An analysis of the starting time in ten great hard X-ray bursts observed with the X-Ray Burst Spectrometer (HXRBS) is presented. It is shown that the impulsive phase of nine of them is composed of a preflash phase, during which the burst is observed up to an energy limit ranging from some tens of keV to 200 keV, followed ten to some tens of seconds afterwards by a flash phase, where the count rate rises simultaneously in all detector channels. For two events strong gamma-ray line emission is observed and is shown to start close to the onset of the flash phase

Klein, K. L.

Magun, A.

Pick, M.

English

NASA Technical Reports Server

N87-21792
A STUDY OF STARTING TIME IN GREAT HARD X-RAY FLARES
K. L. Klein
M. Pick
Observatoire de Paris
Section de Meudon
92195 Meudon, France
A. Magun
Institut flit Angewandte Physik
Sidlerstr. 5
3012 Bern, Switzerland
Abstract
An analysis of the starting time in ten great hard x-ray bursts observed
with HXRBS is presented. It is shown that the impulsivephase of nine of them is
composed of a pre-flashphase, during which the burst is observed up to an energy
limit ranging from some tens of keV to 200 keV, followed ten to some tens of
seconds afterwards by a flash phase, where the count rate rises simultaneously
in all detector channels. For two events strong gamma-ray line emission is
observed and is shown to start close to the onset of the flash phase.
i. Introduction
Hard x-ray and gamma-ray observations from SMMhave shown that energetic
electrons and ions are accelerated since the early stage of solar flares. There
is nevertheless still controversy as to whether high energy electrons and ions
are accelerated simultaneously with electrons of lower energy or whether a
second, distinct step of acceleration is necessary to account for high particle
energies. This controversy arose mainly from analyses of the temporal evolution
of hard x-ray bursts, where in a minority of cases high energy peaks were
observed to lag those at lower energies. This was ascribed to the acceleration
process itself by several authors (e.g. Bai and Ramaty, 1979; Bai and Dennis,
1985), whereas others showed that the interaction of energetic electrons with
the background plasma can account for such delays (Vilmer et al., 1982). In fact
the interaction effects make the peaktime analyses yield ambiguous results. On
the other hand, as hard x-ray emission is the inanediate response of a plasma to
the injection of energetic electrons, the starting time of the burst is not
affected by the above-mentioned ambiguities. It is clear, however, that this
parameter is influenced by the energy-dependent detector threshold.
79
https://ntrs.nasa.gov/search.jsp?R=19870012359 2020-03-20T11:43:57+00:00Z
Studies of the onset phase of the radiation from energetic electrons in
solar flares are still rare. Forrest and Chupp (1983) concluded for two events
that the hard x-ray and gamma-ray observations were compatible, within the
limits of detector sensitivity, with a simultaneous start of photon emission
from 40 keV to 8 MeV. Benz et al. (1983) considered hard x-rays and radio waves
at the very beginning of the impulsive phase. They found in some events evidence
for two components of the impulsive phase, a weak hard x-ray emission lasting up
to one minute, followed by a steep rise within some seconds to the peak count
rate. They termed these phases respectively pre-flash and flash phase.
Subsequently Raoult et al. (1985) showed for a series of energetic hard x-ray
bursts a systematic evolution from the pre-flash phase, associated with metric
type III bursts, to the more energetic flash phase with associated continua in
the decimetre to metre waveband (type V burst ).
In this contribution we present an analysis of the onset phase of hard x-
ray bursts aiming at the precise definition of the starting time of the burst in
each detector channel. The paper focusses on the observational results. Section
2 introduces the method of analysis and the selection criteria for the studied
events. The results are presented in section 3 and discussed in section 4 with
respect to relevant observations in other ranges of the electromagnetic
spectrum.
2. Method
We have analyzed the starting time of ten hard x-ray bursts observed with
the Hard X-ray Burst Spectrometer (}D(RBS; Orwig et al., 1980) on the Solar
Maximum Mission. These events had peak count rates above 7000 s -I integrated
over all detector channels (HXRBS event listing; Dennis et al., 1982). Six of
