Photometric study of the variable star population in the globular cluster NGC 6397

We present the results of a photometric survey for variable stars in the central region of the nearby globular cluster NGC 6397.Time series photometry was obtained for 30 variable objects. The sample includes 12 new objects, of which 6 show periodic lightcurves and 2 are eclipsing binaries of unknown period. Six variables possess certain and three possess likely X-ray counterparts detected with the Chandra observatory. Among them four are cataclysmic variables and one is a foreground eclipsing binary. The cataclysmic variable CV2 exhibited a likely dwarf nova type outburst in May 2003. The cataclysmic variable CV3 was observed at 18.5<V<20.0 during 5 observing runs, but went into a low state in May 2003 when it reached V>22. We have found that thelight curve of the optical companion to the millisecond pulsar PSRJ1740-5340 exhibits noticeable changes of its amplitude on a time scale of a few months. A shallow eclipse with Delta_V=0.03 mag was detected in one of the cluster turnoff stars suggesting the presence of a large planet or brown dwarf in orbit.Comment: 8 pages, 9 figures, submitted to MNRAS on 21 April 200

RR Lyrae Variables in the Globular Cluster M5

We present V-band CCD photometry of 65 RR Lyr variables from the globular cluster M5. We have estimated the basic physical parameters for 16 RRc stars and 26 RRab stars using a Fourier decomposition of the light curves of the variables. The mean values of mass, luminosity, effective temperature and relative helium abundance for the RRc stars are measured to be ${\cal M}=0.54{\cal M}_\odot$, log L = 1.69 T_{eff}=7353 K and Y=0.28, respectively. For the RRab variables the derived mean values of absolute magnitude, metallicity and effective temperature are: M_V=0.81, [Fe/H]=-1.23 and T_{eff}=6465 K. We find that the V amplitude of an RRab star for a given period is a function of metal abundance rather than Oosterhoff type. We find significant problems with the calibration of both the zero point and the scale of the luminosities measured with the Fourier technique. The apparent distance modulus derived from RRc stars is equal to $14.47\pm0.11$ and it is in good agreement with recent determinations. On the other hand distance modulus obtained from the sample of RRab stars (calibrated by the Baade-Wesselink observations of field RR Lyr variables) is significantly smaller and equal to $14.27\pm0.04$Comment: accepted for publication in Astronomy and Astrophysics, 15 pages with 6 figure

The Nature of the Emission Components in the Quasar/NLS1 PG1211+143

We present the study of the emission properties of the quasar PG1211+143, which belongs to the class of Narrow Line Seyfert 1 galaxies. On the basis of observational data analyzed by us and collected from the literature, we study the temporal and spectral variability of the source in the optical/UV/X-ray bands and we propose a model that explains the spectrum emitted in this broad energy range. In this model, the intrinsic emission originating in the warm skin of the accretion disk is responsible for the spectral component that is dominant in the softest X-ray range. The shape of reflected spectrum as well as Fe K line detected in hard X-rays require the reflecting medium to be mildly ionized (xi~500). We identify this reflector with the warm skin of the disk and we show that the heating of the skin is consistent with the classical alpha P_{tot} prescription, while alpha P_{gas} option is at least two orders of magnitude too low to provide the required heating. We find that the mass of the central black hole is relatively small (M_BH~10^7- 10^8 Msun, which is consistent with the Broad Line Region mapping results and characteristic for NLS1 class.Comment: 22 pages, 10 figures, accepted to Ap

Towards equation of state of dark energy from quasar monitoring: reverberation strategy

Context. High-redshift quasars can be used to constrain the equation of state of dark energy. They can serve as a complementary tool to supernovae Type Ia, especially at z > 1

Cluster AgeS Experiment. CCD photometry of SX Phoenicis variables in the globular cluster M 55

We present CCD photometry of SX Phe variables in the field of the globular cluster M 55. We have discovered 27 variables, three of which are probable members of the Sagittarius dwarf galaxy. All of the SX Phe stars in M 55 lie in the blue straggler region of the cluster color-magnitude diagram. Using period ratio information we have identified the radial pulsation modes for one of the observed variables. Inspection of the period-luminosity distribution permits the probable identifications of the pulsation modes for most of the rest of the stars in the sample. We have determined the slope of the period-luminosity relation for SX Phe stars in M 55 pulsating in the fundamental mode. Using this relation and the HIPPARCOS data for SX Phe itself, we have estimated the apparent distance modulus to M 55 to be (m-M)_V=13.86 +- 0.25 mag.Comment: A&A accepted, 11 figure

Cluster AgeS Experiment (CASE): RR Lyrae stars from the globular cluster Omega Centauri as standard candles

New photometry of RRab and RRc stars in Omega Cen is used to calibrate their absolute magnitudes M_V as a function of: a) metallicity; and b) the Fourier parameters of light curves in the V band. The zero point of both calibrations relies on the distance modulus to the cluster derived earlier by the CASE project based on observations of the detached eclipsing binary OGLE GC17. For RRab variables we obtained a relation of M_V=(0.26\pm 0.08)[Fe/H]+(0.91\pm 0.13). A dereddened distance modulus to the LMC based on that formula is $\mu_{0}=18.56\pm 0.14$ mag. The second calibration of M_V, which is based on Fourier coefficients of decomposed light curves, results in the LMC distance of $\mu_{0}=18.51\pm 0.07$ mag.Comment: accepted for publication in MNRAS, 13 pages, 8 figure