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Appendix A: Tables

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

Appendix B.1: NGC 315

NGC 315 is a radio galaxy located in the Zwicky cluster 0107.5+3212. It was classified optically as a type 1.9 LINER by Ho et al. (1997) and as an AGN candidate at X-ray frequencies (González-Martín et al. 2009b).

At radio frequencies (VLBI and VLA) the galaxy shows an asymmetric morphology, with a compact nuclear emission and a one-sided jet (Venturi et al. 1993). The jet can also be observed in X-rays (see Appendix C). Using VLA data, Ishwara-Chandra & Saikia (1999) did not find significant variability over a timescale of ~12 years.

In X-rays, it was observed twice with Chandra in 2000 and 2003 and once with XMM-Newton in 2005. Younes et al. (2011) found variations in Γ (from 1.50.1 to 2.1), and a decreasing in the hard luminosity of 53% between 2003 and 2005. They included the emission of the jet in XMM-Newton data to derive the nuclear spectral parameters. With the same data set, we obtained very similar individual spectral fittings and luminosities (see Tables A.2 and A.3 for Chandra data and Table A.4 for the nuclear region in XMM-Newton data). However, we do not find spectral variations, since SMF0 was used both for Chandra data and when comparing Chandra and XMM-Newton. The difference found with the results reported by Younes et al. (2011) might be due to the different errors. A large set of X-ray observations would be desirable to obtain conclusive results.

XMM-Newton data were used to study short-term variations. From its PSD analysis, González-Martín & Vaughan (2012) did not find them in any of the energy bands (soft, hard, total). From the light curve in the 0.5−10 keV energy band, Younes et al. (2011) reported no variations. We found at 1.6σ confidence level in the 2−10 keV energy band, consistent with no variability.

At UV frequencies, Younes et al. (2012) derived the luminosities from the OM onboard XMM-Newton with UVW2 and UVM2 filters that agree with our results. Variability cannot be studied since OM data are only available at one epoch.

Appendix B.2: NGC 1052

This is the brightest elliptical galaxy in the Cetus I group. Previously classed as a LINER in the pioneering work by Heckman (1980), it was classified optically as a type 1.9 LINER (Ho et al. 1997) and as an AGN candidate at X-ray frequencies (González-Martín et al. 2009b). VLA data show a core-dominated and a two-sided jet structure at radio frequencies (Vermeulen et al. 2003).

NGC 1052 was observed twice with Chandra and five times with XMM-Newton. Long-term variability studies are not found in the literature. We find variations caused by the nuclear power, Norm₂ (49%) and the column density, N_H2 (31%), both at hard energies, in an eight-year period.

González-Martín & Vaughan (2012) studied short term variations from the PSD with XMM-Newton data and did not find variations in any of the energy bands. We analyzed Chandra and XMM-Newton light curves and found no variations. Short-term variations were previously studied with other instruments; Guainazzi et al. (2000) studied BeppoSAX data and did not find short-term variations. The most recent observation in X-rays reported so far is a 100 ks observation taken with Suzaku in 2007, the derived spectral characteristics reported by Brenneman et al. (2009) appear to be similar to those from XMM-Newton, which are compatible with the values in González-Martín et al. (2009b), Brightman & Nandra (2011), and this paper (intrinsic luminosity of log (L(2−10 keV)) ~ 41.5), and no variations along the observation.

In the UV range, Maoz et al. (2005) studied this galaxy with HST ACS and found a decrease by factor of 2 in the flux of the source between the 1997 data reported by Pogge et al. (2000) and their 2002 dataset. We found UV flux variations of a factor of 1.3 using XMM-OM data in a seven-month period.

Appendix B.3: NGC 1961

NGC 1961 is one of the most massive spiral galaxies known (Rubin et al. 1979). It was classified as a type 2 LINER by Ho et al. (1997). MERLIN and EVN data show a core plus two-sided jet structure for this source at radio frequencies (Krips et al. 2007), which makes it a suitable AGN candidate.

This galaxy was observed once with Chandra in 2010 and twice with XMM-Newton in 2011. X-ray variability from these data was not studied before. We did not find variations in a one-month period.

