Discovery of a FR0 radio galaxy emitting at γ -ray energies
aa r X i v : . [ a s t r o - ph . GA ] D ec Mon. Not. R. Astron. Soc. , 000–000 (0000) Printed 6 May 2019 (MN L A TEX style file v2.2)
Discovery of a FR 0 radio galaxy emitting at γ -ray energies Paola Grandi ⋆ , Alessandro Capetti ⋆⋆ and Ranieri D. Baldi § INAF-IASFBO, Via Gobetti 101, I-40129 Bologna, Italy INAF-Osservatorio Astrofisico di Torino, Strada Osservatorio 20, I-10025, Pino Torinese, Italy Department of Physics and Astronomy, The University, Southampton SO17 1BJ, UK
ABSTRACT
We present supporting evidence for the first association of a
Fermi source, 3FGLJ1330.0-3818, with the FR 0 radio galaxy Tol1326-379. FR 0s represent the majority of the local radioloud AGN population but their nature is still unclear. They share the same properties of FR Isfrom the point of view of the nuclear and host properties, but they show a large deficit ofextended radio emission. Here we show that FR 0s can emit photons at very high energies.Tol1326-379 has a GeV luminosity of L > ∼ × erg s − , typical of FR Is, but witha steeper γ -ray spectrum ( Γ = 2 . ± . ). This could be related to the intrinsic jet propertiesbut also to a different viewing angle. Key words: galaxies: active-galaxies:radio-galaxies:individual (Tol1326 − γ -rays:galaxies Radio Galaxies (RGs) has been historically divided into faint edge-darkened FR I and bright edge-brightened FR II (Fanaroff & Riley1974) on the basis of their extended radio morphology withthe transition occurring at, approximately, a radio power ofP
178 MHz ∼ W Hz − sr − . From the optical point of view,radio galaxies are split into Broad Line Radio Galaxies (BLRGs)and Narrow Line Radio Galaxies (NLRGs). NLRGs can be fur-ther classified as High Excitation Galaxies (HEGs) and Low Ex-citation Galaxies (LEGs) based on their optical emission line ra-tios (Buttiglione et al. 2010). While HEGs and BLRGs show al-most exclusively a FR II morphology, LEGs can assume bothradio morphologies. It is believed that in LEGs, the AGN lu-minosity is sustained by hot gas via advection-dominated flow-like/Bondi accretion (Balmaverde, Baldi & Capetti 2008), while inBLRGs and HEGs by a cold geometrically–thin, optically–thickdisk (Best & Heckman 2012; Heckman & Best 2014). Steep Spec-trum Radio Quasars (SSRQs) are similar to BLRGs but more dis-tant and luminous. FRIs and FR IIs are considered the parent pop-ulation of BL LACs (BLs) and Flat Spectrum Radio Quasars (FS-RQs), respectively (Urry & Padovani 1995).Being observed at large angles , the jets of RGs and SS-RQs (collectively named Misaligned AGN: MAGN) do not ben-efit from the strong Doppler amplification typical of blazars. Al-though geometrically disfavored (i.e. less amplified), RGs and SS-RQs have been detected above 100 MeV. The Fermi satellite found ⋆ E-mail:[email protected] ⋆⋆ [email protected] § [email protected]
11 MAGN in only 15 months of Large Area Telescope (LAT) sur-vey (Abdo et al. 2010c) with four of them also detected in the TeVband. Although their number is a tiny fraction of the total
Fermi detected sources, their discovery had a strong impact on the studyof the high energy process in AGN.A clear link, in at least two BLRGs, 3C 111 and 3C 120,has been established between the expulsion of bright super-luminal knots from the radio core and intense γ -ray flares(Grandi, Torresi & Stanghellini 2012; Casadio et al. 2015).The firm detection of GeV emission from the radio lobes innearby RG Centaurus A has shown that extranuclear extended re-gions can be a source of gamma-ray photons, implying the pres-ence of highly energetic particles at large distances from the nu-cleus (Abdo et al. 2010b). Because their radiative lifetimes ( < to10 million years) approach plausible electron transport time scalesacross the lobes, their presence is difficult to explain unless suc-cessive particle acceleration occurs even at large distances from theblack hole.Spectral Energy Distribution (SED) studies of FR I radiogalaxies showed that a pure, one-zone homogeneous, Syn-chrotron Self-Compton (SSC) emitting region is inadequate inreproducing the radio to TeV