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Appendix A: Tables

Appendix B: Notes and comparisons with previous results for individual objects

In this appendix we discuss the general characteristics of the galaxies in our sample at different wavelengths, as well as comparisons with previous variability studies. We recall that long-term UV variability and short-term X-ray variations were studied only for some sources (six and ten sources, see Tables 1 and A.6, respectively), so comparisons are only made in those cases. For the remaining objects, results from other authors are mentioned, when available.

Appendix B.1: MARK 348

MARK 348, also called NGC 262, is an interacting galaxy (with NGC 266, Pogge & Eskridge 1993). It was optically classified as a Seyfert 2 (Koski 1978), while it shows broad lines in polarized light (Miller & Goodrich 1990). It shows a spiral nuclear structure (see HST image in Appendix C.1). VLBI observations showed a compact radio core and jets structure at radio frequencies and revealed variations on timescales from months to years at 6 and 21 cm (Neff & de Bruyn 1983). The XMM-Newton image shows that the soft X-ray emission is very weak in this object (see Appendix C.1), which was classified as a Compton-thin object (e.g., Awaki et al. 2006).

This galaxy was observed twice with XMM-Newton in 2002 and 2013 and once with Chandra in 2010. Recently, Marchese et al. (2014) have compared XMM-Newton and Suzaku data from 2002 and 2008. They fitted the data with a power law component transmitted through three absorbers (one neutral and two ionized), obtaining intrinsic luminosities of log(L(2−10 keV)) = 43.50 and 43.51, respectively. They reported variations attributed to changes in the column density of the neutral and one of the ionized absorbers, together with a variation in the ionization level of the same absorber, on timescales of months. They did not report variations in Γ and/or the continuum of the power law. Variations in the absorbing material on timescales of weeks/months were also reported by Smith et al. (2001) using RXTE data from 1996–97, but accompanied with continuum variations on timescales of ~1 day. They obtained luminosities in the range log(L(2−10 keV)) = [42.90−43.53]. These results agreed with those later reported by Akylas et al. (2002), who analyzed the same observations plus 25 RXTE observations. Our analysis shows that variations between the two XMM-Newton observations are due to changes in the nuclear continuum, but variations in the absorbing material are not required. These differences may be related to the different instruments involved in the analyses.

Awaki et al. (2006) did not find short-term variations from the analysis of the XMM-Newton data from 2002.

In the 14−195 keV energy band, Soldi et al. (2014) estimated a variability amplitude of 25[22–28]% using data from the Swift/BAT 58-month survey.

Appendix B.2: NGC 424

NGC 424 was optically classified as a Seyfert 2 galaxy (Smith 1975), and broad lines have been detected in polarized light (Moran et al. 2000). At radio frequencies, it was observed with VLA at 6 and 20 cm, showing an extended structure (Ulvestad & Wilson 1989). A possible mid-IR variability was reported by Hönig et al. (2012) between 2007 and 2009, but it could also be due to an “observational inaccuracy”. In X-rays, it is a Compton-thick source (Baloković et al. 2014).

It was observed twice with XMM-Newton in 2008 and 2011, and once with Chandra in 2002. Matt et al. (2003) studied XMM-Newton and Chandra data from 2001 and 2002. Both spectra were fitted with a model consisting on two power laws, a cold reflection component (PEXRAV), and narrow Gaussian lines. They report the same luminosity for the two spectra, log(L(2−10 keV)) = 41.68, indicating no variations. LaMassa et al. (2011) studied the same data set. They found no differences between the spectra and therefore fitted the data simultaneously with a simpler model, the 2PL. They estimated an intrinsic luminosity of log(L(2−10 keV)) = 41.56[41.39−41.75]. With the same data set, we did not find variations and obtained similar hard X-ray luminosities (41.85[41.79−41.92]).

We did not find short-term variations from the XMM-Newton light curve from 2008.

Appendix B.3: MARK 573

MARK 573 (also called UCG 1214) is a double-barred galaxy that shows dust lanes (Martini et al. 2001, see also Appendix C.1). It was optically classified as a Seyfert 2 galaxy (Osterbrock & Martel 1993). Observations at 6 cm with VLA showed a triple radio source (Ulvestad & Wilson 1984). A point-like source is observed in hard X-rays, while extended emission can be observed in soft X-rays, aligned with the bars (see Appendix C.1). It was classified as a Compton-thick candidate (Guainazzi et al. 2005b; Bianchi et al. 2010; Severgnini et al. 2012).

