Optical and near-infrared photometry, optical spectroscopy, and soft X-ray and UV monitoring of the changing-look active galactic nucleus NGC 2617 show that it continues to have the appearance of a ...type-1 Seyfert galaxy. An optical light curve for 2010–2016 indicates that the change of type probably occurred between 2010 October and 2012 February and was not related to the brightening in 2013. In 2016, NGC 2617 brightened again to a level of activity close to that in 2013 April. We find variations in all passbands and in both the intensities and profiles of the broad Balmer lines. A new displaced emission peak has appeared in Hβ. X-ray variations are well correlated with UV–optical variability and possibly lead by ∼2–3 d. The K band lags the J band by about 21.5 ± 2.5 d and lags the combined B + J filters by ∼25 d. J lags B by about 3 d. This could be because J-band variability arises from the outer part of the accretion disc, while K-band variability comes from thermal re-emission by dust. We propose that spectral-type changes are a result of increasing central luminosity causing sublimation of the innermost dust in the hollow bi-conical outflow. We briefly discuss various other possible reasons that might explain the dramatic changes in NGC 2617.
Observations of the dwarf nova SS Cyg were made in 2019–2021 at different brightness levels (
) both at the brightness decay stage after the outburst maximum, and in the quite state between ...outbursts. Data were obtained in
(
observations, 3 sets) and
V
(
points, 22 sets) bands. The value of the system’s orbital period obtained in 2019–2021 (
) used in this study is 0.4% less than the value obtained more than a quarter of a century ago (1983–1996). The time resolution between two successive measurements is 6–14 s depending on the equipment used. An extensive database of new observational data allowed us to perform a quantitative analysis of observations. Analysis of the data after taking into account orbital variability and other trends associated with changes in the system’s emission flux during the night showed the presence of cyclic fluctuations in brightness, usually 4–10 events per orbital cycle – flickering. For most series of observations, the Lafleur-Kinman method determined such a value of the oscillation period at which convolution of observations with it showed a single wave. The obtained values of the characteristic flickering times and their amplitudes show their dependence on the average brightness level of the system. With increasing luminosity of the system, both of these quantities decreased linearly. From the size ratios of the binary system components of SS Cyg, it was shown that the source of flickering is located in the interaction region of the gas flow with the near-disk halo: only this region in the SS Cyg system with parameters (
,
,
), defined by the authors earlier, can be eclipsed at large radii disk, and is clearly visible in all other orbital phases of the system.
ABSTRACT
We present the results of photometric and spectroscopic monitoring campaigns of the changing look AGN NGC 2617 carried out from 2016 until 2022 and covering the wavelength range from the ...X-ray to the near-IR. The facilities included the telescopes of the SAI MSU, MASTER Global Robotic Net, the 2.3-m WIRO telescope, Swift, and others. We found significant variability at all wavelengths and, specifically, in the intensities and profiles of the broad Balmer lines. We measured time delays of ∼6 d (∼8 d) in the responses of the Hβ (Hα) line to continuum variations. We found the X-ray variations to correlate well with the UV and optical (with a small time delay of a few days for longer wavelengths). The K-band lagged the B band by 14 ± 4 d during the last three seasons, which is significantly shorter than the delays reported previously by the 2016 and 2017–2019 campaigns. Near-IR variability arises from two different emission regions: the outer part of the accretion disc and a more distant dust component. The HK-band variability is governed primarily by dust. The Balmer decrement of the broad-line components is inversely correlated with the UV flux. The change of the object’s type from Sy1 to Sy1.8 was recorded over a period of ∼8 yr. We interpret these changes as a combination of two factors: changes in the accretion rate and dust recovery along the line of sight.
ABSTRACT
We present optical photometric observations of SN 2018aoq from 2 to 100 d after explosion, and 7 spectra at epochs from 11 to 71 d. The light curves and spectra are typical for SNe II-P. As ...previously reported, SN 2018aoq appears to be of intermediate brightness between subluminous and normal SNe II-P. SN 2018aoq was discovered in Seyfert galaxy NGC 4151, for which the distance is uncertain. We utilized the expanding photosphere method using three sets of filter combinations and velocities derived from the absorption minima of Fe ii lines and obtained a distance of 20.0 ± 1.6 Mpc. The standard candle method applied to SN 2018aoq yields a distance of 16.6 ± 1.1 Mpc. Both values are consistent with the distance measurements for NGC 4151 based on geometric method.
