The optical and x-ray properties of a sample of
Seyfert galaxies that have undergone significant
spectral change
Daymore T.M Makope1
Department of Physics, University of Johannesburg
P.O. Box 524, Auckland Park 2006, Johannesburg, South Africa
E-mail: 201517415@student.uj.ac.za
Hartmut Winkler
Department of Physics, University of Johannesburg
P.O. Box 524, Auckland Park 2006, Johannesburg, South Africa
E-mail: hwinkler@uj.ac.za
Francois van Wyk
Department of Physics, University of Johannesburg
P.O. Box 524, Auckland Park 2006, Johannesburg, South Africa
E-mail: MAY YOU PLEASE WRITE IT DOWN FOR ME.
This paper presents the initial findings of a study of the optical spectra of two sets of data for
about 50 Seyfert galaxies taken 20 years apart. We compared the spectra for the different epochs
and identify some objects where significant spectral changes occurred. We describe the nature of
the spectral changes and collate available information about the x-ray properties of these AGN.
In particular, we probe whether the degree of spectral variability, which may range from relatively
steady to “changing look” AGN, correlates with any x-ray features. We attempt to interpret our
findings in terms of possible physical models of these AGN.
7th Annual Conference on High Energy Astrophysics in Southern Africa - HEASA2019
28 - 30 August 2019
Swakopmund, Namibia
1
Speaker
© Copyright owned by the author(s) under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0).
https://pos.sissa.it/
A search for optical spectroscopic transitions in a sample of Seyfert galaxies
Daymore T.M. Makope
1. Introduction
Active galactic nuclei (AGN) are spiral galaxies with extremely bright nuclei. A massive black
hole at the nucleus of the galaxy is believed to power these galaxies, fed by accretion discs of gas
from its surrounding environment. Seyfert galaxies are a subclass of AGN which have nuclei with
luminosities ranging between 10 8 and 1011 solar luminosities. They have visible and infrared
spectra that show very bright emission lines of hydrogen, helium, nitrogen and oxygen exhibiting
strong Doppler broadening, which implies velocities from 500 km/s to sometimes over 10000
km/s. [1]
These objects are known to show continuum and emission line intensity variations [2], including
emission profile variations on both long and short timescales, with most changes observed in the
broad line region. The unification model for AGN [3] [4] explains the variations using the
orientation of the galaxy. It says that broad emission profiles are observed when the nucleus of
the galaxy is unobscured and are absent or weak when the nucleus of the galaxy is viewed along
a line of sight obscured by a dust torus.
Changing look active galactic nuclei (CL AGN) are a class of AGN that undergo drastic changes
in their spectral properties in the optical region and/or in the X-ray region [5]. They can change
their spectral type within time periods that range from days to decades. This presents a challenge
to the unification model, as the dusty torus should not change its characteristics over short time
intervals [6]. These changes can be variations in the shape of spectral profiles, changes in the
intensity of the lines and disappearance/appearance of (broad) emission lines together with
dramatic continuum flux changes that are orders of magnitude larger than those expected from
typical AGN variability. These variations provide information on the structure and kinematics of
the regions producing the emission line.
Mrk 590, for example, brightened for 23 years, and then dimmed between 2003 and 2014
accompanied by the complete disappearance of the broad Balmer profiles [7]. A long-term
monitoring of the galaxy NGC 3516 [8] showed an increase then complete disappearance of the
broad Balmer lines from a spectrum recorded in 1997 to one obtained in 2018. It was concluded
that the absorbed broad emission lines indicate possible obscuration of the broad line region. The
object NGC 1566 was observed with nearly undetectable broad components between the 1970s
and 1980s [9] but a recent study [10] shows that the object experienced a dramatic outburst in all
wavelengths with an Hβ component a lot stronger than the [OIII] 5007Å component. The paper
proposed tidal star stripping as a possible cause of the outburst.
A few mechanisms have been proposed to explain the observed drastic changes, including
i.
changes in the accretion rate [11] of mass onto the supermassive black hole – depletion
of nuclear material results in objects evolving from high accretion rate to low accretion
rate, which changes the structure of the broad line;
ii.
variable obscuration [12] of the nuclei due to increase acceleration outflow and the
presence of a patchy distribution of dust clouds which results in short-lived drastic
changes; and
iii.
a tidal disruption event [13] which occurs when a star passes close to a black hole and
gets captured and ripped to pieces due to the tidal forces of the massive black hole,
resulting in flares of electromagnetic radiation being observed.
The main objectives of this study are to observe and characterize the changes to the optical spectra
at occurred to a sample of Seyfert galaxies reobserved after a period of 20 years and to use these
findings to test physical models of AGN and to highlight possible examples of a sub-class referred
to as “changing look AGN”.
