A. S. Oliveira

PCA Tomography: how to extract information from data cubes★

Astronomy has evolved almost exclusively by the use of spectroscopic and imaging techniques, operated separately. With the development of modern technologies, it is possible to obtain data cubes in which one combines both techniques simultaneously, producing images with spectral resolution. To extract information from them can be quite complex, and hence the development of new methods of data analysis is desirable. We present a method of analysis of data cube (data from single field observations, containing two spatial and one spectral dimension) that uses Principal Component Analysis (PCA) to express the data in the form of reduced dimensionality, facilitating efficient information extraction from very large data sets. PCA transforms the system of correlated coordinates into a system of uncorrelated coordinates ordered by principal components of decreasing variance. The new coordinates are referred to as eigenvectors, and the projections of the data on to these coordinates produce images we will call tomograms. The association of the tomograms (images) to eigenvectors (spectra) is important for the interpretation of both. The eigenvectors are mutually orthogonal, and this information is fundamental for their handling and interpretation. When the data cube shows objects that present uncorrelated physical phenomena, the eigenvector’s orthogonality may be instrumental in separating and identifying them. By handling eigenvectors and tomograms, one can enhance features, extract noise, compress data, extract spectra, etc. We applied the method, for illustration purpose only, to the central region of the low ionization nuclear emission region (LINER) galaxy NGC 4736, and demonstrate that it has a type 1 active nucleus, not known before. Furthermore, we show that it is displaced from the centre of its stellar bulge.

Assisted stellar suicide in V617 Sagittarii

Context. V617 Sgr is a V Sagittae star – a group of binaries thought to be the galactic counterparts of the Compact Binary Supersoft X-ray Sources – CBSS. Aims. To check this hypothesis, we measured the time derivative of its orbital period. Methods. Observed timings of eclipse minima spanning over 30 000 orbital cycles are presented. Results. We found that the orbital period evolves quite rapidly:

Mapping low- and high-density clouds in astrophysical nebulae by imaging forbidden line emission

P/\dot{P} = 1.1\times10^{6}

SOAR observations of the high-viscosity accretion disc of the dwarf nova V4140 Sagitarii in quiescence and in outburst

years. This is consistent with the idea that V617 Sgr is a wind driven accretion supersoft source. As the binary system evolves with a time-scale of about one million years, which is extremely short for a low mass evolved binary, it is likely that the system will soon end either by having its secondary completely evaporated or by the primary exploding as a supernova of type Ia. Conclusions.

The orbital period of the V Sge star candidate QU Carinae

Emission line ratios have been essential for determining physical parameters such as gas temperature and density in astrophysical gaseous nebulae. With the advent of panoramic spectroscopic devices, images of regions with emission lines related to these physical parameters can, in principle, also be produced. We show that, with observations from modern instruments, it is possible to transform images taken from density-sensitive forbidden lines into images of emission from high- and low-density clouds by applying a transformation matrix. In order to achieve this, images of the pairs of density-sensitive lines as well as the adjacent continuum have to be observed and combined. We have computed the critical densities for a series of pairs of lines in the infrared, optical, ultraviolet and X-rays bands, and calculated the pair line intensity ratios in the highand low-density limit using a four- and five-level atom approximation. In order to illustrate the method, we applied it to Gemini Multi-Object Spectrograph (GMOS) Integral Field Unit (GMOS-IFU) data of two galactic nuclei. We conclude that this method provides new information of astrophysical interest, especially for mapping low- and high-density clouds; for this reason, we call it ‘the ld/hd imaging method’.

