Long-term optical and X-ray variability of the Be/X-ray binary H 1145-619: Discovery of an ongoing retrograde density wave

Tzouvanou

Blinov

et al. 2023

A&A

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Context. GRO J2058+42 is a transient hard X-ray pulsar that occasionally goes into outburst. The optical counterpart is a poorly studied OB-type companion. Aims. We investigate the long-term optical variability of the Be/X-ray binary GRO J2058+42 and the possible connection with periods of enhanced X-ray activity. Methods. We performed an optical spectroscopic and photometric analysis on data collected during about 18 yr. We also present the first optical polarimetric observations of this source. Results. The long-term optical light curves in the BVRI bands and the evolution of the Hα equivalent width display a sinusoidal pattern with maxima and minima that repeat every ∼9.5 yr. The amplitude of this variability increases as the wavelength increases from 0.3 mag in the B band to 0.7 in the I band. The Hα equivalent width varied from about −0.3 to −15 Å. We found a significant decrease in the polarization degree during the low optical state. The intrinsic polarization degree changed by ∼1% from maximum to minimum. The optical maxima occur near periods of enhanced X-ray activity and are followed by a drop in the optical emission. Unlike many other Be/X-ray binaries, GRO J2058+42 does not display V/R variability. Conclusions. The long-term optical variability agrees with the standard model of a Be/X-ray binary, where the circumstellar disk of the Be star grows and dissipates on timescales of 9−10 yr. We find that the dissipation of the disk started after a major X-ray outburst. However, the stability of the Hα line shape as a double-peak profile and the lack of asymmetries suggest the absence of a warped disk and argue against the presence of a highly distorted disk during major X-ray outbursts.

Section: Hα Line Profile Variabilitymentioning

confidence: 89%

Long-term optical variability of the Be/X-ray binary GRO J2058+42

Tzouvanou

Blinov

et al. 2023

A&A

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“…Smooth and slow variations in the optical light curve of Be stars in BeXB of a few tenths of magnitude are common and attributed to the disk formation and dissipation (Reig & Fabregat 2015). The optical and infrared brightening of the Be star normally precedes the appearance of X-ray outbursts in BeXBs (Stevens et al 1997;Coe et al 2006;Reig et al 2007;Camero-Arranz et al 2012;Camero et al 2014;Caballero-García et al 2016;Alfonso-Garzón et al 2017). Although long-term photometric coverage with good cadence is scarce for BeXBs, the available data show that this long-term variability is gradual and slow and contrasts with the sudden increase and subsequent decrease that is observed during the X-ray outburst in Swift J0243.6+6124.…”

Section: Discussionmentioning

confidence: 99%

Optical counterpart to Swift J0243.6+6124

Fabregat

Alfonso-Garzón

2020

A&A

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Context. Swift J0243.6+6124 is a unique system. It is the first and only ultra-luminous X-ray source in our Galaxy. It is the first and only high-mass Be X-ray pulsar showing radio jet emission. It was discovered during a giant X-ray outburst in October 2017. While there are numerous studies in the X-ray band, very little is known about the optical counterpart. Aims. Our aim is to characterize the variability timescales in the optical and infrared bands in order to understand the nature of this intriguing system. Methods. We performed optical spectroscopic observations to determine the spectral type. Long-term photometric light curves together with the equivalent width of the Hα line were used to monitor the state of the circumstellar disk. We used BVRI photometry to estimate the interstellar absorption and distance to the source. Continuous photometric monitoring in the B and V bands allowed us to search for intra-night variability. Results. The optical counterpart to Swift J0243.6+6124 is a V = 12.9, O9.5Ve star, located at a distance of ∼5 kpc. The optical extinction in the direction of the source is AV = 3.6 mag. The rotational velocity of the O-type star is 210 km s−1. The long-term optical variability agrees with the growth and subsequent dissipation of the Be circumstellar disk after the giant X-ray outburst. The optical and X-ray luminosity are strongly correlated during the outburst, suggesting a common origin. We did not detect short-term periodic variability that could be associated with nonradial pulsations from the Be star photosphere. Conclusions. The long-term optical and infrared pattern of variability of Swift J0243.6+6124 is typical of Be/X-ray binaries. However, the absence of nonradial pulsations is unusual and adds another peculiar trait to this unique source.

“…There is also a strong peak at 0.94 c d −1 that appears in all sectors with amplitudes between 0.1 and 0.2%, and a frequency of 1.16 c d −1 with similar amplitudes present only in sectors 10 and 11. Garzón et al 2017). X-ray pulsations with P spin = 292 s were detected in 1977 with Ariel V (White et al 1978).…”

Section: Appendixmentioning

confidence: 97%

“…The X-ray pulsar 4U 1145-619 was discovered in 1972 by Uhuru (Giacconi et al 1972). The optical counterpart of 4U 1145-61 is the V=9 mag, B1 Ve V801 Cen (Stevens et al 1997;Alfonso-Garzón et al 2017). X-ray pulsations with P spin = 292 s were detected in 1977 with Ariel V (White et al 1978).…”

Section: Igr J06074+2205 (Fig B2)mentioning

confidence: 99%

Fast time optical variability in Be/X-ray binaries

Fabregat

2022

A&A

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Context. Classical Be stars, regardless of spectral subtype, display multiperiodic light modulations in the frequency range 0.1-12 c d −1 , when observed with high-cadence and long duration. This behaviour is attributed to non-radial pulsations and/or rotation of the Be star. A similar study for the optical counterparts to Be/X-ray binaries is yet to be carried out. Aims. The main goal of this work is to investigate the fast photometric variability of the optical counterparts to Be/X-ray binaries and compare the general patterns of such variability with the Galactic population of classical Be stars. Methods. The main core of our analysis is based on space-based observations performed by TESS. We analyzed 21 sources with TESS. High-cadence photometry with two ground-based telescopes was also performed for 15 sources. The TESS light curves were created from the full-frame images using the Lightkurve package. The ground-based light curves were obtained through differential photometry between the target and a number of non-variable stars in the same field of view. Standard Fourier analysis and least-squares fitting methods were employed in the frequency analysis. Results. All sources exhibit intra-night light variations with intensity variations of 0.01-0.06 mag in the ground-based observations and up to 5% in flux in TESS observations. This variability manifests itself as multi-periodic signals in the frequency range 0.2-12 c d −1 . We find that the patterns of variability of the Be stars in Be/X-ray binaries agree with that of classical early-type Be stars in terms of the general shape of the periodograms. Based on the general shape and number of peaks in the periodograms, Be/X-ray binaries can be classified into different types. The most common case is the presence of groups of closely-spaced frequencies (67)%, followed by sources that exhibit isolated signals (18%). The remaining type of sources displays frequency spectra characterized by a mixed pattern of stochastic variability and high-frequency peaks. Conclusions. This study reveals that short-term optical photometric variability is a very common, if not ubiquitous, feature intrinsic to the Be optical companions in Be/X-ray binaries. This variability is mainly attributed to pulsations that originate in the stellar interior.