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Astron. Astrophys. 364, 853-858 (2000)

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3. Data analysis

As shown in Fig. 1, the first DCIM has a constant average circular polarization of 3% left-handed. However, the second DCIM changed polarization from -80% right-handed during the rising phase to 14% during the decay phase. Fine structures appear at the beginning during the high polarization phase: spike groups, pulsations, patch and zebra pattern or EEL(FS a, FS b, FS c). We now describe three characteristic fine structures in detail.

3.1. FS a

FS a in Fig. 2a is composed of a continuum on the lower frequency part and two narrow band emissions on the higher frequency part. It is unfortunate that only a part of FS a was recorded, since as it is clear that it continues beyond the 3.80 GHz high frequency limit of the spectrometer, so we do not know exactly what its bandwidth and duration are. However, we can state the following:

  1. FS a has a duration of more than 1 s;

  2. FS a occurs in the range 2.84 GHz-[FORMULA] 3.80 GHz;

  3. In the low frequency range from 2.84 GHz to 3.67 GHz, there is a weaker continuum from the beginning to the end;

  4. In the high frequency range from about 3.67 GHz to 3.80 GHz, two narrow band emissions are present, with bandwidths [FORMULA] 10-20 MHz and 40 MHz for the top (lowest frequency) and bottom (highest frequency) respectively. The corresponding relative bandwidths are [FORMULA]0.004 and 0.011. The frequency separation between the two strips decreases from about 20 MHz at the beginning to zero at the end when they merge into a single strip. The central frequencies of either strip decrease and then increase again simultaneously by about 100 MHz, over the 1 s time period during which we observed them. This corresponds to a relative frequency variation [FORMULA]0.03 which could be due to either a plasma density variation [FORMULA]0.07 (plasma emission), or to a magnetic field variation [FORMULA] 0.03 (electron-cyclotron emission);

  5. FS a keeps the same sense and high degree of polarization all the time (-80% right-handed).

3.2. FS b

This is the most complicated of the three recorded, with lots of detail; it is similar to the "evolving emission line" (EEL) described by Chernov et al. (1998). A pulsating continuum predominates at lower frequencies, and two or three narrow band emissions occur on the high frequency part.

  1. FS b lasts for more than 4 s;

  2. FS b ranges over slightly more than 1.00 GHz bandwidth from 2.72 GHz to 3.78 GHz;

  3. The low frequency continuum is modulated by quasi-periodic pulsations. The intensity fluctuation is given in Fig. 3; we identify a quasi-period of about 275 ms, compared to the continuum pulsation period of 3 minutes at 300 MHz in Chernov et al.'s event. The continuum becomes weaker and weaker from high to low frequencies;

  4. From the beginning to 0801 28.4. UT, two narrow band emissions on the high frequency part, 10-20 MHz wide and 10-20 MHz apart, wiggle by about 50 MHz. At [FORMULA] 0801 28.5. UT (solid triangle marker), and for about 0.1 s, the whole emission splits into five narrow bands, 20 MHz wide and about 10 MHz apart; during this short period, the low frequency part of the continuum is much weaker than nearby. Right afterwards the spectrum is smooth, and about 0.3 s later, at 0801 28.8. UT, the bursts split into three narrow band emissions again, about 30 MHz wide (bottom strip) to about 50 MHz wide (top), corresponding to relative bandwidths [FORMULA] 0.004 to 0.01 respectively. The frequency separation is about 20 MHz. These three strips synchronously wiggle by about 200 MHz on a time scale of 0.5 s - the relative frequency variations [FORMULA] 0.06 corresponds to a plasma density variation [FORMULA]0.1 or to a magnetic field variation [FORMULA]0.06;

  5. FS b has the same sense and degree of polarization as FS a.

3.3. FS c

  1. We have recorded all of this fine structure, which closely follows FS b; the duration is less than 1 s;

  2. The bandwidth is less than 800 MHz;

  3. Instead of a continuum, the low frequency part consists of spikes from 2.70 GHz to 3.40 GHz: durations [FORMULA] 24-40 ms, bandwidth [FORMULA] 40-110 MHz (Isliker & Benz 1994);

  4. A narrow band emission, about 10 MHz wide, is cut off on the high frequency part and wiggles by about 80 MHz (from 3.41 GHz to 3.49 GHz) on a time scale of one second;

  5. The sense and degree of polarization stay the same during the recording, as in FS a and b.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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