Abstract
The population of metastable levels is key to high-precision density diagnostics of astrophysical plasmas. In photoionized plasmas, density is used to infer the distance from the ionizing ...source, which is otherwise difficult to obtain. Perfecting models that compute these populations is thus crucial. The present paper presents a semianalytic hydrogenic approximation for assessing the relative importance of different processes in populating atomic levels. This approximation shows that in the presence of a radiation source, photoexcitations and collisional excitations are both important over a wide range of plasma temperatures and ionizing spectra, while radiative recombination is orders of magnitude weaker. The interesting case of Fe
+21
with a collisional radiative model with photoexcitation demonstrates this effect. The population of the first excited metastable level in Fe
+21
is sensitive to the electron number density in the critical range of
n
e
= 10
12
–10
15
cm
−3
; it was observed to be significantly populated in the X-ray spectrum of the 2005 outburst of the X-ray binary GRO J1655-40. The present model shows that photoexcitation is the predominant process indirectly populating the metastable level. For the photoionized plasma in the GRO J1655-40 outflow, the model indicates a measured value of
n
e
= (2.6 ± 0.5) × 10
13
cm
−3
, implying a distance from the source of
r
= (4.4 ± 0.4) × 10
10
cm. Finally, we show how the computed critical density and distance of Fe
+21
yield the correct ionization parameter of the ion, independent of ionization balance calculations.
Abstract
A large fraction of the baryons at low redshift are undetected and likely reside in the tenuous, hot intergalactic medium (IGM). One way to probe the missing baryons is through their ...absorption of bright sources. The anomalous absorption excess in the X-ray afterglows of gamma-ray bursts (GRBs) has been suggested to result from the missing baryons. In order to test this hypothesis, the present paper employs IllustrisTNG simulations to compute the X-ray absorption effect on cosmological distances. The simulation shows that ionization of H and He in the IGM leaves the metals responsible for >60% of the X-ray opacity of high-
z
sources. The high-
z
asymptotic optical depth at 0.5 keV in the simulation reaches 0.15 ± 0.07, while the GRB afterglow values tend to ≈0.4, implying that the missing baryons can account for a significant fraction of the observed opacity. The remaining discrepancy is ascribed mainly to the low average metallicity in the simulation, which drops from 0.06 solar at
z
= 0 to 0.01 at
z
= 3, and which is below previously measured values.
Abstract
Vela X-1 is the archetypal eclipsing high-mass X-ray binary, composed of a neutron star (NS) accreting the B-star wind. It was observed by nearly all X-ray observatories, often multiple ...times, featuring a rich spectrum of variable emission lines. However, the precise origin of these lines in the binary system remains uncertain. We perform a systematic, orbital-phase-dependent analysis of the reflected Fe K
α
fluorescence line at 6.4 keV using over 100 NS Interior Composition Explorer observations. We resolve the line variability into 500 s time bins and find that it is predominantly due to variation in the ionizing flux, with a moderate underlying phase dependence over the 9-day orbital period. Our analysis reveals a significant reflection component that cannot originate from the companion B star alone. We also find that an appreciable portion of the B-star surface is obscured opposite the eclipse, and this obscuration is not symmetric around the midpoint (phase = 0.5). We argue that an accretion stream, from the B star to the NS and distorted by the orbital motion, is responsible both for the additional fluorescence emission component and for obscuring the B star.
The radio emission in radio-loud quasars originates in a jet carrying relativistic electrons. In radio-quiet quasars (RQQs) the relative radio emission is ∼103 times weaker, and its origin is not ...established yet. We show here that there is a strong correlation between the radio luminosity (LR) and X-ray luminosity (LX) with LR∼ 10−5LX, for the radio-quiet Palomar–Green (PG) quasar sample. The sample is optically selected, with nearly complete radio and X-ray detections, and thus this correlation cannot be due to direct selection biases. The PG quasars lie on an extension of a similar correlation noted by Panessa et al., for a small sample of nearby low-luminosity type 1 active galactic nuclei (AGN). A remarkably similar correlation, known as the Güdel–Benz relation, where LR/LX∼ 10−5, holds for coronally active stars. The Güdel–Benz relation, together with correlated stellar X-ray and radio variability, implies that the coronae are magnetically heated. We therefore raise the possibility that AGN coronae are also magnetically heated, and that the radio emission in RQQ also originates in coronal activity. If correct, then RQQ should generally display compact flat cores at a few GHz due to synchrotron self-absorption, while at a few hundred GHz we should be able to see directly the X-ray emitting corona, and relatively rapid and large amplitude variability, correlated with the X-ray variability, is likely to be seen. We also discuss possible evidence that the radio and X-ray emission in ultraluminous X-ray sources and Galactic black holes may be of coronal origin as well.