We present a variability study of the lowest-luminosity Seyfert 1 nucleus of the galaxy NGC 4395 based on photometric monitoring campaigns in 2017 and 2018. Using 22 ground-based and space ...telescopes, we monitored NGC 4395 with a ∼5-minute cadence during a period of 10 days and obtained light curves in the ultraviolet (UV), V, J, H, and K/Ks bands, as well as narrowband H . The rms variability is ∼0.13 mag in the Swift UVM2 and V filter light curves, decreasing down to ∼0.01 mag in the K filter. After correcting for the continuum contribution to the H narrow band, we measured the time lag of the H emission line with respect to the V-band continuum as - minutes in 2017 and - minutes in 2018, depending on assumptions about the continuum variability amplitude in the H narrow band. We obtained no reliable measurements for the continuum-to-continuum lag between UV and V bands and among near-IR bands, owing to the large flux uncertainty of UV observations and the limited time baseline. We determined the active galactic nucleus (AGN) monochromatic luminosity at 5100 , , after subtracting the contribution of the nuclear star cluster. While the optical luminosity of NGC 4395 is two orders of magnitude lower than that of other reverberation-mapped AGNs, NGC 4395 follows the size-luminosity relation, albeit with an offset of 0.48 dex (≥2.5 ) from the previous best-fit relation of Bentz et al.
We present the results of multicolor (
UBV JHKLM
) photometry (2009–2017) and low-resolution spectroscopy (2016–2017) of the semi-regular variable V1427 Aql = HD 179821, a yellow supergiant with ...gas-dust envelope. The star displays low-amplitude (Δ
V
<0
.
m
2) semi-periodic brightness variations superimposed on a long-term trend. The light curve shape and timescale change from cycle to cycle. There are temperature variations characteristic for pulsations; brightness oscillations with no significant change of color are also observed. The
UBV
data for the 2009–2011 interval are well reproduced by a superposition of two periodic components with
P
= 170
d
and 141
d
(or
P
= 217
d
—the one year alias of
P
= 141
d
). The variation became less regular after 2011, the timescale increased and exceeded 250
d
. Unusual photometric behavior was seen in 2015 when the star brightness increased by 0
.
m
25 in the
V
filter in 130 days and reached the maximum value ever observed in the course of our monitoring since 1990. In 2009–2016 the annual average brightness monotonically increased in
V
,
J
,
K
, whereas it decreased in
U
and
B
. The annual average
U
−
B
,
B
−
V
, and
J
−
K
colors grew, the star was getting redder. The cooling and expanding of the star photosphere along with the increasing of luminosity may explain the long-term trend in brightness and colors. Based on our photometric data we suppose that the photosphere temperature decreased by ~400 K in the 2008–2016 interval, the radius increased by ~24%, and the luminosity grew by ~19%. We review the change of annual average photometric data for almost 30 years of observations. Low-resolution spectra in the λ4000−9000 Å wavelength range obtained in 2016–2017 indicate significant changes in the spectrum of V1427 Aql as compared with the 1994–2008 interval, i.e., the Ba II and near-infraredCa II triplet absorptions have gotten stronger while the OI λ7771-4 triplet blend has weakened that points out the decrease of temperature in the region where the absorptions are formed. The evolutionary stage of the star is discussed. We also compare V1427 Aql with post-AGB stars and yellow hypergiants.
The results of a long-term photometric observations of the cataclysmic variable EX Dra acquired between 2014 and 2016 at the Crimean Station of the Sternberg Astronomical Institute (24 nights, more ...than 10 500 measurements) are presented. The observations were performed using CCD photometers mounted on 50-cm and 60-cm telescopes in the visible and red, during both quiescence and the active state. For completeness, photometric observations obtained at the Ondrejov Observatory in 2010 in the
V
and
R
Johnson filters are also used in the analysis. The new observations of EX Dra are used to derive the orbital period of the system, which agrees well with earlier determinations. A combined model that takes into account the radiation fluxes from the gaseous stream and a hot spot on the lateral surface of the accretion disk is used to determine the parameters of the system components (white dwarf, red dwarf, accretion disk and hot spot, and gaseous stream). Variations of the parameters when the system changes from one activity state to the other are considered. Six light curves displaying unsatisfactory agreement between the observed and theoretical light curves can be successfully fitted using a version of the combined model that includes hot spots on the secondary’s surface. This model is able to qualitatively reproduce a secondary minima in the light curves that exhibits shifts of this minimum from phase 0.5. The parameters of dark spots on the red-dwarf surface were determined. The data obtained indicate that the outbursts in the EX Dra system are related to instability of the matter outflow from the secondary.
