Spectroscopy and photometry of V490 Cep (Cep X-4) in June 2023 X-ray outburst
ATel #16189; V. P. Goranskij (SAI, Moscow University; SAO RAS); E. A. Barsukova, A. S. Vinokurov, A. N. Sarkisyan, E. O. Dedov (SAO RAS)
on 14 Aug 2023; 12:49 UT
Credential Certification: Vitaly Goranskij (goray@sai.msu.ru)
Subjects: Optical, X-ray, Binary, Neutron Star, Star, Variables, Pulsar
The recent X-ray outburst of Be/X-ray binary pulsar Cep X-4 (Ginga 2138+56) was
observed by several observatories at once (ATel#16088,#16171). The outburst started
around 2023 May 26 and finished around July 9, the flux maximum was reached on June 8.
The source is an X-ray pulsar with a donor B1.5Ve star and is associated
with a variable star V490 Cep. We carried out spectral observations and direct
imaging of this star at the decay of the X-ray outburst using the Russian 6 m
telescope BTA with the multimode focal reducer SCORPIO-2. Dates of observations
were 2023 June 21.99 and 28.94 UT, spectral range 3601 - 7312 A, spectral resolution
3.3-3.5 A. Spectrum of V490 Cep is typical of Be-stars containing Balmer lines
in absorption with emission components (H-alpha is in emission only), Ca II 3933
and 3968 A in absorption, and a reach set of Fe II emission lines. Weak He II emission
line with EW ~ 0.2 A is visible in the both spectra. The previous spectrum before
the outburst was taken with BTA/SCORPIO-1 on 2022 August 3, and no He II emission was
seen that time. The equivalent widths of H-beta emission line are 4.13 and 4.10 A,
and those ones of H-alpha are 51.4 and 52.2 in June 21 and 28, consequently. These
values are close to those measured in the spectra, obtained outside the X-ray outburst.
There are many diffuse interstellar bands in the spectrum, which are indicative of
a large interstellar extinction of the star.
CCD UBV(RI)c photometry of V490 Cep was taken during the period between 2023 June 20
and July 22 with different devices at the 6-m BTA telescope and 1-m Zeiss telescope
of SAO RAS, and at the 0.6 m telescope of the Moscow University Crimean Astronomical
Station. A nearby star with the position 21:39:34.09 +56:59:27.3 (2000) was chosen
as a comparison star. The results of measurements are shown in the Table.
| JD hel. | U | B | V | Rc |
Ic | Telescope |
| 2460116.52 | - | 15.468 | 14.495 |
13.630 | 12.821 | SAO 1m |
| 2460117.49 | - | 15.480 | 14.485 |
13.575 | 12.803 | SAO 6m |
| 2460124.43 | - | 15.516 | 14.489 |
13.570 | 12.815 | SAO 6m |
| 2460133.50 | - | 15.477 | 14.500 |
13.633 | 12.830 | SAI 0.6m |
| 2460138.50 | 15.607 | 15.445 | 14.512 |
13.662 | 12.889 | SAI 0.6m |
| 2460139.52 | 15.590 | 15.466 | 14.518 |
13.679 | 12.904 | SAI 0.6m |
| 2460148.43 | 15.632 | 15.446 | 14.501 |
13.662 | 12.898 | SAI 0.6m |
| Standard | 15.03 | 14.00 | 13.06 |
12.37 | 11.84 |
The amplitude of the optical variability during the X-ray outburst decay
was 0.033 mag in the V band, which barely exceeds the systematic errors of data
taken with different telescopes and devices. We continued regular photometric
observations of V490 Cep since 2001, and 6 X-ray outbursts were registered
during that time interval. The star had some brightenings and fadings
on a scale of several years. The total amplitude of variability of V490 Cep
within this time interval was 0.16 mag in the V band. However, this data
shows that the X-ray outbursts are not associated with any level of star
brightness, and are not accompanied by noticeable bhightness changes of the
system in optics.
Spectra and light curve of V490 Cep (Cep X-4)
