27 research outputs found
Common Patterns in the Evolution between the Luminous Neutron Star Low-Mass X-ray Binary Subclasses
The X-ray transient XTE J1701-462 was the first source observed to evolve
through all known subclasses of low-magnetic-field neutron star low-mass X-ray
binaries (NS-LMXBs), as a result of large changes in its mass accretion rate.
To investigate to what extent similar evolution is seen in other NS-LMXBs we
have performed a detailed study of the color-color and hardness-intensity
diagrams (CDs and HIDs) of Cyg X-2, Cir X-1, and GX 13+1 -- three luminous
X-ray binaries, containing weakly magnetized neutron stars, known to exhibit
strong secular changes in their CD/HID tracks. Using the full set of Rossi
X-ray Timing Explorer Proportional Counter Array data collected for the sources
over the 16 year duration of the mission, we show that Cyg X-2 and Cir X-1
display CD/HID evolution with close similarities to XTE J1701-462. Although GX
13+1 shows behavior that is in some ways unique, it also exhibits similarities
to XTE J1701-462, and we conclude that its overall CD/HID properties strongly
indicate that it should be classified as a Z source, rather than as an atoll
source. We conjecture that the secular evolution of Cyg X-2, Cir X-1, and GX
13+1 -- illustrated by sequences of CD/HID tracks we construct -- arises from
changes in the mass accretion rate. Our results strengthen previous suggestions
that within single sources Cyg-like Z source behavior takes place at higher
luminosities and mass accretion rates than Sco-like Z behavior, and lend
support to the notion that the mass accretion rate is the primary physical
parameter distinguishing the various NS-LMXB subclasses.Comment: 20 pages, 14 figures, 5 tables -- matches published version in Ap
On the geometric nature of low-frequency quasi-periodic oscillations in neutron-star low-mass X-ray binaries
We report on a detailed analysis of the so-called ~1 Hz quasi-periodic
oscillation (QPO) in the eclipsing and dipping neutron-star low-mass X-ray
binary EXO 0748-676. This type of QPO has previously been shown to have a
geometric origin. Our study focuses on the evolution of the QPO as the source
moves through the color-color diagram, in which it traces out an
atoll-source-like track. The QPO frequency increases from ~0.4 Hz in the hard
state to ~25 Hz as the source approaches the soft state. Combining power
spectra based on QPO frequency reveals additional features that strongly
resemble those seen in non-dipping/eclipsing atoll sources. We show that the
low-frequency QPOs in atoll sources and the ~1 Hz QPO in EXO 0748-676 follow
similar relations with respect to the noise components in their power spectra.
We conclude that the frequencies of both types of QPOs are likely set by (the
same) precession of a misaligned inner accretion disk. For high-inclination
systems, like EXO 0748-676, this results in modulations of the neutron-star
emission due to obscuration or scattering, while for lower-inclination systems
the modulations likely arise from relativistic Doppler boosting and
light-bending effects.Comment: Updated to published version (ApJ, 812, 80
Discovery of the near-infrared counterpart to the luminous neutron-star low-mass X-ray binary GX 3+1
Using the High Resolution Camera onboard the Chandra X-ray Observatory, we
have measured an accurate position for the bright persistent neutron-star X-ray
binary and atoll source GX 3+1. At a location that is consistent with this new
position we have discovered the near-infrared (NIR) counterpart to GX 3+1 in
images taken with the PANIC and FourStar cameras on the Magellan Baade
Telescope. The identification of this K_s=15.8+-0.1 mag star as the counterpart
is based on the presence of a Br-gamma emission line in a NIR spectrum taken
with the FIRE spectrograph on the Baade Telescope. The absolute magnitude
derived from the best available distance estimate to GX 3+1 indicates that the
mass donor in the system is not a late-type giant. We find that the NIR light
in GX 3+1 is likely dominated by the contribution from a heated outer accretion
disk. This is similar to what has been found for the NIR flux from the brighter
class of Z sources, but unlike the behavior of atolls fainter (Lx ~ 1e36 to
1e37 erg/s) than GX 3+1, where optically-thin synchrotron emission from a jet
probably dominates the NIR flux.Comment: Accepted for publication in Ap
A strongly heated neutron star in the transient Z source MAXI J0556-332
We present Chandra, XMM-Newton, and Swift observations of the quiescent
neutron star in the transient low-mass X-ray binary MAXI J0556-332.