No information in the UV is found for this object in the literature.

Appendix B.4: NGC 2681

The nucleus of this galaxy was optically classified as a type 1.9 LINER (Ho et al. 1997). Classified as an AGN and as a Compton-thick candidate in X-rays (González-Martín et al. 2009b,a), a nuclear counterpart at radio frequencies has not been detected (Nagar et al. 2005).

The source was observed twice with Chandra in January and May 2001. Younes et al. (2011) did not find short-term variations from the analysis of the light curves or long-term variations from the spectral analysis. These results agree with our variability analysis.

At UV frequencies, no variations were found (Cappellari et al. 1999).

Appendix B.5: NGC 2787

The nucleus of NGC 2787 is surrounded by diffuse emission extending up to ~30″ (Terashima & Wilson 2003). It was optically classified as a type 1.9 LINER (Ho et al. 1997) and as an AGN candidate at X-ray frequencies (González-Martín et al. 2009b).

Nagar et al. (2005) detected a radio core with VLA, while evidence of a jet structure has not been found in the literature. Flux variations were obtained at 2 and 3.6 cm on timescales of months (Nagar et al. 2002).

In X-rays, this galaxy was observed twice with Chandra in 2000 (snapshot) and 2004 and once with XMM-Newton in 2004. Younes et al. (2011) found this to be a non-variable object at long-timescales after correcting XMM-Newton data from contamination of X-ray sources. Because of the high contamination from the extranuclear emission in XMM-Newton data, we did not perform a simultaneous fit for this object.

From one Chandra light curve, Younes et al. (2011) calculated an upper limit of , with which our value agrees.

No UV data are found in the literature.

Appendix B.6: NGC 2841

Ho et al. (1997) optically classified NGC 2841 as a type 2 LINER. It was classified as an AGN candidate at X-ray frequencies by González-Martín et al. (2009b). This galaxy shows some X-ray sources in the surroundings (González-Martín et al. 2009b). A core structure was found with VLA by Nagar et al. (2005), without evidence of any jet structure.

NGC 2841 was observed twice with Chandra in 1999 (snapshot) and 2004 and once with XMM-Newton in 2004. We did not use in the analysis the snapshot Chandra data because it does not have a high enough count rate for the spectral analysis. Moreover, since the extranuclear emission in Chandra data contributed with 60% in the 0.5−10.0 keV energy band, we cannot analyze the spectral variations in this source. No information on variability is reported in the literature for this source.

Appendix B.7: NGC 3226

NGC 3226 is a dwarf elliptical galaxy that is strongly interacting with the type 1.5 Seyfert NGC 3227, located at 2′ in projected distance (see Fig. C.19 in González-Martín et al. 2009b). NGC 3226 was optically classified by Ho et al. (1997) as a type 1.9 LINER, and as an AGN candidate at X-ray frequencies by González-Martín et al. (2009b). A compact source is detected with VLA (Nagar et al. 2005), without evidence of any jet structure.

This galaxy was observed twice with Chandra in 1999 and 2001 and four times with XMM-Newton from 2000 to 2006. The possible contamination of NGC 3227 prevents an analysis of long-term variations. We refer to HG13 for details on this subject.

We analyzed one Chandra light curve and obtained below 2σ, consistent with no short-term variations.

UV variations are not found in the literature. We found 11% variations in the UVW1 filter from OM data.

Appendix B.8: NGC 3608

NGC 3608 is a member of the Leo II group, which forms a non-interacting pair with NGC 3607. It was optically classified as a type 2 LINER (Ho et al. 1997). No hard nuclear point source was detected in Chandra images (González-Martín et al. 2009b), thus it was classified in X-rays as a non-AGN candidate, and also it appears to be a Compton-thick candidate (González-Martín et al. 2009a). A compact nuclear source at radio frequencies has not been detected (Nagar et al. 2005).

This galaxy was observed once with Chandra and twice with XMM-Newton in 2000 and 2012. Variability studies are not found at any frequency in the literature. We did not find variations in the 12-year period analyzed.

