Zeki Aslan

INTEGRAL observations of SS433: Results of a coordinated campaign

Results of simultaneous INTEGRAL and optical observations of the galactic microquasar SS433 in May 2003 and INTEGRAL /RXTE observations in March 2004 are presented. Persistent precessional variability with a maximum to minimum uneclipsed hard X-ray flux ratio of ∼ 4 is discovered. The 18-60 keV X-ray eclipse is found to be in phase with optical and near infrared eclipses. The orbital eclipse observed by INTEGRAL in May 2003 is at least two times deeper and apparently wider than in the soft X-ray band. The broadband 2-100 keV X-ray spectrum simultaneously detected by RXTE/INTEGRAL in March 2004 can be explained by bremsstrahlung emission from optically thin thermal plasma with kT ∼ 30 keV. Optical spectroscopy with the 6-m SAO BTA telescope confirmed the optical companion to be an A5-A7 supergiant. For the first time, spectorscopic indications of a strong heating effect in the optical star atmosphere are found. The measurements of absorption lines which are presumably formed on the non-illuminated side of the supergiant yield its radial velocity semi-amplitude Kv = 132 ±9 km/s. The analysis of the observed hard X-ray light curve and the eclipse duration, combined with the spectroscopically determined optical star radial velocity corrected for the strong heating effect, allows us to model SS433 as a massive X-ray binary. Assuming that the hard X-ray source in SS433 is eclipsed by the donor star that exactly fills its Roche lobe, the masses of the optical and compact components in SS433 are suggested to be Mv ≈ 30M⊙ and Mx ≈ 9M⊙, respectively. This provides further evidence that SS433 is a massive binary system with supercritical accretion onto a black hole.

Around-the-Clock Observations of the Q0957+561A,B Gravitationally Lensed Quasar. II. Results for the Second Observing Season

We report on an observing campaign in 2001 March to monitor the brightness of the later arriving Q0957+561B image in order to compare with the previously published brightness observations of the (first-arriving) A image. The 12 participating observatories provided 3543 image frames, which we have analyzed for brightness fluctuations. From our classical methods for time-delay determination, we find a 417.09 ± 0.07 day time delay, which should be free of effects due to incomplete sampling. During the campaign period, the quasar brightness was relatively constant and only small fluctuations were found; we compare the structure function for the new data with structure function estimates for the 1995-1996 epoch and show that the structure function during our observing interval is unusually depressed. We also examine the data for any evidence of correlated fluctuations at zero lag. We discuss the limits of our ability to measure the cosmological time delay if the quasars emitting surface is time resolved, as seems likely.

Solar radius determinations obtained with the CCD astrolabe at TUBITAK National Observatory

This paper measures the solar radius, using the new Tubitak National Observatory astrolabe as well as data acquisition and reduction procedures. The mean values of the solar radius obtained are slightly lower than the other results to which they are compared. We have compared our results with data obtained since 1981, and show that our results are very homogeneous. We hope to extend these measurements to obtain accurate determinations of solar position.

Extensive multiband study of the X-ray rich GRB 050408: a likely off-axis event with an intense energy injection

Aims. Understand the shape and implications of the multiband ligth curve of GRB 050408, an X-ray rich (XRR) burst. Methods. We present a multiband optical light curve, covering the time from the onset of the γ-ray event to several months after, when we only detect the host galaxy. Together with X-ray, millimetre and radio observations we compile what, to our knowledge, is the most complete multiband coverage of an XRR burst afterglow to date. Results. The optical and X-ray light curve is characterised by an early flattening and an intense bump peaking around 6 days after the burst onset. We explain the former by an off-axis viewed jet, in agreement with the predictions made for XRR by some models, and the latter with an energy injection equivalent in intensity to the initial shock. The analysis of the spectral flux distribution reveals an extinction compatible with a low chemical enrichment surrounding the burst. Together with the detection of an underlying starburst host galaxy we can strengthen the link between XRR and classical long-duration bursts.

First simultanous X-ray and optical observations of rapid variability of supercritical accretor SS433

We present results of first simultaneous optical and X-ray observations of peculiar binary system SS433. For the first time, chaotic variability of SS433 in the optical spectral band (R band) on time scales as small as tens of seconds was detected. We find that the X-ray flux of SS433 is delayed with respect to the optical emission by approximately 80 s. Such a delay can be interpreted as the travel time of mass accretion rate perturbations from the jet base to the observed X-ray emitting region. In this model, the length of the supercritical accretion disk funnel in SS433 is ∼10 12 cm.

Around the Clock Observations of the Q0957+561A,B Gravitationally Lensed Quasar

An observing campaign with 10 participating observatories has undertaken to monitor the optical brightness of the Q0957 gravitationally lensed quasar for 10 consecutive nights in 2000 January. The resulting A image brightness curve has significant brightness fluctuations and makes a photometric prediction for the B image light curve for a second campaign planned for 2001 March 12-21. The ultimate purpose is to determine the gravitational lens time delay to a fraction of an hour and to seek evidence of rapid microlensing.

Absolute Magnitudes of M Type Semi-Regular Variables in Hipparcos Catalogue

Visual absolute magnitudes from Hipparcos parallaxes and photometry have been calculated for M giant irregular (L) and semi-regular (SR, SRa, SRb) variables. Positions in the H-R diagram of the variables with relative parallax error < 0.175 indicate that SRb’s tend to be fainter in M v , bluer in B - V, and later in spectral type than the rest of the variables. Semi-regular variables with periods smaller than 10 days as given in Hipparcos obey a clearly defined period and period - colour relations.