Recent Outbursts from the Transient X-Ray Pulsar Cepheus X-4 (GS 2138+56)
Abstract
We report on X-ray observations of the 66 s period transient X-ray pulsar Cep X-4 (GS 2138+56) with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory and with the Rossi X-Ray Timing Explorer (RXTE). Two outbursts from Cep X-4 were observed with BATSE in 1993 June-July and in 1997 July. Pulse frequencies of ν=15.0941+/-0.0002 mHz on 1993 June 25 (MJD 49,163) and ν=15.0882+/-0.0002 mHz on 1997 July 12 (MJD 50,641) were each measured from 2-day spans of BATSE data near each outburst's peak. Cep X-4 showed an average spin down rate of ν˙=(-4.14+/-0.08)×10-14 Hz s-1 between the 1993 and 1997 outbursts. After BATSE could no longer detect Cep X-4, public observations were performed on 1997 July 18 and July 25 with the Proportional Counter Array (PCA) on RXTE. A pulse frequency of ν=15.088+/-0.004 mHz was measured from observations on 1997 July 18 (MJD 50,647). Significant aperiodic noise, with an rms variance of ~18% in the frequency range 0.01-1.0 Hz, was observed on both days. Energy- and intensity-dependent pulse shape variations were also seen in these data. Recently published optical observations associate Cep X-4 with a Be companion star. If all four outbursts observed from Cep X-4 are assumed to occur at the same orbital phase, we find that the orbital period is between 23 and 147.3 days.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1999
- DOI:
- 10.1086/306660
- arXiv:
- arXiv:astro-ph/9808301
- Bibcode:
- 1999ApJ...511..367W
- Keywords:
-
- STARS: BINARIES: CLOSE;
- STARS: NEUTRON;
- STARS: PULSARS: INDIVIDUAL: ALPHANUMERIC: GS 2138+56;
- STARS: PULSARS: INDIVIDUAL: NAME: CEPHEUS X-4;
- X-RAYS: STARS;
- Stars: Binaries: Close;
- Stars: Neutron;
- Stars: Pulsars: Individual: Alphanumeric: GS 2138+56;
- Stars: Pulsars: Individual: Name: Cepheus X-4;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- 19 pages (LaTeX) including 9 figures. Accepted for publication in the Astrophysical Journal