arXiv subjects

Ehud Behar

Publications and source records attributed to Ehud Behar.

At least 145 records · Page 8Linked to original sources

arXiv2013

Stratified Magnetically-Driven Accretion-Disk Winds and Their Relations to Jets

We explore the poloidal structure of two-dimensional (2D) MHD winds in relation to their potential association with the X-ray warm absorbers (WAs) and the highly-ionized ultra-fast outflows (UFOs) in AGN, in a single unifying approach. We present the density $n(r,θ)$, ionization parameter $ξ(r,θ)$, and velocity structure $v(r,θ)$ of such ionized winds for typical values of their fluid-to-magnetic flux ratio, $F$, and specific angular momentum, $H$, for which wind solutions become super-\Alfvenic. We explore the geometrical shape of winds for different values of these parameters and delineate the values that produce the widest and narrowest opening angles of these winds, quantities necessary in the determination of the statistics of AGN obscuration. We find that winds with smaller $H$ show a poloidal geometry of narrower opening angles with their \Alfven\ surface at lower inclination angles and therefore they produce the highest line of sight (LoS) velocities for observers at higher latitudes with the respect to the disk plane. We further note a physical and spatial correlation between the X-ray WAs and UFOs that form along the same LoS to the observer but at different radii, $r$, and distinct values of $n$, $ξ$ and $v$ consistent with the latest spectroscopic data of radio-quiet Seyfert galaxies. We also show that, at least in the case of 3C 111, the winds' pressure is sufficient to contain the relativistic plasma responsible for its radio emission. Stratified MHD disk-winds could therefore serve as a unique means to understand and unify the diverse AGN outflows.

astro-ph.HE↗

arXiv2013

X-ray Absorption of High Redshift Quasars

Soft X-ray photoelectric absorption of high-z quasars has been known for two decades, but has no unambiguous astro-physical context. We construct the largest sample to date of 58 high redshift quasars (z > 0.45) selected from the XMM-Newton archive based on a high photon count criterion (> 1800). We measure the optical depth tau at 0.5 keV, and find that 43% of the quasars show significant absorption. We aim to find which physical parameters of the quasars drive their observed absorption, e.g., redshift, radio luminosity, radio loudness, or the X-ray luminosity. We compare the absorption behavior with redshift with the pattern expected if the diffuse intergalactic medium (IGM) is responsible for the observed absorption, and with a comparison sample of gamma ray burst (GRB) X-ray afterglows. Although the z > 2 quasar opacity is consistent with diffuse IGM absorption, many intermediate z (0.45 < z < 2) quasars are not sufficiently absorbed for this scenario, and are appreciably less absorbed than GRBs. Only 10/37 quasars at z < 2 are absorbed, and only 5/30 radio quiet quasars are absorbed. We find a weak correlation between tau and z, and even a weaker correlation between tau and radio luminosity, which leads to the conclusion that although a diffuse IGM origin for the quasar absorption is unlikely, optical depth does seem to increase with redshift, roughly as (1+z)^{2.2+-0.6}, tending at high-z to tau ~ 0.4, similar to the high-z GRB values. This result can be explained by an ionized and clumpy IGM at z < 2, and a cold, diffuse IGM at higher redshift. If, conversely, ascribed to local absorption at the quasar, and owing to the steep L_x ~ (1+z)^{7.1+-0.5} correlation in the present sample, the host column density scales as N_H ~ L_x^{0.7+-0.1}.

astro-ph.CO↗

arXiv2013

The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission

This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Feedback from supermassive black holes is an essential ingredient in this process and in most galaxy evolution models, but it is not well understood. X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales. Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of supermassive black holes through cosmic time, out to the redshifts where the first galaxies form. X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe. The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions and revolutionize our understanding of the Hot and Energetic Universe. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics.