2
A search for optical spectroscopic transitions in a sample of Seyfert galaxies
Daymore T.M. Makope
In this paper, we present preliminary results of four of the objects analyzed to date and report the
observed spectral changes for each of these objects.
2. Data sample
The four objects with the analysis results presented in this paper are listed in Table 1 below.
Table 1:
Object name RA(2000)
Dec(2000)
Epoch 1
z (epoch 1)
Epoch 2
z (epoch 2)
1E 2124-14
21:27:32.4 -14:46:48
06-Oct-97
0.056872039
04-Jul-19
0.056872039
CTS A08.12
21:32:02.1 -33:42:54
23-May-99 0.030202047
19-Jan-00
0.029727731
CTS A09.25
22:07:44.7 -32:34:56
23-May-99 0.05906449
07-Jun-19
0.059552325
CTS A03.01
20:06:57.9 -34:32:55
06-Oct-97
0.024760575
07-Jun-19
0.024760575
2.1 Observations
The spectra used in this research were obtained using the 1.9 m telescope at the South
African Astronomical Observatory in Sutherland, and the data was obtained 20 years apart. The
first epoch is from the period 1997-1999, and the second epoch started being recorded in 2017
and was finished September 2019. The telescope uses a recently improved Cassegrain that offers
efficient low- to medium-resolution spectrograph. Observers have a selection from 10 diffraction
gratings with different wavelength ranges and resolutions. The total overall wavelength range
coverage is from ~3300Å - 10000Å. The upgraded software on the telescope uses a scripted
Hartmann sequence run to automatically focus the spectrograph camera [14]. When acquiring the
target, the slit position is adjusted depending on the brightness of the target.
2.2 Data Reduction and Analysis
Flatfields were obtained each night before observing and were later used to correct for pixelto-pixel sensitivity variations in the spectrum observed that night. Bias frames and arc spectra
were also collected, and standard stars were observed a few times during each night. Bias frames
were used in removing bias signals during calibration, arc spectra were used to calibrate the
spectra for wavelength and standard star images were used as scaling factors for flux calibration.
Blemishes caused by cosmic rays were removed prior to receiving the data. All data calibrations
were done by H. Winkler.
Spectral analysis was done in the wavelength range 3500Å to 7500Å, after redshiftcorrection of each spectra. The [OIII]5007Å line profile was used to calculate the redshift for each
spectrum. Spectral line profiles were fitted using the Gaussian function
𝐺 (𝑥) = 𝑎 × exp[− (𝑥 − 𝑥0 )2 ⁄2𝑐 2 ]
where a is the height of the peak, c is the width of the peak and x and x0 are the rest and
observed x-position of the peak. Figure 1 below shows the spectrum and fitted Gaussian profiles
for the object CTS A09.25 obtained on 23 May 1999, where the black spectrum is the obtained
raw spectrum from the given data. The fittings used are at the bottom and are as labelled.
3
A search for optical spectroscopic transitions in a sample of Seyfert galaxies
CTS A09.25 - 23 May 1999
Flux (10-17 erg.cm-2s-1Å-1)
CTS A09.25 - 23 May 1999
40
(HeII)4681
Broad Hβ
Narrow Hβ
[OIII]4959
[OIII]5007
1(a)
40
20
1(b
)
broad Hα
narrow Hα
[NII]6583
20
0
0
4800
5100
6400
wavelength
80
Daymore T.M. Makope
6600
CTS A09.25 - 23 May 1999
raw spectrum
overall fit
Flux (10-17 erg.cm-2s-1Å-1)
70
60
6800
wavelength
1(c)
50
40
30
20
10
0
3500
4000
4500
5000
5500
6000
6500
7000
7500
wavelength
Figure 1
The overall sum of the fitted Gaussians is shown together with the raw spectrum in Figure
1(c) above. From the parameters a and c, the FWHM (full-width-at-half-maximum) and intensity
of the peaks were calculated from the equations
𝐹𝑊𝐻𝑀 = 2.355 × 𝑐
and
𝐼 = 𝑎 𝑐 × √2 𝜋
The narrow line region is assumed to be constant over intervals of tens of years [3] as the
narrow-line region is expected to be several light years in diameter [15], therefore we use the
[OIII]5007Å line profile for scaling the spectra from the two epochs for comparison. Therefore,
for the scaling of the spectra, we assume that the intensity of the [OIII]λ5007Å is the same for
both spectra. The Balmer decrement – Hα/Hβ line flux ratio – is calculated for each object. The
luminosities of the narrow and broad line regions were measured from the luminosities of the
[OIII]5007Å and Hβ line profiles, respectivel [16]. Nuclear reddening, electron density and
temperature of the AGN are calculated for some of the objects.