Several studies indicate that radio-loud (RL) active galactic nuclei (AGNs) are produced only by the most massive black holes (BH), M
BH ∼ 108–1010 M⊙. This idea has been challenged by the discovery ...of RL Narrow Line Seyfert 1 (RL NLSy1), having estimated masses of M
BH ∼ 106–107 M⊙. However, these low M
BH estimates might be due to projection effects. Spectropolarimetry allows us to test this possibility by looking at RL NLSy1s under a different perspective, i.e. from the viewing angle of the scattering material. We here report the results of a pilot study of Very Large Telescope spectropolarimetric observations of the RL NLSy1 PKS 2004−447. Its polarization properties are remarkably well reproduced by models in which the scattering occurs in an equatorial structure surrounding its broad-line region, seen close to face-on. In particular, we detect a polarized Hα line with a width of ∼9000 km s−1, ∼6 times broader than the width seen in direct light. This corresponds to a revised estimate of M
BH ∼ 6 × 108 M⊙, well within the typical range of RL AGN. The double-peaked polarized broad Hα profile of the target suggests that the rare combination of the orientation effects and a broad line region dominated by the rotation might account for this class of objects, casting doubts on the virial estimates of BH mass for type-I AGN.
Abstract Supernovae are responsible for the elemental enrichment of the galaxy, and some are postulated to leave behind a black hole. In a stellar binary system, the supernova pollutes its companion, ...and the black hole can accrete back its own debris and emit X-rays. In this sequence of events, which is only poorly understood, winds are ejected and observed through X-ray absorption lines. Measuring abundances of elements in the wind can lead to inferences about the historical explosion and possibly identify the long-gone progenitor of the compact object. Here, we reanalyze the uniquely rich X-ray spectrum of the 2005 outburst of GRO J1655−40. We reconstruct the absorption measure distribution (AMD) of the wind and find that it increases sharply with ionization from H-like O up to H-like Ca and then flattens out. The AMD is then used to measure the relative abundances of 18 different elements. The present abundances are in partial agreement with a previous work with discrepancies mostly for low- Z elements. The overabundance of odd- Z elements hints at a high-metallicity, high-mass (≃25 M ⊙ ) progenitor. Interestingly, the abundances are different from those measured in the companion atmosphere, indicating that the wind entrains lingering ambient supernova debris. This can be expected since the current total stellar mass of the binary (<10 M ⊙ ) is much less than the progenitor mass.
Abstract
We analyze three prototypical black hole X-ray binaries, 4U 1630–472, GRO J1655–40, and H1743–322, in an effort to systematically understand the intrinsic state transition of the observed ...accretion disk winds between
wind-on
and
wind-off
states by utilizing state-of-the-art Chandra/HETGS archival data from multi-epoch observations. We apply our magnetically driven wind models in the context of magnetohydrodynamic (MHD) calculations to constrain (1) their global density slope (
p
), (2) their density (
n
17
) at the foot point of the innermost launching radius, and (3) the abundances of heavier elements (
A
Fe,S,Si
). Incorporating the MHD winds into
xstar
photoionization calculations in a self-consistent manner, we create a library of synthetic absorption spectra given the observed X-ray continua. Our analysis clearly indicates a characteristic bimodal transition of multi-ion X-ray winds; i.e., the wind density gradient is found to steepen (from
p
∼ 1.2–1.4 to ∼1.4–1.5) while its density normalization declines as the source transitions from the
wind-on
to the
wind-off
state. The model implies that the ionized wind remains physically present even in the
wind-off
state, despite its apparent absence in the observed spectra. Supersolar abundances for heavier elements are also favored. Our global multi-ion wind models, taking into account soft X-ray ions as well as Fe K absorbers, show that the internal wind condition plays an important role in wind transitions besides photoionization changes. Simulated XRISM/Resolve and Athena/X-IFU spectra are presented to demonstrate a high fidelity of the multi-ion wind model for a better understanding of these powerful ionized winds in the coming decades.
Abstract The origin of bright X-ray emission lines that appear late in a nova eruption remains largely a puzzle. We present two high-resolution X-ray grating spectra of the classical nova YZ Ret, ...observed 77 and 115 days post-eruption, using XMM-Newton and Chandra, respectively. Both spectra feature resolved emission lines blueshifted by v = –1500 km s −1 and broadened by σ v = 500 km s −1 . The two spectra are well described by a collisionally ionized plasma of kT ∼ 70 eV that dimmed by a factor of ∼40 between the two exposures. The spectra also show narrow radiative recombination continua (RRCs) of C +4 , C +5 , and N +5 , indicating the interaction of the hot ionized plasma with cold electrons of kT ∼ 2 eV. The high- n Rydberg series of C +4 is anomalously bright, allowing us to measure the electron density through continuum lowering, which is in agreement with the He-like N +5 density diagnostic of n e = (1.7 ± 0.4) × 10 11 cm −3 . The high population of these high- n levels constitutes the best evidence to date of charge exchange (CX) with neutral H in an astrophysical ionized plasma. The remarkable fact that the velocity and plasma temperature are the same after 38 days, despite the high density and decreasing flux, is evidence for ongoing heating. We suggest the heating is due to a reverse shock in the nova ejecta, which forms a thin X-ray shell. The narrow RRCs and CX are attributed to direct mixing with cold gas, which overtakes the hot plasma either from the shock front or through the contact discontinuity.
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