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

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4. Discussion

The dynamic spectra shown on Fig. 2a-c are reminiscent of the "zebra pattern" seen in type IV bursts or evolving emission lines (EELs).

4.1. zebra pattern

Zebra patterns are characterised by a number of simultaneous emission strips; the strip bandwidth is narrower than the frequency separation, both being about uniform across the different strips. The strip central frequencies fluctuate in parallel on a typical time scale of a few seconds. The emission strips are usually considered to be harmonics s[FORMULA] of a fundamental frequency [FORMULA], where s represents consecutive integers. Such features as band splitting, strip wiggling and strip disappearance occur occasionally (Kuijpers 1975; Chernov et al. 1998).

4.2. EEL

The EEL event was firstly reported by Chernov et al. (1998) around 300 MHz. They describe it as "right along the high-frequency cut-off line of the pulsating continuum, and following its frequency fluctuations, a narrow band emission, 10-15 MHz wide, oscillates almost sinusoidally in frequency". In addition, the EEL consists of absorption (white) and emission (black) dots with 0.25 s period; Chernov et al. think that the absorption dots result from the quenching of loss-cone instability when new particles are injected inside the current sheet. Because of the EEL being at the highest frequency emitted, it is tempting to locate its source in the densest part of the whole radiating volume, that is inside the current sheet where magnetic reconnection proceeds.

4.3. Difference between zebra pattern and the EEL

Chernov et al. think there are three differences between them:

  1. The EEL mostly consists of one single emission band instead of many in zebra patterns;

  2. The EEL has a larger relative bandwidth than zebra patterns;

  3. It is continuous instead of intermittent.

Moreover, in zebra patterns the frequency separation between strips is larger than the strip bandwidth, and strips wiggle over a frequency interval comparable to the frequency separation between them. In addition, the EEL is right along the high-frequency cut-off line, which results from its source being located in the densest part of the whole radiating volume.

4.4. Our viewpoint

Firstly, we think the FS a is not the zebra pattern at 3.60 GHz because:

  1. The two strips have different bandwidth, 10-20 MHz for the upper and 40 MHz for the lower strip;

  2. The intensity of the lower strip is larger than that of the upper one;

  3. There is some weaker continuum on the low frequency part;

  4. Sometimes during the course of the zebra pattern, one zebra strip disappears while the neighbours approach each other and continue (Kuijpers 1975); however, in FS a, no strip disappears, but two strips combine together at the end;

  5. The strips wiggle by about 130 MHz, much more than the frequency separation between them;

  6. The frequency separation between two strips, which is equal to or smaller than the strip emission band, is changing, from 20 MHz at the beginning to zero at the end.

Although we do not know what zebra pattern could look like around 3.60 GHz, the features above are different from zebra strips around 200-300 MHz. In the event that the FS a was a zebra pattern, we presume that the fundamental frequency would be about 20 MHz (the frequency separation between them), so the same strip emission would have to continuously change its harmonic number s from 190 through 184 to 190 with time, when the strip wiggles from 3.80 GHz through 3.67 GHz to 3.80 GHz again. Therefore, although we also do not know what FS a looks like at frequencies higher than 3.80 GHz, we conclude it is of the EEL kind.

Secondly, the FS b is an EEL with some new properties around 3.60 GHz. The reasons are:

  1. It has a narrow cut-off strip emission at the high frequency all the time, which is the most important reason;

  2. The continuum is modulated with a quasi-period of about 275 ms on the low frequency part;

  3. The three strip emissions have different bandwidth (30 MHz to 50 MHz).

On the other hand, the FS b also has some new features:

  • (I) The bursts split into five narrow band emissions for 84 ms and then these five strips combine together and finally split into three line emissions again;

  • (II) The three strips synchronously wiggle by about 200 MHz on a time scale of 500 ms, which is much larger than the frequency separation (20 MHz) among them;

  • (III) The line emission is continuous with no absorption dots.

  • (IV) The lines have a pseudo-period of their frequency fluctuations, which is about 2 s.

The 5 strips lasting 84 ms are somewhat similar to a short portion of the zebra patterns recorded at lower frequencies. But they are not zebra pattern because the emission band (about 20 MHz) is larger than frequency separation (about 10 MHz). This is also the main reason why the following three line emissions are not zebra strips.

Thirdly, FS c is also an EEL event, since it is right along the high-frequency cut-off line. However, some spike groups on the low frequency part are new features of the FS c.

Finally, we conclude FS a, b and c are EEL events around 3.60 GHz.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001