75 research outputs found
Confirmation of the 62 Day X-Ray Periodicity from M82
Using 400 days of new X-ray monitoring of M82, we confirm the 62 day
periodicity previously reported. In the full data set spanning 1124 days, we
find a period of 62.0 +/- 0.3 days and a coherence, Q = 22.3, that is
consistent with a strictly periodic signal. We estimate that the probability of
chance occurrence of our observed signal is 6E-7. The light curve folded at
this period is roughly sinusoidal and has a peak to peak amplitude of (0.99 +/-
0.10) x 10^-11 erg cm^-2 s^-1. Confirmation of the periodicity strengthens our
previous suggestion that the 62 day modulation is due to orbital motion within
an X-ray binary.Comment: 4 pages, to appear in Ap
Gamma-Ray Observations of GRO J1655-40
The bright transient X-ray source GRO J1655-40 = XN Sco 1994 was observed by the OSSE instrument on the Compton Gamma Ray Observatory (GRO). Preliminary results are reported here. The initial outburst from GRO J1655-40 was detected by BATSE on 27 Jul 1994. OSSE observations were made in five separate viewing periods starting between 4 Aug 1994 and 4 Apr 1995. The first, third, and fifth observations are near the peak luminosity. In the second observation, the source flux had dropped by several orders of magnitude and we can only set an upper limit. The fourth observation is a weak detection after the period of maximum outburst. In contrast with other X-ray novae such as GRO J0422+32, the spectrum determined by OSSE is consistent with a simple power law over the full range of detection, about 50 - 600 keV. The photon spectral index is in the range of -2.5 to 2.8 in all of the observations. We set an upper limit on fractional rms variation \u3c5% in the frequency range 0.01 – 60 Hz. No significant narrow or broad line features are observed at any energy
The Low Quiescent X-Ray Luminosity of the Neutron Star Transient XTE J2123-058
We report on the first X-ray observations of the neutron star soft X-ray
transient (SXT) XTE J2123-058 in quiescence, made by Chandra and BeppoSAX, as
well as contemporaneous optical observations. In 2002, the Chandra spectrum of
XTE J2123-058 is consistent with a power-law model, or the combination of a
blackbody plus a power-law, but it is not well-described by a pure blackbody.
Using the interstellar column density, the power-law fit gives photon index of
3.1 (+0.7,-0.6) and indicates a 0.3-8 keV unabsorbed luminosity of 9(+4,-3)E31
(d/8.5 kpc)^2 ergs/s (90% confidence errors). Fits with models consisting of
thermal plus power-law components indicate that the upper limit on the
temperature of a 1.4 solar mass, 10 km radius neutron star with a hydrogen
atmosphere is kT_eff < 66 eV, and the upper limit on the bolometric luminosity
is L_infinity < 1.4E32 ergs/s, assuming d = 8.5 kpc. Of the neutron star SXTs
that exhibit short (< 1 year) outbursts, including Aql X-1, 4U 1608-522, Cen
X-4, and SAX J1810.8-2609, the lowest temperatures and luminosities are found
for XTE J2123-058 and SAX J1810.8-2609. From the BeppoSAX observation of XTE
J2123-058 in 2000, we obtained an upper limit on the 1-10 keV unabsorbed
luminosity of 9E32 ergs/s. Although this upper limit allows that the X-ray
luminosity may have decreased between 2000 and 2002, that possibility is not
supported by our contemporaneous R-band observations, which indicate that the
optical flux increased significantly. Motivated by the theory of deep crustal
heating by Brown and co-workers, we characterize the outburst histories of the
five SXTs. The low quiescent luminosity for XTE J2123-058 is consistent with
the theory of deep crustal heating without requiring enhanced neutron star
cooling if the outburst recurrence time is >~ 70 years.Comment: 8 pages, accepted by Ap
Strong-field general relativity and quasi-periodic oscillations in x-ray binaries
Quasi-periodic oscillations (QPOs) at frequencies near 1000 Hz were recently
discovered in several x-ray binaries containing neutron stars. Two sources show
no correlation between QPO frequency and source count rate (Berger et al. 1996,
Zhang et al. 1996). We suggest that the QPO frequency is determined by the
Keplerian orbital frequency near the marginally stable orbit predicted by
general relativity in strong gravitational fields (Muchotrzeb-Czerny 1986,
Paczynski 1987, Kluzniak et al. 1990). The QPO frequencies observed from 4U
1636-536 imply that the mass of the neutron star is 2.02 +/- 0.12 solar masses.
