G. L. Israel, Physics Department, University of Rome
S. Mereghetti, Istituto di Fisica Cosmica del C.N.R., Milan; and L. Stella, Brera Astronomical Observatory, Milan, communicate:
"Using
the EXOSAT archival data, we discovered a periodic pulsation from
the direction of the two x-ray sources 4U 0142+614 and RX J0146.9+
6121. The coherent pulsation, at a heliocentric period of 8.68723
+/- 0.00004 s, has been discovered in the Medium Energy (ME)
experiment light curves from a 13-hr observation on 1984 Aug. 27. The
periodicity is visible (with a 8-sigma significance) only for energies
in the range 1-4 keV (i.e., where most of the x-ray flux is
emitted from the optically unidentified source 4U 0142+614). The
peak-to-peak amplitude of the modulation is about 7 percent. During
the same observation, oscillations at 1455 s in an additional
spectral component dominating above 3 keV had been previously reported
(White et al. 1987, MNRAS 226, 645). The ME observations of
1985 November-December show a periodicity at 8.666 +/- 0.001 s in
the range 1-50 keV (Ar + Xe detectors), though at a lower statistical
significance. In the 1985 observations, the additional spectral
component, probably due to the Be system RX J0146.9+6121
(which lies at 25' from 4U 0142+614; Mereghetti, Stella, and De
Nile 1993, A.Ap. 278, L23) was absent. Thus the pulsations likely
originate in the latter source. This is also suggested by ROSAT
HRI data obtained on 1991 Feb. 2, in which a periodicity at P =
8.60 s is detected from 4U 0142+614 with a 2-sigma statistical
significance. Further imaging x-ray data are required to confirm that
the pulsations are not due to another source within the 90' field-of-view
of the EXOSAT ME. If the 8.7-s periodicity is from 4U
0142+614 and the observed spin-up is confirmed, the lack of an optical
counterpart brighter than V about 24 (Steinle et al. 1987, Ap.
Space Sci. 131, 687) suggests that this source is a low-mass x-ray
binary with an unusually soft spectrum at a distance of several
kiloparsecs. The detection of periodic pulsations weakens the
phenomenological criterion that an ultra-soft spectral component is a
signature of accreting black-hole candidates (White and Marshall
1984, Ap.J. 281, 354)."
The discovery of 8.7 second pulsations from the ultrasoft X-ray source 4U 0142+614 1994, ApJ, 433, L25
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