This galaxy was observed four times with Chandra between 2006 and 2010, and once with XMM-Newton in 2004. Bianchi et al. (2010) analyzed the Chandra data from 2006 and did not report flux variations when they compared their results with the analysis of Guainazzi et al. (2005b) of the XMM-Newton spectrum from 2004. Paggi et al. (2012) studied the four Chandra observations, and fitted the nuclear spectrum with a combination of a two-phased photoionized plasma plus a Compton reflection component (PEXRAV), reporting soft X-ray flux variations at 4σ of confidence level that they attributed to intrinsic variations of the source. We did not detect variations for this source, the difference most probably because we did not use two of these observations since they are affected by a pileup fraction greater than 10%.

Ramos Almeida et al. (2008) analyzed the XMM-Newton light curve and found variations of ~300 s. They argue that this is an obscured narrow-line Seyfert 1 galaxy instead of a Seyfert 2, based on near-IR data. We analyzed two Chandra light curves but variations were not found.

Appendix B.4: NGC 788

This galaxy was optically classified as a Seyfert 2 by Huchra et al. (1982). A radio counterpart was detected with VLA data (Nagar et al. 1999). In X-rays, it was classified as a Compton-thin candidate using ASCA data (de Rosa et al. 2012), and it shows a point-like source in the 4.5−8 keV energy band (see Appendix C.1).

It was observed once with Chandra in 2009 and once with XMM-Newton in 2010. Long-term variability analyses of this source were not found in the literature. We did not find variations between the observations.

Variations in this source in the 14–195 keV energy band were studied by Soldi et al. (2014) using data from the Swift/BAT 58-month survey. They report an amplitude of the intrinsic variability of 15[11−19]%.

Appendix B.5: ESO 417-G06

This galaxy was optically classified as a Seyfert 2 galaxy (Maia et al. 2003). A radio counterpart was observed with VLA data (Nagar et al. 1999). It was classified as a Compton-thin candidate (Trippe et al. 2011).

This galaxy was observed twice with XMM-Newton in 2009. Long-term variability studies were not found in the literature. We found spectral variations due to changes in the absorber at hard X-ray energies.

Trippe et al. (2011) report short-term variations of a factor of about two in the count rate in the light curves from Swift/BAT during the 22-month survey.

Appendix B.6: MARK 1066

MARK 1066 is an early-type spiral galaxy (Afanas’ev et al. 1981) showing a double nucleus (Gimeno et al. 2004). It was optically classified as a Seyfert 2 by Goodrich & Osterbrock (1983), and broad lines were not detected in polarized light (Gu & Huang 2002). A radio counterpart showing a jet was found by Ulvestad & Wilson (1989). At X-rays, extended soft emission can be observed, aligned with a nuclear spiral structure observed at optical frequencies, also aligned with the IR emission (see Appendix C.1). Levenson et al. (2001) found this to be a heavily obscured AGN, with N_H> 10²⁴ cm^-2 and an equivalent width of the Fe line ~3 keV using ROSAT and ASCA data; i.e., it was classified as a Compton-thick candidate.

The galaxy was observed once with Chandra in 2003 and once with XMM-Newton in 2005. Variability studies of this object were not found in the literature. We did not find any X-ray variations either.

Appendix B.7: 3C 98.0

Using the optical line measurements in Costero & Osterbrock (1977), 3C 98.0 can be optically classified as a Seyfert 2 (see an optical spectrum in Appendix C.1). A nuclear core plus jet structure was observed at radio frequencies with VLA (Leahy et al. 1997).

It was observed twice with XMM-Newton in 2002 and 2003 and once with Chandra in 2008. Isobe et al. (2005) studied the two XMM-Newton data, and fitted its spectra with a thermal plus a power-law model, reporting X-ray luminosities of log(L(2−10 keV)) = 42.90[42.88−42.93] and 42.66[42.60−42.71], respectively, indicating flux variability. These measurements agree well with ours, where variations due to the nuclear continuum were found.

Awaki et al. (2006) studied short-term variations of the XMM-Newton observation from 2003 and calculated a normalized excess variance of . We did not find short-term variations from one Chandra light curve, where upper limits of the were calculated.

