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Register a new userLong-term Stellar Variability in the Galactic Centre Region
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Added by
Radostin Georgiev Kurtev
Publication date
2018
fields
Physics
and research's language is
English
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We report the detection of variable stars within a 11.5 x 11.5 region near the Galactic centre (GC) that includes the Arches and Quintuplet clusters, as revealed by the VISTA Variables in the Via Lactea (VVV) survey. There are 353 sources that show Ks-band variability, of which the large majority (81%) correspond to red giant stars, mostly in the asymptotic giant branch (AGB) phase. We analyze a population of 52 red giants with long-term trends that cannot be classified into the typical pulsating star categories. Distances and extinctions are calculated for 9 Mira variables, and we discuss the impact of the chosen extinction law on the derived distances. We also report the presence of 48 new identified young stellar object (YSO) candidates in the region.
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We present preliminary results of our analysis on the long-term variations observed in the optical spectrum of the LBV star Eta Carinae. Based on the hydrogen line profiles, we conclude that the physical parameters of the primary star did not change in the last 15 years.
We have observed the central 45 region of the Galaxy at 620 MHz band of the Giant Metrewave Radio Telescope (GMRT) in radio continuum, and measured the polarisation properties of 64 small diameter background extragalactic sources seen through the -6 deg < l < 6 deg, -2 deg < b < 2 deg region with the Australia Telescope Compact Array (ATCA) and the Very Large Array (VLA). Our 620 MHz observations show that Sgr A* is located behind the HII region Sgr A West. Using the ATCA and the VLA observations, we measured the Faraday rotation measure (RM) of the polarised sources. The measured RMs are mostly positive, and show no reversal of sign across the rotation axis of the Galaxy. This rules out any circularly symmetric model of magnetic field in the region. We estimate the magnetic field strength in the region to be ~10 microGauss, which raises doubts against an all pervasive milliGauss field in the central few hundred pc of the Galaxy.
Due to the extreme extinction towards the Galactic centre ($A_{V} sim 30$ mag), its stellar population is mainly studied in the near-infrared (NIR) regime. Therefore, a proper analysis of the NIR extinction curve is necessary to fully characterise the stellar structure and population of the inner part of the galaxy. We studied the dependence of the extinction index ($alpha_lambda$) in the NIR on the line of sight, wavelength, and extinction. We used the GALACTICNUCLEUS imaging survey, a high angular resolution catalogue ($0.2$) for the inner part of the Galaxy in $JHK_s$, and studied the spatial variation in the extinction index. We also applied two independent methods based on red clump stars to compute the extinction index between different bands and its variation with wavelength. We did not detect any significant line-of-sight or extinction variation in $alpha$ within the studied region in the nuclear stellar disc. The extinction index between $JH$ and $HK_s$ differs by $0.19 pm 0.05$. We obtained mean values for the extinction indices $alpha_{JH} = 2.43pm0.03$ and $alpha_{HK_s} = 2.23pm0.03$. The dependence of the extinction index on the wavelength could explain the differences obtained for $alpha_lambda$ in the literature since it was assumed constant for the NIR regime.
Seventeen years of hard X-ray observations with the instruments of the INTEGRAL observatory, with a focus on the Milky Way and in particular on the Galactic Centre region, have provided a unique database for exploration of the Galactic population of low-mass X-ray binaries (LMXBs). Our understanding of the diverse energetic phenomena associated with accretion of matter onto neutron stars and black holes has greatly improved. We review the large variety of INTEGRAL based results related to LMXBs. In particular, we discuss the spatial distribution of LMXBs over the Galaxy and their X-ray luminosity function as well as various physical phenomena associated with Atoll and Z sources, bursters, symbiotic X-ray binaries, ultracompact X-ray binaries and persistent black hole LMXBs. We also present an up-to-date catalogue of confirmed LMXBs detected by INTEGRAL, which comprises 166 objects. Last but not least, the long-term monitoring of the Galactic Centre with INTEGRAL has shed light on the activity of Sgr A* in the recent past, confirming previous indications that our supermassive black hole experienced a major accretion episode just ~100 years ago. This exciting topic is covered in this review too.
We examine whether the spectral energy distribution of optical continuum emission of active galactic nuclei (AGNs) changes during flux variation, based on accurate and frequent monitoring observations of 11 nearby Seyfert galaxies and QSOs carried out in the B, V, and I bands for seven years by the MAGNUM telescope. The multi-epoch flux data in any two different bands obtained on the same night show a very tight linear flux to flux relationship for all target AGNs. The flux of the host galaxy within the photometric aperture is carefully estimated by surface brightness fitting to available high-resolution HST images and MAGNUM images. The flux of narrow emission lines in the photometric bands is also estimated from available spectroscopic data. We find that the non-variable component of the host galaxy plus narrow emission lines for all target AGNs is located on the fainter extension of the linear regression line of multi-epoch flux data in the flux to flux diagram. This result strongly indicates that the spectral shape of AGN continuum emission in the optical region does not systematically change during flux variation. The trend of spectral hardening that optical continuum emission becomes bluer as it becomes brighter, which has been reported by many studies, is therefore interpreted as the domination of the variable component of the nearly constant spectral shape of an AGN as it brightens over the non-variable component of the host galaxy plus narrow lines, which is usually redder than AGN continuum emission.
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