1,212 research outputs found
No periodicity revealed for an "eclipsing" ultraluminous supersoft X-ray source in M81
Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds
are likely white dwarfs that steadily or cyclically burn accreted matter on
their surface, which are promising type Ia supernova progenitors. Observations
of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources
that are generally hotter and more luminous, including some ultraluminous
supersoft sources (ULSs) that are possibly intermediate mass black holes of a
few thousand solar masses. In this paper we report our X-ray spectral and
timing analysis for M81-ULS1, an ultraluminous supersoft source in the nearby
spiral galaxy M81. M81-ULS1 has been persistently supersoft in 17 Chandra ACIS
observations spanning six years, and its spectrum can be described by either a
eV blackbody for a white dwarf, or a
eV multicolor accretion disk for a
intermediate mass black hole. In two observations, the light curves exhibited
dramatic flux drop/rise on time scales of seconds, reminiscent of
eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive
search for periodicity in the reasonable range of 50 ksec to 50 days failed to
reveal an orbital period. The failure to reveal any periodicity is consistent
with the long period ( yrs) predicted for this system given the optical
identification of the secondary with an asymptotic giant star. Also, the
eclipse-like dramatic flux changes in hours are hard to explain under the white
dwarf model, but can in principle be explained by disk temperature changes
induced by accretion rate variations under the intermediate mass black hole
model.Comment: 19 pages, 7 figures, 1 table, to appear in ApJ
Spectral evolution of the microquasar XTE J1550-564 over its entire 2000 outburst
We report on RXTE observations of the microquasar XTE J1550-564 during a ~70
day outburst in April-June 2000. We study the evolution of the PCA+HEXTE
spectra over the outburst. The source transited from an initial Low Hard State
(LS), to an Intermediate State (IS), and then back to the LS. The source shows
an hysteresis effect similar to what is observed in other sources, favoring a
common origin for the state transitions in soft X-ray transients. The first
transition occurs at a ~ constant 2-200 keV flux, which probably indicates a
change in the relative importance of the emitting media. The second transition
is more likely driven by a drop in the mass accretion rate.
In both LS, the spectra are characterized by the presence of a strong
power-law tail (Compton corona) with a variable high energy cut-off. During the
IS, the spectra show the presence of a ~0.8 keV thermal component (accretion
disk). We discuss the apparently independent evolution of the two media, and
show that right after the X-ray maximum on MJD 51662, the decrease of the
source luminosity is due to a decrease of the power-law luminosity, at a
constant disk luminosity. This, together with the detection of radio emission
(with a spectrum typical of optically thin synchrotron emission), may suggest
that the corona is ejected and further detected as a discrete radio ejection.Comment: Accepted for publication in ApJ. 9 pages, 4 figures, abstract
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A Grid of Relativistic, non-LTE Accretion Disk Models for Spectral Fitting of Black Hole Binaries
Self-consistent vertical structure models together with non-LTE radiative
transfer should produce spectra from accretion disks around black holes which
differ from multitemperature blackbodies at levels which may be observed. High
resolution, high signal-to-noise observations warrant spectral modeling which
both accounts for relativistic effects, and treats the physics of radiative
transfer in detail. In Davis et al. (2005) we presented spectral models which
accounted for non-LTE effects, Compton scattering, and the opacities due to
ions of abundant metals. Using a modification of this method, we have tabulated
spectra for black hole masses typical of Galactic binaries. We make them
publicly available for spectral fitting as an Xspec model. These models
represent the most complete realization of standard accretion disk theory to
date. Thus, they are well suited for both testing the theory's applicability to
observed systems and for constraining properties of the black holes, including
their spins.Comment: 7 pages, emulate ApJ, accepted to Ap
The Complex Phase Lag Behavior of the 3-12 Hz Quasi-Periodic Oscillations during the Very High State of XTE J1550-564
We present a study of the complex phase lag behavior of the low-frequency
(<20 Hz) quasi-periodic oscillations (QPOs) in the X-ray transient and
black-hole candidate XTE J1550-564 during its very high state. We distinguish
two different types of low-frequency QPOs, based on their coherence and
harmonic content. The first type is characterized by a 6 Hz QPO with a Q (the
QPO frequency divided by the QPO width) of <3 and with a harmonic at 12 Hz. The
second type of QPO is characterized by a 6 Hz QPO with a Q value of >6 and with
harmonics at 3, 12, 18, and possibly at 9 Hz. Not only the Q values and the
harmonic content of the two types are different, but also their phase lag
behavior. For the first type of QPO, the low energy photons (<5 keV) of both
the 6 Hz QPO and its harmonic at 12 Hz lag the hard energy photons (>5 keV) by
as much as 1.3 radian. The phase lags of the second type of QPO are more
complex. The soft photons (<5 keV) of the 3 and the 12 Hz QPOs lag the hard
photons (>5 keV) by as much as 1.0 radian. However, the soft photons of the 6
Hz QPO precede the hard ones by as much as 0.6 radian. This means that
different harmonics of this type of QPO have different signs for their phase
lags. This unusual behavior is hard to explain when the lags are due to
light-travel-time differences between the photons at different energies, e.g.,
in a Comptonizing region surrounding the area where the QPOs are formed.Comment: Accepted for publication in ApJ Letters, 29 September 199
An Ultraluminous Supersoft X-ray Source in M81: An Intermediate-Mass Black Hole?