them exhibit significant emission in the % to 8 MeV band of the _-Ray
Spectrometer (Chupp, pers. comm. ). For some of the events ISEE 3 data were also
available (courtesy S.R. Kane).
In order to define a starting time of the burst, we computed in each
channel the background count rate (mean value) and the noise (standard deviation
o) in the minutes before burst onset. The starting time was defined by the
instant after which for the first time three successive count rate values
exceeded the background + no - level. We considered separately the cases n=3 and
n-5 in order to have an approximate measure of the uncertainty of our results. In
all channels count rates integrated during 1.024 s were used.
3. Results
Fig. 1 shows the results for the strongest event of our sample, the
neutron flare of 1982 June 3. Crosses give the photon energy where the count rate
exceeds the threshold value, as a function of time: horizontal error bars
delimit the time interval between the 3 _ and 5 o levels' being exceeded (if no
error bars are plotted, these levels are exceeded simultaneously), vertical
error bars represent
8O
the width of the detector channels. For the
' ..... ' ' 1,'.,o',/',"11:41 11:42 purpose of illustration we have also
, , plotted - on a logarithmic scale - the count
HXRBS ,," rate time histories in three HXRBS
s00 1982 June 3 t_'" channels. At low photon energies the burst
/_:ch14 I stares with apparent delays of several
,,." ,'",_ seconds between successive detector
: , , channels. After some tens of seconds the
4oo _ _ :,
r _ _ _ _ ' ;';, • ,_ delays shorten and at high photon energies
_ /"'chtO _ the emission starts nearly simultaneouslyin all channels. This rapid rise of the
i _'_' i cOunt rate" whlch is the first feature Of300 the bu s detected a high energies, is
_ I I[_1 _ also dlscerned at lower energles, but thereI'_,'_ it start is masked by the preceding
_ :'::,;_....,:;;_:::,.m'.::;_ -_ emission. The delays between successive
_=_2o0 --" _ channels, which are particularly imioortant
I ..'" oh2 ] at low energies, are seen to be at least
= t= partly an artefact due to the energy-
5 _ dependent detector threshold.
J
4
,t
j/
.1oo
"It : "1 "_ i i I i I
Fig. 1
Figs, 2.a, b compare the results
obtained with HXRBS and with ISEE 3 for the
1980 June 7 event and the one of fig.
. . .... I 1.Despite of differences introduced by
the detector characte_istlcs, the global
pictures converge: Low-energy emission is
detected first, with an apparent delay between adjacent channels. Some tens of
800
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HXRBS --F- S |
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"; I I I I • I
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1982 June 3
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81
seconds after the first signature at low energies a rapid rise occurs as a
distinctive feature in all channels, and during which the detection threshold is
exceeded in the high energy channels nearly simultaneously. In accordance with
the terminology of Benz et al. (1983), we refer to these two phases as pre-flash
and flash phase, respectively. The onset times of the flash phase observed with
HXRBS and ISEE 3 are seen to coincide within some seconds. Both events exhibit
strong excess emission in the 4-8 MeV photon energy range. This emission
(Forrest and Chupp, 1983; Chupp, pets. comm. ) starts, as indicated by the arrows
in figs. 2.a, b, close to the onset of the flash phase of hard x-rays.
As at low energies the start of the flash phase is masked by the pre-flash
emission, it cannot be analyzed by the method of section 2. We attempted to get
an indication of the starting time by extrapolating the observed rise back to
zero through a straight line. This was possible for four events of our sample.
The starting time of the flash phase found this way in the low energy channels
agreed with that in the high energy channels obtained with the method of section
2 within an uncertainty of +3 s, which cannot be considered as a significant
difference in the frame of the method used here.
:>, 200
o1
0
..c 100
I.-
HXRBS --_
1981 April 26 I
÷
.÷
!, I
11:43 11144 11145 ' 11J:46 ' 'UT
Fig. 3
The two phases have been found
in nine of the ten events studied. The
upper energy limit where pre-flash
emission could be detected ranges
between some tens of keV to 200 keV.
The duration is some seconds to 80 s.
No flash phase signature has been
observed in the "gradual" flare of
1981 April 26 at 11:40 UT (Bai and
Dennis, 1985 ). Fig. 3 shows the
