ACTA ASTRONOMICA
Vol. 48 (1998) pp. 819-824
Detection of SGR 1900+14 Pulse on August 27,1998 by RF-15I Soft/Hard Solar X-ray Photometer
by
J. Sylwester1, Z. Kordylewski1, M. Siarkowski1, S. Gburek1,
1 Space Research Center Polish Academy of Sciences, 51-622 Wroc³aw, Kopernika 11, Poland
2 Astronomical Institute Czech Academy of Sciences, 25165, Ondrcjov, Czech Republic
3 Space Research Institute, Russian Academy of Sciences, 117810, Moscow, Profsoyuznaya 38, Russia
Received November 18, 1998
ABSTRACT
We report detection of the supposed magnetar gamma pulse by the common Czech-Polish solar Soft/Hard X-ray Photometer aboard the INTERBALL-Tail satellite. Timing information presented may allow for better triangulation of the position of the gamma source.
Key words: Gamma rays: bursts - X-rays: bursts
1. Introduction
Unusually intense gamma-ray pulse has been detected by instruments placed aboard at least 7 spacecrafts on August 27, 1998 around 10:22:16 UT. (Cline et al 1998, Hurley et al 1998, Feroci et al 1998). Preliminary analysis of the relative arrival times of this burst at RXTE, Global Geo-Science Wind, Near Earth Asteroid Rendezvous, and Ulysses were used to determine its position in the Aquila constellation. The flash came from the Soft Gamma Repeater SGR 1900+14, which lies about 6 kpc away. The signal was so strong that it saturated detectors on WIND and RXTE and triggered the safety mode automatic shut-off of the NEAR gamma ray instrument. Persistent periodicity of 5.15 ± 0.02 seconds has been reported (Cline et al. 1998) during decay phase of the flash. In this note we present observations of corresponding X-ray burst as recorded by solar Soft/Hard X-ray Photometer RE-151 placed aboard the INTERBALL-Tail spacecraft operating since August 1995. More details on the INTERBALL Project can be found at http://www.iki.rssi.ru/interball.html. The RF-15I instrument has been developed as a common project between the Astronomical Institute, Czech Academy of Sciences, Space Research Center, Polish Academy of Sciences and IKI, Russia.
2. The Instrument
The RF-15I solar X-ray photometer is equipped with two detector systems:
• Proportional argon gas-filled (350 Torr) detector of aperture of 4.5 mm2 equipped with the Be filter of
150 mm thickness. The largest cross-sectional area of the detector for gamma radiation is » 10.5 cm2.
• The Na(I) scintillation detector of 15.2 cm2 aperture and crystal thickness
8 mm.
Detailed description of the instrument is in preparation (Sylwester et al. in preparation). The proportional detector nominally registers the soft X-ray solar flux in the three energy channels 2-3-5-8 keV every 2 seconds. By the time of the reported observations the appropriate boundaries of energy channels moved to » 0.7-1.0-1.7-2.7 keV due to aging of the calibration Fe55 radioactive source.
The scintillation detector registers every 0.125 seconds the hard X-ray flux in five energy bands 10-15-30-60-120-240 keV provided that appropriate rate thresholds are exceeded in each of the upper four energy ranges. At the time of reported observations all these channels have been active due to substantial particle background. In the lowest energy range 10-15 keV, the data are collected every 2 seconds simultaneously with the softer proportional detector
channels.
The normal to the detectors' windows is pointed to the Sun within ±10° thanks to spin-stabilisation of the spacecraft. One satellite revolution takes about 118 seconds.
3. The Observations
At the time of the reported measurements the INTERBALL-tail satellite was on its elongated elliptic orbit (183 000 x 22 000 km) at the distance of about 93 400 km from the Earth center as it is schematically sketched in Fig. 1. Relevant directions are also indicated. The altitude data indicate that the satellite was within the Earth magnetosphere at the time of burst measurements.