data, stimulating the elaborationof more complex models. Stratified jets with different regionsinteracting with each other (Georganopoulos & Kazanas 2003;Ghisellini, Tavecchio & Chiaberge 2005; B¨ottcher & Dermer2010) as well as magnetic reconnection events along the jet(Giannios, Uzdensky & Begelman 2010) or in the vicinity of theblack hole (Khiali, de Gouveia Dal Pino & Sol 2015) have beensuggested as possible sources of γ -ray photons. Hadronic modelsbased on proton-photon interaction (see for a review B¨ottcher2012) have also been explored providing possible connections c (cid:13) Paola Grandi, Alessandro Capetti , Ranieri D. Baldi among AGN, Ultra High Energy Cosmic Rays and neutrinos(Becker & Biermann 2009)All these results are based on the study of the brightest MAGNdiscovered in the first years of the
Fermi-LAT activity, i.e. bright ra-dio sources with flux density of 2Jy or more. The recent publicationof the new
Fermi -LAT point source catalog (3FGL - Acero et al.2015) and the
Fermi -LAT AGN catalogs (3LAC - Ackermann et al.2015) collecting 4 years of data now allow us to extend the studyof GeV radio galaxies to lower fluxes. Although the ’standard’ pic-ture, i.e. the predominance of bright FR Is among MAGN, is con-firmed, we can now enter into the unexplored territory of fainterradio AGN. Indeed we propose here the first association between a3LAC gamma-ray source and a FR 0 galaxy.The cross-match of radio and optical data favored by the ad-vent of large area surveys has surprisingly revealed that the bulk ofthe radio-galaxy population lacks prominent extended radio struc-tures. Best & Heckman (2012) built a sample of radio-galaxies bycross-correlating the Sloan Digital Sky Survey (SDSS), the Na-tional Radio Astronomy Observatory (NRAO) Very Large Array(VLA) Sky Survey (NVSS), and the the Faint Images of the Ra-dio Sky at Twenty centimetres (FIRST) survey datasets. This sam-ple is selected at F . > mJy and it includes RGs up to z ∼ L . ∼ − W Hz − . Mostof them ( ∼
80 %) are unresolved or barely resolved at the 5 ′′ FIRST resolution, corresponding to a limit to their size of ∼ ∼ ) can be classi-fied as LEGS, the same spectroscopic class to which FR Is belong.Finally, the hosts of these two classes are effectively indistinguish-able being in both cases red early-type galaxies with central blackhole masses larger than ∼ M ⊙ (Baldi & Capetti 2009, 2010;Sadler et al. 2014; Baldi, Capetti & Giovannini 2015a).Although FR 0s represent the majority of the low luminosityRGs in the local Universe, they are a puzzling class of AGN com-pletely unexplored at high energies.In this paper, we show that the galaxy Tol1326-379, identifiedas the counterpart of the γ -ray source 3FGLJ1330.0-3818 and clas-sified in the 3LAC as a Flat Spectrum Radio Quasar, is actually thefirst FR 0 radio source discovered in the GeV sky.A cosmology with H = 67 km s − Mpc − , Ω m = 0 . , and Ω Λ = 0 . is assumed. The γ -ray source 3FGLJ1330.0-3818 was detected by Fermi witha significance of ∼ σ , a flux above 1 GeV of F > = (3 . ± . × − phot cm − s − and a power law index Γ = 2 . ± . (Acero et al. 2015).3FGLJ1330.0-3818 is listed in the 3LAC catalog and asso-ciated to the early type galaxy Tol1326-379 at z=0.02843 with aBayesian probability of association of 90%. The Bayesian methodcalculates the posterior probability that a source from a catalog ofcandidates is the counterpart of a γ -ray source detected by the LAT,evaluating the significance of a spatial coincidence between the Figure 1. candidate counterpart and the LAT-detected source. For more de-tails see Abdo et al. (2010a).In order to strengthen the association proposed by the 3LACcatalog, we explored the region around 3FGLJ1330.0-3818 withinthe 95% γ -ray position error ellipse (20 ′ × ′ ) looking for alterna-tive identifications. We considered all radio sources with a NVSSflux density larger than 20 mJy ( ∼ α r ∼ ∼
35 mJy, suggestingthe dominance of the extended radio emission. Furthermore noneof them has a 2MASS counterpart apart from Tol1326-379 whichshows an early-type morphology. This confirms that Tol1326-379is the most likely association.
