69 research outputs found
Hard X-ray Emission and the Ionizing Source in LINERs
We report X-ray fluxes in the 2--10 keV band from LINERs (low-ionization
nuclear emission-line regions) and low-luminosity Seyfert galaxies obtained
with the ASCA satellite. Observed X-ray luminosities are in the range between
4e39 and 5e41 ergs/s, which are significantly smaller than that of the
``classical'' low-luminosity Seyfert 1 galaxy NGC 4051. We found that X-ray
luminosities in 2--10 keV of LINERs with broad Halpha emission in their optical
spectra (LINER 1s) are proportional to their Halpha luminosities. This
correlation strongly supports the hypothesis that the dominant ionizing source
in LINER 1s is photoionization by hard photons from low-luminosity AGNs. On the
other hand, the X-ray luminosities of most LINERs without broad Halpha emission
(LINER 2s) in our sample are lower than LINER 1s at a given Halpha luminosity.
The observed X-ray luminosities in these objects are insufficient to power
their Halpha luminosities, suggesting that their primary ionizing source is
other than an AGN, or that an AGN, if present, is obscured even at energies
above 2 keV.Comment: 11 pages, 3 figures, To appear in the Astrophyscal Jouna
Old and Young X-ray Point Source Populations in Nearby Galaxies
We analyzed 1441 Chandra X-ray point sources in 32 nearby galaxies. The total
point-source X-ray luminosity L_XP is well correlated with B, K, and FIR+UV
luminosities of spiral host galaxies, and with the B and K luminosities for
ellipticals. This suggests an intimate connection between L_XP and both the old
and young stellar populations, for which K and FIR+UV luminosities are proxies
for the galaxy mass M and star-formation rate SFR. We derive proportionality
constants 1.3E29 erg/s/Msol and 0.7E39 erg/s/(Msol/yr), which can be used to
estimate the old and young components from M and SFR, respectively. The
cumulative X-ray luminosity functions for the point sources have quite
different slopes for the spirals (gamma ~= 0.5-0.8) and ellipticals (gamma ~=
1.4), implying *the most luminous point sources dominate L_XP* for the spirals.
Most of the point sources have X-ray colors that are consistent with either
LMXBs or Ultraluminous X-ray sources (ULXs a.k.a. IXOs) and we rule out
classical HMXBs (e.g. neutron-star X-ray pulsars) as contributing much to L_XP.
Thus, for spirals, the ULXs dominate L_XP. We estimate that >~20% of all ULXs
found in spirals originate from the older (pop II) stellar populations,
indicating that many of the ULXs that have been found in spiral galaxies are in
fact pop II ULXs, like those in elliptical galaxies. The linear dependence of
L_XP on the SFR argues for either a steepening in the X-ray luminosity function
of the young (pop I) X-ray source population at L_X >~10^(38.5-39) erg/s, or a
decreasing efficiency for producing all types of young X-ray point sources as
the galaxy SFR increases.Comment: 33 pages AASTEX, ApJ accepted. Please download full version with
figures from http://www.pha.jhu.edu/~colbert/chps_accepted.p
Evidence for Elevated X-ray Emission in Local Lyman Break Galaxy Analogs
In this paper, we study the relationship between the 2-10 keV X-ray
luminosity (L_X), assumed to originate from X-ray binaries (XRBs), and star
formation rate (SFR) in UV-selected z<0.1 Lyman break analogs (LBAs). We
present Chandra observations for four new GALEX-selected LBAs. Including
previously studied LBAs, Haro 11 and VV 114, we find that LBAs demonstrate
L_X/SFR ratios that are elevated by ~1.5sigma compared to local galaxies,
similar to the ratios found for stacked LBGs in the early Universe (z>2). We
show that these LBAs are unlikely to harbor AGN, based on their optical and
X-ray spectra and the spatial distribution of the X-rays in three spatially
extended cases. We expect that high-mass X-ray binaries (HMXBs) dominate the
X-ray emission in these galaxies, based on their high specific SFRs
(sSFRs=SFR/M* > 10^{-9}/yr), which suggest the prevalence of young stellar
populations. Since both LBAs and LBGs have lower dust attenuations and
metallicities compared to similar samples of more typical local galaxies, we
investigate the effects of dust extinction and metallicity on the L_X/SFR for
the broader population of galaxies with high sSFRs (>10^{-10}/yr). The
estimated dust extinctions (corresponding to column densities of
N_H<10^{22}/cm^2) are expected to have insignificant effects on observed
L_X/SFR ratio for the majority of galaxy samples. We find that the observed
relationship between L_X/SFR and metallicity appears consistent with
theoretical expectations from X-ray binary population synthesis models.
Therefore, we conclude that lower metallicities, related to more luminous HMXBs
such as ultraluminous X-ray sources (ULXs), drive the elevated L_X/SFR observed
in our sample of z<0.1 LBAs. The relatively metal-poor, active mode of star
formation in LBAs and distant z>2 LBGs may yield higher total HMXB luminosity
than found in typical galaxies in the local Universe.Comment: 11 pages, 7 figures, Submitted to ApJ (references updated in v2
Chandra Reveals Heavy Obscuration and Circumnuclear Star Formation in Seyfert 2 Galaxy NGC 4968
We present the Chandra imaging and spectral analysis of NGC 4968, a nearby (z
= 0.00986) Seyfert 2 galaxy. We discover extended (1 kpc) X-ray emission
in the soft band (0.5 - 2 keV) that is neither coincident with the narrow line
region nor the extended radio emission. Based on spectral modeling, it is
linked to on-going star formation (2.6-4 M_{\sun} yr). The soft
emission at circumnuclear scales (inner 400 pc) originates from hot gas,
with kT 0.7 keV, while the most extended thermal emission is cooler (kT
0.3 keV). We refine previous measurements of the extreme Fe K
equivalent width in this source (EW = 2.5 keV), which suggests
the central engine is completely embedded within Compton-thick levels of
obscuration. Using physically motivated models fit to the Chandra spectrum, we
derive a Compton-thick column density (
cm) and an intrinsic hard (2-10 keV) X-ray luminosity of
3-8 erg s (depending on the presumed geometry of
the obscurer), which is over two orders of magnitude larger than that observed.
