Ultraluminous X-Ray Sources Fabrika, S. N.; Atapin, K. E.; Vinokurov, A. S. ...
Astrophysical bulletin,
2021/1, Letnik:
76, Številka:
1
Journal Article
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Ultraluminous X-ray sources (ULXs) were identified as a separate class of objects in 2000 based on data from the Chandra X-Ray Observatory. These are unique objects: their X-ray luminosities exceed ...the Eddington limit for a typical stellar-mass black hole. For a long time, the nature of ULXs remained unclear. However, the gradual accumulation of data, new results of X-ray and optical spectroscopy, and the study of the structure and energy of nebulae surrounding ULXs led to the understanding that most of the ultraluminous X-ray sources must be supercritical accretion disks like SS 433. The discovery of neutron stars in a number of objects only increased the confidence of the scientific community in the conclusions obtained, since the presence of neutron stars in such systems clearly indicates a supercritical accretion regime. In this review, we systematize the main facts about the observational manifestations of ULXs and SS 433 in the X-ray and optical ranges and discuss their explanation from the point of view of the supercritical accretion theory.
A search for a correlation between the luminosities of the brightest stars and luminosities of their host galaxies was carried out on archival Hubble Space Telescope (HST)
or
(
V
) and
(
I
) images ...of 150 nearby galaxies. The sample contains only galaxies with ongoing star formation and with known distances we derived with the TRGB-method. We correlated the average absolute luminosities of the three brightest blue and the three brightest red stars with the luminosity of a host. We find a linear relation for both the blue and the red stars in irregular and low-mass spiral galaxies. Their scatters are sufficiently small (
) to make these relations useful for distance determination for low-mass galaxies. We found that all 31 dwarf galaxies
in our sample lack bright massive stars
, probably due to the physical conditions that prevent their birth. For galaxies with higher an average luminosity in the range
, there is an asymmetry in the distribution of the number of galaxies relative to the linear dependence, indicating an increase in the fraction of galaxies with bright stars.
We present the results of studying the spectral and photometric variability of the luminous blue variable star V532 in M33. The photometric variations are traced from 1960 to 2010, spectral ...variations—from 1992 to 2009. The star has revealed an absolute maximum of visual brightness (1992–1994, high/cold state) and an absolute minimum (2007–2008, low/hot state) with a brightness difference of ΔB ≈ 2.3
m
. The temperature estimates in the absolute maximum and absolute minimum were found to be
T
∼ 22000 K and
T
∼ 42000 K, respectively. The variability of the spectrum of V532 is fully consistent with the temperature variations in its photosphere, while both permitted and forbidden lines are formed in an extended stellar atmosphere. Broad components of the brightest lines were found, the broadening of these components is due to electron scattering in the wind parts closest to the photosphere. We measured the wind velocity as a difference between the emission and absorption peaks in the PCyg type profiles. The wind velocity clearly depends on the size of the stellar photosphere or on the visual brightness, when brightness declines, the wind velocity increases. In the absolute minimum a kinematic profile of the V532 atmosphere was detected. The wind velocity increases and its temperature declines with distance from the star. In the low/hot state, the spectral type of the star corresponds to WN8.5h, in the high/cold state—to WN11. We studied the evolution of V532 along with the evolution of AGCar and the massive WR binary HD5980 in SMC. During their visual minima, all the three stars perfectly fit with the WNL star sequence by Crowther and Smith (1997). However, when visual brightness increases, all the three stars form a separate sequence. It is possible that this reflects a new property of LBV stars, namely, in the high/cold states they do not pertain to the
bona fide
WNL stars.
An ongoing SAO RAS program for the 6-m telescope to search for and study luminous blue variable (LBV) candidates in the M31 and M33 galaxies was started in 1997. This paper summarizes the results of ...our observations over the last 20 years. Over this time, we have a general lists of candidates and obtained photometry and spectroscopy for approximately one hundred stars from the list. A detailed study and classification of LBV candidates were carried out.We alsomonitored spectrally and photometrically the confirmed LBV stars in these galaxies. Based on these results, the spectral energy distributions (SEDs) of LBVs and LBV candidates were computed, and the interstellar extinction values, temperature, and luminosity were derived. We propose a new method of confirming the nature of LBV stars. It is based on a special property exhibited only by LBV stars—the constancy of the bolometric luminosity, with except of a rare phase of giant eruption, registered for two historical LBVs in our galaxy and about few dozen extragalactic LBVs. Using this method, we determine the fundamental parameters of these stars: temperature, radius, luminosity, and interstellar extinction. We discuss the nature of several individual LBV stars discovered by us in M33 and the classification of the LBV candidates in M31. Thus, for 20 years of observations we have found four LBV stars in M33; in the M31 galaxy with different methods, we have confirmed seven objects as a LBV class.
ABSTRACT 549 blue stars with Ho; excess in the galaxy M 33 selected as candidates into unique objects of SS 433 and LBV (luminous blue variables) types are studied. These objects were divided by ...morphological types on Hog images in the following way: 81 stars, 154 diffuse nebulae, 180 bubble-type nebulae and 117 common intermediate objects. The diffuse objects are Hll regions with an exciting star. The bubbles are probably the envelopes around WR stars or the remnants of supernovae. Among the stars a group of 20 brightest ones is selected which by their average properties agree satisfactory with the parameters of blue super-supergiants or the objects of LBV type. The interstellar absorption determined by bright stars is equal to AV = 0.93 ± 0.05.
