分类： 其他 分类： 其他 提交时间： 2024-04-25
摘要： Radio-loud quasars (RLQs) are more X-ray luminous than predicted by the X-ray-optical/UV relation (i.e. $L_\mathrm{x}\propto L_\mathrm{uv}^\gamma$) for radio-quiet quasars (RQQs). The excess X-ray emission depends on the radio-loudness parameter ($R$) and radio spectral slope ($\alpha_\mathrm{r}$). We construct a uniform sample of 729 optically selected RLQs with high fractions of X-ray detections and $\alpha_\mathrm{r}$ measurements.We find that steep-spectrum radio quasars (SSRQs; $\alpha_\mathrm{r}\le-0.5$) follow a quantitatively similar $L_\mathrm{x}\propto L_\mathrm{uv}^\gamma$ relation as that for RQQs, suggesting a common coronal origin for the X-ray emission of both SSRQs and RQQs. However, the corresponding intercept of SSRQs is larger than that for RQQs and increases with $R$, suggesting a connection between the radio jets and the configuration of the accretion flow. Flat-spectrum radio quasars (FSRQs; $\alpha_\mathrm{r}>-0.5$) are generally more X-ray luminous than SSRQs at given $L_\mathrm{uv}$ and $R$, likely involving more physical processes. The emergent picture is different from that commonly assumed where the excess X-ray emission of RLQs is attributed to the jets. We thus perform model selection to comparecritically these different interpretations, which prefers the coronal scenario with a corona-jet connection. A distinct jet component is likely important for only a small portion of FSRQs.The corona-jet, disk-corona, and disk-jet connections of RLQs are likely driven by independent physical processes. Furthermore, the corona-jet connection implies that small-scale processesin the vicinity of SMBHs, probably associated with the magnetic flux/topology instead of black-hole spin, are controlling the radio-loudness of quasars.
分类： 其他 提交时间： 2024-04-25
摘要： Rapid flares from blazars in very high energy (VHE) $\gamma$-rays challenge the common understanding of jets of active galactic nuclei (AGNs). The same population of ultra-relativistic electrons is often thought to be responsible for both X-ray and VHE emission. We thus systematically searched for X-ray flares at sub-hour timescales of TeV blazars in the entire Rossi X-ray Timing Explorer archival database. We found rapid flares from PKS 2005-489 and S5 0716+714, and a candidate rapid flare from 1ES 1101-232. In particular, the characteristic rise timescale of PKS 2005-489 is less than half a minute, which, to our knowledge, is the shortest among known AGN flares at any wavelengths. The timescales of these rapid flares indicate that the size of the central supermassive black hole is not a hard lower limit on the physical size of the emission region of the flare. PKS 2005-489 shows possible hard lags in its flare, which could be attributed to particle acceleration (injection), its flaring component has the hardest spectrum when it first appears. For all flares, the flaring components show similar hard spectra with $\Gamma=1.7-1.9$, and we estimate the magnetic field strength $B\sim$ 0.1--1.0 G by assuming synchrotron cooling. These flares could be caused by inhomogeneity of the jets. Models that can only produce rapid $\gamma$-ray flares but little synchrotron activity are less favorable.
分类： 其他 分类： 其他 分类： 其他 提交时间： 2024-04-25
摘要： Fluxes emitted at different wavebands from active galactic nuclei (AGNs) fluctuate at both long and short timescales. The variation can typically be characterized by a broadband power spectrum, which exhibits a red-noise process at high frequencies. The standard method of estimating power spectral density (PSD) of AGN variability is easily affected by systematic biases such as red-noise leakage and aliasing, in particular, when the observation spans a relatively short period and is gapped. Focusing on the high-frequency PSD that is strongly distorted due to red-noise leakage and usually not significantly affected by aliasing, we develop a novel and observable normalized leakage spectrum (NLS), which describes sensitively the effects of leaked red-noise power on the PSD at different temporal frequencies. Using Monte Carlo simulations, we demonstrate how an AGN underlying PSD sensitively determines the NLS when there is severe red-noise leakage and thereby how the NLS can be used to effectively constrain the underlying PSD.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
M. Paolillo
I. E. Papadakis
W. N. Brandt
F. E. Bauer
G. Lanzuisi
V. Allevato
O. Shemmer
X. C. Zheng
D. De Cicco
R. Gilli
B. Luo
M. Thomas
P. Tozzi
F. Vito
Y. Q. Xue
摘要： We study the ensemble X-ray variability properties of Active Galactic Nuclei (AGN) over a large range of timescales (20 ks $\leq T\leq$ 14 yrs), redshift ($0\leq z \lesssim 3$), luminosities ($10^{40}$ erg s$^{-1}\leq L_X\leq 10^{46}$ erg s$^{-1}$) and black hole (BH) masses ($10^6 \leq $M$_\odot \leq 10^9$). We propose the use of the variance-frequency diagram, as a viable alternative to the study of the power spectral density (PSD), which is not yet accessible for distant, faint and/or sparsely sampled AGN. We show that the data collected from archival observations and previous literature studies are fully consistent with a universal PSD form which does not show any evidence for systematic evolution of shape or amplitude with redshift or luminosity, even if there may be differences between individual AGN at a given redshift or luminosity. We find new evidence that the PSD bend frequency depends on BH mass and, possibly, on accretion rate. We finally discuss the implications for current and future AGN population and cosmological studies.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
T. K. Garratt
J. E. Geach
Y. Tamura
K. E. K. Coppin
M. Franco
Y. Ao
C. -C. Chen
C. Cheng
D. L. Clements
Y. S. Dai
H. Dannerbauer
T. R. Greve
B. Hatsukade
H. S. Hwang
L. Jiang
K. Kohno
M. P. Koprowski
M. J. Michalowski
M. Sawicki
D. Scott
摘要： We present 850um imaging of the XMM-LSS field observed for 170 hours as part of the James Clerk Maxwell Telescope SCUBA-2 Large eXtragalactic Survey (S2LXS). S2LXS XMM-LSS maps an area of 9 square degrees, reaching a moderate depth of 1-sigma ~ 4 mJy/beam. This is the largest contiguous area of extragalactic sky mapped by JCMT at 850um to date. The wide area of the S2LXS XMM-LSS survey allows us to probe the ultra-bright (S_850um > 15 mJy), yet rare submillimetre population. We present the S2LXS XMM-LSS catalogue, which comprises 40 sources detected at >5-sigma significance, with deboosted flux densities in the range of 7 mJy to 48 mJy. We robustly measure the bright-end of the 850um number counts at flux densities >7 mJy, reducing the Poisson errors compared to existing measurements. The S2LXS XMM-LSS observed number counts show the characteristic upturn at bright fluxes, expected to be motivated by local sources of submillimetre emission and high-redshift strongly lensed galaxies. We find that the observed 850um number counts are best reproduced by model predictions that include either strong lensing or source blending from a 15 arcsec beam, indicating that both may make an important contribution to the observed over-abundance of bright single dish 850um selected sources. We make the S2LXS XMM-LSS 850um map and >5-sigma catalogue presented here publicly available.
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