We present new theoretical period-luminosity-metallicity (PLZ) relations for RR Lyræ stars (RRLs) at Spitzer and WISE wavelengths. The PLZ relations were derived using nonlinear, time-dependent ...convective hydrodynamical models for a broad range of metal abundances (Z = 0.0001-0.0198). In deriving the light curves, we tested two sets of atmospheric models and found no significant difference between the resulting mean magnitudes. We also compare our theoretical relations to empirical relations derived from RRLs in both the field and in the globular cluster M4. Our theoretical PLZ relations were combined with multi-wavelength observations to simultaneously fit the distance modulus, 0, and extinction, AV, of both the individual Galactic RRL and of the cluster M4. The results for the Galactic RRL are consistent with trigonometric parallax measurements from Gaia's first data release. For M4, we find a distance modulus of 0 = 11.257 0.035 mag with AV = 1.45 0.12 mag, which is consistent with measurements from other distance indicators. This analysis has shown that, when considering a sample covering a range of iron abundances, the metallicity spread introduces a dispersion in the PL relation on the order of 0.13 mag. However, if this metallicity component is accounted for in a PLZ relation, the dispersion is reduced to ∼0.02 mag at mid-infrared wavelengths.
Abstract
Heavy ion signatures of coronal mass ejections (CMEs) indicate that rapid and strong heating takes place during the eruption and early stages of propagation. However, the nature of the ...heating that produces the highly ionized charge states often observed in situ is not fully constrained. An MHD simulation of the Bastille Day CME serves as a test bed to examine the origin and conditions of the formation of heavy ions evolving within the CME in connection with those observed during its passage at L1. In particular, we investigate the bimodal nature of the Fe charge state distribution, which is a quintessential heavy ion signature of CME substructure, as well as the source of the highly ionized plasma. We find that the main heating experienced by the tracked plasma structures linked to the ion signatures examined is due to field-aligned thermal conduction via shocked plasma at the CME front. Moreover, the bimodal Fe distributions can be generated through significant heating and rapid cooling of prominence material. However, although significant heating was achieved, the highest ionization stages of Fe ions observed in situ were not reproduced. In addition, the carbon and oxygen charge state distributions were not well replicated owing to anomalous heavy ion dropouts observed throughout the ejecta. Overall, the results indicate that additional ionization is needed to match observation. An important driver of ionization could come from suprathermal electrons, such as those produced via Fermi acceleration during reconnection, suggesting that the process is critical to the development and extended heating of extreme CME eruptions, like the Bastille Day CME.
We present our project to calibrate the RR Lyræ period-luminosity-metallicity relation using a sample of Galactic calibrators in the halo and globular clusters.
Accretion disks are among the most important and well-studied objects in astrophysics. Disks play a critical role in both star formation and later stellar evolution, and are the site of planet ...formation. Compact objects accreting from a stellar companion may result in luminous and rapidly evolving UV or X-ray disks for white dwarf (WD) or stellar-mass black hole and neutron star (BH or NS) accretors, respectively. At the largest scales, accretion onto supermassive black holes is known to power active galactic nuclei (or, AGN). These are among the most powerful phenomena in the observable universe and their activity can have a profound effect on star formation and evolution of the host galaxy. Despite the astrophysical importance of the accretion process, as well as considerable observational and theoretical efforts, critical questions regarding specific physical mechanisms remain unanswered. There is broad theoretical consensus that angular momentum and mass transfer within the disk are likely mediated via magnetic processes; however, observational signatures of specific physical mechanisms are few and largely indirect, complicating efforts to characterize underlying processes. Likewise, magnetic processes may be central to the physics of the X-ray corona, potentially a cloud of hot in AGN and accreting NSs/stellar-mass BHs that imparts additional energy to disk photons via inverse Compton scattering. Our understanding of the geometry, energetics, and underlying physics of the X-ray Corona is limited and requires knowledge of its compactness - a difficult property to constrain observationally.In this dissertation, I demonstrate how high-resolution X-ray spectroscopic studies of accreting black holes and neutron stars can address some of these issues. Specifically, I characterize absorption phenomena occurring above the disk's surface, such as disk winds and atmospheres, using astrophysical plasma models in order to probe the physical processes that underlie the accretion disk. We find that the highly ionized disk winds (outflowing at 0.1%-1% c) in BH candidate 4U 1630-472, for instance, are likely magnetic; the radial structure of these outflows may be indicative of magnetically driven accretion (Chapter II). Moreover, I present the discovery of gravitationally redshifted disk atmospheres in a sample of short-period neutron star systems. Like disk winds in BH systems, the location of these absorbers means that the atmosphere may require strong magnetic pressure support (Chapter III). Finally, I developed a new methodology for constraining the size of the central emitting regions of these systems, providing new angles on the physical nature of these emitters (Chapter IV).
