Superfast Photometry with MANIA Complex

V. F. Shvartsman, I. N. Bernstein, G. M. Beskin, V. N. Komarova, S. I. Neizvestny, V. L. Plokhotnichenko, M. Yu. Popova, A. V. Zhuravkov

Special Astrophysical Observatory, Nizhniy Arkhyz, Karachai-Circassia, 357147, Russia, E-mail: beskin@sao.stavropol.su

Abstract:

We present a hard/software MANIA complex for carrying out superfast photometry. A new type of ``time-code'' converter ``Quantochron'' allows us to measure arrival times of photons registered by a photodetector with time resolutions up to 20 nsec in channels (space, colors, polarization, etc.) simultaneously. Special software was developed to investigate variability on a time scale of sec by statistical analysis of the time interval distribution between detected photons and ``classical'' light curves. The results of some observations are presented.

1. Introduction

Research of fast optical variability of relativistic and variable objects has being carried out at the Special Astrophysical Observatory of the Russian Academy of Science since 1972 in frames of the MANIA (Multichannel Analysis of Nanosecond Intensity Alterations) experiment. Its goal is to study the energy transformation in strong gravitational and/or magnetic fields which is manifested by very fast optical variability on time scales from up to sec (Shvartsman 1977). Special hard- and software is used to investigate nonstationary processes on flare stars, pulsars, X-ray binaries, etc.

2. The Complex Description

To study such a fast variability a special photometrical hard/software complex was created which consists of:

• a photometer (Vikul'ev et al. 1991);

• the ``Quantochron 3--16''---a special time-to-code converter capable of encoding arrival times of each detected photon (Zhuravkov et al. 1992);

• special data acquisition and reduction software (Plokhotnichenko 1983, 1992).

• the rate of data registration with the PC AT 286-486 is up to 370,000 q/s (in monochannel mode up to 750,000 q/s);

• 16-bit registers are used to register the arrival times of photons in channels simultaneously;

• the accuracy of time measuring is ns, the dead time is 20 ns; the equipment supports an external synchronization by a precise standard time service.

3. Data Analysis Software

3.1.y₂-, d₂-function Methods

While studying variability on a time scale smaller than the mean interval between photocounts the classical methods for light curve analysis are not effective due to very small fluxes and, therefore, very large data sizes. For these purposes a special, so called -function, method was worked out (Shvartsman 1977).

The -function method is intended for variability analyses on the time scale smaller than the mean interval between photocounts. It is based on statistical analysis of the time intervals between photons. The -function is defined as:

where is the fraction of intervals of duration from to in the flux from the object; is the same for the standard flux.

The -function method is used on times larger that the mean interval between photons and is an analog of the variances method. The -function is defined as:

where and are the sample dispersions of the number of photocounts and in the window of duration of the object and standard flux respectively and is the expectation value of the .

As a standard flux we can use either the flux from a comparison star or the constant Poisson flux with the mean flux equal to the object's one. It can be shown that there is a connection between the variability parameters (amplitude, filling factor, time, etc.), and the forms of the - and -functions allow us to estimate the parameters. In the presence of a variability the -, -functions show an increase on times less than the characteristic time of the variability.

If a variability is absent we can find the upper limits for the relative power of a variable component with 99.9% probability ( level) on timescales from up to sec. The limits are calculated based on the dispersion of the -, -functions of the comparison star. The algorithms of analysis are described in (Plokhotnichenko 1983, 1992).

3.2. The ``Fresnel Lense''-Like Method (FLL) for Period Search

The special FLL method for period search was developed (Plokhotnichenko 1992). In this method we choose a ``test'' period and build a set of consecutive light curves with it. We will sum all these light curves afterwards. But due to the difference between the test and the real periods the light curve details will not coincide in phase and they will be broadened in the summarized light curve. To avoid this we summarize the light curves taking all possible values of phase shift and get a set of final summed light curves. Then by means of statistical methods, we find the light curve with the most significant deviation from the noise (poisson) light curve. The period this curve is folded with is the closest to the true one.

3.3. Period Fine Fitting and Pulsar Investigation Methods

These methods are used in the case of a pulsar period known with sufficient accuracy to build a pulsar light curve with a well-shaped profile. Using data obtained in close moments of time we build a set of light curves. The uncertainties in the period cause the phase shift in each light curve relative to the first one. The value of this shift is calculated by the minimization of the squared difference of each couple. We express this phase shift as a function of time, expanding it into Taylor series. Finally we calculate the values of pulsar frequency with its two first derivatives which are used to build the pulsar light curve with a high time resolution. This light curve is used afterwards in search for fine structure and in investigation of pulsar photometric features.

3.4. LCA package

To study variability in a wide time range by classical methods (light curves, FFT and correlation analysis) a special software system was created---Light Curve Analyses (LCA) system for ``Quantochron'' data analysis. It runs under X Windows on Linux (UNIX) systems using the XView library. It allows working with the data in an interactive mode.

4. Some Results

• Nonthermal optical flares from A0620-00 and MXB1735-44 with durations from to sec were detected (Shvartsman et al. 1989a; Beskin et al. 1994).

• Fast optical variability of Nova Per 1992 on time scales from to sec in the high optical state was revealed (Bartolini et al. 1994).

• More than 100 flares during 35 hours of monitoring of flare stars with time resolution of sec were registered and analyzed (Shvartsman et al. 1988a).

• The light curve of the Crab pulsar with a time resolution of sec was analyzed (Shvartsman et al. 1988).

• 40 candidates for single black holes were investigated. The upper limits on the number density of the black holes relative to one of the normal stars near the Sun were found (Shvartsman et al. 1989).

5. Some Future Plans

• The new two-head (6-channel) photometer for simultaneous observations of an object and comparison star will be built.

• The testing and fitting of the 2D-photometer based on the microchannel plates detector will be finished. Some test observations have been carried out already.

• The new ``Quantochron 3--24'' is being developed now with 24-bit storage registers for holding coordinate information.

Acknowledgments:

This work was partially supported by ESO's Support Programme for Central and East Europe (grant A-02-023), by the Scientific and Educational Center ``Cosmion'', by the Russian Ministry of Science and by the Russian Foundation of Fundamental Research (grant 95-02-0368).

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