Exploratory Spectroscopy of Magnetic Cataclysmic Variables Candidates and Other Variable Objects

We report the analysis of 22 B-band light curves of the dwarf nova V4140 Sgr obtained with SOI/SOAR during two nights along the decline of a superoutburst in 2006 Sep 12-24 and in quiescence over 50 days following the superoutburst. Three-dimensional eclipse mapping of the outburst light curves indicates that the accretion disc is elliptical (eccentricity e=0.13) and that superhump maximum occurs when the mass donor star is aligned with the bulge of the elliptical disc. The accretion disc is geometrically thin both in outburst and in quiescence; it fills the primary Roche lobe in outburst and shrinks to about half this size in quiescence. The stability of the eclipse shape, width and depth along quiescence and the derived disc surface brightness distribution indicate that the quiescent accretion disc is in a high-viscosity, steady-state. Flickering mapping of the quiescent data reveal that the low-frequency flickering arises from an azimuthally-extended stream-disc impact region at disc rim and from the innermost disc region, whereas the high-frequency flickering originates in the accretion disc. Assuming the disc-related flickering to be caused by fluctuations in the energy dissipation rate induced by magneto-hydrodynamic turbulence (Gertseema & Achterberg 1992), we find that the quiescent disc viscosity parameter is large (alpha ~ 0.2-0.4) at all radii. The high-viscosity quiescent disc and the inferred low disc temperatures in superoutburst are inconsistent with expectations of the disc-instability model, and lead to the conclusion that the outbursts of V4140 Sgr are powered by mass transfer bursts from its donor star.

Nature and chemical abundances of a sample of Lyman-α emitter objects at high redshift

Close Binary Supersoft X-ray Sources (CBSS) are considered strong candidates to SN Ia progenitors, but very few CBSS are known in our Galaxy. The galactic counterparts of the CBSS may be the V Sge stars, not detected in X-rays due to the strong absorption by the interstellar gas. Nevertheless, the number of members in the V Sge class is still small. In order to find new members we selected QU Car for photometric and spectroscopic observations. The orbital period of this system was published in the literature as 10.9 h, determined from radial velocity data taken in 1979-1980, but posterior analysis of data taken in 2006-2007 did not confirm this period. We analysed the high variability of its emission line profiles with the Temporal Variance Spectrum (TVS) technique. Besides, we recovered the 10.9 h orbital period from the radial velocities of the He II 4686 A emission line and, for the first time, detected what may be the orbital modulation in the photometric data. This photometric modulation is present only in the lower brightness state data, when the flickering is attenuated. The inclusion of QU Car in the V Sge class is supported by many features like high/low states, strong winds, nebular lines and He II 4686 A/Hbeta line ratios. However, the non-detection of the characteristic O VI 3811-34 A lines in its spectrum claims against this classification. These lines, though, may be highly variable so additional spectra analysed with the TVS technique can, possibly, solve this question.

Optical identification of the transient supersoft X-ray source RX J0527.8-6954, in the LMC

The increasing number of synoptic surveys made by small robotic telescopes, such as the photometric Catalina Real-Time Transient Survey (CRTS), represents a unique opportunity for the discovery of variable sources and improves the statistical samples of such classes of objects. Our goal is the discovery of magnetic Cataclysmic Variables (mCVs). These are rare objects, which probe interesting accretion scenarios controlled by the white dwarf magnetic field. In particular, improved statistics of mCVs would help to address open questions on their formation and evolution. We performed an optical spectroscopy survey to search for signatures of magnetic accretion in 45 variable objects selected mostly from the CRTS. In this sample we found 32 CVs, 22 being mCV candidates from which 13 are previously unreported as such. If the proposed classifications are confirmed, it would represent an increase of 4% in the number of known polars and 12% in the number of known IPs. A fraction of our initial sample was classified as extragalactic sources or other types of variable stars by the inspection of the identification spectra. Despite the inherent complexity in identifying a source as a mCV, variability-based selection followed by spectroscopic snapshot observations has proved to be an efficient strategy for their discoveries, being a relatively inexpensive approach in terms of telescope time.

The nature of the LINER in the galaxy NGC 404

We built a grid of photoionization models and compiled already available observational emission line intensities (

The Active Nucleus in NGC 4579

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