A new type of transient high-energy source in the direction of the Galactic Centre

Abstract

* Universities Space Research Association, 4950 Corporate Drive, Suite100, Huntsville, Alabama 35806, USA¢NASA Marshall Space Flight Center, ES-84, Huntsville, Alabama 35812,USA$Astronomical Institute Anton Pannekoek and Center for High-EnergyAstrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands§Physics Department, University of Alabama in Huntsville, Huntsville,Alabama 35899, USAIIDepartment of Physics, Center for Space Research, MIT 37-627,Cambridge, Massachusetts 02139, USASotJaCES of high-energy (>20keV) bursts fall into two distincttypes: the non-repeating /-ray bursters I, several thousand ofwhich have been detected but whose origin remains unknown,and the soft y-ray repeaters (SGRs), of which there are onlythree 2. The SGRs are known to be associated with supernovaremnants, suggesting that the burst events most probably origi-nate from young neutron stars 3. Here we report the detection of athird type of transient high-energy source. On 2 December 1995,we observed the onset of a sequence of hard X-ray bursts from adirection close to that of the Galactic Centre 4. The intervalbetween bursts was initially several minutes, but after two days,the burst rate had dropped to about one per hour and has beenlargely unchanged since then. More than 1,000 bursts have nowbeen detected, with remarkably similar light curves and inten-sities; this behaviour is unprecendented among transient X-rayand ,/-ray sources. We suggest that the origin of these burstsmight be related to the spasmodic accretion of material onto aneutron star.Visual inspection of the X-ray intensity records of the eightLarge Area Detectors (LADs) of BATSE 5 showed that bursts ofhard X-rays were emitted in rapid succession from a source in thegeneral direction of the Galactic Centre beginning 1995 Decem-ber 2 12:17 UT (Fig. 1). The bursts came at intervals of about 5minutes; they lasted for typically 30 seconds and reached a peakintensity of _300countss _ in the 25-611keV energy band.Spectral analysis of the events showed significant emission to60 kcV in virtually all bursts; in _ 17% of them, emission wasdetected to about 75 keV. The outbursts were also registered bythe Spectroscopy Detectors _ which look into the same sky direc-tions as the LADs and are sensitive down to _ 10 keV. The burstsusually show a clear single peak; in many cases a tail of extendedemission can be seen. The burst which we could unambiguouslyidentify as the first in this sequence of rapid hard X-ray burstsoccurred on December 2 08:22 UT. Between December 2 17:1111and 20:00 UT, the burst intervals clustered around (172 + 15)s.After 20:00 UT the intervals became more erratic and longer.Figure 2 shows the behaviour of the burst interval evolution duringthe first 4 days of emission. Between 4 and 16 December the burstdetection rate remained remarkably constant at an average of 18events per day (about one-third of the bursts were missed becauseof data gaps).We determined the location of the individual events from therelative strengths of the signals detected in different BATSEdetectors, using an algorithm that includes a detailed descriptionof the energy/angular detector response and the contribution fromX-rays scattered off the Earth atmosphere% Initially, the burstswere seen in two detectors only; to improve the constraints on thelocation, the Compton Gamma Ray Observatory (CGRO) wasreoriented on December 8 (10:57 trr, after which the events wereregistered by four detectors. The BATSE locations combined with(1) Earth occultation constraints of the source, (2) a galacticlongitude constraint from observations with the Oriented Scintil-lation Spectroscopy Experiment (OSSE) on CGRO v, and (3)Interplanetary Network (IPN) triangulation results _', producedan _ 2# × 6' error box centred on right ascension 17 h 44 min 28 s,declination -28' 45.0' (equinox 20110.0). A 3-arcmin error boxrecently m obtained with the X-Ray Timing Explorer (XTE) liesentirely within the IPN location. This error box does not containknown bright X-ray sources_LAs part of a current project .2we made a comprehensive searchfor events in the 20-60 keV energy, range in archival BATSE data,covering the period 13 January 1993 to 24 December 1993 (345FIG. 1 The BATSE(25-60 keY) intensity recordsobtained during three consecutive orbits of CGROon 2 December 1995 between 13:50 and 17:30UT;the data are displayed with a time resolution of4s. (The time scale starts at 00:00 UTon 2December 1995).