelationispossibleifthebroadlineregionisassumed (roughly corresponding to four Fermi orbits, thus ensuring to havea disc-like geometry (but see Marconi et al. 2008). that each source is observed at least twice for each tem- (cid:13)c 2011RAS,MNRAS000,1–7 Gamma-ray variability of radio-loud narrow-line Seyfert 1 galaxies 3 Figure 1.Upper panels: lightcurves of the four RL-NLS1fordetections withTS>10. Fluxes aregiven inunits of 10−6 phcm−2 s−1 intherange0.1–100GeV.Verticalerrorbarscorrespondto1σ errors,whilehorizontalbarscorresponds tothetimebinning(15days for PMN J0948+0022, 1H 0323+342 and PKS 1502+036, 30 days for PKS 2004-447). Upper limits (TS<10) at 2σ level are denoted by arrows. Middle panels: photon indices. Vertical error bars correspond to 1σ errors. In both panels horizontal dashed lines are the integrated (over the entire periodof 26 months) flux value and photon index respectively. Lower panels: TS values (plus symbols) and numberofcounts (trianglesymbols,valuesontherightaxis),thehorizontaldottedlineisthethreshold(TS=10). poral bin). Then, we considered all combinations of non- 3 RESULTS overlappingtimebinswiththefollowing characteristics: (1) both time bins havea significant detection, TS>10; (2) the Fig.1showsthelightcurves(upperpanels)ofthefourRL- fluxdifferenceisgreaterthanthegreatestfluxerrorinvolved NLS1overtheentireperiodof26months.Verticalerrorbars at the 3σ level; (3) the count difference is significant at the are 1σ errors on fluxes, while horizontal bars are equal to 3σ level, assuming a Poisson statistic; (4) the number of thetimebinning(15days,exceptPKS2004-447forwhichis countsineachbinmustbegreaterthan3.Forsuchpairsof 30days)forallpointswithasignificantdetection(TS>10). binswe computed thee-folding timescale as: Points without a significant detection are upper limits de- notedbyarrows. Middlepanelsshowthephotonindicesfor τ = ti−tj (1) points with significant detection, assuming a simple power ij lnF /F law model (F(E) ∝ EΓ) for each source in the range 0.1 – (cid:12) i j(cid:12) (cid:12) (cid:12) 100GeV.Horizontaldashedlinesinbothupperandmiddle where i and j are theindice(cid:12)s of thein(cid:12)volved time bins(i> (cid:12) (cid:12) panels are theintegrated (over26 months) fluxand photon j). The associated error (at 3σ level) is computed through index respectively. Lower panels show the TS value (plus error propagation: symbol) and number of counts (triangle symbol) for each 2 2 1/2 bin with significant detection. The integrated luminosity ∆τ = τij (∆t )2+ τ ∆Fi + τ ∆Fj and photon indices, as well as the χ2 values and DOF in ij ti−tj " ij (cid:18) ij Fi (cid:19) (cid:18) ij Fj (cid:19) # thehypotesisof constant flux,are reported in Table 1. The (2) minimume-folding variabilitytimescale computedwith Eq. where∆t ishalfthewidth ofthewidertimebininvolved, 1 and 2 is reported in Table 1. The detailed view (zoom) ij ∆F and ∆F are the 3σ error on fluxes. We compute the on the time bins involved in the computation of the mini- i j minimum e-folding variability timescale as τ = min(τ ), mum e-folding variability timescale are shown in Fig. 2. To ij and associate the corresponding error ∆τ using Eq. 2. The compute τ, points with different time binnings are consid- reliability of Eq. 2 has been assessed by simulating 4 se- ered and are denoted with different horizontal bar lengths ries (respectively for t , F , t and F ) of 10000 normally in Fig. 2 (e.g. for PMN J0948+0022, one time binning is i i j j distributed values. The resulting values of τ are normally 0.23 days and the other is 7.5 days. The bar corresponding distributedandthe3σ confidenceintervalsarecorrectly es- to the shorter time bin is barely visible). Similarly, vertical timated by ∆τ when the mean and standard deviation of barsreferto1σ errorsintheflux.Thesquaresymbolsiden- thesimulated data are used in Eq. 2. tify the points that fulfill the constraints described in Sect. (cid:13)c 2011RAS,MNRAS000,1–7 4 G. Calderone et al. Figure 2. Detail view on the bins (denoted by squares) involved in the computation of the e-folding minimum variability timescale. Units and meaning of symbols arethe same as inFig. 