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 πΊ (π₯) = π × exp[− (π₯ − π₯0 )2 ⁄2π 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 (ο΄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 (ο΄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 πΉππ»π = 2.355 × π and πΌ = π π × √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]λ5007β« 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. 4 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]π5007Å 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]π5007Å 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]π6548Å 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, HπΎ and HπΏ only broadened, with an evident blueshift. The HeII π4686Å 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 πππ ππ−2 π −1 in July 2007 to 1.44 × 10−11 πππ ππ−2 π −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. 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