Ultraluminous supersoft X-ray sources (ULSSS) exhibit supersoft spectra with
blackbody temperatures of 50-100 eV and bolometric luminosities above
erg s, and are possibly intermediate mass black holes (IMBHs) of
or massive white dwarfs that are progenitors of type Ia
supernovae. In this letter we report our optical studies of such a source in
M81, M81-ULS1, with HST archive observations. M81-ULS1 is identified with a
point-like object, the spectral energy distribution of which reveals a blue
component in addition to the companion of an AGB star. The blue component is
consistent with the power-law as expected from the geometrically-thin accretion
disk around an IMBH accretor, but inconsistent with the power-law as expected
from the X-ray irradiated flared accretion disk around a white dwarf accretor.
This result is strong evidence that M81-ULS1 is an IMBH instead of a white
dwarf.Comment: 12 pages, 1 table, 3 figure
XMM-Newton observations of the spiral galaxy M74 (NGC 628)
The face-on spiral galaxy M74 (NGC 628) was observed by XMM on 2002 February
2. In total, 21 sources are found in the inner 5' from the nucleus (after
rejection of a few sources associated to foreground stars). Hardness ratios
suggest that about half of them belong to the galaxy. The higher-luminosity end
of the luminosity function is fitted by a power-law of slope -0.8. This can be
interpreted as evidence of ongoing star formation, in analogy with the
distributions found in disks of other late-type galaxies. A comparison with
previous Chandra observations reveals a new ultraluminous X-ray transient (L_x
\~ 1.5 x 10^39 erg/s in the 0.3--8 keV band) about 4' North of the nucleus. We
find another transient black-hole candidate (L_x ~ 5 x 10^38 erg/s) about 5'
North-West of the nucleus. The UV and X-ray counterparts of SN 2002ap are also
found in this XMM observation.Comment: submitted to ApJL. Based on publicly available data, see
http://xmm.vilspa.esa.es/external/xmm_news/items/sn_2002_ap/index.shtm
Electric Polarization Induced by a Proper Helical Magnetic Ordering in a Delafossite Multiferroic CuFe1-xAlxO2
Multiferroic CuFe1-xAlxO2 (x=0.02) exhibits a ferroelectric ordering
accompanied by a proper helical magnetic ordering below T=7K under zero
magnetic field. By polarized neutron diffraction and pyroelectric measurements,
we have revealed a one-to-one correspondence between the spin helicity and the
direction of the spontaneous electric polarization. This result indicates that
the spin helicity of the proper helical magnetic ordering is essential for the
ferroelectricity in CuFe1-xAlxO2. The induction of the electric polarization by
the proper helical magnetic ordering is, however, cannot be explained by the
Katsura-Nagaosa-Balatsky model, which successfully explains the
ferroelectricity in the recently explored ferroelectric helimagnets, such as
TbMnO3. We thus conclude that CuFe1-xAlxO2 is a new class of magnetic
ferroelectrics.Comment: 4 pages, 4 figure
The 1996 Soft State Transitions of Cygnus X-1
We report continuous monitoring of Cygnus X-1 in the 1.3 to 200 keV band
using ASM/RXTE and BATSE/CGRO for about 200 days from 1996 February 21 to 1996
early September. During this period Cygnus X-1 experienced a hard-to-soft and
then a soft-to-hard state transition. The low-energy X-ray (1.3-12 keV) and
high-energy X-ray (20-200 keV) fluxes are strongly anti-correlated during this
period. During the state transitions flux variations of about a factor of 5 and
15 were seen in the 1.3-3.0 keV and 100-200 keV bands, respectively, while the
average 4.8-12 keV flux remains almost unchanged. The net effect of this
pivoting is that the total 1.3-200 keV luminosity remained unchanged to within
about 15%. The bolometric luminosity in the soft state may be as high as 50-70%
above the hard state luminosity, after color corrections for the luminosity
below 1.3 keV. The blackbody component flux and temperature increase in the
soft state is probably caused by a combination of the optically thick disk mass
accretion rate increase and a decrease of the inner disk radius.Comment: 18 pages, 1 PostScript figure. Accepted for ApJ
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