We studied both components of a slightly overlooked visual binary HR 1847 spectroscopically to determine its basic physical and orbital parameters. Basic stellar parameters were determined by ...comparing synthetic spectra to the observed echelle spectra, which cover both the optical and near-IR regions. New observations of this system used the Ondřejov and Rozhen 2-m telescopes and their coudé spectrographs. Radial velocities from individual spectra were measured and then analysed with the code FOTEL to determine orbital parameters. The spectroscopic orbit of HR 1847A is presented for the first time. It is a single-lined spectroscopic binary with a B-type primary, a period of 719.79 days, and a highly eccentric orbit with e=0.7. We confirmed that HR 1847B is a Be star. Its Hα emission significantly decreased from 2003 to 2008. Both components have a spectral type B7-8 and luminosity class IV-V.
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Context.
During its all-sky survey, the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) observatory has uncovered a growing number of ...X-ray transients associated with the nuclei of quiescent galaxies. Benefitting from its large field of view and excellent sensitivity, the eROSITA window into time-domain X-ray astrophysics yields a valuable sample of X-ray selected nuclear transients. Multi-wavelength follow-up enables us to gain new insights into understanding the nature and emission mechanism of these phenomena.
Aims.
We present the results of a detailed multi-wavelength analysis of an exceptional repeating X-ray nuclear transient, eRASSt J045650.3−203750 (hereafter J0456−20), uncovered by SRG/eROSITA in a quiescent galaxy at a redshift of
z
∼ 0.077. We aim to understand the radiation mechanism at different luminosity states of J0456−20, and provide further evidence that similar accretion processes are at work for black hole accretion systems at different black hole mass scales.
Methods.
We describe our temporal analysis, which addressed both the long- and short-term variability of J0456−20. A detailed X-ray spectral analysis was performed to investigate the X-ray emission mechanism.
Results.
Our main findings are that (1) J0456−20 cycles through four distinctive phases defined based on its X-ray variability: an X-ray rising phase leading to an X-ray plateau phase that lasts for abouttwo months. This is terminated by a rapid X-ray flux drop phase during which the X-ray flux can drop drastically by more than a factor of 100 within one week, followed by an X-ray faint state for about two months before the X-ray rising phase starts again. (2) The X-ray spectra are generally soft in the rising phase, with a photon index ≳3.0, and they become harder as the X-ray flux increases. There is evidence of a multi-colour disk with a temperature of
T
in
∼ 70 eV in the inner region at the beginning of the X-ray rising phase. The high-quality
XMM-Newton
data suggest that a warm and hot corona might cause the X-ray emission through inverse Comptonisation of soft disk seed photons during the plateau phase and at the bright end of the rising phase. (3) J0456−20 shows only moderate UV variability and no significant optical variability above the host galaxy level. Optical spectra taken at different X-ray phases are constant in time and consistent with a typical quiescent galaxy with no indication of emission lines. (4) Radio emission is (as yet) only detected in the X-ray plateau phase and rapidly declines on a timescale of two weeks.
Conclusions.
J0456−20 is likely a repeating nuclear transient with a tentative recurrence time of ∼223 days. It is a new member of this rare class. We discuss several possibilities to explain the observational properties of J0456−20. We currently favour a repeating partial tidal disruption event as the most likely scenario. The long-term X-ray evolution is explained as a transition between a thermal disk-dominated soft state and a steep power-law state. This implies that the corona can be formed within a few months and is destroyed within a few weeks.
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We report the spectroscopic identification of close to 1000 hydrogen-rich (DA) white dwarfs discovered in the course of the Anglo-Australian 2dF QSO Redshift Survey (2QZ). We measured the atmospheric ...parameters of a subsample of 201 stars based on good-quality Balmer line spectra and we determine the 2QZ population characteristics. Over ten per cent of the sample are potential ZZ Ceti variables. We estimate the DA white dwarf birthrate (bDA= 0.5–1.0 × 10−12 yr−1 pc−3) and the scaleheight (h= 220–300 pc) of the population in the thin disc of the Galaxy by combining information acquired in the Palomar–Green, AAT–UVX and new 2QZ surveys. White dwarfs appear related to G and late-F progenitors. A catalogue of 201 DA white dwarfs is presented.
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The New Luyten Two-Tenths catalog contains a large number of high proper motion white dwarf candidates that remain to be spectroscopically confirmed. We present new spectroscopic observations, as ...well as SDSS archival spectra of 49 white dwarf candidates selected from the revised NLTT catalog of Salim & Gould. Of these, 34 are cool DA white dwarfs with temperatures ranging from approximately 5000 up to 11,690 K, and 11 are DC white dwarfs with temperatures ranging from 4300 K (NLTT 18555) up to 11,000 K. Three of the DA white dwarfs also display abundances of heavy elements (NLTT 3915, NLTT 44986, and NLTT 43806), and one is a cool magnetic white dwarf (NLTT 44447) with an estimated magnetic field strength of 1.3 MG. We also present a new cool DQ white dwarf (NLTT 31347) with an estimated temperature of 6250 K. We supplement our sample with SDSS ugriz photometry for a fraction of the newly identified white dwarfs. A kinematical study of this sample of white dwarfs, characterized by proper motions ranging from 0".136 to 0".611 yr super(-1), suggest that they belong to the thin disk population.