The optical spectrum of Tol1326-379 from the 6dF Galaxy Survey(Jones et al. 2004, 2009) is presented in Fig. 2, left panel. It doesnot show any evidence for broad emission lines, arguing againstits identification as FSRQ; on the other hand, the equivalent width(EW) of the emission lines largely exceed the limit of 5 ˚A com-monly adopted for BL Lac objects (Stickel et al. 1991). We ob-tained, in particular, EW(H α ) ∼ ± ∼ ± β =0.10, log [N II]/H α =-0.03, log [O I]/H α =-0.40, and log [S II]/H α =0.01) are characteris-tic of a LEG spectrum (Kewley et al. 2006; Buttiglione et al. 2010).This lends further weight against its association with a FSRQ, be-cause such sources always show a high ionization optical spectrum(Shaw et al. 2012).Unfortunately the 6dFGS spectrum is not flux calibrated. In c (cid:13) , 000–000 iscovery of a FR 0 radio galaxy emitting at γ -ray energies Figure 2.
Left: optical spectrum of Tol1326-379 from the 6dF Galaxy Survey (Jones et al. 2004). Right: radio spectrum of Tol1326-379. Data are fromliterature (Healey et al. 2007; Wright et al. 1994; Condon et al. 1998; Feain et al. 2009; Mauch et al. 2003). The 20 GHz flux density was measured with theAustralian Telescope Compact Array (see text for the details). The dotted line is the single power fit to the data having a slope of 0.37. order to obtain the emission lines luminosity we need to rely onan indirect estimate. We derived the flux in the J band Two MicronAll Sky Survey (2MASS) image from a synthetic aperture of 6 . ′′ × − ergcm − s − and a luminosity of 4 × erg s − .In order to assess the accuracy of such a procedure, we per-formed the same analysis on a group of 7 early-type emission linegalaxies in common between the 6dFGS and the SDSS survey. Ourline measurements agree with those provided by the SDSS databasewithin a factor of 4.From its 2MASS image, we derived a total K magnitude of11.22, corresponding to a luminosity of 1.0 × solar luminos-ity. The tight correlation between M BH and the near-infrared bulgeluminosity proposed by Marconi & Hunt (2003) allows us to esti-mate (within a factor 2) the black hole mass of M BH = 2 × M ⊙ .Using the relation L bol =3500 L [O III] measured byHeckman et al. (2004), we obtain a bolometric luminosityL bol = 44 . erg s − (with an uncertainty of 0.4 dex) for thissource. The Eddington-scaled accretion rate of Tol1326-379is ˙ L = L bol /L Edd ∼ × − , a value typical of LEGs(Best & Heckman 2012). We collected the radio flux density data from the NASA/ IPAC In-frared Science Archive and Extragalactic Database (NED). To thesedata we added our own measurement at 20 GHz from the Aus-tralian Telescope Compact Array (ATCA); the observations wereobtained on Oct 27 th ′′ ); we perform a Gaussian fit using ’jmfit’ task of AIPS software to estimate its flux density of 23.2 ± . mJy. Theavailable radio data cover the frequency range from 843 MHz to 20GHz (see Fig. 2, right panel). None of the available radio imagesshows extended emission and in particular Tol1326-379 is reportedby Healey et al. (2007) as a point source in their CRATES cata-log at 8.4 GHz with an angular resolution of ∼ ′′ provided bythe ATCA observation. The overall spectral slope is α = 0 . butthe data points show a large scatter. This in part due to the sourcevariability; indeed its flux density varied from 53.2 to 71.5 mJy inthe NVSS and ATCA observations, respectively, two measurementsobtained at the same frequency and with similar spatial resolution.In addition, the observations have rather different spatial resolu-tion, typically ∼ ′′ for the four lower frequencies and of a fewarcsec for the data at 8.4 and 20 GHz. The radio spectrum measuredbetween these two high frequency (and high resolution) measure-ments is even flatter than the overall value, α . −
20 GHz = 0 . .The core dominance R parameter defined as the ratio between the4.85 GHz and 1.4 GHz flux densities is log R = − . .Summarizing, Tol1326-379 fulfills all the spectro-photometric requirements for a FR 0 classification listed byBaldi, Capetti & Giovannini (2015a). It is an early-type galaxywith a black hole mass larger than M ⊙ , a low excitationoptical spectrum, and a high radio core dominance. FurthermoreTol1326-379 , given its radio luminosity at 1.4 GHz of 2 × erg s − , falls within the region typical of FR 0s in the diagramshown in Fig.3 that compares the total radio and emission lineluminosities. It also shows, as characteristic of this class of sources,a large deficit (a factor of ∼ The NRAO Astronomical Image Processing System (AIPS) is a packageto support the reduction and analysis of data taken with radio telescopes.c (cid:13) , 000–000
Paola Grandi, Alessandro Capetti , Ranieri D. Baldi
Figure 3.
