ترغب بنشر مسار تعليمي؟ اضغط هنا

Unveiling the physics of low luminosity AGN through X-ray variability: LINER versus Seyfert 2

100   0   0.0 ( 0 )
 نشر من قبل Lorena Hern\\'andez-Garc\\'ia
 تاريخ النشر 2016
  مجال البحث فيزياء
والبحث باللغة English




اسأل ChatGPT حول البحث

X-ray variability is very common in active galactic nuclei (AGN), but these variations may not occur similarly in different families of AGN. We aim to disentangle the structure of low ionization nuclear emission line regions (LINERs) compared to Seyfert 2s by the study of their spectral properties and X-ray variations. We assembled the X-ray spectral parameters and variability patterns, which were obtained from simultaneous spectral fittings. Major differences are observed in the X-ray luminosities, and the Eddington ratios, which are higher in Seyfert 2s. Short-term X-ray variations were not detected, while long-term changes are common in LINERs and Seyfert 2s. Compton-thick sources generally do not show variations, most probably because the AGN is not accesible in the 0.5--10 keV energy band. The changes are mostly related with variations in the nuclear continuum, but other patterns of variability show that variations in the absorbers and at soft energies can be present in a few cases. We conclude that the X-ray variations may occur similarly in LINERs and Seyfert 2s, i.e., they are related to the nuclear continuum, although they might have different accretion mechanisms. Variations at UV frequencies are detected in LINER nuclei but not in Seyfert 2s. This is suggestive of at least some LINERs having an unobstructed view of the inner disc where the UV emission might take place, being UV variations common in them. This result might be compatible with the disappeareance of the torus and/or the broad line region in at least some LINERs.