Interpretation of the 4.1 keV absorption line observed from 4U 1636-536 (Waki
et al. 1984) as due to Fe XXV ions then implies a neutron star radius of 9.6
+/-0.6 km.Comment: 4 pages, uses aas2pp4.sty, submitted to ApJ
The Low Quiescent X-Ray Luminosity of the Transient X-Ray Burster EXO 1747-214
We report on X-ray and optical observations of the X-ray burster EXO
1747-214. This source is an X-ray transient, and its only known outburst was
observed in 1984-1985 by the EXOSAT satellite. We re-analyzed the EXOSAT data
to derive the source position, column density, and a distance upper limit using
its peak X-ray burst flux. We observed the EXO 1747-214 field in 2003 July with
the Chandra X-ray Observatory to search for the quiescent counterpart. We found
one possible candidate just outside the EXOSAT error circle, but we cannot rule
out the possibility that the source is unrelated to EXO 1747-214. Our
conclusion is that the upper limit on the unabsorbed 0.3-8 keV luminosity is L
< 7E31 erg/s, making EXO 1747-214 one of the faintest neutron star transients
in quiescence. We compare this luminosity upper limit to the quiescent
luminosities of 19 neutron star and 14 black hole systems and discuss the
results in the context of the differences between neutron stars and black
holes. Based on the theory of deep crustal heating by Brown and coworkers, the
luminosity implies an outburst recurrence time of >1300 yr unless some form of
enhanced cooling occurs within the neutron star. The position of the possible
X-ray counterpart is consistent with three blended optical/IR sources with
R-magnitudes between 19.4 and 19.8 and J-magnitudes between 17.2 and 17.6. One
of these sources could be the quiescent optical/IR counterpart of EXO 1747-214.Comment: 7 pages, accepted by the Astrophysical Journa
Discovery of a Hard X-Ray Source, SAX J0635+0533, in the Error Box of the Gamma-Ray Source 2EG 0635+0521
We have discovered an x-ray source, SAX J0635+0533, with a hard spectrum
within the error box of the GeV gamma-ray source in Monoceros, 2EG J0635+0521.
The unabsorbed x-ray flux is 1.2*10^-11 erg cm^-2 s^-1 in the 2-10 keV band.
The x-ray spectrum is consistent with a simple powerlaw model with absorption.
The photon index is 1.50 +/- 0.08 and we detect emission out to 40 keV. Optical
observations identify a counterpart with a V-magnitude of 12.8. The counterpart
has broad emission lines and the colors of an early B type star. If the
identification of the x-ray/optical source with the gamma-ray source is
correct, then the source would be a gamma-ray emitting x-ray binary.Comment: Accepted to the Astrophysical Journal, 8 page
A burst chasing x-ray polarimeter
Gamma-ray bursts are one of the most powerful explosions in the universe and have been detected out to distances of almost 13 billion light years. The exact origin of these energetic explosions is still unknown but the resulting huge release of energy is thought to create a highly relativistic jet of material and a power-law distribution of electrons. There are several theories describing the origin of the prompt GRB emission that currently cannot be distinguished. Measurements of the linear polarization would provide unique and important constraints on the mechanisms thought to drive these powerful explosions. We present the design of a sensitive, and extremely versatile gamma-ray burst polarimeter. The instrument is a photoelectric polarimeter based on a time-projection chamber. The photoelectric time-projection technique combines high sensitivity with broad band-pass and is potentially the most powerful method between 2 and 100 keV where the photoelectric effect is the dominant interaction process. We present measurements of polarized and unpolarized X-rays obtained with a prototype detector and describe the two mission concepts; the Gamma-Ray Burst Polarimeter (GRBP) for the U.S. Naval Academy satellite MidSTAR-2, and the Low Energy Polarimeter (LEP) onboard POET, a broadband polarimetry concept for a small explorer mission
Uncovering the Nature of the X-Ray Transient 4U 1730–22: Discovery of X-Ray Emission from a Neutron Star in Quiescence with Chandra
The X-ray transient 4U 1730-22 has not been detected in outburst since 1972, when a single ~200 day outburst was detected by the Uhuru satellite. This neutron star or black hole X-ray binary is presumably in quiescence now, and here we report on X-ray and optical observations of the 4U 1730-22 field designed to identify the system's quiescent counterpart. Using the Chandra X-ray Observatory, we have found a very likely counterpart. The candidate counterpart, CXOU J173357.5-220156, is close to the center of the Uhuru error region and has a thermal spectrum. The 0.3-8 keV spectrum is well described by a neutron star atmosphere model with an effective temperature of 131 ± 21 eV. For a neutron star with a 10 km radius, the implied source distance is 10^(+12)_(-4) kpc, and the X-ray luminosity is 1.9 × 10^(33) (d/10 kpc)^2 ergs s^(-1). Accretion from a companion star is likely required to maintain the temperature of this neutron star, which would imply that it is an X-ray binary, and therefore almost certainly the 4U 1730-22 counterpart. We do not detect an optical source at the position of the Chandra source down to R > 22.1, and this is consistent with the system being a low-mass X-ray binary at a distance greater than a few kpc. If our identification is correct, 4U 1730-22 is one of the 5 most luminous of the 20 neutron star transients that have quiescent X-ray luminosity measurements. We discuss the results in the context of neutron star cooling and the comparison between neutron stars and black holes in quiescence
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