Issue |
A&A
Volume 581, September 2015
|
|
---|---|---|
Article Number | A93 | |
Number of page(s) | 7 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/201526846 | |
Published online | 11 September 2015 |
Research Note
The stability of the optical flux variation gradient for 3C 120⋆
1
Astronomisches Institut, Ruhr–Universität Bochum, Universitätsstraße
150, 44801
Bochum, Germany
e-mail: ramolla@astro.rub.de
2
Instituto de Astronomía, Universidad Católica del
Norte, Avenida Angamos 0610,
Casilla 1280, Antofagasta, Chile
Received: 29 June 2015
Accepted: 6 August 2015
New B- and V-band monitoring in 2014–2015 reveals that the Seyfert 1 Galaxy, 3C 120, has brightened by a magnitude of 1.4, compared to our campaign that took place in 2009–2010. This allowed us to check for the debated luminosity and time-dependent color variations claimed for SDSS quasars. For our 3C 120 data, we find that the B/V flux ratio of the variable component in the bright epoch is indistinguishable from the faint one. We do not find any color variability on different timescales ranging from about 1 to 1800 days. We suggest that the luminosity and time-dependent color variability is an artifact caused by analyzing the data in magnitudes instead of fluxes. The flux variation gradients of both epochs yield consistent estimates of the host galaxy contribution to our 7''̣5 aperture. These results confirm that the optical flux variation gradient method works well for Seyfert galaxies.
Key words: galaxies: active / galaxies: Seyfert / galaxies: nuclei
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2015
1. Introduction
The UV to optical color ratio of the total AGN, measured in magnitude units, becomes bluer as the AGN brightens (e.g. Meusinger et al. 2011). Some studies attribute this change in color to the spectral hardening of the variable component (Giveon et al. 1999; Wilhite et al. 2005; Wamsteker et al. 1990; Webb & Malkan 2000). However, there is ample evidence to suggest that the color of the variable component stays constant and that the corresponding flux variation gradient is offset from the origin of the flux-flux diagram (Choloniewski 1981; Winkler et al. 1992; Winkler 1997; Paltani & Walter 1996; Sakata et al. 2010). In this case, the “bluer when brighter” total observed fluxes, which are measured in a finite aperture, are explained by the superimposition of a constant red host galaxy (including non-varying emission lines) and a varying blue AGN. Even if the measured host contribution is small, compared to the required offset of the flux variation gradient (FVG), there is no significant curvature seen in the flux-flux diagrams (Sakata et al. 2011).
The constancy of the optical colors of the variable component is physically plausible if the variable emission has a hot thermal origin, as expected for an accretion disk (AD). In this case, fluxes in the optical range lie on the Rayleigh-Jeans tail, and they scale almost linearly with temperature.
Recently, Sun et al. (2014) report time-dependent g/r color variability of the SDSS Stripe 82 quasar sample. A blue color at variations inside <30 days (with smaller amplitudes) gradually changes to redder colors on larger timescales >1000 days (and larger amplitudes). Furthermore, Sun et al. (2014) claim that the FVG method lacks rigor and is, therefore, not valid. Consequently, one also expects to find similar behavior for Seyfert galaxies, the less luminous siblings of quasars.
Using data from 2009–2010, Pozo Nuñez et al. (2012) performed B-, V-band monitoring of the Seyfert 1 Galaxy 3C 120 including dense daily observations over a period of five months. Here we report new B-, V-band results from a six-month monitoring project in 2014 and 2015. The total brightening (by about 1.4 mag) and the dense time-sampling of observations allow us to check whether or not brightness and time-dependent color variations are present.
2. Observations and data reduction
The new photometric data at Johnson B and V was observed between 27 August 2014 and 3 March 2015 at the Universitätssternwarte Bochum, near Cerro Armazones. We combined the RoBoTT telescope data (Pozo Nuñez et al. 2012) with new data from the BEST II (Kabath et al. 2009).
All data has been corrected for the latest revision of galactic foreground extinction1 by Schlafly & Finkbeiner (2011) and the corresponding lightcurves are displayed in Figs.1 and 2.