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arXiv2012

Variability of a stellar corona on a time scale of days

Elemental abundance effects in active coronae have eluded our understanding for almost three decades, since the discovery of the First Ionization Potential (FIP) effect on the sun. The goal of this paper is to monitor the same coronal structures over a time interval of six days and resolve active regions on a stellar corona through rotational modulation. We report on four iso-phase X-ray spectroscopic observations of the RS CVn binary EI Eri with XMM-Newton, carried out approximately every two days, to match the rotation period of EI Eri. We present an analysis of the thermal and chemical structure of the Ei Eri corona as it evolves over the six days. Although the corona is rather steady in its temperature distribution, the emission measure and FIP bias both vary and seem to be correlated. An active region, predating the beginning of the campaign, repeatedly enters into our view at the same phase as it rotates from beyond the stellar limb. As a result, the abundances tend slightly, but consistently, to increase for high FIP elements (an inverse FIP effect) with phase. We estimate the abundance increase of high FIP elements in the active region to be of ~75% over the coronal mean. This observed fractionation of elements in an active region on time scales of days provides circumstantial clues regarding the element enrichment mechanism of non-flaring stellar coronae.

astro-ph.SR↗

arXiv2012

Accretion and outflow of gas in Markarian 509

A major uncertainty in models for photoionised outflows in AGN is the distance of the gas to the central black hole. We present the results of a massive multiwavelength monitoring campaign on the bright Seyfert 1 galaxy Mrk 509 to constrain the location of the outflow components dominating the soft X-ray band. Mrk 509 was monitored by XMM-Newton, Integral, Chandra, HST/COS and Swift in 2009. We have studied the response of the photoionised gas to the changes in the ionising flux produced by the central regions. We were able to put tight constraints on the variability of the absorbers from day to year time scales. This allowed us to develop a model for the time-dependent photoionisation in this source. We find that the more highly ionised gas producing most X-ray line opacity is at least 5 pc away from the core; upper limits to the distance of various absorbing components range between 20 pc up to a few kpc. The more lowly ionised gas producing most UV line opacity is at least 100 pc away from the nucleus. These results point to an origin of the dominant, slow (v<1000 km/s) outflow components in the NLR or torus-region of Mrk 509. We find that while the kinetic luminosity of the outflow is small, the mass carried away is likely larger than the 0.5 Solar mass per year accreting onto the black hole. We also determined the chemical composition of the outflow as well as valuable constraints on the different emission regions. We find for instance that the resolved component of the Fe-K line originates from a region 40-1000 gravitational radii from the black hole, and that the soft excess is produced by Comptonisation in a warm (0.2-1 keV), optically thick (tau~10-20) corona near the inner part of the disk.

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arXiv2012

Toward a Unified AGN Structure

We present a unified model for the structure and appearance of accretion powered sources across their entire luminosity range from galactic X-ray binaries to luminous quasars, with emphasis on AGN and their phenomenology. Central to this model is the notion of MHD winds launched from the accretion disks that power these objects. These winds provide the matter that manifests as blueshifted absorption features in the UV and X-ray spectra of a large fraction of these sources; furthermore, their density distribution in the poloidal plane determines the "appearance" (i.e. the column and velocity structure of these absorption features) as a function of the observer inclination angle. This work focuses on just the broadest characteristics of these objects; nonetheless, it provides scaling laws that allow one to reproduce within this model the properties of objects spanning a very wide luminosity range and viewed at different inclination angles, and trace them to a common underlying dynamical structure. Its general conclusion is that the AGN phenomenology can be accounted for in terms of three parameters: The wind mass flux in units of the Eddington value, $\dot m$, the observer's inclination angle $θ$ and the logarithmic slope between the O/UV and X-ray fluxes $α_{OX}$. However, because of a significant correlation between $α_{OX}$ and UV luminosity, we conclude that the AGN structure depends on only two parameters. Interestingly, the correlations implied by this model appear to extend to and consistent with the characteristics of galactic X-ray sources, suggesting the presence of a truly unified underlying structure for accretion powered sources.

