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Opis: Context. The timescale of a microlensing event scales as a square root of a lens mass. Therefore, long-lasting events are important candidates for massive lenses, including black holes. Aims. Here, we present the analysis of the Gaia18cbf microlensing event reported by the Gaia Science Alerts system. It exhibited a long timescale and features that are common for the annual microlensing parallax effect. We deduce the parameters of the lens based on the derived best fitting model. Methods. We used photometric data collected by the Gaia satellite as well as the follow-up data gathered by the ground-based observatories. We investigated the range of microlensing models and used them to derive the most probable mass and distance to the lens using a Galactic model as a prior. Using a known mass-brightness relation, we determined how likely it is that the lens is a main-sequence (MS) star. Results. This event is one of the longest ever detected, with the Einstein timescale of tE = 491.41−84.94+128.31 days for the best solution and tE = 453.74−105.74+178.69 days for the second best. Assuming Galaxy priors, this translates to the most probable lens masses of ML = 2.65−1.48+5.09 M⊙ and ML = 1.71−1.06+3.78 M⊙, respectively. The limits on the blended light suggest that this event was most likely not caused by a MS star, but rather by a dark remnant of stellar evolution.
Ključne besede: gravitational lensing: micro, Galaxy: stellar content, stars: black holes, stars: neutron, Astrophysics - Solar and Stellar Astrophysics
Objavljeno v RUNG: 13.11.2023; Ogledov: 604; Prenosov: 4
Celotno besedilo (9,87 MB)
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Opis: Context. We present the wavelength-resolved reverberation mapping (RM) of combined Mg II and UV Fe II broad-line emissions for two intermediate-redshift (z ∼ 1), luminous quasars, HE 0413-4031 and HE 0435-4312, monitored by the Southern African Large Telescope (SALT) and 1m class telescopes between 2012 and 2022. Aims. Using a wavelength-resolved technique, we aim to disentangle the Mg II and Fe II emission regions and to build a radius–luminosity (R–L) relation for UV Fe II emission, which has so far remained unconstrained. Methods. We applied several time-delay methodologies to constrain the time delays for total Mg II and Fe II emissions. In addition, wavelength-resolved RM is performed to quantify the inflow or outflow of broad-line region (BLR) gas around the supermassive black hole and to disentangle the emission and the emitting regions based on lines produced in proximity to each other. Results. The mean total FeII time delay is nearly equal to the mean total MgII time delay for HE 0435-4312, suggesting the co-spatiality of their emission regions. However, in HE 0413-4031, the mean FeII time delay is found to be longer than the mean MgII time delay, suggesting that FeII emission is produced at greater distances from the black hole. The UV FeII R–L relation is updated with these two quasars (now four in total) and compared with the optical FeII relation (20 sources), which suggests that the optical FeII emission region is located further than the UV FeII region by a factor of 1.7–1.9, that is, RFeII-opt ∼ (1.7 − 1.9)RFeII-UV. Conclusion. Wavelength-resolved reverberation is an efficient way to constrain the geometry and structure of the BLR. We detected a weak pattern in the time delay versus wavelength relation, suggesting that the MgII broad line originates from a region slightly closer to the SMBH than the UV FeII pseudo continuum, although the difference is not very significant. Comparison of MgII, UV, and optical FeII R–L relations suggests that the difference may be greater for lower-luminosity sources, possibly with the MgII emission originating further from the SMBH. In the future, more RM data will be acquired, allowing better constraints on these trends, in particular the UV FeII R–L relation.
Ključne besede: accretion, accretion disks, emission lines, photometric, spectroscopic
Objavljeno v RUNG: 13.11.2023; Ogledov: 640; Prenosov: 4
Celotno besedilo (2,92 MB)
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Opis: Context. Thanks to the advent of large-scale optical surveys, a diverse set of flares from the nuclear regions of galaxies has recently been discovered. These include the disruption of stars by supermassive black holes at the centers of galaxies – nuclear transients known as tidal disruption events (TDEs). Active galactic nuclei (AGN) can show extreme changes in the brightness and emission line intensities, often referred to as changing-look AGN (CLAGN). Given the physical and observational similarities, the interpretation and distinction of nuclear transients as CLAGN or TDEs remains difficult. One of the obstacles of making progress in the field is the lack of well-sampled data of long-lived nuclear outbursts in AGN. Aims. Here, we study PS16dtm, a nuclear transient in a Narrow Line Seyfert 1 (NLSy1) galaxy, which has been proposed to be a TDE candidate. Our aim is to study the spectroscopic and photometric properties of PS16dtm, in order to better understand the outbursts originating in NLSy1 galaxies. Methods. Our extensive multiwavelength follow-up that spans around 2000 days includes photometry and spectroscopy in the UV/optical, as well as mid-infrared (MIR) and X-ray observations. Furthermore, we improved an existing semiempirical model in order to reproduce the spectra and study the evolution of the spectral lines. Results. The UV/optical light curve shows a double peak at ∼50 and ∼100 days after the first detection, and it declines and flattens afterward, reaching preoutburst levels after 2000 days of monitoring. The MIR light curve rises almost simultaneously with the optical, but unlike the UV/optical which is approaching the preoutburst levels in the last epochs of our observations, the MIR emission is still rising at the time of writing. The optical spectra show broad Balmer features and the strongest broad Fe II emission ever detected in a nuclear transient. This broad Fe II emission was not present in the archival preoutburst spectrum and almost completely disappeared +1868 days after the outburst. We found that the majority of the flux of the broad Balmer and Fe II lines is produced by photoionization. We detect only weak X-ray emission in the 0.5−8 keV band at the location of PS16dtm, at +848, +1130, and +1429 days past the outburst. This means that the X-ray emission continues to be lower by at least an order of magnitude, compared to archival, preoutburst measurements. Conclusions. We confirm that the observed properties of PS16dtm are difficult to reconcile with normal AGN variability. The TDE scenario continues to be a plausible explanation for the observed properties, even though PS16dtm shows differences compared to TDE in quiescent galaxies. We suggest that this event is part of a growing sample of TDEs that show broad Balmer line profiles and Fe II complexes. We argue that the extreme variability seen in the AGN host due to PS16dtm may have easily been misclassified as a CLAGN, especially if the rising part of the light curve had been missed. This implies that some changing look episodes in AGN may be triggered by TDEs. Imaging and spectroscopic data of AGN with good sampling are needed to enable testing of possible physical mechanisms behind the extreme variability in AGN.
Ključne besede: nuclear transients, supermassive black holes, tidal disruption events, active galactic nuclei
Objavljeno v RUNG: 24.01.2023; Ogledov: 1224; Prenosov: 18
Celotno besedilo (2,75 MB)
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