![]() |
Fig. 1 Lightcurves in the 2009–2010 epoch obtained with the RoBoTT telescope. All fluxes are corrected for galactic foreground extinction. |
![]() |
Fig. 2 Combined lightcurves in the 2014–2015 epoch, obtained with two different telescopes. Filled circles correspond to BEST II, while open triangles represent the RoBoTT observations. All fluxes are corrected for galactic foreground extinction. |
3. Results
3.1. Flux-flux diagrams
![]() |
Fig. 3 B
versus V flux variations of 3C 120 in the
|
Total B- versus V-band fluxes for the two epochs are shown in Fig. 3, with one that had low luminosity in 2009–2010 and one that had high luminosity in 2014–2015. Between the two epochs, the AGN luminosity increased by a magnitude of about 1.4 in both filters. The slope of AGN variability is well matched by a linear relation with ΓBV = 0.979 ± 0.005.
We consider the host colors of 3C 120 by Sakata et al.
(2010) in a aperture, corrected for galactic
foreground extinction (Schlafly & Finkbeiner
2011). Assuming that these will be similar when looked at in our
aperture, this allows us to compute our
own host fluxes by measuring the center of gravity of the area that is encased by the cone
of AGN slope ΓBV-fitting uncertainties and Sakata’s
host color ΓBV,Host (inside the red circle of Fig.
3). The results for the B- and V-band host fluxes (Table
1) agree well with the values of Sakata. We also
fit the slopes of the individual epochs of 2009–2010 and 2014–2014 separately. For both
epochs the slope is slightly flatter than the combined one, but they agree within the
errors.
The offset of the combined slope (2009−2015) to the individual epoch slopes could imply that the short-term variability is redder than the long-term one. A more likely physical explanation, however, is based on the systematically different instrumental point spread functions (PSF). The PSF of the RoBoTT is slightly larger than that of the BEST II. The host also has a different spatial flux distribution at B and V. This leads to a larger host galaxy contribution in the 2009−2010 lightcurves, and the host galaxy contribution is larger in V than in B. In the net effect, compared to the 2014−2015 data, the 2009−2010 data appear slightly shifted in the FVG diagram toward the right, resulting in a marginally steeper overall ΓBV. We believe that these negligible effects do not alter the basic results and conclusions on the stability of the FVG method because for all three data sets (2009−2010, 2014−2015, and 2009−2015) the host galaxy contribution is in excellent agreement within the errors. In B-band we measure a minimum fB,AGN = 3.59 mJy and a maximum of fB,AGN = 12.69 mJy. Correspondingly, the increase in V ranges from a minimum of fV,AGN = 3.57 mJy to a maximum of fV,AGN = 12.76 mJy.
Results of the FVG analysis, separated into epochs and filter sets B and V.
3.2. Search for timescale-dependent AGN colors
![]() |
Fig. 4 Scheme of the color slope |
The aim of this section is to investigate potential, timescale-dependent variability that has recently been observed by Sun et al. (2014) in a sample of Stripe 82 SDSS quasars. The ensemble color variability of their quasars was not constant for all observed redshifts, showing rather blue slopes on short timescales of <30 days that turn to redder slopes on longer timescales.
Here we improve their approach in some aspects. Instead of using magnitudes, we make use of fluxes fν directly. As pointed out by Kokubo et al. (2014) and references therein, fitting a straight line in magnitude–magnitude space (e.g. Sun et al. 2014) relies heavily on the contamination of the baseline flux by host galaxies and emission lines.
The color of an arbitrary flux variation in
B- and
V-band in
the time interval τ = |
tj −
ti | is computed using
Eq. (1). The geometry is explained in Fig.
4.
(1)Like Sun et
al. (2014), we restrict ourselves to the range of slopes
. The slopes on the other half circle
represent flux decreases and are rotated by 180° to their equivalent slopes for a flux increase. The offset of
45° is chosen to equally
weigh cases of both bluer (i) when brighter (
); and (ii) redder, when brighter
(
), slopes in the averaging process. The
angle of 45° represents no
color change.
Our photometric measurements in all observed epochs offer
for many combinations of ti and tj, with the lowest
sampling interval of τ =
1 day. It is useful to average all
inside bins with this size. The average of
these bins is described by
(2)The results of this approach for the
B- and
V-band
fluxes are shown in Fig. 5, together with the
structure function S(τ), as defined by Eq. (3). The latter is an indicator of the strength
of the variability on a specific time scale τ. The black solid lines enclose the average
photometric uncertainty of each τ bin.
