841 research outputs found
Survey of Optimization Models for Power System Operation and Expansion Planning With Demand Response
A Persistent Disk Wind in GRS 1915+105 with NICER
The bright, erratic black hole X-ray binary GRS 1915+105 has long been a
target for studies of disk instabilities, radio/infrared jets, and accretion
disk winds, with implications that often apply to sources that do not exhibit
its exotic X-ray variability. With the launch of NICER, we have a new
opportunity to study the disk wind in GRS 1915+105 and its variability on short
and long timescales. Here we present our analysis of 39 NICER observations of
GRS 1915+105 collected during five months of the mission data validation and
verification phase, focusing on Fe XXV and Fe XXVI absorption. We report the
detection of strong Fe XXVI in 32 (>80%) of these observations, with another
four marginal detections; Fe XXV is less common, but both likely arise in the
well-known disk wind. We explore how the properties of this wind depends on
broad characteristics of the X-ray lightcurve: mean count rate, hardness ratio,
and fractional RMS variability. The trends with count rate and RMS are
consistent with an average wind column density that is fairly steady between
observations but varies rapidly with the source on timescales of seconds. The
line dependence on spectral hardness echoes known behavior of disk winds in
outbursts of Galactic black holes; these results clearly indicate that NICER is
a powerful tool for studying black hole winds.Comment: Accepted for publication in ApJL. Comments welcom
The Large-Scale Structure of the X-ray Background and its Cosmological Implications
A careful analysis of the HEAO1 A2 2-10 keV full-sky map of the X-ray
background (XRB) reveals clustering on the scale of several degrees. After
removing the contribution due to beam smearing, the intrinsic clustering of the
background is found to be consistent with an auto-correlation function of the
form (3.6 +- 0.9) x 10^{-4} theta^{-1} where theta is measured in degrees. If
current AGN models of the hard XRB are reasonable and the cosmological
constant-cold dark matter cosmology is correct, this clustering implies an
X-ray bias factor of b_X ~ 2. Combined with the absence of a correlation
between the XRB and the cosmic microwave background, this clustering can be
used to limit the presence of an integrated Sachs-Wolfe (ISW) effect and
thereby to constrain the value of the cosmological constant, Omega_Lambda <
0.60 (95 % C.L.). This constraint is inconsistent with much of the parameter
space currently favored by other observations. Finally, we marginally detect
the dipole moment of the diffuse XRB and find it to be consistent with the
dipole due to our motion with respect to the mean rest frame of the XRB. The
limit on the amplitude of any intrinsic dipole is delta I / I < 5 x 10^{-3} at
the 95 % C.L. When compared to the local bulk velocity, this limit implies a
constraint on the matter density of the universe of Omega_m^{0.6}/b_X(0) >
0.24.Comment: 15 pages, 8 postscript figures, to appear in the Astrophysical
Journal. The postscript version appears not to print, so use the PDF versio
The BeppoSAX view of the hard X-ray background
First results on a medium-deep X-ray survey in the "new" 5-10 keV band
carried out with the MECS detectors onboard BeppoSAX are presented. The High
Energy Llarge Area Survey (HELLAS) is aimed to directly explore a band where
the energy density of the X-ray background is more than twice than that in the
soft (0.5-2.0 keV) band. The optical identification follow-up of the first ten
HELLAS hard X-ray sources indicate that Active Galactic Nuclei are the dominant
population at 5-10 keV fluxes of the order of 10e-13 cgs. We discuss the
implications of these findings for the AGN synthesis models for the XRB.Comment: 6 pages, 4 figures, uses psfig.sty. Accepted for publication in
Advances in Space Research, Proceedings of the 32nd Scientific Assembly of
COSPA
The BeppoSAX High Energy Large Area Survey. IV. On the soft X-ray properties of the hard X-ray-selected HELLAS sources
We present a comprehensive study of the soft X-ray properties of the BeppoSAX
High-Energy Large Area Survey (HELLAS) sources. A large fraction (about 2/3) of
the hard X-ray selected sources is detected by ROSAT. The soft X-ray colors for
many of these objects, along with the 0.5-2 keV flux upper limits for those
undetected in the ROSAT band, do imply the presence of absorption. The
comparison with the ROSAT Deep Survey sources indicates that a larger fraction
of absorbed objects among the HELLAS sources is present, in agreement with
their hard X-ray selection and the predictions of the X-ray background
synthesis models. Another striking result is the presence of a soft
(additional) X-ray component in a significant fraction of absorbed objects.Comment: 11 pages, LateX, 6 figures (included), aa.cls, graphicx.sty and
lscape.sty macros. Accepted by A&
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NGC 7582: The Prototype Narrow-Line X-ray Galaxy
NGC 7582 is a candidate prototype of the Narrow Line X-ray Galaxies (NLXGs)
found in deep X-ray surveys. An ASCA observation shows the hard (> 3 keV) X-ray
continuum of NGC 7582 drops 40% in ~6 ks, implying an AGN, while the soft band
(< 3 keV) does not drop in concert with the hard continuum, requiring a
separate component. The X-ray spectrum of NGC 7582 also shows a clear 0.5-2 keV
soft (kT = 0.8 (+0.9,-0.3) keV or Gamma = 2.4 +/- 0.6; L(X) = 6 x 10**40 ergs
s**-1) low--energy component, in addition to a heavily absorbed [N(H) = (6 +/-
2)\times 10**22 cm**-2 ] and variable 2-10 keV power law [Gamma = 0.7
(+0.3,-0.4); L(X) = (1.7-2.3) x 10**42 ergs s**-1]. This is one of the flattest
2-10 keV slopes in any AGN observed with ASCA. (The ROSAT HRI image of NGC 7582
further suggests extent to the SE.)