Context.
Ultraprecise space photometry enables us to reveal light variability even in stars that were previously deemed constant. A large group of such stars show variations that may be rotationally ...modulated. This type of light variability is of special interest because it provides precise estimates of rotational rates.
Aims.
We aim to understand the origin of the light variability of K2 targets that show signatures of rotational modulation.
Methods.
We used phase-resolved medium-resolution X-shooter spectroscopy to understand the light variability of the stars KIC 250152017 and KIC 249660366, which are possibly rotationally modulated. We determined the atmospheric parameters at individual phases and tested the presence of the rotational modulation in the spectra.
Results.
KIC 250152017 is a HgMn star, whose light variability is caused by the inhomogeneous surface distribution of manganese and iron. It is only the second HgMn star whose light variability is well understood. KIC 249660366 is a He-weak, high-velocity horizontal branch star with overabundances of silicon and argon. The light variability of this star is likely caused by a reflection effect in this post-common envelope binary.
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Context. Ultraprecise space photometry enables us to reveal light variability even in stars that were previously deemed constant. A large group of such stars show variations that may be rotationally ...modulated. This type of light variability is of special interest because it provides precise estimates of rotational rates. Aims. We aim to understand the origin of the light variability of K2 targets that show signatures of rotational modulation. Methods. We used phase-resolved medium-resolution X-shooter spectroscopy to understand the light variability of the stars KIC 250152017 and KIC 249660366, which are possibly rotationally modulated. We determined the atmospheric parameters at individual phases and tested the presence of the rotational modulation in the spectra. Results. KIC 250152017 is a HgMn star, whose light variability is caused by the inhomogeneous surface distribution of manganese and iron. It is only the second HgMn star whose light variability is well understood. KIC 249660366 is a He-weak, high-velocity horizontal branch star with overabundances of silicon and argon. The light variability of this star is likely caused by a reflection effect in this post-common envelope binary.
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Context.
Very precise satellite photometry has revealed a large number of variable stars whose variability is caused either by surface spots or by binarity. Detailed studies of such variables provide ...insights into the physics of these objects.
Aims.
We study the nature of the periodic light variability of the white dwarf EPIC 206197016 that was observed by the K2 mission.
Methods.
We obtain phase-resolved medium-resolution spectroscopy of EPIC 206197016 using X-shooter spectrograph at VLT to understand the nature of the white dwarf variability. We use non-local thermodynamical equilibrium model atmospheres to determine stellar parameters at individual phases.
Results.
EPIC 206197016 is a hot DA white dwarf with
T
eff
= 78 kK. The analysis of the spectra reveals periodic radial velocity variations that can result from gravitational interaction with an invisible secondary whose mass corresponds to a red dwarf. The close proximity of the two stars where the semimajor axis is about 3
R
⊙
results in the irradiation of the companion with temperatures more than twice as high on the illuminated side compared to the nonilluminated hemisphere. This effect can explain the observed light variations. The spectra of the white dwarf show a particular feature of the Balmer lines called the Balmer line problem, where the observed cores of the lower Balmer lines are deeper than predicted. This can be attributed to either weak pollution of hydrogen in the white dwarf atmosphere by heavy elements or to the presence of a circumstellar cloud or disk.
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Aims.We verify the nature of emission-line stars in the field of the open cluster NGC 6910. Methods.Spectroscopy in the Hα region was obtained. Raw CCD frames of spectra of all stars fainter than V = ...9 mag observed by us are significantly affected by nebular emission originating in the surrounding $\ion{H}{ii}$ region IC 1318. After careful data reduction and subtraction of the nebular radiation we succeeded in obtaining reliable stellar spectra. Results.We confirm that the star NGC 6910 37 is a Be star, and we have corrected the classification of V1973 Cyg from an Ae star to a normal A type star. Since the diffuse interstellar bands do not appear in the spectrum of this star while being present in the other stars we observed, we confirm that V1973 Cyg is a foreground object with respect to IC 1318 and NGC 6910. We also find that the Hα line in HD 194279 has a P Cygni profile and the Hα line profile is variable in HD 229196.
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We present BV(RI)C and JHKs photometry and low- and high-resolution spectroscopy of the 11th mag G-type star TYC 2627-638-1. Our investigation reveals that the target is separated into two young, ...early-G-type main-sequence (or late pre-main-sequence) stars, which are most probably bound and form a wide binary system. A substellar body orbits the brighter component as implied by radial velocity variations. The brighter component possibly also has a faint, later type stellar companion. Both components of the wide binary have clear emission cores in the Ca ii H&K lines and filled-in Hα absorption, indicating that both stars are chromospherically active. Our photometric time series reveals clear but only a few hundredths of a magnitude amplitude rotational modulation, which is most likely due to cool starspots. Two distinct periods, near 3.5 and 3.7 days, are found in the brightness variations. Photometry obtained separately of the two components of the wide binary show that these periodicities belong to the brighter star. The fainter component shows a much slower light variation of about 0.3 mag. amplitude. In addition, long-term changes in the brightness of both stars are seen. The spectral energy distribution shows a strong near-infrared excess in the fainter component of the wide binary.
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