(3)Analyzing photometric fluxes with large
uncertainties σi,σj,
compared to the difference in fluxes with
, can cause a bias on Θij. We assume a
case of flux that is constant except for noise that is described by different photometric
uncertainties for B- and V-band with σfB ≫
σfV. Then, referring to
the sketch in Fig. 4, we have ΔfB ≫
Δfv for samples of
measurements drawn from these distributions. As a result, we obtain
.
![]() |
Fig. 5 Top: small dots represent |
The structure function S(τ) for the short timescales in
Fig. 5 shows that for τ< 20 days the
variations are still on the order of typical photometric errors. As a consequence, the
distribution of is very broad in this region. The shape of
the structure function is very similar for both bands with a slightly larger variation in
amplitudes around 60 days for
the V-band.
For comparison purposes, we plot the previous linear fit result of ΓBV = 0.979 as
solid red line. In this early epoch, the majority of slopes
is about 3° higher than the linear approximation but
with only low significance and large scatter. From 20 to 90 days, the scatter decreases and the
average cannot be distinguished from the linear fit. A potential contribution of
~5% Hβ at 25 days (Pozo
Nuñez et al. 2012) may introduce additional flux to the V-band and shifts the slope
downward in this region. However, there is no significant trace of this effect. After
90 days, the number of data
points in the τ bins decreases naturally, causing larger scatter
and a minimally lower average slope of 42°. In summary, the plot reveals no clear difference in
on timescales τ within a single
observation epoch of 3C 120.
In Fig. 6, we restrict the plot to 1640 <τ< 1960, so
that all have one data point in our early epoch of
2009–2010 and one in the late epoch of 2014–2015. In total, all angles
are consistent with our linear fit result,
taking the photometric uncertainties into consideration. Within the errors, there appears
a positive slope from 44° to
45° for the considered
range of τ.
This is, however, consistent with a potential host flux offset between the two epochs that
originates from different PSFs as explained in Sect. 3.1.
4. Discussion and conclusions
Our results show a highly linear evolution of B versus V fluxes for 3C 120 on all timescales – from days to several years. Additionally, the color is highly constant, despite the AGN undergoing a strong increase of 1.4 mag in flux over a duration of five years.
Calculating their time dependent colors θ(τ) in magnitude-magnitude space,
Sun et al. (2014) report a significant change of
color for g-
and r-band data
of a large SDSS quasar sample. The colors change from short timescale (small amplitude) blue
colors to long timescale (large amplitude) red colors. Considering a total observed flux
f(t) =
fHost +
fAGN(t), composed of
constant host galaxy component and variable AGN, the difference in magnitude space is
(4)This means that a constant host causes a
reduction in the observed magnitude difference. Because the host galaxy SED is red, compared
to that of the AGN, the magnitude difference will be reduced more for the red
Δmr(τ)
compared to the blue Δmg(τ) and therefore
θ(τ) =
arctan(Δmr(τ) /
Δmg(τ)) turns to
artificially blue colors. Since the structure function of the SDSS quasars rises towards
long timescales (Sun et al. 2014), we simultaneously
observe large amplitudes of flux variations. Because the host galaxy contribution is reduced
in such cases, θ(τ) will be of a redder color. In
our flux-flux approach, however, this issue is completely avoided. Therefore, we suggest
that the luminosity and time-dependent color variability observed by Sun et al. is an
artifact of data analysis in magnitudes instead of fluxes.
The intrinsic brightness change of any astronomical object may be caused by a change in the area and/or the temperature T
of the emitting surface. While an increase in area would simply cause a constant color, the temperature change always has a degree of curvature. Only in the Rayleigh-Jeans approximation is the flux again proportional to the temperature T. Using the blue side λ of the Johnson B-band in hc<λkBT, the temperature T must be higher than 36 000 K, because it is at the lower end of typical effective AD temperatures. The existence of a linear relationship between two (optical) fluxes shows that the variable component is hot enough to be approximated by Rayleigh Jeans.
Another mechanism for a brightness change is variable extinction. In this case, the extinction law influences the color of the varying component, with the bluer band usually more affected than the red band. As a result, extinction can provide a different color slope than a temperature fluctuation in the AD. However, extinction variations resulting from dust clouds that move into the line of sight are random events and should occur with diverse amplitude and timescales. Because they are independent of the variations in the AD with different its color, non-linear behavior of the total fluxes should then be observed in individual objects. Averaging a large sample of AGN flux variations, such random disturbances of the slope in the flux-flux diagrams will be washed out.