The large Fe K EW suggests a spherical covering geometry, which could
be confirmed with X-ray measurements above 10 keV. NGC 4968 is similar to other
active galaxies that exhibit extreme Fe K EWs (i.e., 2 keV) in that
they also contain ongoing star formation. This work supports the idea that gas
associated with nuclear star formation may increase the covering factor of the
enshrouding gas and play a role in obscuring AGN.Comment: 11 pages, 8 figures, 4 tables. Accepted for publication in Ap
On the Nature of the X-ray Emission from the Ultraluminous X-ray Source, M33 X-8: New Constraints from NuSTAR and XMM-Newton
We present nearly simultaneous NuSTAR and XMM-Newton observations of the
nearby (832 kpc) ultraluminous X-ray source (ULX) M33 X-8. M33 X-8 has a 0.3-10
keV luminosity of LX ~ 1.4 x 10^39 erg/s, near the boundary of the
"ultraluminous" classification, making it an important source for understanding
the link between typical Galactic X-ray binaries and ULXs. Past studies have
shown that the 0.3-10 keV spectrum of X-8 can be characterized using an
advection-dominated accretion disk model. We find that when fitting to our
NuSTAR and XMM-Newton observations, an additional high-energy (>10 keV)
Comptonization component is required, which allows us to rule out single
advection-dominated disk and classical sub-Eddington models. With our new
constraints, we analyze XMM-Newton data taken over the last 17 years to show
that small (~30%) variations in the 0.3-10 keV flux of M33 X-8 result in
spectral changes similar to those observed for other ULXs. The two most likely
phenomenological scenarios suggested by the data are degenerate in terms of
constraining the nature of the accreting compact object (i.e., black hole
versus neutron star). We further present a search for pulsations using our
suite of data; however, no clear pulsations are detected. Future observations
designed to observe M33 X-8 at different flux levels across the full 0.3-30 keV
range would significantly improve our constraints on the nature of this
important source.Comment: Accepted for publication in ApJ (15 pages, 4 tables, 6 figures
X-ray Properties of the Weak Seyfert 1 Nucleus in NGC 4639
We obtained observations of NGC 4639 with ASCA in order to investigate its
mildly active Seyfert 1 nucleus at hard X-ray energies. Koratkar et al. (1995)
have previously shown that the nucleus is a pointlike source in the ROSAT soft
X-ray band. We detected in the 2-10 keV band a compact central source with a
luminosity of 8.3E+40 erg/s. Comparison of the ASCA data with archival data
taken with the Einstein and ROSAT satellites shows that the nucleus varies on
timescales of months to years. The variability could be intrinsic, or it could
be caused by variable absorption. More rapid variability, on a timescale of
\~10^4 s, may be present in the ASCA data. The spectrum from 0.5 to 10 keV is
well described by a model consisting of a lightly absorbed (N_H = 7.3E+20
cm^-2) power law with a photon index of 1.68. We find no evidence for
significant emission from a thermal plasma; if present, it can account for no
more than 25% of the flux in the 0.5-2.0 keV band. The limited photon
statistics of our data do not allow us to place significant limits on the
presence of iron K emission. (abridged)Comment: To appear in The Astrophysical Journal. LaTex, 18 pages including
embedded figures and table
The Broadband XMM-Newton and NuSTAR X-ray Spectra of Two Ultraluminous X-ray Sources in the Galaxy IC 342
We present results for two Ultraluminous X-ray Sources (ULXs), IC 342 X-1 and
IC 342 X-2, using two epochs of XMM-Newton and NuSTAR observations separated by
7 days. We observe little spectral or flux variability above 1 keV
between epochs, with unabsorbed 0.3--30 keV luminosities being
erg s for IC 342 X-1 and
erg s for IC 342 X-2, so that both were
observed in a similar, luminous state. Both sources have a high absorbing
column in excess of the Galactic value. Neither source has a spectrum
consistent with a black hole binary in low/hard state, and both ULXs exhibit
strong curvature in their broadband X-ray spectra. This curvature rules out
models that invoke a simple reflection-dominated spectrum with a broadened iron
line and no cutoff in the illuminating power-law continuum. X-ray spectrum of
IC 342 X-1 can be characterized by a soft disk-like black body component at low
energies and a cool, optically thick Comptonization continuum at high energies,
but unique physical interpretation of the spectral components remains
challenging. The broadband spectrum of IC 342 X-2 can be fit by either a hot
(3.8 keV) accretion disk, or a Comptonized continuum with no indication of a
seed photon population. Although the seed photon component may be masked by
soft excess emission unlikely to be associated with the binary system, combined
with the high absorption column, it is more plausible that the broadband X-ray
emission arises from a simple thin blackbody disk component. Secure
identification of the origin of the spectral components in these sources will
likely require broadband spectral variability studies.Comment: 12 pages, 11 figures, 5 Tables, Accepted for publication in The
Astrophysical Journa
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