GRO J1744-28
Abstract
S. Sazonov and R. Sunyaev, Space Research Institute, Moscow, on behalf of the Granat/WATCH Team, report: "On Jan. 14-15 the Galactic Center region was observed by the Granat/WATCH instrument. At the edge of the field of view, a strong flux, 4.4 +/- 0.3 Crab (8-20 keV), was detected from the direction of the new transient x-ray burster and pulsar GRO J1744-28, discovered by BATSE. The WATCH data show that the source's spectrum is much flatter in the standard x-ray band than in the BATSE sensitivity range. It is similar to typical spectra of accreting x-ray pulsars. At the distance of the Galactic Center, the luminosity of GRO J1744-28 in the 8-20 keV band is approximately 4 x 10**38 erg/s, a value that exceeds the Eddington luminosity for a neutron star. However, super-Eddington luminosity was observed earlier in SMC X-1 and several other x-ray pulsars. GRANAT/WATCH will continue to observe GRO J1744-28 in scanning mode, while observational conditions are improving daily." J. Swank, NASA Goddard Space Flight Center, reports, for the X-Ray Timing Explorer instrument and operations team: "XTE observed the new galactic transient GRO J1744-28 (IAUC 6272, 6275, 6276, 6284, 6285, 6286, 6290) for 11 hours on Jan. 18-19. A source pulsing at 2.1 Hz, as well as nine bursts 2-8 times brighter than the persistent emission, was observed from a position within the error box reported by Hurley et al. (IAUC 6286). The XTE best-fit position, from scans across the source, is R.A. = 17h44m.5 +- 0m.2, Decl. = -28d45' +/- 2' (equinox 2000.0), where the errors are systematic. A sinusoidal pulse shape was observed, and the burst flux was sometimes strongly modulated. Following each burst, the flux is depressed and takes a few minutes to recover. The spectra of both the pulsed and the burst flux are characteristic of bright accreting pulsars; they are consistent with a power law with number index about 1.2 and a rollover above 13 keV with an e-folding energy of 14 keV. The absorbing column density is less than 10**22 cm**-2. The low column density implies that the source is probably closer than the galactic center and that at least the non-burst luminosity probably does not exceed the Eddington limit. The non-burst flux was about 2 times 10**-7 erg cm**-2 s**-1 (2-100 keV). These observations were taken during the period of initial instrument verification, so the calibrations are preliminary. See http://heasarc.gsfc.nasa.gov/docs/xte/SOF/TOONews.html for further information about results, data and future observations."
- Publication:
-
International Astronomical Union Circular
- Pub Date:
- January 1996
- Bibcode:
- 1996IAUC.6291....1S