These observations make it clear that the hard X-ray emission of NGC 7582,
the most "narrow-line" of the NLXGs, is associated with an AGN. The strong
suggestion is that all NLXGs are obscured AGNs, as hypothesized to explain the
X-ray background spectral paradox. The separate soft X-ray component makes NGC
7582 (and by extension other NLXGs) detectable as a ROSAT source.Comment: text: Latex2e 10 pages, including 1 table, and 2 postscript figures
via psfi
A NICER Discovery of a Low-Frequency Quasi-Periodic Oscillation in the Soft-Intermediate State of MAXI J1535-571
We present the discovery of a low-frequency Hz quasi-periodic
oscillation (QPO) feature in observations of the black hole X-ray binary MAXI
J1535-571 in its soft-intermediate state, obtained in September-October 2017 by
the Neutron Star Interior Composition Explorer (NICER). The feature is
relatively broad (compared to other low-frequency QPOs; quality factor
) and weak (1.9% rms in 3-10 keV), and is accompanied by a weak
harmonic and low-amplitude broadband noise. These characteristics identify it
as a weak Type A/B QPO, similar to ones previously identified in the
soft-intermediate state of the transient black hole X-ray binary XTE J1550-564.
The lag-energy spectrum of the QPO shows increasing soft lags towards lower
energies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum).
This large phase shift has similar amplitude but opposite sign to that seen in
Rossi X-ray Timing Explorer data for a Type B QPO from the transient black hole
X-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of the
Type B QPO in GX 339-4 pointed towards a precessing jet-like corona
illuminating the accretion disk as the origin of the QPO signal. We suggest
that this QPO in MAXI J1535-571 may have the same origin, with the different
lag sign depending on the scale height of the emitting region and the observer
inclination angle.Comment: Accepted for publication in ApJ Letter
X-Rays from NGC 3256: High-Energy Emission in Starburst Galaxies and Their Contribution to the Cosmic X-Ray Background
The infrared-luminous galaxy NGC3256 is a classic example of a merger induced
nuclear starburst system. We find here that it is the most X-ray luminous
star-forming galaxy yet detected (~10^42 ergs/s). Long-slit optical
spectroscopy and a deep, high-resolution ROSAT X-ray image show that the
starburst is driving a "superwind" which accounts for ~20% of the observed soft
(kT~0.3 keV) X-ray emission. Our model for the broadband X-ray emission of
NGC3256 contains two additional components: a warm thermal plasma (kT~0.8 keV)
associated with the central starburst, and a hard power-law component with an
energy index of ~0.7. We find that the input of mechanical energy from the
starburst is more than sufficient to sustain the observed level of emission. We
also examine possible origins for the power-law component, concluding that
neither a buried AGN nor the expected population of high-mass X-ray binaries
can account for this emission. Inverse-Compton scattering, involving the
galaxy's copious flux of infrared photons and the relativistic electrons
produced by supernovae, is likely to make a substantial contribution to the
hard X-ray flux. Such a model is consistent with the observed radio and IR
fluxes and the radio and X-ray spectral indices. We explore the role of
X-ray-luminous starbursts in the production of the cosmic X-ray background
radiation. The number counts and spectral index distribution of the faint radio
source population, thought to be dominated by star-forming galaxies, suggest
that a significant fraction of the hard X-ray background could arise from
starbursts at moderate redshift.Comment: 31 pages (tex, epsf), 8 figures (postscript files), accepted for
publication in Part 1 of The Astrophysical Journa
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