We did not find any long-term UV variations in the UVW1 filter.

Appendix B.8: MARK 3

It was optically classified as a Seyfert 2 galaxy (Khachikian & Weedman 1974, see an optical spectrum in Appendix C.1). Broad lines have been found in polarized light (Miller & Goodrich 1990). A high resolution image at 2 cm with VLA data shows a double nucleus at radio frequencies (Ulvestad & Wilson 1984). This galaxy shows extended soft X-ray emission perpendicular to the IR emission and a point-like source at hard X-rays (see Appendix C.1). It is also a Compton-thick source (Bassani et al. 1999; Goulding et al. 2012), with a column density of 1.1 × 10²⁴ cm^-2 measured with BeppoSAX (Cappi et al. 1999).

It was observed 11 times with XMM-Newton between 2000 and 2012, and once with Chandra in 2012. Bianchi et al. (2005b) report variations of the normalization of the absorbed power law when comparing the XMM-Newton from 2001 with Chandra and BeppoSAX data. Guainazzi et al. (2012) studied the X-ray variability of this nucleus during 12 years of observations with Chandra, XMM-Newton, Suzaku, and Swift satellites. Their analysis was performed in the 4−10 keV energy band. To estimate the luminosities, they fit a pure reflection model plus Gaussian lines to the spectra individually, and report a variability dynamical range greater than 70%. They also used alternative models to fit the data, variations found independently of the model used. They estimated the shortest variability timescale to be ~64 days from the measurement between two statistically inconsistent measures. From our analysis, variations due to the nuclear continuum were found, with an upper limit of the variability timescale of about five months, thus in agreement with the results presented by Guainazzi et al. (2012).

Short-term variations from XMM-Newton data were found neither by González-Martín & Vaughan (2012) nor by Cappi et al. (2006) from light curves from 2000 and 2001, respectively.

Soldi et al. (2014) reported an amplitude of the intrinic variability of 35[26−46]% in the 14−195 keV energy band using data from the Swift/BAT 58-month survey.

Appendix B.9: MARK 1210

This galaxy, also called the Phoenix galaxy or UGC 4203, was optically classified as a Seyfert 2 by Dessauges-Zavadsky et al. (2000). Broad lines have been observed in polarized light using spectropolarimetric data (Tran et al. 1992; Tran 1995). The HST image shows a nuclear spiral structure (see Appendix C.1). A very compact radio counterpart was found with VLA at 3.5 cm, with no evidence of a jet structure (Falcke et al. 1998). At X-rays, a point like source is observed in the 4.5−8.0 keV energy band (see Appendix C.1). It was classified as a Compton-thick candidate by Bassani et al. (1999). Furthermore, Guainazzi et al. (2002) classified this galaxy as a changing-look AGN because transitions from Compton-thick (ASCA data) to Compton-thin (XMM-Newton data) were found.

MARK 1210 was observed with Chandra six times between 2004 and 2008, and once with XMM-Newton in 2001. Matt et al. (2009) used Suzaku data from 2007 to study this source (caught in the Compton-thin state), and compared with previous observations from ASCA and XMM-Newton. They fit the spectra with a power law, a Compton reflection, and a thermal (MEKAL) component, and found a change in the absorber, which was about a factor of 2 higher in Suzaku data. They obtained intrinsic X-ray luminosities of log(L(2−10 keV)) = 42.87 and 43.04 for Suzaku and XMM-Newton data. Risaliti et al. (2010) simultaneously fit the five Chandra observations from 2008 using a model consisting on a doubled temperature plus power law to account for the soft energies, an absorbed power law, and a constant cold reflection component (PEXRAV). They concluded that variations are found in both the intrinsic flux and in the absorbing column density. They reported a variability time scale of ~15 days, whereby they estimated the physical parameters of the absorbing material, concluding that they are typical of the broad line region (BLR). Their result agrees well with ours.

Awaki et al. (2006) studied short-term variations from the XMM-Newton data and found .

Soldi et al. (2014) used data from the Swift/BAT 58-month survey to account for the variability amplitude (S_v = 24 [15−32] %) in the 14–195 keV energy band.