Logarithm of the radio vs. [O III] luminosities (both in erg s − units) for the SDSS/NVSS sub-sample ( . < z < . ) analyzed byBaldi & Capetti (2009) and mainly composed of FR 0 sources. The solidline reproduces the line-radio correlation followed by FR Is of the 3CRsample (green stars). The dashed line marks the boundary of the loca-tion of Seyfert galaxies (e.g. Whittle 1985). FR 0s show a strong deficitof total radio emission, occupying the region to the left of the FR Is(Baldi, Capetti & Giovannini 2015a). Tol1326-379 (red point) falls into theFR 0s area. We explored the position of Tol1326-379 in a plot where the radioluminosity at 1.4 GHz of all the blazars and MAGN of the 3LACclean sample (with known optical classification and redshift) arereported as a function of the gamma-ray luminosities (see Fig. 4,left panel). We also add 3C 111 and Cen B (not found in the cleansample because of their low galactic latitudes) and 3C 120 alreadyreported in the 15 month-sample of MAGN and that has recentlyundergone strong flares (Casadio et al. 2015). As the origin (jet,lobes) of the FornaxA γ -ray emission is unclear, we prefer not toinclude it.We derived for all sources in the 3LAC catalog the k-corrected1.4 GHz rest frame flux density by assuming α r = 0 for blazarsand 0.8 for MAGN. For Tol1326-379 we assume the observedvalue of α r = 0 . . As expected, different classes lie in differ-ent zones of the diagram with FSRQs at higher luminosities thanBL Lacs. We recover the well known radio γ -ray correlation forblazars (Ghirlanda et al. 2010; Ackermann et al. 2011). On aver-age, MAGN are offset from the blazar strip showing a radio excesswith respect to BL Lacs and FSRQs with similar γ -ray luminosi-ties. This is due to the additional contribution from extranuclearradio emission. Tol1326-379 falls into the low luminosity tail ofthe blazar strip. It is, together with IC 310, the least powerful radiogalaxy with γ -ray detection.To further explore the nature of our source, the γ -ray spectralslope ( Γ ) of blazars and MAGN of the clean 3LAC sample wasplotted in Fig. 4, right panel, as a function of the rest frame isotropicluminosity above 1 GeV ( L > in ergs cm − s − ). As is wellknown, different classes of AGN occupy different locations in the Γ - L > plane (Abdo et al. 2010c). FSRQs and SSRQs are in In some cases the 3LAC radio flux density was provided at different radiofrequencies (for example at 20 GHz or at 843 MHz). The above reportedspectral radio slopes were also adopted to convert the listed flux to the 1.4GHz flux. the upper right part of the diagram (high luminosities and steepspectra), BL Lacs in the bottom right side (low luminosities andflat spectra), while RGs are all at low luminosities, but with a largescatter in spectral indeces. It is evident that Tol1326-379 falls intothe general radio-galaxy area and not into the FSRQs region.