Based on the directional scheme and satellite attitude data we conclude that before entering the detectors the investigated radiation pulse had to cross substantial amount of satellite shielding and structure, estimated as a few cm aluminum, and in addition, about 1 mm of steel cover for the proportional detector.
Approximately 60 hours before the reported pulse a strong Xl.O long duration solar flare has been observed which caused substantial increase of the SEP particle
Fig. 1. Schematic diagram showing the position of INTERBALL spacecraft at the time of gamma pulse. The indicated spacecraft distance is actual i.e., not projected on the equatorial plane.
flux propagating through the Earth vicinity. GOES satellite measured high levels of energetic particles most probably related with this flare. Also the RF-151 detector background rates were high (up to 2000 cts/s, more than hundred times the usual level) at the time of the SGR1900+14 burst detection. At the time of the burst, a soft, very small solar flare in progress can also be seen on GOES X-ray records.
In Fig. 2 we present plots of the rates recorded in all eight energy channels of RF-151. In Fig. 3, the rates corresponding to the four highest energy channels are plotted with the time scale expanded around the main peak. The particle background and solar signal have been subtracted in this case. Background levels have been interpolated based on the appropriate signals recorded immediately before and after the burst.
The most direct information which can be derived from the reported observations is burst timing. The system of time stamping of the telemetry records for RF-151 allows for absolute time alignment of the measurements with the accuracy of 0.03125 seconds, provided there is no delay due to data package queuing for delivery to the telemetry system. The telemetry may sometimes be bottled up due to large data transfers from the other instruments on board of the satellite. This effect caused that the overall uncertainty of the data time stamp was £0.1875 seconds at the time of the burst. We determined that the front of the burst arrived to the spacecraft between: 10h22m15.s6875 and 10h22m15.s8125 UT, at the same time (to within the 0.125 seconds time resolution) in all harder energy
channels.
Fig. 2. Time behavior of the signal in all RF-151 energy channels covering approximately 20 minutes before and after the SGR 1900+14 burst. The burst is distinguished in all energy channels except for the 10-15keVband. It is seen most clearly in the higher energy bands. Significant pre-and post-burst rates are related to the enhanced particle background in the vicinity of the INTERBALL spacecraft.
Fig. 3. Background subtracted signal for the four highest energy channels of RF-15I. The width of time bin is 125ms. The upper uncertainty levels (dashed) are drawn at the 5a for each energy channel respectively. Uncertainties are derived from pre- and post-burst background level fluctuations.
In softer channels the arrival time spans the period between 10h22m15.s0625 and 10h22m17.s0625 UT due to lower sampling cadence (each 2 seconds).
Fig. 4. Energy spectra for two instants when statistically significant rales have been recorded. Overall softening of the source spectrum is observed. One should have in mind that the burst radiation had to cross the detectors and the payload plate backwards, working against a few cm of the metal structure.
In Fig. 4 we plot "energy spectra" of the pulse for two time intervals when the signal was statistically significant. It is noticeable that the spectrum softens with time during the burst. The shape of the spectra shown is not typical for solar flares which provide the argument against solar origin of the observed spike. Inspection of the INTERBALL data obtained in situ from available magnetospheric energetic particle detectors reveals no dramatic changes at the time of the registered burst which helps rejecting the particle origin of the spike in question.
We were not able to confirm the burst signal periodicity, since after initial prominent spike the burst signal became to weak relative to enhanced particle background
level.
Acknowledgements.
This contribution has been supported by Polish KEN grant 2.P03C.006.13.
REFERENCES
Cline, T.L., Mazets, E.P., and Golenetskii, S.V. 1998, IAU Circ., #7002.
Hurley, K., Cline, T., Mazets, E., and Golenetskii, S. 1998, IAU Circ., #7004.
Feroci, M., Soffitta, P., Costa, E., Frontera, F., Tavani, M., and Cline, T., 1998, IAU Circ., #7005.