Appendix B.10: NGC 3079

This galaxy was optically classified as a Seyfert 2 (Ho et al. 1997, based on the spectra presented in Appendix C.1). Broad lines were not detected in polarized light (Gu & Huang 2002). The HST image shows dust lanes (Appendix C.1). A water maser and parsec-scale jets were observed at radio frequencies with VLBI (Trotter et al. 1998). The X-ray image in the 0.6−0.9 keV energy band shows strong diffuse emission, while a point-like source is detected in the 4.5−8.0 keV energy band (see Appendix C.1). It has been classified as a Compton-thick object with BeppoSAX data (N_H = 10²⁵ cm^-2, Comastri 2004) and evidence was also found at lower energies (Cappi et al. 2006; Akylas & Georgantopoulos 2009; Brightman & Nandra 2011a).

It was observed once with Chandra and once with XMM-Newton, both in 2001. We did not find variability studies of this source in the literature. We did not study its variability because the extranuclear emission in Chandra data was too high to properly compare XMM-Newton and Chandra observations.

It is worth noting that NGC 3079 is classified as a Compton-thin candidate in this work but it has been classified as a Compton-thick candidate by Cappi et al. (2006) using the same XMM-Newton observation. Since these data have the lowest signal-to-noise ratio, this mismatch is most probably due to a problem related to the sensitivity of the data, because we used only data from the pn detector, while they combined pn, MOS1, and MOS2 data in their study; i.e., Cappi et al. (2006) data have a higher signal-to noise. We notice that cross-calibration uncertainties between pn and MOS cameras may add systematic to statistical uncertainties that can be misinterpreted as possible intrinsic variability due to large error bars (Kirsch et al. 2004; Ishida et al. 2011; Tsujimoto et al. 2011), thus preventing us from doing a variability analysis.

Appendix B.11: IC 2560

This galaxy was optically classified as a Seyfert 2 (Fairall 1986, see an optical spectrum in Appendix C.1). In hard X-rays it shows a point-like source (see Appendix C.1). It was classified as a Compton-thick object (Baloković et al. 2014).

IC 2560 was observed once with XMM-Newton in 2003 and once with Chandra in 2004. Variability studies were not found in the literature. We do not report X-ray variations for this source, either at short or long term.

Appendix B.12: NGC 3393

NGC 3393 was optically classified as a Seyfert 2 (Diaz et al. 1988, see an optical spectrum in Appendix C.1). A radio counterpart was found using VLA data, the galaxy showing a double structure (Morganti et al. 1999). The HST image shows a nuclear spiral structure aligned with the soft X-ray emission, where the spiral structure can also be appreciated; this emission is perpendicular to the disk emission, observed at optical wavelengths and aligned with the IR emission (see Appendix C.1). A point-like source is observed at hard X-rays (see Appendix C.1). It is a Compton–thick object observed by BeppoSAX (N_H> 10²⁵ cm^-2, Comastri 2004).

This galaxy was observed once with XMM-Newton in 2003 and six times with Chandra between 2004 and 2012. Variability studies were not found in the literature. We did not find X-ray variations, whether on the short or the long term.

Appendix B.13: NGC 4507

The nucleus of this galaxy was optically classified as a Seyfert 2 (Corbett et al. 2002, see an optical spectrum in Appendix C.1). Broad lines have been detected in polarized light (Moran et al. 2000). A radio counterpart was observed with VLA data (Morganti et al. 1999). In X-rays, it shows a point-like source in the hard energy band (see Appendix C.1), and it is a Compton-thin source (Bassani et al. 1999; Braito et al. 2013).

NGC 4507 was observed six times with XMM-Newton between 2001 and 2010, and once with Chandra in 2010. Matt et al. (2004) studied Chandra and XMM-Newton data from 2001. They fit the XMM-Newton spectrum with a composite of two power laws, a Compton reflection component (PEXRAV), plus ten Gaussian lines, and the Chandra spectrum with a power law plus a Gaussian line (only in the 4–8 keV spectral range). They found that the luminosity of the Chandra data was about twice that of XMM-Newton. Marinucci et al. (2013) studied five observations from XMM-Newton in 2010. They fit the spectra with two photoionized phases using Cloudy, a thermal component, an absorbed power law, and a reflection component. They report variations of the absorber on timescales between 1.5−4 months. Braito et al. (2013) studied XMM-Newton, Suzaku, and BeppoSAX data spanning around ten years to study the X-ray variability of the nucleus. They fit the spectra with the model that best represents the Suzaku data, composed of two power laws, a PEXRAV component, and eight Gaussian lines, and found that variations are mainly due to absorption but also due to the intensity of the continuum level. They also fit the spectra with the mytorus model¹¹ and obtained similar results, although the continuum varied less. We found variations in the absorber and the normalization of the power law, in agreement with the results by Braito et al. (2013).