The 3LAC catalog associates the γ -ray source 3FGLJ1330.0-3818to the galaxy Tol1326-379. We confirm that no other flat radiosource brighter than Tol1326-379 at 5 GHz is present with the 95%error circle position of 3FGLJ1330.0-3818. Although flat in theradio, Tol1326-379 is not a FSRQ but a FR 0 radio galaxy, the firstsource of this new radio class with a γ -ray counterpart.Several observations support this conclusion: • Tol1326-379 is an early-type galaxy at z=0.02843; • it does not show any evidence for broad emission lines, argu-ing against its identification as FSRQs; • the high values of the line equivalent widths exclude the pos-sibility that it is a BL Lac; • its line ratios are typical of LEGs; • its estimated black hole mass of M BH = 2 × M ⊙ andaccretion rate ˙ L = L bol /L Edd ∼ × − are characteristic ofradio-loud AGN (Chiaberge & Marconi 2011) associated with lowefficiency accretion flows; • it is unresolved in the radio images, showing a high core dom-inance and a flat radio spectrum; • when put in a radio versus [O III] luminosity plot, it falls intothe FR 0 region and not in the FR I area.In Fig. 5, we present the SED of Tol1326-379. Besides theradio and Fermi observations already discussed, we complementedit with the WISE, Galex, and ROSAT data.Tol1326-379 is detected at high significance by the WISE satel-lite in all four bands and its colors are W1-W2=0.38 ± ± , are corrected forgalactic extinction assuming E(B-V)=0.0678. In the X-ray band,Tol1326-379 was only detected by the PSPC instrument (0.1-2.4keV) onboard of the ROSAT satellite. Its count rate reported in theROSAT All-Sky Survey Faint Source Catalog is 0.029 ± . c/s.This can be converted to an unabsorbed flux of F . − . keV =(7 . ± . × − erg s − cm − if a power law with spectralslope Γ x = 2 and Galactic absorption of N H = 5 . × cm − are assumed. Due to the low spatial resolution of the UV and X-rayimages, and the lack of any spectral information, the correspond-ing measurements should be considered as upper limits, as theyinclude the host galaxy emission. In particular, its soft X-ray lu-minosity is compatible with that expected from the hot corona ofan early-type galaxy with the luminosity estimated above for oursource (Fabbiano, Kim & Trinchieri 1992). http://galex.stsci.edu/GR6/ (cid:13) , 000–000 iscovery of a FR 0 radio galaxy emitting at γ -ray energies Figure 4.
Left: the radio luminosity of the 3 LAC FR I radio galaxies (green circles), FR II radio sources (magenta squares), BL Lacs (open blue triangles),FSRQs (open black squares), and SSRQs (cyan crosses) is plotted as a function of the γ -ray luminosity between 1 GeV and 100 GeV. MAGN, extended at 1.4GHz, are more luminous in radio than blazars with similar γ -ray luminosity. Tol1326-379 (red open diamond) has a typical FRI γ -ray luminosity but falls intothe low luminosity tail of the blazar strip. Right: γ -ray spectral slope versus 1-100 GeV luminosity. Tol1326-379 is located into the MAGN region and outsidethe FSRQs zone. For clarity, error-bars are reported for non-blazars only. All the luminosities are in erg s − . The SED of Tol1326-379 is compared to those of Centaurus Aand M87, the prototype nearby FR I radio galaxies. Despite of thesimilar radio luminosity, M 87 is less luminous than Tol1326-379by a factor of 30 at 1 GeV and has a flat SED in the γ -ray domain.On the contrary, Centaurus A and Tol1326-379 are quite similarin shape but the former source is about two orders of magnitudefainter.Balmaverde & Capetti (2006) found that the SEDs of the FR Inuclei differ from those of BL Lacs. This is partly related tothe frequency shift in the SED due to the relativistic beaming,but also to differences in the emitting regions. Indeed, due tothe presence of a velocity stratification with the relativistic jets(Kovalev et al. 2007; Nagai et al. 2014) in BL Lacs we are see-ing the regions of the jet with the highest Doppler factor (theso-called jet spine) while in FR Is the emission could be dom-inated by the slower jet layers. The SED differences might bewitnessing the diverse physical conditions in these two regions(Chiaberge et al. 2000; Ghisellini, Tavecchio & Chiaberge 2005;Tavecchio & Ghisellini 2014) leading to a dependence of theirshape on the viewing angle.This effect might account for the SED differences betweenM 87 and Tol1326-379 and, on the other hand, the similarities withCentaurus A. Indeed, M 87 jet is seen at a rather small angle fromthe line of sight with θ ∼ − ◦ (Acciari et al. 2009), whileCen A lies close to the plane of the sky, θ ∼ − ◦ (Tingay et al.1998). We argue that the SED shapes are, similarly to what is de-rived from the