Photometric observations of the variable star ASASSN-13cx acquired in the course of a program of studies of cataclysmic variables and their parameters recently carried out at the Sternberg ...Astronomical Institute (SAI) are presented. The star was observed with the 50-cm and 60-cm telescopes of the SAI Crimean Astronomical Station and a CCD photometer (∼1800 images in the
V
and
Rc
filters) during the variable’s outburst of August–September 2014 and in a period of quiescence in October–November 2016. The ASASSN-13cx system is confirmed to be a SU UMa variable. Parameters of the system are derived from eight light curves using a “composite” model that takes into account the presence of a hot spot on the lateral surface of the geometrically thick disk and of a region of enhanced energy release near the disk edge, at the base of the gas flow (the so-called “hot line”). Parameters of the system for three light curves during the outburst were obtained in the framework of a “spiral” model that additionally takes into account the presence of geometric perturbations on the accretion-disk surface. The parameters of ASASSN-13cx determined using these models provide good accuracy in reproducing the system’s light curves in both states. The basic parameters of the system have been determined for the first time: the component mass ratio
q
=
M
1
/
M
2
= 7.0 ± 0.2, the orbital inclination
i
= 79.9°−80.1°, the distance between the components’ centers of mass
a
0
= 0.821(1)
R
ʘ
, and the sizes and temperatures of the stars:
R
1
= 0.0124(5)
a
0
= 0.0102(4)
R
ʘ
,
T
1
= 12 500 ± 280 K, 〈
R
2
〉 = 0.236(4)
a
0
= 0.194(3)
R
ʘ
,
T
2
= 2550 ± 400 K, corresponding to M4–9V for the spectral type of the secondary. Parameters of the accretion disk have been derived for both activity states. The mass of matter in the accretion disk increased by almost a factor of two during ∼400 orbital periods in quiescence.
Super Li-rich stars form a very small and enigmatic group whose existence cannot be explained in terms of the standard stellar evolution theory. The goal of our study is to check the reality of this ...group of cool giants based on an independent technique. We have carried out such a check using the K giant HD 77361 (HR 3597), which has previously been assigned to this rare type, as an example. We have redetermined the effective temperature
T
eff
and surface gravity log
g
for this star. We have applied two different methods, photometric and spectroscopic, to estimate
T
eff
(the accuracy of the Li-abundance determination depends significantly on this parameter). The value of log
g
has been found from the highly accurate parallax of this nearby star. To apply the photometric method of determining
T
eff
, we have performed
U
BV
observations of the star, which yielded
V
= 6.18 ± 0.03,
B
−
V
= 1.13 ± 0.01, and
U
−
B
= 1.18±0.05. The following parameters of the star have been found: effective temperature
T
eff
= 4370± 100 K, surface gravity log
g
= 2.30 ± 0.10, iron abundance log
ϵ
(Fe) = 7.49 ± 0.14, microturbulence
V
t
= 1.1 ± 0.2 km s
−1
, rotational velocity
V
sin
i
= 4.5 km s
−1
, and mass
M
= 1.3 ± 0.2
M
⊙
The lithium abundance has been determined from a non-LTE analysis of three Li I lines: the resonance line at 6707.8 Å and the subordinate lines at 6103.6 and 8126.4 Å (the latter in a blend with a CN molecular line). We have found a high lithium abundance, log
ϵ
(Li) = 3.75 ± 0.11, which exceeds considerably the initial abundance log
ϵ
(Li) = 3.2 ± 0.1 for young stars in the solar neighborhood. Thus, we have confirmed that the K giant HD 77361 actually belongs to the type of super Li-rich stars. It is noted that a high lithium abundance in such cool giants is inconsistent with predictions of the standard stellar evolution theory and may suggest a recent synthesis of lithium in these stars.
The results of photometric observations of the dwarf nova GY Cnc in the
Rc
filter acquired in 2013–2015 (~3900 orbital cycles, 19 nights in total) are presented, including observations during its ...outburst in April 2014. The binary’s orbital elements have been refined. The orbital period has changed only insignificantly during the ~30 000
P
orb
since the earlier observations; no systematic O–C variations were detected, only fluctuations within 0.004
d
on time scales of 1500–2000
P
orb
. A “combined” model is used to solve for the parameters of GY Cnc during two states of the system. The flux from the white dwarf is negligible due to the star’s small size. The temperature of the donor star,
T
2
~ 3667 K (Sp M0.2 V), varies between 3440 and 3900 K (Sp K8.8–M1.7 V). The semi-major axis of the disk is
a
~ 0.22a
0
, on average. In quiescence,
a
varies within ~40%. The disk has a considerable eccentricity (
e
~ 0.2−0.3) for
a
< 0.2a
0
. The disk shape becomes more circular (
e
< 0.1) with increasing
a
. The outburst of GY Cnc was associated with increased luminosity of the disk due to the parameter α
g
(related to the viscosity of the disk material) decreasing to 0.1–0.2 and the temperature in the inner parts of the disk increasing twofold, to
T
in
~ 95 000 K. These changes were apparently due to the infall of matter onto the surface of the white dwarf as the outburst developed. All parameters of the accretion disk in quiescence display considerable variations about their mean values.
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