1 (upper and lower panels). Flux symbols have been changed to × fora clearer visibility.SeealsoTable2. Table 1. Data and results of the analysis on the four RL-NLS1 sources. Columns are: (1) name of the source; (2) redshift; (3) luminosity distance; (4) radio loudness; (5) integrated γ-ray luminosity (0.1 – 100 GeV) over the entire period (26 months) with errors at 1σ level; (6) photon index with errors at 1σ level; (7) χ2 and (8) DOFcomputed on the light curves of Fig. 1 inthe null hypotesis of constant flux equal to theintegratedflux;(9)minimume-foldingvariabilitytimescalewitherrorat3σ level. Source z DL R Lγ Γ χ2 DOF τ [Gpc] [1045 ergs−1] [days] PMNJ0948+0022 0.59 3.40 1000 250.00±13.04 −2.851±0.007 528 33 3.3± 2.5 1H0323+342 0.06 0.27 151 0.16± 0.04 −2.807±0.010 2575 5 17.7±14.4 myPKS1502+036 0.41 2.20 1549 41.45± 4.10 −2.708±0.007 385 21 12.0± 9.0 PKS2004-447 0.24 1.20 6320 3.85± 0.70 −2.650±0.006 4032 4 28.4±18.5 Table 2.Quantitiesinvolvedinthecomputationoftheminimume-foldingvariabilitytimescale. Source ta ∆tb Fγc N/∆t TS [days] [days] [10−6 phcm−2 s−1] [ctsdays−1] PMNJ0948+0022 703.20 0.23 2.578±0.584 67.5 38 709.40 7.50 0.389±0.058 7.0 53 1H0323+342 102.40 0.94 0.622±0.117 21.5 21 135.70 30.00 0.095±0.034 2.6 29 PKS1502+036 198.00 0.94 0.674±0.058 15.8 16 225.70 30.00 0.067±0.025 1.9 41 PKS2004-447 75.66 30.00 0.001±0.001 0.1 16 214.40 7.50 0.197±0.006 3.5 11 a MJD-54682 b Timebinning. c Fluxintherange0.1–100GeV. (cid:13)c 2011RAS,MNRAS000,1–7 Gamma-ray variability of radio-loud narrow-line Seyfert 1 galaxies 5 2 and provide the value of τ. Notice, as an example, that the requirement that the source is detected with high sig- the point at time ∼706 (MJD - 54682, PMN J0948+0022) nificance, TS> 10 (rule (1) in Sect. 2). Since the shortest isexcludedfromthecomputationsinceitsfluxvalueiscom- width of the time bins scales linearly with the inverse of patible with nearby points at 3σ level (rule (2) of Sect. 2). the photon flux, we may use narrower time bins (and de- The quantities involved in the calculation of τ (time, time tect shorter timescales) on brighter sources. The accuracy binning, photon flux, number of counts and TS values) are of τ is improved if we compute the e-folding timescale us- reported in Table 2. ing non-contiguous time bins, since the exponential law is betterconstrainedbydistantpointsratherthancloserones. On the other hand, if the bins are contiguous, the error ∆τ may be significantly greater than τ itself, i.e. the mea- 4 DISCUSSION surewouldbeuseless.Theissuerelatedtothecomputation A statistically significant variability over the entire period of τ with non-contiguous time bins is that we are deliber- of 26 months is present for all sources. The significance of ately ignoring what lies between the two time bins, even this variability on timescales of <∼2 years is supported by if we have a significant measure. An example is given in the chi-squared test performed against the null hypotesis Fig. 2 for PMN J0948+0022 (upper-left panel) where the of constant flux (last two columns of Table 1). This rules minimum e-folding timescale is computed ignoring the flux out the possibility that the γ-ray emission is due to a star- measurement at time ∼ 706 (MJD - 54682). The reason burst activity. Thus, the data support the hypotesis that is that the flux measure has a rather big error bar, which γ-ray photons are associated to the presence of a jet. We does not allow us to consider it statistically incompatible cannotexcludethepossibility ofastarburstactivitybutits at the 3σ level with neighbouring flux measures (rule (2) contribution would be negligible compared to the jet emis- in Sect. 2). This example suggests that the intrinsic vari- sion,sincetheγ-rayluminosities(Table1)foundinourRL- ability timescale, i.e. a measure of “how fast” the source is NLS1 are at least four order of magnitude greater than the able to change its flux significantly, may be shorter than archetypal starburst galaxy M82 hosting a quiescent black our measure of τ, although we cannot assess it on a firm hole(Gaffney et al.1993)andwhoseγ-rayluminosityinthe statistical basis. This is a consequence of the fact that we 0.1 – 100 GeV range is ∼1040 erg s−1 (Abdoet al. 2010a). are assuming a particular function (the exponential) and The next step in our analysis has been to estimate the that we are using only two flux measurements instead of minimum timescale variability for each source. Among the all available data. If