Observations of the source made during outburst (with the Rossi X-ray Timing
Explorer) reveal tracks in its X-ray color--color and hardness--intensity
diagrams that closely resemble those of the neutron-star Z sources, suggesting
that MAXI J0556-332 had near- or super-Eddington luminosities for a large part
of its ~16 month outburst. A comparison of these diagrams with those of other Z
sources suggests a source distance of 46+/-15 kpc. Fits to the quiescent
spectra of MAXI J0556-332 with a neutron-star atmosphere model (with or without
a power-law component) result in distance estimates of 45+/-3 kpc, for a
neutron-star radius of 10 km and a mass of 1.4 Msun. The spectra show the
effective surface temperature of the neutron star decreasing monotonically over
the first ~500 days of quiescence, except for two observations that were likely
affected by enhanced low-level accretion. The temperatures we obtain for the
fits that include a power-law (kT_eff=184-308 eV) are much higher than those
seen for any other neutron star heated by accretion, while the inferred cooling
(e-folding) timescale (~200 days) is similar to other sources. Fits without a
power-law yield higher temperatures (kT_eff=190-336 eV) and a shorter e-folding
time (~160 days). Our results suggest that the heating of the neutron-star
crust in MAXI J0556-332 was considerably more efficient than for other systems,
possibly indicating additional or more efficient shallow heat sources in its
crust.Comment: Accepted for publication: 2014, ApJ, 795, 131 (13 pages
Evidence for simultaneous jets and disk winds in luminous low-mass X-ray binaries
Recent work on jets and disk winds in low-mass X-ray binaries (LMXBs)
suggests that they are to a large extent mutually exclusive, with jets observed
in spectrally hard states and disk winds observed in spectrally soft states. In
this paper we use existing literature on jets and disk winds in the luminous
neutron star (NS) LMXB GX 13+1, in combination with archival Rossi X-ray Timing
Explorer data, to show that this source is likely able to produce jets and disk
winds simultaneously. We find that jets and disk winds occur in the same
location on the source's track in its X-ray color-color diagram. A further
study of literature on other luminous LMXBs reveals that this behavior is more
common, with indications for simultaneous jets and disk winds in the black hole
LMXBs V404 Cyg and GRS 1915+105 and the NS LMXBs Sco X-1 and Cir X-1. For the
three sources for which we have the necessary spectral information, we find
that the simultaneous jets/winds all occur in their spectrally hardest states.
Our findings indicate that in LMXBs with luminosities above a few tens of
percent of the Eddington luminosity, jets and disk winds are not mutually
exclusive, and that the presence of disk winds does not necessarily result in
jet suppression.Comment: Updated to match published version (2016, ApJ, 830, L5
The Variable Quiescent X-ray Emission of the Neutron Star Transient XTE J1701-462
We present the results of continued monitoring of the quiescent neutron star
low-mass X-ray binary XTE J1701-462 with Chandra and Swift. A new Chandra
observation from 2010 October extends our tracking of the neutron star surface
temperature from ~800 days to ~1160 days since the end of an exceptionally
luminous 19 month outburst. This observation indicates that the neutron star
crust may still be slowly cooling toward thermal equilibrium with the core;
another observation further into quiescence is needed to verify this. The shape
of the overall cooling curve is consistent with that of a broken power law,
although an exponential decay to a constant level cannot be excluded with the
present data. To investigate possible low-level activity, we conducted a
monitoring campaign of XTE J1701-462 with Swift during 2010 April-October.