temporal evolution of its threshold
energy. The emission exhibits a slow
rise in all detector channels up to
250 keV. In the two highest channels
there seems to be some fine structure
disturbing the slow rise, but the
count rate statistics is too poor to
yield clear evidence for flash-phase
like features.
4. Discussion
We have shown that nine out of our sample of ten hard x-ray bursts with
peak count rates above 7000 s-1 exhibit two components of the impulsive phase:
-a pre-flash phase restricted to lower energies up to 200 keVand lasting
some seconds to 80 s,
-a flash phase during which the emission in all detector channels rises
simultaneously within an uncertainty of some seconds.
In their study of 45 events with KXRBS count rates above i000 s -I Benz et al.
(1983) found evidence for these two phases in seven cases from visual inspection
of the count rate time histories integrated from 26 keV to 461 keV. Two of these
events (on 1980 March 29, cf. fig. 1 of Benz et al. ) belong to the sample
discussed in this contribution and confirm the identification of pre-flash and
flash phase with the features found in our analysis. The presence of these two
phases seems to be a much more frequent phenomenon in the great hard x-ray bursts
discussed here than in the smaller events of Benz et al. (1983), probably
because the pre-flash emission is too weak to be detected in events with low peak
count rate.
In the two events where we dispose of precise onset times measured with
the gamma-ray spectrometer, the rise of emission in the 4 to 8 MeV band occurs
close to the onset of the flash phase of energetic electrons. Forrest and Chupp
( 1983 ), comparing starting times of the 40 keV to 8 MeV emission in the 1980 June
7 and June 21 events, concluded on their simultaneous rise in all channels of the
gama-ray spectrometer. This is, however, only valid for the flash phase, in
agreement with our results. The pre-flash emission was not detected in their
study. For the 1982 June 3 flare detailed observations of low-energy photons are
also available. Centimetric radio observations at Bern University show a
spectrum peaking at low frequencies during the pre-flash phase. With the rise of
the flash phase the spectral maximum starts a rapid drift to high frequencies.
In the metre waveband the pre-flash hard x-rays are accompanied by fast-drift
bursts. Near the onset of the flash phase the starting frequency increases
rapidly - in accordance with the observations reported by Benz et al. (1983) -
and a continuum emission extending over all the band 150 MHz to 470 MHz covered
by the Nanqay radio spectrograph is established some seconds after the start of
the flash phase in hard x-rays. Raoult et al. (1985) have shown that this is a
typical evolution of radio emission associated with impulsive hard x-ray
bursts.
These independent observations suggest that the pre-flash and flash phase
as defined by the method of section 2 are physically significant phenomena
occurring during the early stage of a flare, despite the problems introduced by
the detector properties into an analysis based on low count rates. These
problems affect the question of time delays between different photon energies.
The reality and physical meaning of these - evolution of a single mechanism of
acceleration/injectlon or action of different processes - can only be
investigated through a thorough simulation of the detector response to model
spectra.
Acknowledgements
The authors are indepted to Brian Dennis and the HXRBS team for providing
the data used for this communication and to Sharad Kane for having made ISEE 3
observations available. K.-L.K. acknowledges helpful discussions with B. Dennis
and D. Forrest on the subject of this paper. This research was supportedbyCNES
and CNRS. One of the authors (K.-L.K.) acknowledges financial support byanESA
fellowship.
83
References
Bai, T., and Dennis, B.: 1985, Astrophys. J. 292, 699
Bai, T., and Ramaty, R.: 1979, Astrophys. J. 227, 1072
Benz, A.O., Barrow, C.H., Dennis, B.R., Pick, M., Raoult, A., and Slmnett, G.:
1983, Solar Phys. 83, 267
Dennis, B.R. et al.: 1982, NASA TM 84998
Forrest, D.J., and Chupp, E.L.: 1983, Nature 30_5_, 291
Orwig, L.E., Frost, K.J., and Dennis, B.R. : 1980, Solar Phys. 65, 25
Raoult, A., Pick, M., Dennis, B.R., and Kane, S.R._ 1985, Astrophys. J. 29__9,
1027
Vilmer, N., Kane, S.R., and Trotter, G. : 1982, Astron. Astrophys. i0_88, 306


A study of starting time in great hard X-ray flares

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