comparison between BL Lacs and FR Is, mainlydriven by the jet viewing angle. Indeed, by excluding Centaurus A,all the FR Is have flat γ -ray spectra and are seen at small angles (seeTable 1). The only exception is 3C 120, that has a slope comparableto that of Cen A (and Tol1326-379), but smaller inclination angle.3C 120 is, however, a peculiar radio galaxy. Although classified asFR I, it has optical-UV-X-ray properties typical of FR IIs. It shows broad optical emission lines (Tadhunter et al. 1993), an UV bump,and an K α iron line in the X-ray spectrum (Zdziarski & Grandi2001; Ogle et al. 2005; Kataoka et al. 2007; Chatterjee et al. 2011),all clear signatures of the presence of an efficient accretion disk.Indeed, in Fig. 4 (right panel) 3C 120 falls between 3C 111( Γ =2.79,L γ =43.21), and Pictor A ( Γ =2.49,L γ =42.65), two (FR II)BLRGs detected by Fermi .Although we do not have any direct estimate for the orientationof Tol1326-379, we speculate that it is oriented at a large angle andthis causes the similarity between its SED and that of Cen A.Alternatively, if we are seeing the jet of Tol1326-379 at asmall angle, its SED would be intrinsically different from thoseof the FR Is. This would be the first indication of a discrepantproperty between these two classes (other than the paucity of ex-tended radio emission in FR 0) and might be an important clue tounderstand their nature. Interesting enough, the Compton peak inTol1326-379 appears to be more prominent than the Synchrotronone. This generally occurs in FSRQs, where a surplus of seed pho-tons coming from the accretion disk, the broad line region and thetorus, contributes to the high energy emission by External Comp-ton (Sikora et al. 2009). Incidentally we note that Tol1326-379 ischaracterized by a steep γ -ray spectrum ( Γ γ = 2 . ± . ), moresimilar to that generally observed in FSRQs (and their misalignedpopulation, i.e BLRGs and SSRQs) than in BL Lacs (and FRIs).It is, however, improbable that an External Compton mechanismis responsible for the cooling of the jet particles of Tol1326-379.Its nuclear environment is poor in photons as indicated by the lowaccretion rate. It is then more plausible that its γ -ray luminosity issustained by different jet components that mutually interact ampli-fying the IC emission as suggested for FR Is. The excess of γ -rayradiation could then reflect different physical conditions of the highenergy dissipation regions (i.e. of the spine and/or the layers).Progress in our understanding of the properties of Tol1326-379 c (cid:13) , 000–000 Paola Grandi, Alessandro Capetti , Ranieri D. Baldi
Figure 5. can come from a better definition of its SED. In particular, the in-formation in the X-rays can be improved with a firm detection andwith a measurement of the spectral slope in this band. This mightbe used to test the indication that the Compton peak in Tol1326-379is more prominent than the synchrotron one.Since a large diversity of spectral behavior among this class ofsources, as already observed in blazars, cannot be excluded, otherobservations of γ -rays emitting FR 0s are necessary to consolidatethe overall picture. ACKNOWLEDGMENTS
We are grateful to F. Paresce for carefully reading the paper anduseful comments which improved the final version. We thank thereferee for constructive comments/suggestions on the manuscript.This research made use of the NASA/ IPAC Infrared ScienceArchive and Extragalactic Database (NED), which are operated bythe Jet Propulsion Laboratory, California Institute of Technology,under contract with the National Aeronautics and Space Adminis-tration. Part of this work is based on archival data, software or on-line services provided by the Italian Space Agency (ASI) ScientificData Center (ASDC).
Table 1.
Jet inclination angle from literature (Ref.) , γ -rays spectral slope( Γ ) and luminosity L γ [ > GeV] (erg s − ) of the FR I radio galaxies.Source θ Γ L γ Ref.3C 78 ◦ ± < ◦ ± ◦ ± + < ◦ ± a − ◦ ± . < ◦ ± ◦ ± ∼ ◦ ± ◦ ± ◦ ± a,b < ◦ ± ◦ ± a – Cen B and 3C 120 are not in the clean 3LAC sample. b – Cen B: Jet inclination upper limit estimated using the jet/counterjet fluxratio J ∼ at 4.8 GHz provided by Jones, Lloyd & McAdam (2001). Pos-sible γ -ray contribution from the lobes (Katsuta et al. 2013). (1) Kharb et al. (2009), (2) Schulz et al. (2015), (3) Walker, Romney & Benson (1994), (4)Giovannini et al. (2001), (5) Jorstad et al. (2005), (6) Venturi et al. (2000), (7) Bondi et al. (2000), (8)Lara et al. (2004), (9) Acciari et al. (2009), (10) Tingay et al. (1998), (11) Jones, Lloyd & McAdam(2001), (12) Migliori et al. (2011). REFERENCES
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