We present the results of our optical identifications of four hard X-ray sources from the Swift all-sky survey. We obtained optical spectra for each of the program objects with the 6-m BTA telescope ...(Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz), which allowed their nature to be established. Two sources (SWIFT J2237.2+6324 and SWIFT J2341.0+7645) are shown to belong to the class of cataclysmic variables (suspected polars or intermediate polars). The measured emission line width turns out to be fairly large (
FWHM
∼ 15–25 Å), suggesting the presence of extended, rapidly rotating (
v
≃ 400–600 km s
−1
) accretion disks in the systems. Apart from line broadening, we have detected a change in the positions of the line centroids for SWIFT J2341.0+7645, which is most likely attributable to the orbital motion of the white dwarf in the binary system. The other two program objects (SWIFT J0003.3+2737 and SWIFT J0113.8+2515) are extragalactic in origin: the first is a Seyfert 2 galaxy and the second is a blazar at redshift
z
= 1.594. Apart from the optical spectra, we provide the X-ray spectra for all sources in the 0.6–10 keV energy band obtained from XRT/Swift data.
A total of 185 luminous blue variable star (LBV) candidates with V<18
·
m
5 are selected based on the results of aperture photometry. The primary selection criterion is that the prospective candidate ...should be a blue star with H
α
emission. In order not to overlook appreciably reddened LBV candidates, we compose an additional list of 25 red (0
·
m
35<
B
−
V
< 1
·
m
2,
V
< 18
·
m
5) emission star candidates. A comparison with the list of known variables in the M33 galaxy showed 29% of our selected candidates to be photometrically variable. We also find our list to agree well with the lists of emission-line objects obtained in earlier papers using different methods.
We present two new luminous blue variable (LBV) candidate stars discovered in the M33 galaxy. We identified these stars as massive star candidates at the final stages of evolution, presumably with a ...notable interstellar extinction. The candidates were selected from the Massey et al. catalog based on the following criteria: emission in
H
α
, V<18
./m
5 and 0
.
m
35 < (
B - V
) < 1
.
m
2. The spectra of both stars reveal a broad and strong
H
α
emission with extended wings (770 and 1000 kms
−1
). Based on the spectra we estimated the main parameters of the stars. Object N45901 has a bolometric luminosity log(L/L
⊙
) = 6.0–6.2 with the value of interstellar extinction A
V
= 2.3 ± 0.1. The temperature of the star’s photosphere is estimated as T⋆ ∼ 13000–15000 K, its probable mass on the Zero Age Main Sequence is M∼ 60–80 M
⊙
. The infrared excess in N 45901 corresponds to the emission of warm dust with the temperature Twarm ∼ 1000 K, and amounts to 0.1%of the bolometric luminosity. A comparison of stellar magnitude estimates from different catalogs points to the probable variability of the object N45901. Bolometric luminosity of the second object, N125093, is log(L/L
⊙
) = 6.3 − 6.6, the value of interstellar extinction is A
V
= 2.75 ± 0.15. We estimate its photosphere’s temperature as T⋆∼ 13000–16000K, the initial mass as M ∼ 90–120 M
⊙
. The infrared excess in N125093 amounts to 5–6% of the bolometric luminosity. Its spectral energy distribution reveals two thermal components with the temperatures T
warm
∼ 1000K and T
cold
∼ 480 K. The Ca II
λλ
7291, 7323 lines, observed in LBV-like stars Var A and N93351 in M33 are also present in the spectrum of N 125093. These lines indicate relatively recent gas eruptions and dust activity linked with them. High bolometric luminosity of these stars and broad
H
α
emissions allow classifying the studied objects as LBV candidates.
We present the results of our stellar photometry and spectroscopy for the new Local Group galaxy VV124 (UGC4879) obtainedwith the 6-m BTAtelescope. The presence of a fewbright supergiants in the ...galaxy indicates that the current star formation process is weak. The apparent distribution of stars with different ages in VV 124 does not differ from the analogous distributions of stars in irregular galaxies, but the ratio of the numbers of young and old stars indicates that VV 124 belongs to the rare Irr/Sph type of galaxies. The old stars (red giants) form the most extended structure, a thick disk with an exponential decrease in the star number density to the edge. Definitely, the young population unresolvable in images makes a great contribution to the background emission from the central galactic regions. The presence of young stars is also confirmed by the OIII emission line visible in the spectra that belongs to extensive diffuse galactic regions. The mean radial velocity of several components (two bright supergiants, the unresolvable stellar population, and the diffuse gas) is
υ
h
= −70 ± 15 km s
−1
and the velocity with which VV 124 falls into the Local Group is
υ
LG
= −12 ± 15 km s
−1
. We confirm the distance to the galaxy (
D
= 1.1 ± 0.1 Mpc) and the metallicity of red giants (Fe/H = −1.37) found by Kopylov et al. (2008). VV 124 is located on the periphery of the Local Group approximately at the same distance from M31 and our Galaxy and is isolated from other galaxies. The galaxy LeoA nearest to it is 0.5 Mpc away.
Photometric and spectroscopic observations of the nearby type-IIP supernova 2004dj are presented. The Ni mass in the envelope of SN 2004dj was estimated from the light curve to be -0.02M {circled ...dot}. This estimate is confirmed by modeling the Ha luminosity. The Ha emission line exhibits a strong asymmetry characterized by the presence of a blue component in the line with a shift of -1600 km s at the early nebular phase. A similar asymmetry was found in the Hb, O I, and Ca II lines. The line asymmetry is interpreted as being the result of asymmetric Ni ejecta. The Ha profile and its evolution are reproduced in the model of an asymmetric bipolar Ni structure for a spherical hydrogen distribution. The mass of the front Ni jet is comparable to that of the central component and twice that of the rear Ni jet. We point out that the asymmetric bipolar structure of Ni ejecta is also present in SN 1999em, a normal type-IIP supernova.
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