We did not find short-term variations from the analysis of one XMM-Newton and another Chandra light curve.

Soldi et al. (2014) report an amplitude of the intrinsic variability of 20[16−24]% in the 14−195 keV energy band using data from the Swift/BAT 58-month survey.

Appendix B.14: NGC 4698

This galaxy was optically classified as a Seyfert 2 (Ho et al. 1997, see their spectra in Appendix C.1). González-Martín et al. (2009b) classified it as a LINER, but Bianchi et al. (2012) reconfirmed the Seyfert 2 classification using optical observations with the NOT/ALFOSC/Gr7. A radio counterpart was found by Ho & Ulvestad (2001) at 6 cm with VLA data. Georgantopoulos & Zezas (2003) state that this is an atypical Seyfert 2 galaxy because it showed no absoption and lacks the broad line region. The Chandra image revealed point-like sources around the nucleus, which can be ultraluminous X-ray sources (ULX), the closest located at ~30″ from the nucleus. In X-rays, González-Martín et al. (2009b) classified it as an AGN candidate, and Bianchi et al. (2012), based on the log(L_X/L_{[O III]}) ratio, classified it as a Compton-thick candidate.

This galaxy was observed twice with XMM-Newton in 2001 and 2010, and once with Chandra in 2010. Bianchi et al. (2012) compared the XMM-Newton spectra and did not find spectral variations, in agreement with the results reported by us.

We did not find any UV variations in the UVM2 filter.

Appendix B.15: NGC 5194

NGC 5194, also known as M 51, is interacting with NGC 5195. Optical and radio observations show extended emissions to the north and south of the nucleus, resulting from outflows generated by the nuclear activity (Ford et al. 1985). The extended emission can be observed in soft X-ray energies (top-left image in Appendix C.1). Moreover, the HST image shows a dusty nuclear spiral structure that can also be observed at IR frequencies (see Appendix C.1). This galaxy was optically classified as a Seyfert 2 (Ho et al. 1997, see their optical spectra in Appendix C.1). Broad lines were not detected in polarized light (Gu & Huang 2002). A point-like source is detected at hard X-ray energies (see Appendix C.1). Around the nucleus, it shows at least seven ultraluminous X-ray sources (ULX), the nearest one located at ~28″ from the nucleus (Dewangan et al. 2005). It was classified as a Compton-thick source using BeppoSAX data, with N_H = 5.6 × 10²⁴ cm^-2 (Comastri 2004; see also Terashima & Wilson 2001; Dewangan et al. 2005; Cappi et al. 2006]).

This galaxy was observed ten times with Chandra between 2000 and 2012 and six times with XMM-Newton between 2003 and 2011. LaMassa et al. (2011) studied three Chandra observations between 2000 and 2003. They simultaneously fit these spectra with the ME2PL model, with spectral values in very good agreement with our SMF0 fitting and estimated a luminosity of log(L(2−10 keV)) = 38.95[38.42,39.45]. They did not report variability between the observations. This result agrees with ours.

Fukazawa et al. (2001) did not find any short-term variability from BeppoSAX data. We studied six Chandra light curves and did not find short-term variations either.

UV variations were not detected from the UVW2 and UVM2 filters, but variations were found in the UVW1 filter. However, since this is a Compton-thick source, variations are not expected, so it is most probable that the UV emission does not come from the nucleus. Therefore the variations might be related with, for example, circumnuclear star formation.

Appendix B.16: MARK 268

This galaxy was optically classified as a Seyfert 2 by Komossa & Schulz (1997). A radio counterpart was detected with VLA data at 6 cm with a weaker component 1.1 kpc away from the nucleus (Ulvestad & Wilson 1984). XMM-Newton data show a compact source at hard X-rays (see Appendix C.1) .

It was observed twice with XMM-Newton in 2008. Variability studies were not found in the literature. We did not find variations, but we notice that observations were obtained separated by only two days.

UV variations are not found from the UVM2 and the UVW1 filters.