Abstract
Analyses of absorption from disk winds and atmospheres in accreting compact objects typically treat the central emitting regions in these systems as point sources relative to the absorber. ...This assumption breaks down if the absorbing gas is located within a few × 1000
GM
/
c
2
, in which case a small component of the absorber’s Keplerian motion contributes to the velocity width of absorption lines. Here, we demonstrate how this velocity-broadening effect can be used to constrain the sizes of central engines in accreting compact objects via a simple geometric relationship, and develop a method for modeling this effect. We apply this method to the Chandra/HETG spectra of three ultracompact and short-period neutron star X-ray binaries in which evidence of gravitationally redshifted absorption, owing to an inner-disk atmosphere, has recently been reported. The significance of the redshift is above 5
σ
for XTE J1710−281 (this work) and 4U 1916−053, and is inconsistent with various estimates of the relative radial velocity of each binary. For our most sensitive spectrum (XTE J1710−281), we obtain a 1
σ
upper bound of 310 km s
−1
on the magnitude of this geometric effect and a central engine of size
R
CE
< 60
GM
/
c
2
(or < 90
GM
/
c
2
at the 3
σ
level). These initial constraints compare favorably to those obtained via microlensing in quasars and approach the sensitivity of constraints via relativistic reflection in neutron stars. This sensitivity will increase with further exposures, as well as the launch of future microcalorimeter and grating missions.
Extinction and the Dimming of KIC 8462852 Meng, Huan Y. A.; Rieke, George; Dubois, Franky ...
The Astrophysical journal,
10/2017, Volume:
847, Issue:
2
Journal Article
Peer reviewed
Open access
To test alternative hypotheses for the behavior of KIC 8462852, we obtained measurements of the star over a wide wavelength range from the UV to the mid-infrared from 2015 October through 2016 ...December, using Swift, Spitzer and AstroLAB IRIS. The star faded in a manner similar to the long-term fading seen in Kepler data about 1400 days previously. The dimming rate for the entire period reported is 22.1 9.7 mmag yr−1 in the Swift wavebands, with amounts of 21.0 4.5 mmag in the ground-based B measurements, 14.0 4.5 mmag in V, and 13.0 4.5 in R, and a rate of 5.0 1.2 mmag yr−1 averaged over the two warm Spitzer bands. Although the dimming is small, it is seen at 3 by three different observatories operating from the UV to the IR. The presence of long-term secular dimming means that previous spectral energy distribution models of the star based on photometric measurements taken years apart may not be accurate. We find that stellar models with K and best fit the Swift data from UV to optical. These models also show no excess in the near-simultaneous Spitzer photometry at 3.6 and 4.5 m, although a longer wavelength excess from a substantial debris disk is still possible (e.g., as around Fomalhaut). The wavelength dependence of the fading favors a relatively neutral color (i.e., , but not flat across all the bands) compared with the extinction law for the general interstellar medium ( ), suggesting that the dimming arises from circumstellar material.
The very small accretion disks in ultracompact X-ray binaries are special laboratories in which to study disk accretion and outflows. We report on three sets of new (250 ks total) and archival (50 ...ks) Chandra/HETG observations of the "dipping" neutron star X-ray binary 4U 1916-053, which has an orbital period of P 50 minutes. We find that the bulk of the absorption in all three spectra originates in a disk atmosphere that is redshifted by v 220-290 km s−1, corresponding to the gravitational redshift at a radius of R ∼ 1200 GM/c2. This shift is present in the strongest, most highly ionized lines (Si xiv and Fe xxvi), with a significance of 5 . Absorption lines observed during dipping events (typically associated with the outermost disk) instead display no velocity shifts and serve as a local standard of rest, suggesting that the redshift is intrinsic to an inner disk atmosphere and not due to radial motion in the galaxy or a kick. In two spectra, there is also evidence of a more strongly redshifted component that would correspond to a disk atmosphere at R ∼ 70 GM/c2; this component is significant at the 3 level. Finally, in one spectrum, we find evidence of a disk wind with a blueshift of . If real, this wind would require magnetic driving.