Appendix B.9: NGC 3718

NGC 3718 has a distorted gas and a dusty disk, maybe caused by the interaction with a close companion (Krips et al. 2007). It was optically classified as a type 1.9 LINER (Ho et al. 1997). It shows a point-like source in the 4.5−8.0 keV energy band (see Fig. B.1), and therefore we can classify it as an AGN candidate following González-Martín et al. (2009b).

At radio frequencies, NGC 3718 was observed with the VLA by Nagar et al. (2005), and with MERLIN at 18 cm by Krips et al. (2007), where it shows a core and a compact jet. Nagar et al. (2002) reported radio variability at 2 cm with VLA data, although the result “is not totally reliable”.

In X-rays, this galaxy was observed once with Chandra in 1999 and twice with XMM-Newton in 2004. Younes et al. (2011) studied all the available data for this object and reported it as variable. When jointly fit Chandra and XMM-Newton data, we found spectral variations in Norm₂ (37%).

Younes et al. (2011) did not find short-term variations from the analysis of the light curves. We did not analyze short-term variations because the length of the observations is <30 ks.

At UV frequencies, Younes et al. (2012) studied this galaxy with XMM-Newton, but the nucleus was not detected, so they estimated upper limits for the flux in one epoch.

Appendix B.10: NGC 4261

Ho et al. (1997) optically classified this galaxy as a type 2 LINER. González-Martín et al. (2009b) classified it as an AGN candidate at X-ray frequencies. NGC 4261 contains a pair of symmetric kpc-scale jets (Birkinshaw & Davies 1985) and a nuclear disk of dust roughly perpendicular to the radio jet (Ferrarese et al. 1996).

It was observed twice with Chandra, in 2000 and 2008, and with XMM-Newton in another three epochs from 2001 to 2007. Long-term variability studies are not found in the literature. We did not find variations in six years period.

Sambruna et al. (2003) found variations of 3−5 ks in the 2−10 keV and 0.3−8.0 keV energy bands in the light curve from 2001, and argued in favor of these variations being more closely related to the inner X-ray jet than to an advection-dominated accretion flow (ADAF), since the expected timescale for the light-crossing time of an ADAF was ~2 orders of magnitude longer than the observed variability timescale. In HG13 we analyzed the same observation in the 0.5−10 keV band and reported it as non-variable. However, we notice that at 1σ confidence level. In the present paper we did not analyze this light curve since the net exposure time is shorter than 30 ks. Other light curves were studied. González-Martín & Vaughan (2012) did not find short-term variations from the PSD analysis of XMM-Newton data. In the present study we analyzed two Chandra observations and cannot confirm rapid variations in this source, since upper limits for the were obtained in both cases.

No information from the UV is found in the literature. We found variation of a 10% (33%) in the UVW1(UVW2) filter.

Appendix B.11: NGC 4278

The north-northwest side of NGC 4278 is heavily obscured by large-scale dustlanes, whose distribution shows several dense knots interconnected by filaments (Carollo et al. 1997). It is an elliptical galaxy with a relatively weak, broad H_α line, which caused Ho et al. (1997) to classify it optically as a type 1.9 LINER. It was classified at X-ray frequencies as an AGN candidate (González-Martín et al. 2009b).

A two-sided jet is observed at radio frequencies wih VLBA and VLA (Giroletti et al. 2005). Nagar et al. (2002) reported radio variability at 2 and 3.6 cm with VLA data. However, these results “are not totally reliable”.

In X-rays this galaxy was observed on nine occasions with Chandra from 2000 to 2010 and once with XMM-Newton in 2004. Brassington et al. (2009) used six Chandra observations and found 97 variable sources within NGC 4278, in a 4′ elliptical area centered on the nucleus, none of them within the aperture we used for the nuclear extraction. Pellegrini et al. (2012) studied Chandra observations of NGC 4278 and found an X-ray luminosity decrease by a factor of ~18 between 2005 and 2010. Younes et al. (2010) detected a factor of ~3 flux increase on a timescale of a few months and a variation of a factor of 5 between the faintest and brightest observations (separated by about three years). We used three of these observations (others were affected by pileup or did not meet the minimum count number), and found that our spectral fittings agreed well with theirs, although we found weaker variations in luminosities.