Appendix B.17: MARK 273

Also called UGC 8696, this galaxy is an ultraluminous infrared galaxy with a double nucleus that was optically classified as a LINER (Veilleux et al. 1995), but later reclassified as a Seyfert 2 from better S/N data (Kim et al. 1998). Optical spectra are presented in Appendix C.1, together with an HST image which shows dust lanes. VLBA observations showed a radio counterpart (e.g., Carilli & Taylor 2000). Extended emission to the south is observed in soft X-rays, while it shows a point-like source at hard energies (Appendix C.1). It was classified as a Compton-thick candidate (Teng et al. 2009).

It was observed once with Chandra in 2000, and five times with XMM-Newton between 2002 and 2013. Balestra et al. (2005) fit the Chandra and XMM-Newton spectra with a composite of three thermal plus an absorbed PL components and found similar spectral parameters, except in the value of the column densities (41[35−47] and 69[50−85] × 10²² cm^-2, respectively). This result is compatible with ours, with N_H2 being responsible for the observed variations. In the same sense, Teng et al. (2009) studied Suzaku data from 2006 and found spectral variations when comparing with Chandra and XMM-Newton data. They attributed the changes to the covering fraction of the absorber.

We found no short-term variations from the Chandra light curve or UV variations from the UVW1 filter.

Appendix B.18: Circinus

It was optically classified as a Seyfert 2 galaxy (Oliva et al. 1994), and it shows broad lines in polarized light (Oliva et al. 1998). The HST image shows dust lanes (Appendix C.1). ATCA observations show a radio counterpart, a water maser, and large radio lobes (Elmouttie et al. 1998). Circinus is a Compton-thick source (Bassani et al. 1999), which in fact was observed by BeppoSAX (N_H = 4.3 × 10²⁴ cm^-2, Matt et al. 1999).

This galaxy was observed eight times with Chandra between 2000 and 2010, and twice with XMM-Newton in 2001 and 2014. The most comprehensive analysis of this source has recently been done by Arévalo et al. (2014), who analyzed 26 observations from NuSTAR, Chandra, XMM-Newton, Swift, Suzaku, and BeppoSAX satellites spanning 15 years and the energy range 2–79 keV. They used different models to fit the data, based on PEXMON, MyTorus, and Torus models (in XSPEC). Since different appertures were used for the analysis, they decontaminated the extranuclear emission. They conclude that the nucleus did not show variations, in agreement with our result when comparing Chandra data. Moreover, Arévalo et al. (2014) find that extranuclear sources included in the larger apertures showed variations (an ultraluminous X-ray source and a supernova remnant), also in agreement with our results when comparing XMM-Newton data, where the extranuclear sources were included, and we found variations in both the normalizations at soft and hard energies.

We analysed one Chandra light curves, but variations were not detected.

The analysis of light curves from the Swift/BAT 58-month survey by Soldi et al. (2014) showed a small variability amplitude of 11[10−12]% in the 14−195 keV energy band.

Appendix B.19: NGC 5643

This galaxy was optically classified as a Seyfert 2 (Phillips et al. 1983, see an optical spectrum in Appendix C.1), and broad lines were not detected in polarized light (Gu & Huang 2002). The HST image shows a nuclear spiral structure (see Appendix C.1). VLA data show a nuclear counterpart alongside fainter features extending to the east and west at radio frequencies (Morris et al. 1985). The XMM-Newton image shows a compact source at hard X-ray energies. This is a Compton-thick object observed with BeppoSAX (N_H> 10²⁵ cm^-2, Comastri 2004).

It was observed twice with XMM-Newton in 2003 and 2009, and once with Chandra in 2004. Matt et al. (2013) analyzed the two observations from XMM-Newton, who found that the spectra are reproduced well by reflection from warm and cold matter. The spectral parameters were consistent with the same values for the two observations. Thus, variations are not observed. These results agree well with ours, where variations are not found.

Appendix B.20: MARK 477

This object was classified as a Seyfert 2 (Veron et al. 1997), and broad lines have been detected in polarized light (Tran et al. 1992; Tran 1995). The HST image reveals a structure around the nucleus, which could be a spiral or a circumnuclear ring (see Appendix C.1). A nuclear counterpart was found at 6 cm using VLA data (Ulvestad & Wilson 1984). It was classified as a Compton-thick candidate (Bassani et al. 1999).