Analyses of absorption from disk winds and atmospheres in accreting compact objects typically treat the central emitting regions in these systems as point sources relative to the absorber. This ...assumption breaks down if the absorbing gas is located within \(few \times 1000\cdot GM/{c}^{2}\), in which case a small component of the absorber's Keplerian motion contributes to the velocity-width of absorption lines. Here, we demonstrate how this velocity-broadening effect can be used to constrain the sizes of central engines in accreting compact objects via a simple geometric relationship, and develop a method for modeling this effect. We apply this method on the Chandra/HETG spectra of three ultra-compact and short period neutron star X-ray binaries in which evidence of gravitationally redshifted absorption, owing to an inner-disk atmosphere, has recently been reported. The significance of the redshift is above \(5\sigma\) for XTE J1710\(-\)281 (this work) and 4U 1916\(-\)053, and is inconsistent with various estimates of the relative radial velocity of each binary. For our most sensitive spectrum (XTE J1710\(-\)281), we obtain a 1\(\sigma\) upper bound of 310 \(\text{km}\) \(\text{s}^{-1}\) on the magnitude of this geometric effect and a central engine of size \({R}_{CE} < 60 ~ GM/{c}^{2}\) (or, \(< 90 ~ GM/{c}^{2}\) at the \(3\sigma\) level). These initial constraints compare favorably to those obtained via microlensing in quasars and approach the sensitivity of constraints via relativistic reflection in neutron stars. This sensitivity will increase with further exposures, as well as the launch of future microcalorimeter and grating missions.
The mechanisms that drive disk winds are a window into the physical processes that underlie the disk. Stellar-mass black holes are an ideal setting in which to explore these mechanisms, in part ...because their outbursts span a broad range in mass accretion rate. We performed a spectral analysis of the disk wind found in six Chandra/HETG observations of the black hole candidate 4U~1630\(-\)472, covering a range of luminosities over two distinct spectral states. We modeled both wind absorption and extended wind re-emission components using PION, a self-consistent photoionized absorption model. In all but one case, two photoionization zones were required in order to obtain acceptable fits. Two independent constraints on launching radii, obtained via the ionization parameter formalism and the dynamical broadening of the re-emission, helped characterize the geometry of the wind. The innermost wind components (\(r \simeq {10}^{2-3}\) \(GM/{c}^{2}\)) tend towards small volume filling factors, high ionization, densities up to \(n \simeq {10}^{15-16} {\text{cm}}^{-3}\), and outflow velocities of \(\sim 0.003c\). These small launching radii and large densities require magnetic driving, as they are inconsistent with numerical and analytical treatments of thermally driven winds. Outer wind components (\(r \simeq {10}^{5}\) \(GM/{c}^{2}\)) are significantly less ionized and have filling factors near unity. Their larger launching radii, lower densities (\(n \simeq {10}^{12} {\text{cm}}^{-3}\)), and outflow velocities (\(\sim 0.0007c\)) are nominally consistent with thermally driven winds. The overall wind structure suggests that these components may also be part of a broader MHD outflow and perhaps better described as magneto-thermal hybrid winds.
The very small accretion disks in ultra-compact X-ray binaries (UCXBs) are special laboratories in which to study disk accretion and outflows. We report on three sets of new (250 ks total) and ...archival (50 ks) Chandra/HETG observations of the "dipping" neutron-star X-ray binary 4U 1916\(-\)053, which has an orbital period of \(P\simeq 50\)~minutes. We find that the bulk of the absorption in all three spectra originates in a disk atmosphere that is redshifted by \(v\simeq 220-290\) \(\text{km}\) \(\text{s}^{-1}\), corresponding to the gravitational redshift at radius of \(R \sim 1200\) \(GM/{c}^{2}\). This shift is present in the strongest, most highly ionized lines (Si XIV and Fe XXVI), with a significance of 5\(\sigma\). Absorption lines observed during dipping events (typically associated with the outermost disk) instead display no velocity shifts and serve as a local standard of rest, suggesting that the redshift is intrinsic to an inner disk atmosphere and not due to radial motion in the galaxy or a kick. In two spectra, there is also evidence of a more strongly redshifted component that would correspond to a disk atmosphere at \(R \sim 70\) \(GM/{c}^{2}\); this component is significant at the 3\(\sigma\) level. Finally, in one spectrum, we find evidence of disk wind with a blue shift of \(v = {-1700}^{+1700}_{-1200}\) \(\text{km}\) \(\text{s}^{-1}\). If real, this wind would require magnetic driving.
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