اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدParsec-scale radio morphology and variability of a changing-look AGN: the case of Mrk 590
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We investigate the origin of the parsec-scale radio emission from the changing-look active galactic nucleus (AGN) of Mrk 590, and examine whether the radio power has faded concurrently with the dramatic decrease in accretion rates observed between the 1990s and the present. We detect a compact core at 1.6 GHz and 8.4 GHz using new Very Long Baseline Array observations, finding no significant extended, jet-like features down to $sim$1 pc scales. The flat spectral index ($alpha_{1.6}^{8.4} = 0.03$) and high brightness temperature ($T_{rm b} sim 10^{8},rm K$) indicate self-absorbed synchrotron emission from the AGN. The radio to X-ray luminosity ratio of ${rm log}(L_{rm R}/L_{rm X}) sim -5$, similar to that in coronally active stars, suggests emission from magnetized coronal winds, although unresolved radio jets are also consistent with the data. Comparing new Karl G. Jansky Very Large Array measurements with archival and published radio flux densities, we find $46%$, $34%$, and (insignificantly) $13%$ flux density decreases between the 1990s and the year 2015 at 1.4 GHz, 5 GHz and 8.4 GHz respectively. This trend, possibly due to the expansion and fading of internal shocks within the radio-emitting outflow after a recent outburst, is consistent with the decline of the optical-UV and X-ray luminosities over the same period. Such correlated variability demonstrates the AGN accretion-outflow connection, confirming that the changing-look behaviour in Mrk 590 originates from variable accretion rates rather than dust obscuration. The present radio and X-ray luminosity correlation, consistent with low/hard state accretion, suggests that the black hole may now be accreting in a radiatively inefficient mode.
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Broad Balmer emission lines in active galactic nuclei (AGN) may display dramatic changes in amplitude, even disappearance and re-appearance in some sources. As a nearby galaxy at a redshift of z = 0.0264, Mrk 590 suffered such a cycle of Seyfert type
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t followup observations to the original discovery and characterization of this changing look active galactic nucleus (AGN). The new Chandra and HST observations from 2014 show that Mrk 590 is awakening, changing its appearance again. While the source continues to be in a low state, its soft excess has re-emerged, though not to the previous level. The UV continuum is brighter by more than a factor of two and the broad MgII emission line is present, indicating that the ionizing continuum is also brightening. These observations suggest that the soft excess is not due to reprocessed hard X-ray emission. Instead, it is connected to the UV continuum through warm Comptonization. Variability of the Fe K-alpha emission lines suggests that the reprocessing region is within about 10 light years or 3 pc of the central source. The AGN type change is neither due to obscuration, nor due to one-way evolution from type-1 to type-2, as suggested in literature, but may be related to episodic accretion events.
We examine the long-term optical/near-infrared (NIR) flux variability of a changing-look active galactic nucleus (AGN) Mrk 590 between 1998 and 2007. Multi-band multi-epoch optical/NIR photometry data from the SDSS Stripe 82 database and the Multicol
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ocular Telescope, we find that the activity in the nucleus of Mrk 590 has diminished so significantly that the continuum luminosity is a factor of 100 lower than the peak luminosity probed by our long baseline observations. Furthermore, the broad emission lines, once prominent in the UV/optical spectrum, have all but disappeared. Since AGN type is defined by the presence of broad emission lines in the optical spectrum, our observations demonstrate that Mrk 590 has now become a changing look AGN. If classified by recent optical spectra, Mrk 590 would be a Seyfert ~1.9-2, where the only broad emission line still visible in the optical spectrum is a weak component of Halpha. As an additional consequence of this change, we have definitively detected UV narrow-line components in a Type 1 AGN, allowing an analysis of these emission-line components with high-resolution COS spectra. These observations challenge the historical paradigm that AGN type is only a consequence of the line of sight viewing angle toward the nucleus in the presence of a geometrically-flattened, obscuring medium (i.e., the torus). Our data instead suggest that the current state of Mrk 590 is a consequence of the change in luminosity, which implies the black hole accretion rate has significantly decreased.
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