The source was observed twice with ASCA in December 1995; variations were not found when fitting a scattered power law plus a narrow line (Levenson et al. 2001).

It was observed twice with XMM-Newton in 2010. We did not find variations between these observations.

Kinney et al. (1991) studied UV variability of this source with HST, but variations were not found. We did not find UV flux variations from the UVW1 filter.

Appendix B.21: IC 4518A

This galaxy was optically classified as a Seyfert 2 galaxy (Zaw et al. 2009). The 2MASS image shows two interacting galaxies (see Appendix C.1). It is a Compton-thin source (Bassani et al. 1999; de Rosa et al. 2008).

It was observed twice with XMM-Newton in 2006. Variability analyses were not found in the literature. However, comparing the luminosities obtained by de Rosa et al. (2012) and Pereira-Santaella et al. (2011) of log(L(2−10 keV)) = 42.60 and 42.34 for the different spectra, their results are suggestive of flux variability. In fact, these luminosities agreed with our estimates. Our analysis shows that this variability is related with the nuclear continuum.

Appendix B.22: ESO 138-G01

Alloin et al. (1992) optically classified this galaxy as a Seyfert 2. It shows a jet-like morphology at radio frequencies (Morganti et al. 1999). The XMM-Newton image shows a compact source at hard X-ray energies (see Appendix C.1). It was classified as a Compton-thick candidate (Collinge & Brandt 2000).

This galaxy was observed three times with XMM-Newton in 2007 and 2013. Variability analyses were not found in the literature. We did not find any X-ray variations.

Appendix B.23: NGC 6300

NGC 6300 is a barred spiral galaxy, whose Seyfert 2 classification at optical frequencies was derived from the data reported in Phillips et al. (1983). The HST image shows dust lanes (see Appendix C.1). A nuclear counterpart was found at radio frequencies, without any jet structure (Ryder et al. 1996). NGC 6300 was classified as a changing-look AGN, observed in the Compton-thick state with RXTE in 1997 and in the Compton-thin state with BeppoSAX in 1999 (Guainazzi 2002).

The galaxy was observed once with XMM-Newton in 2001 and five times with Chandra during 2009. Guainazzi (2002) found variations due to a difference in the normalization of the power law when comparing BeppoSAX and RXTE data. All the observations analyzed in this work caught the object in the thin state. Variations in the normalizations at soft and hard energies were found when comparing Chandra and XMM-Newton data.

Matsumoto et al. (2004) and Awaki et al. (2005, 2006) studied the light curve from XMM-Newton data and found rapid variations at hard energies.

Variations in the 14−195 keV energy band were analyzed by Soldi et al. (2014) using data from the Swift/BAT 58-month survey, who estimated an intrinsic variability amplitude of 17[14−20]%.

Appendix B.24: NGC 7172

NGC 7172 is an early type galaxy located in the HCG 90 group, which shows dust lanes (Sharples et al. 1984, see also Appendix C.1). Optically classified as a Seyfert 2 (see an optical spectrum in Appendix C.1), no broad lines have been observed in polarized light (Lumsden et al. 2001). A radio core was detected with VLA data (Unger et al. 1987). At IR frequencies, Sharples et al. (1984) found variations on timescales of about three months. The nucleus of this galaxy is not detected at UV frequencies with the OM (see Table A.1). Even if Chandra data are available for this source, they suffer from strong pileup. The XMM-Newton image shows a compact source (see Appendix C.1).

Guainazzi et al. (1998) first reported X-ray flux variations in this source using ASCA data. They found short-term variations (hours) from the analysis of a light curve from 1996 and long-term variations when comparing the flux of these data with previous data from 1995, when it was about three times brighter. Risaliti (2002) studied two BeppoSAX observations taken in October 1996 and November 1997 and fit the data with an absorbed power law, a thermal component, a cold reflection, a warm reflection, and a narrow Gaussian line. They reported very similar spectral parameters for the two spectra.

This galaxy was observed once with Chandra in 2000 and three times with XMM-Newton between 2002 and 2007. LaMassa et al. (2011) analyzed the XMM-Newton spectra by fitting the data with the ME2PL model and needed to fit the normalization of the power law independently. They report luminosities of log(L(2−10 keV)) = 42.960.03 (for the spectrum from 2007) and 42.610.03 (for the other two spectra). These results agree well with our SMF1.

