Sains Malaysiana 49(1)(2020): 189-199

http://dx.doi.org/10.17576/jsm-2020-4901-23

Calon-calon Nukleus Galaksi Aktif Berubah-Rupa pada z ≤ 0.02 dalam Katalog Swift-BAT ke-105 Bulan

(Changing-Look Active Galactic Nuclei Candidates at z ≤ 0.02 in the 105-Month Swift-BAT Catalogue)

MUHAMMAD EZZAT ABDUL HAMID & ADLYKA ANNUAR*

School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia

Received: 1 August 2019/Accepted: 8 October 2019

ABSTRAK

Nukleus galaksi aktif berubah-rupa (CL AGN) adalah sebuah objek astrofizik yang jarang ditemui. Objek ini mengalami perubahan fluks dan spektra yang dramatik dalam skala masa beberapa tahun atau dekad. Dalam kertas ini, kami mengkaji sampel AGN berhampiran dengan kita yang terletak pada z ≤ 0.0200 dalam katalog Swift-BAT ke-105 bulan. Berdasarkan lengkung cahaya sinar-X bertenaga tinggi, kami mendapati bahawa 27 daripada 174 AGN; i.e. (15.51 ± 0.03)%,  dalam sampel kami menunjukkan perubahan fluks yang signifikan berdasarkan ujian statistik. Jumlah ini menunjukkan pertambahan sebanyak dua kali ganda daripada jumlah CL AGN yang dikenal pasti dalam alam semesta setakat ini. Ini mencadangkan bahawa objek tersebut adalah calon CL AGN. Kami juga mengkaji faktor fizikal yang mungkin menyumbang kepada perubahan yang dilihat pada calon tersebut berdasarkan bentuk lengung cahaya dan kajian lepas dan mendapati bahawa bagi satu objek, perubahan yang dilihat berkemungkinan besar disebabkan oleh kejadian gangguan pasang surut. Sementara itu bagi tiga calon, kajian lepas menunjukkan bahawa mereka mengalami perubahan spektra optik atau sinar-X. Ini mencadangkan bahawa calon ini mempunyai struktur torus yang tidak sekata. Hal ini bertentangan dengan apa yang dicadangkan oleh model satuan AGN, menunjukkan bahawa model ini mungkin tidak lagi sesuai digunakan sebagai gambaran umum struktur fizikal sesebuah AGN. Namun begitu, data dan kajian lanjut perlu dijalankan untuk mengenal pasti faktor ini dengan lebih terperinci dan bagi mengkaji faktor untuk calon CL AGN yang selebihnya dalam kajian kami yang selanjutnya.

Kata kunci: Lohong hitam; nukleus galaksi aktif (AGN); sinar X; Swift-BAT

ABSTRACT

Changing-look active galactic nuclei (CL AGN) are relatively rare objects, identified based upon their dramatic flux and spectral changes within a timescale of years or decades. In this paper, we study a sample of nearby AGN within z ≤ 0.0200 in the 105-month Swift-BAT catalogue to identify CL AGN using their hard X-ray lightcurves. Based on this, we find that 27 out of 174 AGN; i.e. (15.51 ± 0.03)%, show statistically significant flux changes, suggesting that they are candidates for CL AGN. This number further doubles up the number of CL AGN that we know so far in our universe. We investigate the physical factors that could contribute to the changes seen in these AGN based on the pattern of their lightcurves and past studies, and find that for one object, tidal disruption event is likely the cause. Meanwhile for three of the candidates, past studies have shown that they underwent significant optical or X-ray spectral changes, suggesting that the torus distributions in these AGN are patchy. This is in contradiction with that suggested by the AGN unified model, indicating that this model might no longer be relevant to illustrate the general physical structure of an AGN. However, further data and studies need to be conducted to investigate these factors in more detail. This is also true for the remaining of the CL AGN candidates we identify which we could not investigate the cause due to lack of data and information. This will be our next step for this work.

Keywords: Active galactic nuclei (AGN); black holes; Swift-BAT; X-ray

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*Corresponding author; email: adlyka@ukm.edu.my