821 research outputs found
X-ray Constraints on Accretion and Starburst Processes in Galactic Nuclei I. Spectral Results
The results of a 0.4-10.0 keV ASCA spectral analysis of a sample of
low-luminosity AGN (LLAGN; M51, NGC 3147, NGC 4258), low-ionization nuclear
emission line regions (LINERs; NGC 3079, NGC 3310, NGC 3998, NGC 4579, NGC
4594) and starburst galaxies (M82, NGC 253, NGC 3628 and NGC 6946) are
presented. In spite of the heterogeneous optical classifications of these
galaxies, the X-ray spectra are fit well by a ``canonical'' model consisting of
an optically-thin Raymond-Smith plasma ``soft'' component with T ~ 7 x 10^6 K
and a ``hard'' component that can be modeled by either a power-law with a
photon index ~ 1.7 or a thermal bremsstrahlung with T ~ 6 x 10^7 K. The
soft-component 0.4-10 keV instrinsic luminosities tend to be on the order
10^39-40 ergs/s while the hard-component luminosities tend to be on the order
of 10^40-41 ergs/s. The detection of line emission is discussed. An analysis of
the short-term variability properties was given in Ptak et al. (1998) and
detailed interpretation of these results will be given in Paper II. (abridged)Comment: Accepted for Jan. 99 issue of ApJS. 35 pages with embedded postscript
figures. 8 large tables included externally as postscript file
Time Variability of Nonthermal X-ray Stripes in Tycho's Supernova Remnant with Chandra
Analyzing Chandra data of Tycho's supernova remnant (SNR) taken in 2000,
2003, 2007, 2009, and 2015, we search for time variable features of synchrotron
X-rays in the southwestern part of the SNR, where stripe structures of hard
X-ray emission were previous found. By comparing X-ray images obtained at each
epoch, we discover a knot-like structure in the northernmost part of the stripe
region became brighter particularly in 2015. We also find a bright filamentary
structure gradually became fainter and narrower as it moved outward. Our
spectral analysis reveal that not only the nonthermal X-ray flux but also the
photon indices of the knot-like structure change from year to year. During the
period from 2000 to 2015, the small knot shows brightening of and
hardening of . The time variability can be explained
if the magnetic field is amplified to and/or if
magnetic turbulence significantly changes with time.Comment: 8 pages, 3 figures, 2 tables, accepted for publication in Ap
A Chandra View of the Normal SO Galaxy NGC 1332: II: Solar Abundances in the Hot Gas and Implications for SN Enrichment
We present spectral analysis of the diffuse emission in the normal, isolated,
moderate-Lx S0 NGC 1332, constraining both the temperature profile and the
metal abundances in the ISM. The characteristics of the point source population
and the gravitating matter are discussed in two companion papers. The diffuse
emission comprises hot gas, with an ~isothermal temperature profile (~0.5 keV),
and emission from unresolved point-sources. In contrast with the cool cores of
many groups and clusters, we find a small central temperature peak. We obtain
emission-weighted abundance contraints within 20 kpc for several key elements:
Fe, O, Ne, Mg and Si. The measured iron abundance (Z_Fe=1.1 in solar units;
>0.53 at 99% confidence) strongly excludes the very sub-solar values often
historically reported for early-type galaxies but agrees with recent
observations of brighter galaxies and groups. The abundance ratios, with
respect to Fe, of the other elements were also found to be ~solar, although
Z_o/Z_Fe was significantly lower (<0.4). Such a low O abundance is not
predicted by simple models of ISM enrichment by Type Ia and Type II supernovae,
and may indicate a significant contribution from primordial hypernovae.
