17 research outputs found
Nature of the Soft ULX in NGC 247: Super-Eddington Outflow and Transition between the Supersoft and Soft Ultraluminous Regimes
We report on XMM-Newton/Chandra/Swift/Hubble Space Telescope observations of the ultraluminous X-ray source (ULX) in NGC 247, which is found to make transitions between the supersoft ultraluminous (SSUL) regime with a spectrum dominated by a cool (~0.1 keV) blackbody component and the soft ultraluminous (SUL) regime with comparable luminosities shared by the blackbody and power-law components. Multi-epoch observations revealed an anti-correlation between the blackbody radius and temperature, R_(bb) ∝ T_(bb)^(-2.8±0.3), ruling out a standard accretion disk as the origin of the soft X-ray emission. The soft X-ray emission is much more variable on both short and long timescales in the SSUL regime than in the SUL regime. We suggest that the SSUL regime may be an extension of the ultraluminous state toward the high accretion end, being an extreme case of the SUL regime, with the blackbody emission arising from the photosphere of thick outflows and the hard X-rays being emission leaked from the embedded accretion disk via the central low-density funnel or advected through the wind. However, the scenario that the supersoft ULXs are standard ULXs viewed nearly edge-on cannot be ruled out. Flux dips on a timescale of 200 s were observed. The dips cannot be explained by an increase of absorption, but could be due to the change of accretion rate or related to thermal fluctuations in the wind or disk. The optical emission of NGC 247 ULX exhibits a blackbody spectrum at a temperature of 19,000 K with a radius of 20 R⊙, likely arising from an OB supergiant companion star
A Luminous X-ray Flare From The Nucleus of The Dormant Bulgeless Spiral Galaxy NGC 247
NGC 247 is a nearby late-type bulgeless spiral galaxy that contains an
inactive nucleus. We report a serendipitous discovery of an X-ray flare from
the galaxy center with a luminosity up to 2*10^39 erg/s in the 0.3-10 keV band
with XMM-Newton. A Chandra observation confirms that the new X-ray source is
spatially coincident with the galaxy nucleus. The XMM-Newton data revealed a
hard power-law spectrum with a spectral break near 3-4 keV, no pulsations on
timescales longer than 150 ms, and a flat power spectrum consistent with
Poisson noise from 1 mHz to nearly 10 Hz. Follow-up observations with Swift
detected a second flux peak followed by a luminosity drop by factor of almost
20. The spectral and temporal behaviors of the nuclear source are well
consistent with the scenario that the flare was due to an outburst of a
low-mass X-ray binary that contains a stellar-mass black hole emitting near its
Eddington limit at the peak. However, it cannot be ruled out that the sudden
brightening in the nucleus was due to accretion onto a possible low-mass
nuclear black hole, fed by a tidally disrupted star or a gas cloud; the MAXI
observations limit the peak luminosity of the flare to less than ~10^43 erg/s,
suggesting that it is either a low mass black hole or an inefficient tidal
disruption event (TDE).Comment: accepted for publication in Ap
Chandra and HST Observations of the Supersoft ULX in NGC 247: Candidate for Standard Disk Emission
We report on multiwavelength observations of the supersoft ultraluminous
X-ray source (ULX) in NGC 247 made with the Chandra X-ray Observatory and
Hubble Space Telescope (HST). We aligned the X-ray and optical images using
three objects present on both and identified a unique, point-like optical
counterpart to the ULX. The X-ray to optical spectrum is well fitted with an
irradiated disk model if the extinction measured for Cepheids in NGC 247 is
used. Assuming only Galactic extinction, then the spectrum can be modeled as a
standard thin accretion disk. Either result leads to the conclusion that a disk
interpretation of the X-ray spectrum is valid, thus the source may be in the
X-ray thermal state and contain an intermediate mass black hole of at least 600
solar masses. In contrast to other supersoft ULXs which are transient and
exhibit a luminosity temperature relation inconsistent with a disk
interpretation of the X-ray emission, the NGC 247 ULX has a relatively steady
flux and all available X-ray data are consistent with emission from a disk in
the thermal state.Comment: 9 pages, 5 figures, accepted for publication in Ap
Black Hole Powered Nebulae and a Case Study of the Ultraluminous X-ray Source IC342 X-1
We present new radio, optical, and X-ray observations of three Ultraluminous
X-ray sources (ULXs) that are associated with large-scale nebulae. We report
the discovery of a radio nebula associated with the ULX IC342 X-1 using the
Very Large Array (VLA). Complementary VLA observations of the nebula around
Holmberg II X-1, and high-frequency Australia Telescope Compact Array (ATCA)
and Very Large Telescope (VLT) spectroscopic observations of NGC5408 X-1 are
also presented. We study the morphology, ionization processes, and the
energetics of the optical/radio nebulae of IC342 X-1, Holmberg II X-1 and
NGC5408 X-1. The energetics of the optical nebula of IC342 X-1 is discussed in
the framework of standard bubble theory. The total energy content of the
optical nebula is 6 x 10^52 erg. The minimum energy needed to supply the
associated radio nebula is 9.2 x 10^50 erg. In addition, we detected an
unresolved radio source at the location of IC342 X-1 at VLA scales. However,
our Very Long Baseline Interferometry (VLBI) observations using the European
VLBI Network likely rule out the presence of any compact radio source at
milli-arcsecond (mas) scales. Using a simultaneous Swift X-ray Telescope