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arXiv2012

Broad Band Photometric Reverberation Mapping of NGC 4395

We present results of broad band photometric reverberation mapping (RM) to measure the radius of the broad line region, and subsequently the black hole mass (M$_{\rm BH}$), in the nearby, low luminosity active galactic nuclei (AGN) NGC 4395. Using the Wise Observatory's 1m telescope equipped with the SDSS g$'$, r$'$ and i$'$ broad band filters, we monitored NGC 4395 for 9 consecutive nights and obtained 3 light curves each with over 250 data points. The g$'$ and r$'$ bands include time variable contributions from H$β$ and H$α$ (respectively) plus continuum. The i$'$ band is free of broad lines and covers exclusively continuum. We show that by looking for a peak in the difference between the cross-correlation and the auto-correlation functions for all combinations of filters, we can get a reliable estimate of the time lag necessary to compute M$_{\rm BH}$. We measure the time lag for H$α$ to be $3.6 \pm 0.8 $ hours, comparable to previous studies using the line resolved spectroscopic RM method. We argue that this lag implies a black hole mass of M$_{\rm BH} = (4.9 \pm 2.6) \times 10^{4}$ \Msun .

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arXiv2011

X-Ray Absorption Analysis of NGC3516: Appearance of Fast Components with Increased Source Flux

By analyzing the X-ray spectra of NGC 3516 from 2001 and 2006 obtained with the HETGS spectrometer on board the Chandra observatory, we find that the kinematic structure of the outflow can be well represented by four outflow components intrinsic to NGC 3516. The outflow velocities of the different components are 350 +-100 km s-1, 1500 +-150 km s-1, 2600 +-200 km s-1 and 4000 +-400 km s-1 for components 1, 2, 3 and 4, respectively. A local component at z = 0 could be confused with intrinsic component 3. Components 1 and 2 have a broad range of ionization manifested by absorption from 23 different charge states of Fe. Component 3 and 4 are more highly ionized and show absorption from only 9 different charge states of Fe, however we were able to reconstruct the absorption measure distribution (AMD) for all four. The total column density of each component is NH = (1.8+- 0.5) X10^22 cm-2, NH = (2.5+- 0.3) X10^22 cm-2, NH = (6.9+- 4.3) X10^22 cm-2 and NH = (5.4+- 1.2) X10^22 cm-2, respectively. The fast components 3 and 4 appear only in the high state of 2006 and not in 2001, while the slower components persist during both epochs. On the other hand, there is no significant absorption variability within days during 2001 or during 2006. We find that covering factor plays a minor role for the line absorption.

astro-ph.GA↗

arXiv2011

Reverberation Mapping of the Intermediate Mass Nuclear Black Hole in SDSS J114008.71+030711.4

We present the results of a reverberation mapping (RM) campaign on the black hole (BH) associated with the active galactic nucleus (AGN) in SDSS J114008.71+030711.4 (hereafter GH08). This object is selected from a sample of 19 candidate intermediate mass BHs (M_{BH} < 10^{6} Msun) found by Greene & Ho 2004 in the Sloan Digital Sky Survey (SDSS). We used the Hobby-Eberly Telescope to obtain 30 spectra over a period of 178 days in an attempt to resolve the reverberation time lag (tau) between the continuum source and the broad line region (BLR) in order to determine the radius of the BLR (R_{BLR}) in GH08. We measure tau to be 2 days with an upper limit of 6 days. We estimate the AGN luminosity at 5100 Angstroms to be approximately 1.1 x 10^{43} erg s^{-1} after deconvolution from the host galaxy. The most well calibrated R_{BLR}-L relation predicts a time lag which is 4 times larger than what we measure. Using the measured Hβ full-width-at-half-maximum of 703 (+/-) 110 km s^{-1} and an upper limit for R_{BLR} = 6 light days, we find M_{BH} < 5.8 x 10^{5} Msun as an upper limit to the BH virial mass in GH08, which implies super-Eddington accretion. Based on our measured M_{BH} we propose that GH08 may be another candidate to add to the very short list of AGNs with M_{BH} < 10^{6} Msun determined using RM.

astro-ph.CO↗

arXiv2011

Galactic Scale Absorption Outflow in the Low Luminosity Quasar IRAS~F04250-5718: HST/COS Observations

