publications
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Refereed Journal Articles
2024
- Discovery of Six Low-z, High-luminosity, and High-mass QuasarsSeyit Hökelek, Enes Selam Kaçan, and Nurten Filiz AkPublications of the Astronomical Society of the Pacific, Nov 2024
This study presents the discovery of six low-redshift quasars, identified through spectral observations conducted with the RTT-150 telescope. These quasars, with redshifts ranging from 0.3 to 0.6, were selected incorporating their placement in the color–color diagrams and detection by ROSAT 2RXS, focusing on those resembling the spectral energy distribution of a quasar. Our analysis includes detailed modeling of their continuum and emission line properties, which allowed us to estimate their bolometric luminosities, central black hole masses, and Eddington ratios. The findings reveal that these quasars exhibit exceptionally high luminosities and high masses for their redshift values, making them rare examples in the low-redshift quasar population. The results contribute to our understanding of quasar characteristics and their role in probing the structure and evolution of the nearby universe. This research underscores the importance of continued exploration of low-redshift quasars to enhance our knowledge of cosmic evolution and the dynamics of supermassive black holes in the early universe.
Conference Proceedings
2025
- Spectral Verification of 6 Low-z Quasars with RTT-150Seyit Hökelek, Enes Selam Kacan, and Nurten Filiz AkTurkish Journal of Astronomy and Astrophysics, Jul 2025
This paper presents the discovery of 6 quasars that have been spectrally confirmed for the first time using the RTT-150 telescope and the TFOSC spectrometer. The full study is published in Hökelek vd. (2024). Detailed modeling of the continuum and emission line properties of these quasars with redshifts (z) ranging from 0.3 to 0.6, determined for the first time in this study, was performed to assess their bolometric luminosities, central black hole masses and Eddington ratios. The results show that these quasars have exceptionally high luminosities and high masses compared to known quasars with the same z values.
- Modeling Blazar TEDs with Simple RegressionBerker Utku, Nurten Filiz Ak, and Seyit HökelekTurkish Journal of Astronomy and Astrophysics, Jul 2025
Blazars are a subclass of active galactic nuclei characterized by jet structures oriented perpendicular to the accretion disk surrounding the central black hole. Unlike other active galaxies, the jets of blazars are aligned nearly directly toward the observer. As a result, their optical spectra lack distinct line-like features, making them unsuitable for determining distances using traditional methods. The most significant observational insights into blazars come from their spectral energy distributions (SEDs). In this study, the spectral energy distributions of selected blazars were constructed and analyzed using archival data. Instead of employing detailed modeling of the SEDs, simple methods were applied to determine two fundamental parameters: the peak flux and the peak frequency. These straightforward approaches provide practical and rapid results, making them suitable for statistical studies of blazars. The outcomes of three different approaches are compared in this study, and it is concluded that these fast methods hold promise for enhancing our understanding of the physical nature and intrinsic properties of blazars.
- Bayesian Radial Velocity Solver: BRaVeEmre Barbaros, Seyit Hökelek, Hasan Ak, and 1 more authorTurkish Journal of Astronomy and Astrophysics, Jul 2025
The analysis of the time-dependent radial velocity curves of double stars allows direct determination of the physical parameters of the components. The program BRaVe, which we developed to obtain the most accurate orbital modeling, obtains orbital parameters and errors with high precision. BRaVe includes an optimization routine based on Bayesian statistics. This routine aims to obtain the basic orbital parameters of the considered double star system in the most precise way. Capella A was chosen as the reference star for the BRaVe code. Radial velocities were calculated from high-resolution spectra from the Polarbase data archive using the Cross Corelation Function (CCF) method and the orbital parameters were solved using BRaVe. The results obtained in this study are presented and compared with the literature.
2023
- AGN Studies of TUG-SRG ColloborationNurten Filiz Ak, Seyit Hökelek, Tuğba Boztepe, and 8 more authorsTurkish Journal of Astronomy and Astrophysics, Dec 2023
A protocol signed between Russia and Turkey endures a collaboration over the data from the Mikhail Pavlinsky ART-XC space telescope. The TUG-SRG workgroup has been built to organize collaborative studies among Turkish researchers. This proceeding briefs the procedures running within the TUG-SRG workgroup in a specific example of an Active Galactic Nuclei~(AGN) candidate. The proceeding precedes the analysis of the RTT-150 optical spectra, ZTF light curve, and SED modeling for SRGAJ03p76. The first spectroscopic confirmation of the optical counterpart of this hard X-ray source reveals SRGAJ03p76 to be a \z\approx0.2 quasar. AGN studies will continue within the TUG\-\SRG group to pursue optical identification of the hard X-ray sources.
- Quasar Wind Variabilty Prediction with Machine Learning MethodsSeyit Hökelek and Nurten Filiz AkTurkish Journal of Astronomy and Astrophysics, Dec 2023
Approximately 20% of the quasars exhibit the Broad Absorption Lines (BALs) as a tracer of the outflows from the accretion disk of the central supermassive black holes. The time dependent variability of the BALs caries important information to understand the formation and evolution of the quasar winds. In this study, using a sample of BALs that have at least two epoch spectra, we develop a model to predict the variability of the winds based on the BAL parameters measured form their first epoch observation. We compare a few machine learning methods from which the Linear Discriminant method results the best accuracy. Even the most accurate method produces insufficient models to predict the BAL variability from the BAL parameters measured from the first epoch. This suggests that the internal properties of the absorbing gas carried by the outflows have, if any, a weak connection with the variations of BALs.
2022
- Quasars Discovered by RTT-150 TFOSC Spectral ObservationsSeyit Hökelek and Nurten Filiz AkTurkish Journal of Astronomy and Astrophysics, Dec 2022
This study presents the first spectroscopic observations of four quasar candidates. We select the quasar candidates considering the high candidacy probabilities calculated from the SDSS and WISE photometric colors. We obtained the long-slit spectrum of each candidate with RTT-150 telescope and TFOSC spectrometer in two nights of January and February 2021. The reduced spectra shows both broad and narrow emission features confirming that all four of the candidates are members of quasar class. We present the calculation results of \z \d_L and \M_i(z=2) parameters for four quasars.
- Determining Redshift in Quasars: zFinderEnes Selam Kaçan, Seyit Hökelek, and Nurten Filiz AkTurkish Journal of Astronomy and Astrophysics, Dec 2022
This study presents the algorithm and the test results for the zFinder pipeline code that is developed to measure the redshift (z) of the quasar spectra. The methodological approach of the zFinder code considers both the and Spearman parameters to define the accordance parameter where majority of the existing studies adopt the cross-correlation. The approach of comparing the entire observed spectra with a quasar template increases the precision of the measured z value by examining both the continuum and the strong lines on each spectra. We test the "zFinder" results for 150 quasars selected from SDSS DR16 quasar catalog of which z values are measured by spec1d pipeline of SDSS. We show that the z values are in a general agreement with and the average difference between zFinder and is "0.0018". There is one quasar spectra with measurements of "z=6.6.48" from "spec1d" and "z = 0.38" from "zFinder". Further investigations of the spectra showed that the "zFinder" gives the more accurate value of "z" and the "spec1d" misclassified the strong emission lines of the quasar.