our detector were able to monitor the many methods to measure a timescale variability we chose flux evolution continuously, we could have employed more the e-folding timescale (Eq. 1). The main advantage of this sophisticated methods, such as the Fourier analysis, in or- methodisthatitallowsacomputationofthetimescaleusing derto estimate theintrinsic minimum timescale variability. justtwofluxmeasurements;itdoesnotrequireanyfittingor Actually, our light curves do not allow a continuous mon- minimising procedure. Furthermore the resulting timescale itoring of the sources with time binning of 30 days, and iswelldefinedandcanbeusedtocomparedifferentsources. we are thus forced to rely on the e-folding timescale. As a Finally, the error on the timescale is easily computed us- consequence, our measure of τ should be considered as an ing analytical error propagation (Eq. 2). The underlying upper limit for the intrinsic minimum timescale variability. assumption is that the flux evolve according to an (either The sources display an e-folding minimum timescale vari- increasingordecreasing)exponentiallaw.Althoughthisas- ability in therange of 3– 30 days. Inparticular, variability sumptionisnotalwaysjustified(e.g.inthepresenceofflar- oftheorderof∼dayshasbeenfoundforPMNJ0948+0022, ingepisodes) itisofcommon practicesinceweoften donot and of the order of ∼tens of days for 1H 0323+342, PKS know the actual law which drives the evolution of the flux. 1502+036andPKS2004-447.Theminimummeasuredvari- Furthermore,therequirementofjusttwofluxmeasurements ability timescales in blazars can be as low as 200 seconds canbeeasilyaccomplishedevenforweaksourcessuchasthe at very high energies (E>200 GeV) and 800 seconds at X- onesanalyzedinthiswork.Therulestoselectthefluxmea- rays (see Aharonian et al. 2007, and references therein). At surementsusedinthecomputationofthee-foldingtimescale Fermi/LAT energies variability on scales of few hours has havebeen described in Sect.2: rules(2) and (3) areneeded been detected in several blazars (e.g. Foschini et al. 2008; toensurethatthefluxinthetwotemporalbinsareactually Tavecchio et al. 2010; Foschini et al. 2010a). In particular, different,otherwiseourassumptionofexponentiallyvarying thebrightblazars3C454.3 andPKS1510-089 showed vari- flux cannot be justified. Rules (1) and (4) ensure that the ability on scales of 3–6 hourswith flux∼10−5 ph cm−2 s−1 flux measures and associated errors are reliable. The rela- (Tavecchio et al. 2010), that is 1 – 2 orders of magnitude tive error ∆τ/τ of our measure of the minimum e-folding greater than the flux of the sources analyzed here. As dis- timescale depends on several factors. The main sources of cussed above, we expect to measure longer timescales on uncertainties are the errors in the flux measurements ∆F weaker sources, as a consequence of the longer time inte- and thewidth of thetimebins∆t.The ratio ∆F/F can be grationrequiredtosignificantlydetectthesource.Ourmin- made smaller by increasing the number of counts detected imum timescale estimates are indeed approximately 1 – 2 inawidertimebin(greater∆t).Viceversa,anarrowertime ordersofmagnitudelongercomparedtothoseoftheabove- bin results in a smaller number of counts and consequently mentioned blazars. Therefore, we cannot exclude the possi- a poor accuracy in the flux measurement. The best achiev- bility that variability in RL-NLS1 is as fast as the one ob- able accuracy on τ (that is the smaller value of ∆τ/τ) is servedin someof themost luminousblazars. Thedetection thusdeterminedbythetrade-offbetweenthenarrowness of of shorter timescales on our sources is challenging due to thetimebinandtheaccuracyoffluxmeasurements.Notice theirweakness.Ground-basedCherenkovtelescopesmaybe however that the narrowness of the time bin is limited by moresuitedtosearchforshortertimescalevariabilities,ifthe (cid:13)c 2011RAS,MNRAS000,1–7 6 G. Calderone et al. spectrumofRL-NLS1extendtoveryhighenergies.Inprin- of similarities in the SED shape of PMN J0948+0022 (also ciple variability can also be measured using the flux upper analyzed here) and the archetypal blazar 3C 273. 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Gamma-ray variability of radio-loud narrow-line Seyfert 1 galaxies

Similar Gamma-ray variability of radio-loud narrow-line Seyfert 1 galaxies