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A search for optical spectroscopic transitions in a sample of Seyfert galaxies
Daymore T.M. Makope
These findings will be used to test physical models of AGN and to highlight possible
examples of new CL AGN cases.
3.Results
Results for each object are given below.
3.1 1E 2124-14
2.5
relative flux
1.0
2.0
1997
0.0
4700
relative flux
1.0
4800
0.5
0.0
4700
4900
5000
relative flux
0.5
5100
2019
4800
4900
1.5
1.0
1997
2019
0.5
5000
0.0
6400
5100
wavelength
6450
6500
6550
6600
6650
6700
wavelength
In 1997, the spectrum showed strong Balmer profiles with line intensity ratio
[OIII]𝜆5007Å to Hβ ~ 0.543. Noticeable Hβ and Hα profiles strengths can be used to
classify the object as a Type 1 Seyfert galaxy. The spectrum from 2019 can be classified
as a Type 1.9 Seyfert galaxy. The Balmer profiles have decreased significantly, with only
a weak broad component of the Hα line profile remaining. The strength of the line is now
comparable to the [OIII]𝜆5007Å and it now shows a prominent [NII]6583Å line profile
which was almost embedded in the Hα profile in 1997. The once strong and broad Hß
profile has completely vanished suggesting a new changing look case for the object 1E
2124-14.
3.2 CTS A08.12
relative flux
1.0
relative flux
1.0
0.8
0.6
0.4
0.0
6200
0.2
0.0
0.4
3750
4000
4250
4500
wavelength
4700
4750
4800
4850
4900
4950
5000
5050
5100
wavelength
2019
0.6
0.0
6200
0.2
4000
wavelength
4250
4500
6500
6600
6700
6800
0.0
4700
4750
4800
4850
4900
6600
6700
6800
2019
0.5
0.4
3750
6400
wavelength
1.0
0.8
0.0
6300
0.0
1.0
2019
0.2
1999
0.5
1999
relative flux
1999
0.2
relative flux
relative flux
relative flux
0.4
4950
wavelength
5000
5050
5100
6300
6400
6500
wavelength
The 2019 spectrum shows a significantly strengthened Hα component that has engulfed
the once prominent [NII]𝜆6548Å component. We must note that it’s only this component
5
A search for optical spectroscopic transitions in a sample of Seyfert galaxies
Daymore T.M. Makope
that has strengthened, the rest of the Balmer profiles Hß, H𝛾 and H𝛿 only broadened, with
an evident blueshift. The HeII 𝜆4686Å almost vanishes completely. The broad Hβ line
profile now extends under the [OIII] doublet. There is a noticeable increase in the overall
continuum flux accompanied by a noticeable strengthening of the narrow components of
the Balmer profiles. Data from the Swift archive shows that the X-ray flux for the object
decreased from 7.5 × 10−11 𝑒𝑟𝑔 𝑐𝑚−2 𝑠 −1 in July 2007 to 1.44 × 10−11 𝑒𝑟𝑔 𝑐𝑚−2 𝑠 −1
in March 2013.
3.3 CTS A09.25
Unlike object A0812, object A0925 shows both strengthening and broadening of the
Balmer profiles. These profiles were weaker than the [OIII]λ5007Å profile in the first
epoch. The Balmer lines from the 1999 spectrum shows very defined narrow
components. However, in 2019, all the Balmer lines became much broader than before.
The Hα profile has become the stronger line in the spectrum with the [NII] λ6548Å line
profile blended into the strong Hα left wing. The Hα and Hγ line profiles maintain their
structures with two distinct peaks. The Hβ profile changed drastically over the 20 years,
showing strong wings in the new spectra which extend under the [OIII] doublet. There
is an increase in the strength of the (HeII)λ4686Å line profile which could be due to the
blending by Fe II lines in the region. Swift data shows X-ray flux in the 0.3-10 keV to
be 5.0 × 10−12 𝑒𝑟𝑔 𝑐𝑚−2 𝑠 −1 in July 2008 and 4.1 × 10−12 𝑒𝑟𝑔 𝑐𝑚−2 𝑠 −1 in October
2012.
3.4 CTS A03.12
Object CTS A03.12 shows an increase in the overall continuum intensity accompanied
by strengthening of the Balmer profiles. The intensity of the Hβ and Hα profiles increased
by more than three magnitudes with a noticeable broadening. The Hγ and Hδ profiles
which were once weakly visible now have intensities comparable to the [OIII] 𝜆5007Å
profile.
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A search for optical spectroscopic transitions in a sample of Seyfert galaxies
Daymore T.M. Makope
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