We present absorption line analysis of the outflow in the quasar IRAS F04250-5718. Far-ultraviolet data from the Cosmic Origins Spectrograph onboard the Hubble Space Telescope reveal intrinsic narrow absorption lines from high ionization ions (e.g., C IV, N V, and O VI) as well as low ionization ions (e.g., C II and Si III). We identify three kinematic components with central velocities ranging from ~-50 to ~-230 km/s. Velocity dependent, non-black saturation is evident from the line profiles of the high ionization ions. From the non-detection of absorption from a metastable level of C II, we are able to determine that the electron number density in the main component of the outflow is < 30 per cubic cm. Photoionization analysis yields an ionization parameter log U ~ -1.6 +/- 0.2, which accounts for changes in the metallicity of the outflow and the shape of the incident spectrum. We also consider solutions with two ionization parameters. If the ionization structure of the outflow is due to photoionization by the active galactic nucleus, we determine that the distance to this component from the central source is > 3 kpc. Due to the large distance determined for the main kinematic component, we discuss the possibility that this outflow is part of a galactic wind.

astro-ph.CO↗

arXiv2011

Have the missing cosmic baryons been found?

The angular power spectrum and polarization of the cosmic microwave background radiation (CMB), the relative abundances of primordial hydrogen, deuterium and helium isotopes, and the large-scale structure of the universe all indicate that 4.5% of the current mass density of the universe consists of baryons. However, only a small fraction of these baryons can be accounted for in stars and gas inside galaxies, galaxy groups and galaxy clusters, and in spectral-line absorbing gas in the intergalactic medium (IGM). Too hot to show up in Lyman-absorption, too cool to cause detectable spectral distortions of the cosmic microwave background radiation, and too diffused to emit detectable X-rays, about 90% of the cosmic baryons remain missing in the local universe (redshift z~0). Here, we report on prevalent, isotropic, source independent, and fairly uniform soft X-ray absorption along the lines of sight to high-z gamma-ray bursts (GRBs) and quasars. It has the magnitude, redshift and energy dependence that are expected from a hot IGM that contains the missing cosmological baryons and has a mean metallicity similar to that in the intracluster medium (ICM) of galaxy clusters.

astro-ph.CO↗

arXiv2011

Can the soft X-ray opacity towards high redshift sources probe the missing baryons?

Observations with the Swift satellite of X-ray afterglows of more than a hundred gamma ray bursts (GRBs) with known redshift reveal ubiquitous soft X-ray absorption. The directly measured optical depth τat a given observed energy is found to be constant on average at redshift z > 2, i.e., <τ(0.5 keV) >_{z > 2} = 0.40+/- 0.02. Such an asymptotic optical depth is expected if the foreground diffuse intergalactic medium (IGM) dominates the absorption effect, and if the metallicity of the diffuse IGM reaches ~ 0.2 - 0.4 solar at z = 0. To further test the IGM absorption hypothesis, we analyze the 12 highest S/N (> 5000 photon) z > 2 quasar spectra from the XMM-Newton archive, which are all extremely radio loud (RLQs). The quasar optical depths are found to be consistent with the mean GRB value. The four lowest-z quasars (2 < z < 2.5), however, do not show significant absorption. The best X-ray spectra of radio-quiet quasars (RQQs) at z > 2 provide only upper limits to the absorption, which are still consistent with the RLQs, albeit with much lower S/N (< 1000 photons at z ~ 4). Lack of quasar absorption poses a challenge to the smooth IGM interpretation, and could allude to the opacity being rather due to the jets in RLQs and GRBs. However, the jet absorbing column would need to appear in RLQs only at z > 2.5, and in GRBs to strongly increase with z in order to produce the observed tendency to a constant mean τ. High X-ray spectral resolution can differentiate between an absorber intrinsic to the source that produces discernible spectral lines, and the diffuse IGM that produces significant absorption, but no discrete features.