Our results are consistent with variations that stem from temperature changes in the AD, where the B- and V-bands are placed in the Rayleigh-Jeans tail of the thermal emission. There is no evidence for a short timescale, bluer color variation that may be caused by variable extinction in the line of sight.
In summary, our finding makes a good case for the use of the rest-frame optical FVG method on different timescales of low luminosity AGN. To further confirm this result, more longterm observations of varying Seyferts (and also quasars) with high photometric precision and dense (daily) temporal sampling will be required.
Online material
Appendix A: Measured fluxes
Galactic foreground extinction corrected B-band fluxes from 2009 until 2015.
Galactic foreground extinction corrected V-band fluxes from 2009 until 2015.
Acknowledgments
This research made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA). This publication is supported as a project of the Nordrhein-Westfälische Akademie der Wissenschaften und der Künste, in the framework of the academy program of the Federal Republic of Germany and the state of Nordrhein-Westfalen. This work is supported by the DFG Program (HA 3555/12-1). The observations on Cerro Armazones benefited from the support of guardians, Hector Labra, Gerardo Pino, Roberto Munoz, and Francisco Arraya. We thank the referee, Ian Glass, for his helpful comments and careful review of the manuscript.
References
- Choloniewski, J. 1981, Acta Astron., 31, 293 [NASA ADS] [Google Scholar]
- Giveon, U., Maoz, D., Kaspi, S., Netzer, H., & Smith, P. S. 1999, MNRAS, 306, 637 [NASA ADS] [CrossRef] [Google Scholar]
- Kabath, P., Erikson, A., Rauer, H., et al. 2009, A&A, 506, 569 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Kokubo, M., Morokuma, T., Minezaki, T., et al. 2014, ApJ, 783, 46 [NASA ADS] [CrossRef] [Google Scholar]
- Meusinger, H., Hinze, A., & de Hoon, A. 2011, A&A, 525, A37 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Paltani, S., & Walter, R. 1996, A&A, 312, 55 [NASA ADS] [Google Scholar]
- Pozo Nuñez, F., Ramolla, M., Westhues, C., et al. 2012, A&A, 545, A84 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Sakata, Y., Minezaki, T., Yoshii, Y., et al. 2010, ApJ, 711, 461 [NASA ADS] [CrossRef] [Google Scholar]
- Sakata, Y., Morokuma, T., Minezaki, T., et al. 2011, ApJ, 731, 50 [NASA ADS] [CrossRef] [Google Scholar]
- Schlafly, E. F., & Finkbeiner, D. P. 2011, ApJ, 737, 103 [NASA ADS] [CrossRef] [Google Scholar]
- Sun, Y.-H., Wang, J.-X., Chen, X.-Y., & Zheng, Z.-Y. 2014, ApJ, 792, 54 [NASA ADS] [CrossRef] [Google Scholar]
- Wamsteker, W., Rodriguez-Pascual, P., Wills, B. J., et al. 1990, ApJ, 354, 446 [NASA ADS] [CrossRef] [Google Scholar]
- Webb, W., & Malkan, M. 2000, ApJ, 540, 652 [NASA ADS] [CrossRef] [Google Scholar]
- Wilhite, B. C., Van den Berk, D. E., Kron, R. G., et al. 2005, ApJ, 633, 638 [NASA ADS] [CrossRef] [Google Scholar]
- Winkler, H. 1997, MNRAS, 292, 273 [NASA ADS] [CrossRef] [Google Scholar]
- Winkler, H., Glass, I. S., van Wyk, F., et al. 1992, MNRAS, 257, 659 [NASA ADS] [Google Scholar]
All Tables
All Figures
![]() |
Fig. 1 Lightcurves in the 2009–2010 epoch obtained with the RoBoTT telescope. All fluxes are corrected for galactic foreground extinction. |
In the text |
![]() |
Fig. 2 Combined lightcurves in the 2014–2015 epoch, obtained with two different telescopes. Filled circles correspond to BEST II, while open triangles represent the RoBoTT observations. All fluxes are corrected for galactic foreground extinction. |
In the text |
![]() |
Fig. 3 B
versus V flux variations of 3C 120 in the
|
In the text |
![]() |
Fig. 4 Scheme of the color slope |
In the text |
![]() |
Fig. 5 Top: small dots represent |
In the text |
![]() |
Fig. 6 Same as Fig. 5, here for the high values of τ between the epochs of 2009–2010 and 2014–2015. |
In the text |
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