measurement, we estimate an upper limit on the mass of the black hole in IC342
X-1 using the "fundamental plane" of accreting black holes and obtain M_BH <
(1.0\pm0.3) x 10^3 M_Sun. Arguing that the nebula of IC342 X-1 is possibly
inflated by a jet, we estimate accretion rates and efficiencies for the jet of
IC342 X-1 and compare with sources like S26, SS433, IC10 X-1.Comment: 11 pages, 8 figures, accepted for publication in Ap
Compact Optical Counterparts of Ultraluminous X-ray Sources
Using archival Hubble Space Telescope (HST) imaging data, we report the
multiband photometric properties of 13 ultraluminous X-ray sources (ULXs) that
have a unique compact optical counterpart. Both magnitude and color variation
are detected at time scales of days to years. The optical color, variability,
and X-ray to optical flux ratio indicate that the optical emission of most ULXs
is dominated by X-ray reprocessing on the disk, similar to that of low mass
X-ray binaries. For most sources, the optical spectrum is a power-law, with in the range 1.0 to 2.0 and the optically
emitting region has a size on the order of 1e12 cm. Exceptions are NGC 2403 X-1
and M83 IXO 82, which show optical spectra consistent with direct emission from
a standard thin disk, M101 ULX-1 and M81 ULS1, which have X-ray to optical flux
ratios more similar to high-mass X-ray binaries, and IC 342 X-1, in which the
optical light may be dominated by the companion star. Inconsistent extinction
between the optical counterpart of NGC 5204 X-1 and the nearby optical nebulae
suggests that they may be unrelated.Comment: accepted by ApJ, 14 page
The Nature of the UV/optical Emission of the Ultraluminous X-Ray Source in Holmberg II
We report on UV and X-ray spectroscopy and broad-band optical observations of
the ultraluminous X-ray source in Holmberg II. Fitting various stellar spectral
models to the combined, non-simultaneous data set, we find that normal
metallicity stellar spectra are ruled out by the data, while low metallicity, Z
= 0.1 Z_{\odot}, late O-star spectra provide marginally acceptable fits, if we
allow for the fact that X-ray ionization from the compact object may reduce or
eliminate UV absorption/emission lines from the stellar wind. By contrast, an
irradiated disk model fits both UV and optical data with chi^2/dof=175.9/178,
and matches the nebular extinction with a reddening of
E(B-V)=0.05^{+0.05}_{-0.04}. These results suggest that the UV/optical flux of
Holmberg II X-1 may be dominated by X-ray irradiated disk emission.Comment: 9 pages, 1 table, 6 figures, accepted by Ap
Performance Testing of a Large-Format Reflection Grating Prototype for a Suborbital Rocket Payload
The soft X-ray grating spectrometer on board the Off-plane Grating Rocket
Experiment (OGRE) hopes to achieve the highest resolution soft X-ray spectrum
of an astrophysical object when it is launched via suborbital rocket. Paramount
to the success of the spectrometer are the performance of the reflection
gratings populating its reflection grating assembly. To test current grating
fabrication capabilities, a grating prototype for the payload was fabricated
via electron-beam lithography at The Pennsylvania State University's Materials
Research Institute and was subsequently tested for performance at Max Planck
Institute for Extraterrestrial Physics' PANTER X-ray Test Facility. Bayesian
modeling of the resulting data via Markov chain Monte Carlo (MCMC) sampling
indicated that the grating achieved the OGRE single-grating resolution
requirement of at the 94% confidence level.
The resulting posterior probability distribution suggests that this
confidence level is likely a conservative estimate though, since only a finite
parameter space was sampled and the model could not constrain the upper
bound of to less than infinity. Raytrace simulations of the system found
that the observed data can be reproduced with a grating performing at
. It is therefore postulated that the behavior of the obtained
posterior probability distribution can be explained by a finite
measurement limit of the system and not a finite limit on . Implications
of these results and improvements to the test setup are discussed.Comment: 25 pages, 16 figures, preprint of an article accepted for publication
in the Journal of Astronomical Instrumentation \copyright 2020 [copyright
World Scientific Publishing Company]
[https://www.worldscientific.com/worldscinet/jai
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Comprehensive line-spread function error budget for the Off-plane Grating Rocket Experiment
The Off-plane Grating Rocket Experiment (OGRE) is a soft x-ray grating spectrometer to be flown on a suborbital rocket. The payload is designed to obtain the highest-resolution soft x-ray spectrum of Capella to date with a resolution goal of R ( λ / Δλ ) > 2000 at select wavelengths in its 10 to 55 Å bandpass of interest. The optical design of the spectrometer realizes a theoretical maximum resolution of R ≈ 5000, but this performance does not consider the finite performance of the individual spectrometer components, misalignments between components, and in-flight pointing errors. These errors all degrade the performance of the spectrometer from its theoretical maximum. A comprehensive line-spread function (LSF) error budget has been constructed for the OGRE spectrometer to identify contributions to the LSF, to determine how each of these affects the LSF, and to inform performance requirements and alignment tolerances for the spectrometer. In this document, the comprehensive LSF error budget for the OGRE spectrometer is presented, the resulting errors are validated via raytrace simulations, the implications of these results are discussed, and future work is identified