Awaki et al. (2006) analyzed the XMM-Newton light curve from 2002. They did not find significant variability when computing the normalized excess variance.

At higher energies, Beckmann et al. (2007) reported an intrinsic variability of within 20 days using Swift/BAT data, and using data from the Swift/BAT 58-month survey, Soldi et al. (2014) report a variability amplitude of 28[25−31]%, both in the 14−195 keV energy band.

Appendix B.25: NGC 7212

This galaxy is interacting with a companion (see the 2MASS image in Appendix C.1). It was optically classified as a Seyfert 2 galaxy (Veilleux & Osterbrock 1987, see an optical spectrum in Appendix C.1). Broad lines were detected in polarized light (Tran et al. 1992). At radio wavelengths, a nuclear counterpart was found with the interacting galaxy (Falcke et al. 1998). A point-like source is detected at hard X-rays (see Appendix C.1). It was classified as a Compton-thick candidate (Severgnini et al. 2012).

It was observed once with Chandra in 2003 and once with XMM-Newton in 2004. Bianchi et al. (2006) report the same fluxes for the two spectra, also in agreement with our results.

Appendix B.26: NGC 7319

NGC 7319 is a spiral galaxy located in the Stephan’s Quintet, a group composed by six galaxies including a core of three galaxies (Trinchieri et al. 2003). These three galaxies were also observed at radio wavelengths with VLA (Aoki et al. 1999) and later with MERLIN (Xanthopoulos et al. 2004), revealing a jet structure in NGC 7319. It has been optically confirmed as a Seyfert 2 (Rodríguez-Baras et al. 2014, see an optical spectrum in Appendix C.1). The nucleus of this galaxy is not detected at UV frequencies with the OM (see Table A.1). In X-rays, a point-like source is observed in the 4.5−8.0 keV energy band, and it shows extended emission at soft X-ray energies (Appendix C.1).

It was observed twice with Chandra in 2000 and 2007, and once with XMM-Newton in 2001. We did not find any variability studies in the literature. We found variations in the nuclear power of the nucleus, accompained by absorber variations at soft energies.

One Chandra and the XMM-Newton light curves were analyzed, but short-term variations were not detected.

Appendix C: Images

Appendix C.1: Optical spectra, and X-ray, 2MASS and optical HST images

In this appendix we present images at different wavelengths for each energy and the optical spectrum when available from NED. In X-rays we extracted Chandra data in four energy bands: 0.6−0.9 keV (top left), 1.6−2.0 keV (top middle), 4.5−8.0 keV (top right), and 0.5−10.0 keV (bottom left). The csmooth task included in CIAO was used to adaptatively smooth the three images in the top panels (i.e., the images in the 0.5−10.0 keV energy band are not smoothed), using a fast Fourier transform algorithm and a minimum and maximum significance level of the signal-to-noise of 3 and 4, respectively. When data from Chandra was not available, XMM-Newton images were extracted in the same energy bands, and the asmooth task was used to adaptatively smooth the images. At infrared frequencies, we retrieved an image from 2MASS in the K_s filter¹². At optical frequencies we used images from HST¹³, preferably in the F814W filter, but when it was not available we retrieved an image in the F606W filter. HST data have been processed following the sharp dividing method to show the internal structure of the galaxies (Marquez & Moles 1996). The red squares in the bottom images represent the area covered by the HST image (presented in the bottom right panel when available). In all images the gray levels extend from twice the value of the background dispersion to the maximum value at the center of each galaxy. We used IRAF¹⁴ to estimate these values.

Appendix C.2: Chandra and XMM-Newton images

In this appendix we present the images from Chandra (left) and XMM-Newton (right) that were used to compare the spectra from these two instruments in the 0.5−10 keV band. In all cases, the gray scales extend from twice the value of the background dispersion to the maximum value at the center of each galaxy.

Appendix D: Light curves

This appendix provides the plots corresponding to the light curves. Three plots per observation are presented, corresponding to soft (left), hard (middle), and total (right) energy bands. Each light curve has a minimum of 30 ks (i.e., 8 h) exposure time, while long light curves are divided into segments of 40 ks (i.e., 11 h). Each segment is enumerated in the title of the light curve. Count rates versus time continua are represented. The solid line represents the mean value, dashed lines the 1σ from the average.

© ESO, 2015