While the different Chandra observations did not show short-term variability, during the XMM-Newton observation Younes et al. (2010) found a flux increase of a 10% in few hours. With the same dataset, HG13 obtained a 3% variation in the same time range, the difference being most probably due to the different apertures used for the analysis (10″ vs. 25″).

In the UV, Cardullo et al. (2008) found that the luminosity increased by a factor of 1.6 in about six months using data from HST WFPC2/F218W.

Appendix B.12: NGC 4374

NGC 4374 is one of the brightest giant elliptical galaxies in the center of the Virgo cluster. Optically classified as a type type 2 LINER (Ho et al. 1997), at X-ray frequencies it is a Compton-thick AGN candidate (González-Martín et al. 2009a,b).

It shows a core-jet structure at radio frecuencies, with two-sided jets emerging from its compact core (Xu et al. 2000). Nagar et al. (2002) reported flux variations at 3.6 cm with VLA, and variations at 2 cm that “are not fully reliable”.

This galaxy was observed four times with Chandra, twice in 2000 (ObsID 401 is a snapshot) and twice in 2005, and once with XMM-Newton in 2011. No information about variability in X-ray or UV is found in the literature. Here we report strong variations at hard energies (73% in Norm₂).

We analyzed two Chandra light curves, one of them with below 2σ confidence level, which is compatible with no variations.

Appendix B.13: NGC 4494

NGC 4494 is an elliptical galaxy located in the Coma I cloud. It was optically classified as a type 2 LINER (Ho et al. 1997), and at X-rays as an AGN candidate (González-Martín et al. 2009b). The nucleus of this galaxy was not detected in radio with VLA data (Nagar et al. 2005).

This galaxy was observed twice with Chandra in 1999 (snapshot) and 2001 and once with XMM-Newton in 2001. Variability analyses are not found in the literature. We report the source as variable at X-ray frequencies.

Appendix B.14: NGC 4636

NGC 4636 was optically classified as a type 1.9 LINER by Ho et al. (1997). At X-rays it does not show emission at hard energies and therefore was classified as a non-AGN candidate (González-Martín et al. 2009b). It was also classified as a Compton-thick candidate (González-Martín et al. 2009a).

At radio frequencies, it shows a compact core with VLA data (Nagar et al. 2005). Recently, Giacintucci et al. (2011) found bright jets at radio frequencies. No variations were found at 2 cm with VLA data (Nagar et al. 2002).

This galaxy was observed four times with Chandra data between 1999 and 2003, and three times with XMM-Newton between 2000 and 2001. O’Sullivan et al. (2005) studied the X-ray morphology of the galaxy and suggest that it can be the result of a past AGN that is actually quiescent. Long-term variations were not found in the present analysis.

González-Martín & Vaughan (2012) did not find short-term variations from the analysis of XMM-Newton light curves. From one XMM-Newton light curve, we found at 1.4σ confidence level. We did not find long term variations.

No UV variability studies are found in the literature. Our analysis lets us conclude that it is variable at UV frequencies.

Appendix B.15: NGC 4736

NGC 4736 is a Sab spiral galaxy, member of the Canes Venatici I cloud (CVn I) (de Vaucouleurs 1975). Optically classified as a type 2 LINER (Ho et al. 1997), it is an AGN candidate at X-ray frequencies (González-Martín et al. 2009b). Nagar et al. (2005) reported an unresolved nuclear source at its nucleus, using 0.15″ resolution VLA data, without evidence of any jet structure.

This galaxy was observed three times with Chandra between 2000 and 2008 and three times with XMM-Newton between 2002 and 2006. No long-term variability information is found in the literature. In the present work we did not find any variation in a four-year period.