Short-term flares - presumably arising from episodic low-level accretion - were
observed up to a luminosity of ~1e35 erg/s, ~20 times higher than the normal
quiescent level. We conclude that flares of this magnitude are not likely to
have significantly affected the equilibrium temperature of the neutron star and
are probably not able to have a measurable impact on the cooling curve.
However, it is possible that brighter and longer periods of low-level activity
have had an appreciable effect on the equilibrium temperature.Comment: 14 pages, 4 figures - matches published version except for a slight
difference in the title due to an ApJ proofreader's mistak
XTE J1701-462 and its Implications for the Nature of Subclasses in Low-Magnetic-Field Neutron Star Low-Mass X-Ray Binaries
We report on an analysis of RXTE data of the transient neutron star low-mass
X-ray binary (NS-LMXB) XTE J1701-462, obtained during its 2006-2007 outburst.
The X-ray properties of the source changed between those of various types of
NS-LMXB subclasses. At high luminosities the source switched between two types
of Z source behavior and at low luminosities we observed a transition from Z
source to atoll source behavior. These transitions between subclasses primarily
manifest themselves as changes in the shapes of the tracks in X-ray color-color
and hardness-intensity diagrams, but they are accompanied by changes in the kHz
quasi-periodic oscillations, broad-band variability, burst behavior, and/or
X-ray spectra. We find that the low-energy X-ray flux is a good parameter to
track the gradual evolution of the tracks in color-color and hardness-intensity
diagrams, allowing us to resolve the evolution of the source in greater detail
than before and relate the observed properties to other NS-LMXBs. We further
find that during the transition from Z to atoll, characteristic behavior known
as the atoll upper banana can equivalently be described as the final stage of a
weakening Z source flaring branch, thereby blurring the line between the two
subclasses. Our findings strongly suggest that the wide variety in behavior
observed in NS-LXMBs with different luminosities can be linked through changes
in a single variable parameter, namely the mass accretion rate, without the
need for additional differences in the neutron star parameters or viewing
angle. We briefly discuss the implications of our findings for the spectral
changes observed in NS LMXBs and suggest that, contrary to what is often
assumed, the position along the color-color tracks of Z sources is not
determined by the instantaneous mass accretion rate.Comment: Submitted to ApJ, comments are welcome. 13 pages, 8 figure
The Long-Term Variability of the X-Ray Sources in NGC 6946 and NGC 4485/4490
We analyze data from five Chandra observations of the spiral galaxy NGC 6946
and from three Chandra observations of the irregular/spiral interacting galaxy
pair NGC 4485/4490, with an emphasis on investigating the long-term variability
exhibited by the source populations. We detect 90 point sources coincident with
NGC 6946 down to luminosities of a few times 10^36 erg/s, and 38 sources
coincident with NGC 4485/90 down to a luminosity of ~1x10^37 erg/s. Twenty-five
(15) sources in NGC 6946 (NGC 4485/90) exhibit long-term (i.e., weeks to years)
variability in luminosity; 11 (4) are transient candidates. The single
ultraluminous X-ray source (ULX) in NGC 6946 and all but one of the eight ULXs
in NGC 4485/90 exhibit long-term flux variability. Two of the ULXs in NGC
4485/90 have not been identified before as ultraluminous sources. The
widespread variability in both systems is indicative of the populations being
dominated by X-ray binaries, and this is supported by the X-ray colors of the
sources. The distribution of colors among the sources indicates a large
fraction of high-mass X-ray binaries in both systems. The shapes of the X-ray
luminosity functions of the galaxies do not change significantly between
observations and can be described by power laws with cumulative slopes ~0.6-0.7
(NGC 6946) and ~0.4 (NGC 4485/90).Comment: 26 pages, 9 figures, 15 tables - to appear in the August 2008 issue
of ApJS - new version corrects a few typo