分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present a long-term and intraday variability study on optical multiwaveband ($U\!BV\!RI$) data from the blazar AO 0235+164 collected by various telescopes for $\sim$44 years (1975--2019). The blazar was found to be significantly variable over the years in all wavebands with a variation of about six magnitudes between its low and active states. The variations in the different wavebands are highly correlated without any time lag. We did not observe any significant trend in color variation with time, but we observed a bluer-when-brighter trend between the $B-I$ color index and the $R$-magnitude. Optical $BV\!R$-band spectral energy distributions always show a convex shape. Significant intraday variability was frequently seen in the quasi-simultaneous observations of AO\,0235+164 made on 22 nights in $R$ and $V$-bands by the CASLEO and CAHA telescopes during 1999--2019. We also estimated the central supermassive black-hole mass of $7.9\times10^7 M_{\odot}$ by analyzing the broad Mg II emission line in AO 0235+164's spectrum. We briefly explore the probable physical scenarios responsible for the observed variability.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The radio emission in radio-quiet quasars (RQQs) has been a long mystery and its physical origin remains unclear. In a previous work we find UV/optical more variable quasars have stronger X-ray emission, indicating a link between disc turbulence and X-ray corona heating. In this work, for the first time, we investigate the relation between UV/optical variability and the radio emission in RQQs selected from SDSS stripe 82 and FIRST radio survey. We median stack the FIRST images and detect clear signals from RQQs in the co-added images of individually radio non-detected sources. Controlling the effects of other parameters, including redshift, black hole mass, bolometric luminosity and Eddington ratio, we find more variable RQQs, which are known to be X-ray relatively brighter, show tentatively weaker radio emission, contrary to the linear X-ray/radio correlation if the radio emission is from or driven by the corona. The discovery also suggests that if the radio emission in RQQs is driven by AGN activity (such as weak jet), the underlying driving process is independent to the disc turbulence which drives UV/optical variability and probably also corona heating. Alternatively, the radio emission could be due to star formation in the host galaxies.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Dwarf galaxies are characterised by a very low luminosity and low mass. Because of significant accretion and ejection activity of massive black holes, some dwarf galaxies also host low-luminosity active galactic nuclei (AGNs). In a few dwarf AGNs, very long baseline interferometry (VLBI) observations have found faint non-thermal radio emission. SDSS J090613.77+561015.2 is a dwarf AGN owning an intermediate-mass black hole (IMBH) with a mass of $M_{BH} = 3.6^{+5.9}_{-2.3} \times 10^5 M_{sun}$ and showing a rarely-seen two-component radio structure in its radio nucleus. To further probe their nature, i.e. the IMBH jet activity, we performed additional deep observations with the European VLBI Network (EVN) at 1.66 GHz and 4.99 GHz. We find the more diffuse emission regions and structure details. These new EVN imaging results allow us to reveal a two-sided jet morphology with a size up to about 150 mas (projected length $\sim$140 pc) and a radio luminosity of about $3\times10^{38}$ erg s$^{-1}$. The peak feature has an optically thin radio spectrum and thus more likely represents a relatively young ejecta instead of a jet base. The EVN study on SDSS J090613.77+561015.2 demonstrates the existence of episodic, relatively large-scale and powerful IMBH jet activity in dwarf AGNs. Moreover, we collected a small sample of VLBI-detected dwarf AGNs and investigated their connections with normal AGNs. We notice that these radio sources in the dwarf AGNs tend to have steep spectra and small linear sizes, and possibly represent ejecta from scaled-down episodic jet activity.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Compact radio active galactic nuclei (compact radio AGN) are compact ($\leq$ 20 kpc), powerful radio sources. Currently, the preferred scenario is that they are at the early stage of AGN evolution. At present, the research of compact radio AGN mainly focuses on the radio band, other bands have not been extensively studied. We present the systemic optical properties and X-ray emission studies for compact radio AGN, to investigate the accretion properties, AGN evolution and their X-ray origin. We find that compact radio AGN have various accretion modes indicated by the accretion rate analysis. In the radio power-linear size diagram they generally follow the evolutionary trend towards large-scale radio galaxies with increasing linear size and decreasing accretion rate. Their hard X-ray emission may be from jet based on the radio/X-ray relation and fundamental plane of black hole activity.