قيم البحث

اقرأ أيضاً

In this work, which is a continuation of Castello-Mor et al. (2016), we present new X-ray and infrared (IR) data for a sample of active galactic nuclei (AGN) covering a wide range in Eddington ratio over a small luminosity range. In particular, we ri gorously explore the dependence of the optical-to-X-ray spectral index $alpha_{OX}$ and the IR-to-optical spectral index on the dimensionless accretion rate, $dot{mathcal{M}}=dot{m}/eta$ where $dot{m}=L_{AGN}/L_{Edd}$ and $eta$ is the mass-to-radiation conversion efficiency, in low and high accretion rate sources. We find that the SED of the faster accreting sources are surprisingly similar to those from the comparison sample of sources with lower accretion rate. In particular: i) the optical-to-UV AGN SED of slow and fast accreting AGN can be fitted with thin AD models. ii) The value of $alpha_{OX}$ is very similar in slow and fast accreting systems up to a dimensionless accretion rate $dot{mathcal{M}}_{c}sim$10. We only find a correlation between $alpha_{OX}$ and $dot{mathcal{M}}$ for sources with $dot{mathcal{M}} > dot{mathcal{M}}_{c}$. In such cases, the faster accreting sources appear to have systematically larger $alpha_{OX}$ values. iii) We also find that the torus in the faster accreting systems seems to be less efficient in reprocessing the primary AGN radiation having lower IR-to-optical spectral slopes. These findings, failing to recover the predicted differences between the SEDs of slim and thin ADs within the observed spectral window, suggest that additional physical processes or very special geometry act to reduce the extreme UV radiation in fast accreting AGN. This may be related to photon trapping, strong winds, and perhaps other yet unknown physical processes.
277 - W. Yuan 2004
We present the first result of a programme to search for large flux variations in the X-ray sources of the XMM Serendipitous Survey compared to previous ROSAT observations. An increase in X-ray flux by a factor >10 was discovered from the nucleus of the galaxy NGC7589 on a timescale of less than 5 years. The 0.4-10keV XMM spectrum can be approximated by a power-law with photon index of 1.7-1.8, though it seems to flatten above 5keV, suggesting a possible complex model, such as partial covering or disc reflection. A classification based on an analysis of its optical spectrum places NGC7589 in the Seyfert region, but close to the Seyfert-LINER border-line on the AGN diagnostic diagrams. We classify NGC7589 as either Seyfert1.9 or LINERI, in the light of the detection of a broad H_alpha line, which makes NGC7589 an AGN in the low-luminosity regime. We interpret the observed variability in terms of either changes in covering factor of absorbing gas in the AGN, or variability in the intrinsic X-ray luminosity. Should the latter be the case, the inferred Eddington accretion rate increased from the radiatively inefficient accretion dominated regime to a value close to the putative critical value, at which a transition of the accretion mode is supposed to take place. This possibility presents a new prospect of studying accretion physics in the central black holes of external galaxies by direct observing changes of `spectral state, as is common in stellar black hole binary systems.
301 - G. Younes 2010
(Abridged) Based on UV to X-ray and radio to UV flux ratios, some argue that low ionization emission line regions (LINERs) and low luminosity AGN (LLAGN) are a scaled-down version of their more luminous predecessors Seyfert galaxies. Others, based on the lack of X-ray short (hours) time-scale variability, the non-detection of an iron line at 6.4 keV, and the faint UV emission, suggest the truncation of the classical thin accretion disk in the inner regions of the AGN where a radiatively inefficient accretion flow (RIAF) structure forms. We investigate the LINER-Seyfert connection by studying the unabsorbed, AGN powered, LINER galaxy NGC 4278. We analyzed one XMM-Newton and seven Chandra X-ray observations of NGC 4278 spread over a three year period. We detected a flux increase by a factor of ~3 on a ~3 months time-scale and by a factor of 5 between the faintest and the brightest observation separated by ~3 years. During only the XMM-Newton observation, where the highest flux level is detected, we found a 10% flux increase on a ~1 hour time-scale. A combination of an absorbed power law (N(H)~10^20 cm^-2, Gamma~2.2) plus a thermal component (kT~0.6 keV) were able to fit the Chandra spectra. The XMM-Newton spectra, where the highest X-ray flux is detected, are well fitted with a single absorbed power-law. No Fe K(alpha) emission line is detected at 6.4 keV. We constructed SEDs based on simultaneous or quasi simultaneous observations and compared them to LINER, radio-loud, and radio-quiet quasar SEDs. We find that at a low X-ray flux the NGC 4278 SED resembles that of typical LINER sources where the radio to X-ray emission can be considered as originating from a jet and/or RIAF, whereas at a high X-ray flux, NGC 4278 SED is more like a low luminosity Seyfert SED. Consequently, NGC 4278 could exhibit both LINER and Seyfert nuclear activity depending on the strength of its X-ray emission.
Seyfert 1.8/1.9 are sources showing weak broad H-alpha components in their optical spectra. We aim at testing whether Seyfert 1.8/1.9 have similar properties at UV and X-ray wavelengths to Seyfert 2. We use the 15 Seyfert 1.8/1.9 in the Veron Cetty a nd Veron catalogue with public data available from the Chandra and/or XMM-Newton archives at different dates, with timescales between observations ranging from days to years. Our results are homogeneously compared with a previous work using the same methodology applied to a sample of Seyfert 2 (Hernandez-Garcia et al. 2015). X-ray variability is found in all 15 nuclei over the aforementioned ranges of timescales. The main variability pattern is related to intrinsic changes in the sources, which are observed in ten nuclei. Changes in the column density are also frequent, as they are observed in six nuclei, and variations at soft energies, possibly related to scattered nuclear emission, are detected in six sources. X-ray intraday variations are detected in six out of the eight studied sources. Variations at UV frequencies are detected in seven out of nine sources. A comparison between the samples of Seyfert 1.8/1.9 and 2 shows that, even if the main variability pattern is due to intrinsic changes of the sources in the two families, these nuclei exhibit different variability properties in the UV and X-ray domains. In particular, variations in the broad X-ray band on short time-scales (days/weeks), and variations in the soft X-rays and UV on long time-scales (months/years) are detected in Seyfert 1.8/1.9 but not in Seyfert 2. Overall, we suggest that optically classified Seyfert 1.8/1.9 should be kept separated from Seyfert 2 galaxies in UV/X-ray studies of the obscured AGN population because their intrinsic properties might be different.
We investigate the photoionised X-ray emission line regions (ELRs) within the Seyfert 2 galaxy NGC 1068, to determine if there are any characteristic changes between observations taken fourteen years apart. We compare XMM-Newton observations collecte d in 2000 and 2014, simultaneously fitting the reflection grating spectrometer (RGS) and EPIC-pn spectra of each epoch, for the first time, with the photoionisation model, PION, in SPEX. We find that four PION components are required to fit the majority of the emission lines in the spectra of NGC 1068, with $log xi=1-4$, $log N_H>26 m^{-2}$, and $v_{out}=-100$ to $-600 kms^{-1}$ for both epochs. Comparing the ionisation state of the components shows almost no difference between the two epochs, while there is an increase in the total column density. To estimate the locations of these plasma regions from the central black hole we compare distance methods, excluding the variability arguments as there is no spectral change between observations. Although the methods are unable to constrain the distances, the locations are consistent with the narrow line region, with the possibility of the higher ionised component being part of the broad line region, but we cannot conclude this for certain. In addition, we find evidence for emission from collisionally ionised plasma, while previous analysis had suggested that collisional plasma emission was unlikely. However, although PION is unable to account for the FeXVII emission lines at 15 and 17 AA, we do not rule out that photoexcitation is a valid processes to produce these lines too. NGC 1068 has not changed, both in terms of the observed spectra or from our modelling, within the 14 year time period between observations. This suggests that the ELRs are fairly static relative to the 14 year time frame between observations, or there is no dramatic change in the black hole variability.
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا