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Register a new userOrbital parameters of the high-mass X-ray binary 4U 2206+54
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We present new radial velocities of the high-mass X-ray binary star 4U 2206+54 based on optical spectra obtained with the Coude spectrograph at the 2m RCC telescope at the Rozhen National Astronomical Observatory, Bulgaria in the period November 2011 -- July 2013. The radial velocity curve of the HeI $lambda$6678 AA line is modeled with an orbital period P$_{orb}$ = 9.568~d and an eccentricity of $e$ = 0.3. These new measurements of the radial velocity resolve the disagreements of the orbital period discussions.
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The source 4U 2206+54 is one of the most enigmatic high-mass X-ray binaries. In spite of intensive searches, X-ray pulsations have not been detected in the time range 0.001-1000 s. A cyclotron line at ~30 keV has been suggested by various authors but never detected with significance. The stellar wind of the optical companion is abnormally slow. The orbital period, initially reported to be 9.6 days, disappeared and a new periodicity of 19.25 days emerged. Our new long and uninterrupted RXTE observations allow us to search for long (~1 hr) pulsations for the first time. We have discovered 5560-s pulsations in the light curve of 4U 2206+54. Initially detected in RXTE data, these pulsations are also present in INTEGRAL and EXOSAT observations. The average X-ray luminosity in the energy range 2-10 keV is 1.5 x 10^{35} erg s^{-1} with a ratio Fmax/Fmin ~ 5. This ratio implies an eccentricity of ~0.4, somewhat higher than previously suggested. The source also shows a soft excess at low energies. If the soft excess is modelled with a blackbody component, then the size and temperature of the emitting region agrees with its interpretation in terms of a hot spot on the neutron star surface. The source displays variability on time scales of days, presumably due to changes in the mass accretion rate as the neutron star moves around the optical companion in a moderately eccentric orbit.
The X-ray binary 4U 1954+31 has been classified as a Low Mass X-ray Binary (LMXB) containing a M giant and a neutron star (NS). It has also been included in the rare class of X-ray symbiotic binaries (SyXB). The Gaia parallax, infrared colors, spectral type, abundances, and orbital properties of the M star demonstrate that the cool star in this system is not a low mass giant but a high mass M supergiant. Thus, 4U 1954+31 is a High Mass X-ray Binary (HMXB) containing a late-type supergiant. It is the only known binary system of this type. The mass of the M I is 9$^{+6}_{-2}$ M$_odot$ giving an age of this system in the range 12 - 50 Myr with the NS no more than 43 Myr old. The spin period of the NS is one of the longest known, 5 hours. The existence of M I plus NS binary systems is in accord with stellar evolution theory, with this system a more evolved member of the HMXB population.
The X-ray binary system 4U 2206+54 hides many mysteries. Among them, the surprising behavior of both of its components: the O9.5 dwarf star BD+53$^circ$2790 and a slowly rotating neutron star. BD+53$^circ$2790 misled the astronomers showing itself very likely as a Be star. However, a deeper spectral analysis and more intense monitoring, revealed that the real picture was a bit more complicated: a) Although it shows evidence of a circumstellar envelope, its observable properties differ from those typical envelopes in Be stars. b) Comparison with spectral standards and models indicates a possible over-abundance in He. This would open the possibility to link the behavior of BD+53$^circ$2790 to the He-rich class of stars. c) UV spectra shows an abnormally slow and dense wind for an O9.5V star. d) Spectral classification in the IR wavelength region suggest a more likely supergiant nature of the source, in contradiction with the optical classification. e) The presence of an intense magnetic field is under investigation. BD+53$^circ$2790 stands as a perfect laboratory for testing stellar structure, as well as wind and evolutionary theories. The observable properties of this source in a wide range of spectral bands are discussed, and some interpretations outlined.
We modelled optical light curves of Sco~X-1 obtained by the Kepler space telescope during K2 mission. Modelling was performed for the case of the strong heating of the optical star and accretion disc by X-rays. In the considered model the optical star fully filled its Roche lobe. We investigated the inverse problem in wide ranges of values of model parameters and estimated following parameters of Sco X-1: the mass ratio of components $q=M_x/M_v=3.6$ ($3.5-3.8$), where $M_x$ and $M_v$ were masses of the neutron and optical stars correspondingly, the orbital inclination was $i=30^{circ}$ ($25^{circ}-34^{circ}$). In the brackets uncertainties of parameters $q$ and $i$ were shown, they originated due to uncertainties of characteristics of the physical model of Sco X-1. The temperature of non-heated optical star was $T_2 = 2500-3050$ K, its radius was $R_2=1.25R_{odot}=8.7times 10^{10}$ cm, and its bolometric luminosity was $L_{bol}=(2.1-4.6)times 10^{32}$ erg s$^{-1}$. The mass of the star was $M_vsimeq 0.4M_{odot}$. The contribution of the X-ray heated accretion disc dominated in the total optical emission of Sco~X-1. The transition between low and high states occurred due to the increase of X-ray luminosity by a factor $2-3$.
There are very few confirmed black holes with a mass that could be $sim! 4, M_odot$ and no neutron stars with masses greater than $sim! 2, M_odot$, creating a gap in the observed distribution of compact star masses. Some black holes with masses between 2 and $4, M_odot$ might be hiding among other X-ray sources, whose masses are difficult to measure. We present new high-speed optical photometry of the low-mass X-ray binary V1408 Aql (= 4U 1957+115), which is a persistent X-ray source thought to contain a black hole. The optical light curve of V1408~Aql shows a nearly sinusoidal modulation at the orbital period of the system superimposed on large night-to-night variations in mean intensity. We combined the new photometry with previously-published photometry to derive a more precise orbital period, $P = 0.388893(3)$ d, and to better define the orbital light curve and night-to-night variations. The orbital light curve agrees well with a model in which the modulation is caused entirely by the changing aspect of the heated face of the secondary star. The lack of eclipses rules out orbital inclinations greater than $65^{circ}$. Our best models for the orbital light curve favor inclinations near $13^{circ}$ and black hole masses near $3, M_odot$ with a 90% upper bound of $6.2, M_odot$, and a lower bound of $2.0, M_odot$ imposed solely by the maximum mass of neutron stars. We favor a black hole primary over a neutron star primary based on evidence from the X-ray spectra, the high spin of the compact object, and the fact that a type I X-ray burst has not been observed for this system. Although uncertainties in the data and the models allow higher masses, possibly much higher masses, the compact star in V1408~Aql is a viable candidate for a black hole lying in the mass gap.
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