Non Cosmic Bursts detected with the PCA

3 December 1998

We are aware of 3 occasions in 1998 November where PCA light curves show odd behaviour. We believe that these "bursts" are likely to be related to solar activity. The purpose of this note is to present the evidence that the these bursts (a) originate outside the PCA (i.e. are not related to detector breakdown), and (b) that these bursts are are not astrophysical in origin. We thank Angel Butts and Rudy Wijnands for bringing these events to our attention.

A list, likely to be incomplete, of affected times (in MET seconds and calendar date) and obs-id is:

We discuss each event below

November 5, 1998

Figure 1 shows the rate measured in each of the 5 PCU during two orbits. The point marked A indicates the beginning of the anomalous event; the points marked B represent the earth limb, which is quite bright (this is unusual) at this time; the point marked C is where PCU 0 was commanded off. There is also an upward step function which corresponds to the crossing of the other earth limb. Of particular note here is that all 5 detectors detected a sharp and burst-like rise in count rate at the same time. This requires that they are responding to an external stimulus rather than breaking down. PCU 0, for reasons that are still not understood, shows a different, and larger response to this stimulus. The fact that the gross detector behaviour is different rules out a cosmic origin for this event. While discrepant behaviour in one PCU is usually a sign of breakdown, the fact that all detectors had an increase in activity at the same time, argues that this was due to an external source.

November 22, 1998

Figure 2 again shows the rate measured in each of the 5 PCU for a fraction of one orbit. In this event, each PCU responds similarly, indicating that the event is of some external origin.

Figure 3 shows the same interval. The lower trace shows the sum of the Good Event rate from the 5 detectors while the upper trace shows the remaining event rate. These rates are all from the Standard 1 data. The burst clearly appears in the remaining rate, with a peak of about 4000 count/sec correlated with a peak in the Good Event rate of ~3000 count/sec. (These rates correspond to about 800 count/sec/PCU and 600 count/sec/PCU). The coincidence rates are much too high to be associated with chance coincidences of cosmic events detected through the collimator ( Jahoda et al. 1998 ).

We collected a spectrum for this burst, using the data from the first 500 seconds of Figure 3 as a background. The spectrum is extraorinarily hard, being detected in all PCA channels.
Figure 4 shows a simple fit, suitable for characterizing the spectral shape of the observed "good events". The XSPEC fit is described as

  ---------------------------------------------------------------------------
  ---------------------------------------------------------------------------
  mo = ( powerlaw[1] )wabs[2] + powerlaw[3]
  Model Fit Model Component  Parameter  Unit     Value
  par   par comp
    1    1    1   powerlaw   PhoIndex            1.947     +/-   0.000
    2    2    1   powerlaw   norm                38.97     +/-   0.000
    3    3    2   wabs       nH       10^22      7464.     +/-   0.000
    4    4    3   powerlaw   PhoIndex           0.7237     +/-   0.000
    5    5    3   powerlaw   norm               1.0137E-02 +/-   0.000
  ---------------------------------------------------------------------------
  ---------------------------------------------------------------------------

We interpret the heavily absorbed power law as representative of a bright burst of gamma-rays, observed through the back or side of the detector. Since a large fraction of gamma rays (above the Xenon edge) will produce an escape photon which may be absorbed elsewhere in the detector, this is a plausible explanation for the similarly sized coincidence rate. The second, hard, power-law is an ad hoc description of many other processes, such as gamma-ray interactions in the detector walls which lead to an x-ray photon detected in the main volume or Compton scattering of higher energy photons. We do not intend, however, to over interpret the parameterization (made, after all, with a matrix that assumed that the X-rays came through the collimator - a situation we believe to be false here) but present it as it is similar to to the November 23 event, and potentially to other events of which we are not yet aware.

This burst apparently corresponds in time to a burst observed in BATSE's lowest channel and to a solar flare observed by the GEOS satellites (R. Wijnands, personnal communication).

November 23, 1998

Figure 5 shows Standard 1 data. Both the Good rates and the combind propane rates show a sharp rise as the line of sight crosses the limb of the earth (marked A); both the Good rate and the remaining rates show burst like behaviour (beginning near mark B). The spectrum of the burst behaviour (using data from the end of the light curve as background) is quite similar to the November 22 event with the following XSPEC characterization:

  ---------------------------------------------------------------------------
  ---------------------------------------------------------------------------
  mo = ( powerlaw[1] )wabs[2] + powerlaw[3]
  Model Fit Model Component  Parameter  Unit     Value
  par   par comp
    1    1    1   powerlaw   PhoIndex            3.276     +/-  0.1126
    2    2    1   powerlaw   norm                1219.     +/-   559.3
    3    3    2   wabs       nH       10^22      7528.     +/-   228.4
    4    4    3   powerlaw   PhoIndex            1.004     +/-  0.9942E-01
    5    5    3   powerlaw   norm               1.8072E-03 +/-  0.6760E-03
  ---------------------------------------------------------------------------
  ---------------------------------------------------------------------------

The spectrum of the limb crossing is likely thermal and has a narrow Iron line with an equivalent width greater than 2 keV. We did not look at this carefully, and the background spectrum used for the burst is unsuitable here as it contains a significant source contribution not present in the Earth looking data.

Conclusions

None of these bursts appear to have a cosmic origin; the bright limb of the earth, the report of a solar flare, and the indication that these bursts are observed through the detector body suggest a solar origin. We emphasize that our list of events is likely to be incomplete, and that these events could become more common as solar activity increases in the next year or two.

Notes prepared by Keith Jahoda (keith.jahoda_at_gsfc.nasa.gov) and Tod Strohmayer (tod.strohmayer_at_gsfc.nasa.gov)
December 3, 1998