39 research outputs found
The Orbital Period of the Be/Neutron Star Binary RX J0812.4-3114
We present the results of Rossi X-ray Timing Explorer observations of the Be
star X-ray binary system RX J0812.4-3114. A light curve obtained with the RXTE
All-Sky Monitor shows that the source is currently in an active state with
outbursts occurring at approximately 80 day intervals. The source underwent a
transition from an inactive state to this regular outburst state early in 1998.
An observation of RX J0812.4-3114 was obtained with the RXTE Proportional
Counter Array close to the time of a predicted maximum in March 1999 and strong
pulsations were detected at a period of 31.88 seconds. This confirms the result
of an earlier PCA observation by Reig & Roche which was serendipitously also
obtained near the predicted maximum flux of the 80 day period and also near the
start of the current active state. We interpret the periodicity in the ASM
light curve as indicating the orbital period of RX J0812.4-3114 with outbursts
occurring around periastron passage
The Orbit and Position of the X-ray Pulsar XTE J1855-026 - an Eclipsing Supergiant System
A pulse timing orbit has been obtained for the X-ray binary XTE J1855-026
using observations made with the Proportional Counter Array on board the Rossi
X-ray Timing Explorer. The mass function obtained of ~16Mo together with the
detection of an extended near-total eclipse confirm that the primary star is a
supergiant as predicted. The orbital eccentricity is found to be very low with
a best fit value of 0.04 +/- 0.02. The orbital period is also refined to be
6.0724 +/- 0.0009 days using an improved and extended light curve obtained with
RXTE's All Sky Monitor. Observations with the ASCA satellite provide an
improved source location of R.A. = 18h 55m 31.3s}, decl. = -02o 36' 24.0"
(2000) with an estimated systematic uncertainty of less than 12". A
serendipitous new source, AX J1855.4-0232, was also discovered during the ASCA
observations.Comment: Accepted for publication in the Astrophysical Journa
Rossi X-ray Timing Explorer Observations of the X-ray Pulsar EXO 1722-363 - a Candidate Eclipsing Supergiant System
Observations made of the X-ray pulsar EXO 1722-363 using the Proportional
Counter Array and All Sky Monitor on board the Rossi X-ray Timing Explorer
reveal the orbital period of this system to be 9.741 +/- 0.004 d from periodic
changes in the source flux. The detection of eclipses, together with the values
of the pulse and orbital periods, suggest that this source consists of a
neutron star accreting from the stellar wind of an early spectral type
supergiant companion. Pulse timing measurements were also obtained but do not
strongly constrain the system parameters. The X-ray spectra can be well fitted
with a model consisting of a power law with a high energy cutoff and, for some
spectra, a blackbody component with a temperature of approximately 0.85 keV.Comment: Accepted for publication in The Astrophysical Journal. 27 pages
including 10 figure
RXTE Observations of the Be star X-ray Transient X0726-260 (4U0728-25) - Orbital and Pulse Periods
Rossi X-ray Timing Explorer (RXTE) All Sky Monitor observations of the
transient Be star X-ray source X0726-260 suggest a 34.5 day period. This is
apparently confirmed by a serendipitous RXTE Proportional Counter Array (PCA)
slew detection of the source on 1997 May 5, near the time of a predicted flux
maximum. A subsequent 5000 second pointed observation of X0726-260 with the
RXTE PCA detector was carried out on 1997 June 7, when X0726-260 was predicted
to be bright again, and this revealed pulsations at a period of 103.2 seconds.
If the 34.5 day period is orbital, then the pulse period is surprisingly long
compared to that predicted by the correlation between orbital period and spin
period observed for other Be/neutron star systems. A possible similarity with
GROJ2058+42 is briefly discussed.Comment: 7 pages LateX, 7 figures. To be published in Astrophysical Journal
Letter
4U2206+54 - an Unusual High Mass X-ray Binary with a 9.6 Day Orbital Period but No Strong Pulsations
Rossi X-ray Timing Explorer All-Sky Monitor observations of the X-ray source
4U2206+54, previously proposed to be a Be star system, show the X-ray flux to
be modulated with a period of approximately 9.6 days. If the modulation is due
to orbital variability then this would be one of the shortest orbital periods
known for a Be star X-ray source. However, the X-ray luminosity is relatively
modest whereas a high luminosity would be predicted if the system contains a
neutron star accreting from the denser inner regions of a Be star envelope.
Although a 392s pulse period was previously reported from EXOSAT observations,
a reexamination of the EXOSAT light curves does not show this or any other
periodicity. An analysis of archival RXTE Proportional Counter Array
observations also fails to show any X-ray pulsations. We consider possible
models that may explain the properties of this source including a neutron star
with accretion halted at the magnetosphere and an accreting white dwarf.Comment: Accepted for publication in the Astrophysical Journa