It harbors a plethora of discrete X-ray sources in and around its nucleus (see Appendix C). Eracleous et al. (2002) studied Chandra data from 2000. They found a very dense cluster of ten discrete sources in the innermost 400 × 400 pc of the galaxy. They studied the brightest four sources (namely X-1 to X-4) and found that spectra are well described by a single power-law with photon indices in the range 1.1−1.8, and 2−10 keV luminosities between 4−9 × 10³⁹ erg s^-1. They also studied short-term variability from the analysis of the light curves. They estimated the normalized excess variance () of the nucleus of NGC 4736 (X-2), and reported it as variable. The other sources also showed short-term variations (see Table 5 in Eracleous et al. 2002). They argued that there is no evidence for the presence of an AGN and concluded that this LINER spectrum could be the result either of a current or recent starburst or of an AGN. However, they noted that X-2 is the only source with an UV counterpart detected by HST. González-Martín et al. (2009b) assigned X-2 to the nucleus of the galaxy, since it coincides with the 2MASS near-IR nucleus within 0.82″.

By studying BeppoSAX and ROSAT data, Pellegrini et al. (2002) excluded variations of the 2−10 keV flux higher than ~50% on timescales on the order of one day. Comparing data from both instruments, they did not find variations between 1995 and 2000. They concluded that the X-ray emission is caused by a recent starburst in NGC 4736. However, they mentioned that an extremely low-luminosity AGN could still be present, because of a compact nonthermal radio source that is coincident with an X-ray faint central point source.

González-Martín & Vaughan (2012) studied the PSD of the XMM-Newton data from 2006 and found no short-term variations.

We analyzed Chandra and XMM-Newton light curves. Variations were found, but were throughout below the 2σ confidence level, in agreement with Eracleous et al. (2002).

At UV frequencies, Maoz et al. (2005) found long-term variations between 1993 and 2003, the nucleus being 2.5 times brighter in 2003. From the OM data, we found variations of 66% in the U filter between 2002 and 2006.

Appendix B.16: NGC 5195

NGC 5195 is tidally interacting with a companion SB0 galaxy NGC 5194 (M51). It was optically classified as a type 2 LINER by Ho et al. (1997). It shows a point-like source in the 4.5−8.0 keV energy band (see Fig. B.2), and therefore we can classify it as an AGN candidate following González-Martín et al. (2009b). A radio counterpart was found by Ho & Ulvestad (2001) with VLA data at 6 and 20 cm, without any jet indications.

This source was observed eight times with Chandra between 2000 and 2012 and five times with XMM-Newton between 2003 and 2011. Terashima & Wilson (2004) studied Chandra data from 2000 and 2001 and found neither long term, nor short-term variability in the full, soft, or hard energy bands. Since the Chandra observations from 2000 and 2001 were rejected from our sample because of the low number counts, we cannot compare our spectral fittings with theirs. However, our estimate of the luminosity in Chandra data agrees with their results. We found this object to be variable on long-timescales, while short-term variations were not detected.

At UV frequencies, no references are found in the literature. We found variations in the UVW1 filter.

Appendix B.17: NGC 5813

NGC 5813 is one of the galaxies in the group catalog compiled by de Vaucouleurs (1975), with NGC 5846 being the brightest member of the group. It was classified as a type 2 LINER by Ho et al. (1997). The X-ray morphology is extremely diffuse, with very extended emission at softer energies and without emission above 4 keV, which caused González-Martín et al. (2009b) to classify it as a non-AGN candidate. It was also classified as Compton-thick candidate (González-Martín et al. 2009a). At radio frequencies, it shows a compact core (Nagar et al. 2005) and a jet-like structure (Randall et al. 2011).

This source was observed nine times with Chandra between 2005 and 2011 and three times with XMM-Newton between 2005 and 2009. Variability studies at X-ray and UV frequencies are not reported in the literature. We did not find either long-term or short-term variations in X-rays. UV variations were found in the UVW1 filter.

Appendix B.18: NGC 5982

NGC 5982 is the brightest galaxy in the LGG 402 group, which is composed of four members (Garcia 1993). Recently, Vrtilek et al. (2013) found a compact radio core in the position of the source using GMRT 610 MHz observations, which indicates that this is an AGN-like object; jets were not detected.

This galaxy was observed twice with XMM-Newton in 2011 and 2012. Variability studies are not reported in the literature. We found variations in the nuclear power (50%) in a one-year period, while UV variations were not found.

Appendix C: 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 levels extend from twice the value of the background dispersion to the maximum value at the center of each galaxy.

© ESO, 2014