Revisiting Chandra observations of the moderate-Lx, isolated elliptical NGC
720, we obtain similar abundance constraints. Adopting standard SNIa and SNII
metal yields, our abundance ratio constraints imply 73+/-5% and 85+/-6% of the
Fe enrichment in NGC 1332 and NGC 720, respectively, arises from SNIa. Although
these results are sensitive to the considerable systematic uncertainty in the
SNe yields, they are in good agreement with observations of more massive
systems. These two moderate-Lx early-type galaxies reveal a consistent pattern
of metal enrichment from cluster scales to moderate Lx/Lb galaxies. (abridged)Comment: 12 pages, 4 figures, accepted for publication in ApJ. Minor changes
to match published versio
Scaling laws of solar and stellar flares
In this study we compile for the first time comprehensive data sets of solar
and stellar flare parameters, including flare peak temperatures T_p, flare peak
volume emission measures EM_p, and flare durations t_f from both solar and
stellar data, as well as flare length scales L from solar data. Key results are
that both the solar and stellar data are consistent with a common scaling law
of EM_p ~ T_p^4.7, but the stellar flares exhibit ~250 times higher emission
measures (at the same flare peak temperature). For solar flares we observe also
systematic trends for the flare length scale L(T_p) ~ T_p^0.9 and the flare
duration t_F(T_p) ~ T_p^0.9 as a function of the flare peak temperature. Using
the theoretical RTV scaling law and the fractal volume scaling observed for
solar flares, i.e., V(L) ~ L^2.4, we predict a scaling law of EM_p ~ T_p^4.3,
which is consistent with observations, and a scaling law for electron densities
in flare loops, n_p ~ T_p^2/L ~ T_p^1.1. The RTV-predicted electron densities
were also found to be consistent with densities inferred from total emission
measures, n_p=(EM_p/q_V*V)^1/2, using volume filling factors of q_V=0.03-0.08
constrained by fractal dimensions measured in solar flares. Our results affect
also the determination of radiative and conductive cooling times, thermal
energies, and frequency distributions of solar and stellar flare energies.Comment: 9 Figs., (paper in press, The Astrophsycial Journal
Spectrum of Relativistic and Subrelativistic Cosmic Rays in the 100 pc Central Region
From the rate of hydrogen ionization and the gamma ray flux, we derived the
spectrum of relativistic and subrelativistic cosmic rays (CRs) nearby and
inside the molecular cloud Sgr B2 near the Galactic Center (GC). We studied two
cases of CR propagation in molecular clouds: free propagation and scattering of
particles by magnetic fluctuations excited by the neutral gas turbulence. We
showed that in the latter case CR propagation inside the cloud can be described
as diffusion with the coefficient cm s. For
the case of hydrogen ionization by subrelativistic protons, we showed that
their spectrum outside the cloud is quite hard with the spectral index
. The energy density of subrelativistic protons ( eV cm)
is one order of magnitude higher than that of relativistic CRs. These protons
generate the 6.4 keV emission from Sgr B2, which was about 30\% of the flux
observed by Suzaku in 2013. Future observations for the period after 2013 may
discover the background flux generated by subrelativistic CRs in Sgr B2.
Alternatively hydrogen ionization of the molecular gas in Sgr B2 may be caused
by high energy electrons. We showed that the spectrum of electron
bremsstrahlung is harder than the observed continuum from Sgr B2, and in
principle this X-ray component provided by electrons could be seen from the
INTEGRAL data as a stationary high energy excess above the observed spectrum
.Comment: 42 pages, 6 figures, accepted by Ap
Suzaku Observation of HESS J1825-137: Discovery of Largely-Extended X-rays near from PSR J1826-1334
We observed the brightest part of HESS J1825-137 with the Suzaku XIS, and
found diffuse X-rays extending at least up to 15' (~ 17 pc) from the pulsar PSR
J1826-1334. The spectra have no emission line, and are fitted with an absorbed
power-law model. The X-rays, therefore, are likely due to synchrotron emission
from a pulsar wind nebula. The photon index near at the pulsar (r<1.5') is 1.7
while those in r=1.5-16 are nearly constant at Gamma=2.0. The spectral energy
distribution of the Suzaku and H.E.S.S. results are naturally explained by a
combined process; synchrotron X-rays and gamma-rays by the inverse Compton of
the cosmic microwave photons by high-energy electrons in a magnetic field of 7
micro G. If the electrons are accelerated at the pulsar, the electrons must be
transported over 17 pc in the synchrotron life time of 1900 yr, with a velocity
of > 8.8 times 10^3 km s^{-1}.Comment: 9 pages, 10 figure
Discovery of Bright Variable X-ray Sources in NGC 1569 with Chandra
From the analysis of a ~100 ks Chandra observation of the dwarf starburst
galaxy NGC 1569, we have found that the X-ray point sources, CXOU
043048.1+645050 and CXOU 043048.6+645058, showed significant time variability.
