ASCA Observations of GRO J1744-28

We report the ASCA results of the bursting X-ray pulsar \gro, which was observed in February 1996 and March 1997. The source flux in the 2--10 keV band was

2.0\times10^{-8}

erg/sec/cm

^2

in 1996 and

5.0\times10^{-9}

erg/sec/cm

^2

in 1997. We detected 12 and 17 Type II bursts during the two observations with mean bursting intervals of about 27 min and 37 min. Each burst is followed by an intensity dip with the depleted flux depending on the burst fluence. The energy spectra are approximated by an absorbed power law with additional structure around 6--7 keV\@. Constant absorption column,

(5-6)\times10^{22}

cm

^{-2}

, independent of the observation dates and emission phases (persistent, burst and dip) is interpreted as an interstellar absorption. The source may be actually located near the Galactic center, at a distance of 8.5 kpc. The structure in the energy spectrum at 6--7 keV is most probably due to iron and maybe reproduced by a disk line model with additional broadening mechanism.

Note:

28 pages 11 figures. ApJ accepted

References(6)

Figures(0)

[1]

We detected 10 and 17 giant Type II bursts during / and II, with mean burst intervals of about 27 min and 37 min, respectively. The burst fluence is found to have good correlation with the total flux deficiency in the following dip. This correlation is interpreted that the average mass accretion rate is constant, but accretion instability makes the bursts and dips. The burst intervals do not change very much in spite of a factor of 4 decrease in X-ray luminosity between observation I and II. Thus the burst fluence also decreases in accordance with the persistent flux

Observation

[2]

The absorption column is found to be constant at NH = (5 - 6) × 1022 H cm-2 regardless of the observation date and the source status (persistent, burst and dip). This strongly indicates that the column density corresponds to the interstellar absorption, and the source is actually located near the Galactic center, at a distance of 8.5 kpc

[3]

The persistent X-ray luminosity in the first and the second observations are (1.6 - 2.0) × 1038 erg s-1 and

4.1 - 4.2) × 1037 erg s-1, respectively. The burst peak fluxes at the first observation exceed the Eddington limit of a neutron star by a factor of 10, if the radiation is spherically symmetric. 4. The energy spectra of GRO J1744-28 are represented approximately by an absorbed power law with a broad line at ∼6.7 keV. The shape of the spectra are almost - 20 - the same for the different observation dates and types (burst, dip and persistent emission). However, the spectra is slightly harder in observation II (Γ ∼ 1.0) than in observation I (Γ ∼ 1.2), and is harder during bursts than in persistent phase in each set of observations

[5]

The energy spectrum of the pulsed component is harder than that of the non-pulsed component. We consider that the different spectral hardness indicates different emission regions, e.g. pulsed component from the polar caps and non-pulsed component from the whole neutron star surface

[6]

The presence of the iron feature is clearly seen in all energy spectra and is also indicated by the decrease of pulse fraction at 6-7 keV. The feature is well reproduced by a disk line model. However, some line broadening mechanism is needed to make the disk line parameters consistent to the system parameters of GRO J1744-28. We express our thanks to all the ASCA team members for many efforts of the fabrication of the satellite, launching, daily operation, soft ware developments and calibrations. M.N. and Y.M. are financially supported by the Japan society for the promotion of science. - 21 -