During this observation, the X-ray flux of CXOU 043048.1+645050 increased by 10
times in only 2 x 10^4 s. Since the spectrum in its bright phase was fitted
with a disk blackbody model with kT_in ~0.43 keV and the bolometric luminosity
is L_bol ~10^38 ergs s^-1, this source is an X-ray binary with a stellar mass
black-hole. Since the spectrum in its faint phase was also fitted with a disk
blackbody model, the time variability can be explained by a change of the
accretion rate onto the black hole. The other variable source, CXOU
043048.6+645058, had a flat spectrum with a photon index of ~1.6. This source
may be an X-ray binary with an X-ray luminosity of several x 10^37 ergs s^-1.
In addition, three other weak sources showed possible time variability. Taking
all of the variability into account may suggest an abundant population of
compact X-ray sources in NGC 1569.Comment: 15 pages including 4 Postscript figures; accepted for publication in
ApJ
An X-ray Mini-survey of Nearby Edge-on Starburst Galaxies II. The Question of Metal Abundance
(abbreviated) We have undertaken an X-ray survey of a far-infrared flux
limited sample of seven nearby edge-on starburst galaxies. Here, we examine the
two X-ray-brightest sample members NGC 253 and M 82 in a self-consistent
manner, taking account of the spatial distribution of the X-ray emission in
choosing our spectral models. There is significant X-ray absorption in the disk
of NGC 253. When this is accounted for we find that multi-temperature thermal
plasma models with significant underlying soft X-ray absorption are more
consistent with the imaging data than single-temperature models with highly
subsolar abundances or models with minimal absorption and non-equilibrium
thermal ionization conditions. Our models do not require absolute abundances
that are inconsistent with solar values or unusually supersolar ratios of the
alpha-burning elements with respect to Fe (as claimed previously). We conclude
that with current data, the technique of measuring abundances in starburst
galaxies via X-ray spectral modeling is highly uncertain.
Based on the point-like nature of much of the X-ray emission in the PSPC
hard-band image of NGC 253, we suggest that a significant fraction of the
``extended'' X-ray emission in the 3-10 keV band seen along the disk of the
galaxy with ASCA and BeppoSAX (Cappi et al.) is comprised of discrete sources
in the disk, as opposed to purely diffuse, hot gas. This could explain the low
Fe abundances of ~1/4 solar derived for pure thermal models.Comment: (accepted for publication in the Astrophysical Journal
A deep X-ray observation of M82 with XMM-Newton
We report on the analysis of a deep (100 ks) observation of the starburst
galaxy M82 with the EPIC and RGS instruments on board the X-ray telescope
XMM-Newton. The broad-band (0.5-10 keV) emission is due to at least three
spectral components: i) continuum emission from point sources; ii) thermal
plasma emission from hot gas; iii) charge exchange emission from neutral metals
(Mg and Si). The plasma emission has a double-peaked differential emission
measure, with the peaks at ~0.5 keV and ~7 keV. Spatially resolved spectroscopy
has shown that the chemical absolute abundances are not uniformly distributed
in the outflow, but are larger in the outskirts and smaller close to the galaxy
centre. The abundance ratios also show spatial variations. The X-ray derived
Oxygen abundance is lower than that measured in the atmospheres of red
supergiant stars, leading to the hypothesis that a significant fraction of
Oxygen ions have already cooled off and no longer emit at energies > ~0.5 keV.Comment: Accepted for publication in MNRAS. 20 pages, 15 figures, LaTeX2
- …