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arXiv2010

Comparing shocks in planetary nebulae with the solar wind termination shock

We show that suprathermal particles, termed pick-up ions (PUIs), might reduce the postshock temperature of the fast wind and jets in some planetary nebulae (PNs) and in symbiotic systems. The goal is to explain the finding that the temperature of the hot bubble formed by the post-shock gas in some PNs and symbiotic nebulae is lower, sometimes by more than an order of magnitude, than the value expected from simple hydrodynamical calculations. Although various explanations have been proposed, there is as yet no prefered solution for this low tempeature problem. PUIs have been invoked to explain the low temperature behind the termination shock of the solar wind. While in the case of the solar wind the neutral atoms that turn into PUIs penetrate the pre-shock solar wind region from the interstellar medium (ISM), in PNs the PUI source is more likely slowly moving clumps embedded in the fast wind or jets. These clumps are formed by instabilities or from backflowing cold gas. Our estimates indicate that in young PNs these PUIs will thermalize before leaving the system. Only in older PNs whose sizes exceed ~5000 AU and for which the fast-wind mass loss rate is below ~10^{-7} Mo/yr do we expect the PUIs to be an efficient carrier of energy out of the postshock region (the hot bubble).

astro-ph.SR↗

arXiv2010

Modeling High-Velocity QSO Absorbers with Photoionized MHD Disk-Winds

We extend our modeling of the ionization structure of magnetohydrodynamic (MHD) accretion-disk winds, previously applied to Seyfert galaxies, to a population of quasi-stellar-objects (QSOs) of much lower X-ray-to-UV flux ratios, i.e. smaller $α_{\rm ox}$ index, motivated by UV/X-ray ionized absorbers with extremely high outflow velocities in UV-luminous QSOs. We demonstrate that magnetically-driven winds ionized by a spectrum with $α_{\rm ox} \simeq -2$ can produce the charge states responsible for \civ ~and \fexxv/\fexxvi ~absorption in wind regions with corresponding maximum velocities of $v$(\civ) $\lsim 0.1c$ and $v({\rm \fexxv}) \lsim 0.6 c$ (where $c$ is the speed of light) and column densities $N_H \sim 10^{23}-10^{24}$ cm$^{-2}$, in general agreement with observations. In contrast to the conventional radiation-driven wind models, {\it high-velocity flows are always present in our MHD-driven winds} but manifest in the absorption spectra only for $α_{\rm ox} \lsim -2$, as larger $α_{\rm ox}$ values ionize the wind completely out to radii too large to demonstrate the presence of these high velocities. We thus predict increasing velocities of these ionized absorbers with decreasing (steeper) $α_{\rm ox}$, a quantity that emerges as the defining parameter in the kinematics of the AGN UV/X-ray absorbers.

astro-ph.HE↗

arXiv2010

XMM-Newton detection of a transient X-ray source in the vicinity of V838 Monocerotis

We report the XMM-Newton/EPIC detection in 2008 March of a luminous (L_X ~ 10^32-33 erg/s), variable X-ray source in the vicinity (within ~6") of the enigmatic star V838 Mon, which underwent a spectacular outburst in early 2002. Spectral modeling of the XMM-Newton X-ray source indicates the presence of two plasma components with characteristic temperatures of TX ~ 2x10^6K and ~1.5x10^7K, attenuated by an absorbing column (N_H ~ 4 x 10^21cm^-2) that is consistent with the visual extinction measured toward V838 Mon (A_V ~ 2). No such luminous source was detected in the immediate vicinity of V838 Mon in Chandra/ACIS-S observations obtained about one year after outburst or, most recently, in 2010 January. The two XMM source spectral components appear to be marginally spatially resolved, with the spatial centroid of the hard component lying closer to (within ~2" of) the position of V838 Mon than the broad-band source or the soft source component; however, if there are two sources at or near V838 Mon, the Chandra nondetections would imply that both of them are variable. An alternative is that there is a single variable source, and that the apparent spatial separation may actually be due to photon-counting statistics or is perhaps instrumental in origin. We consider constraints placed by the X-ray detection and nondetections on a stellar merger model for the 2002 V838 Mon outburst, in which the spun-up merger product drives a powerful magnetic dynamo. Alternatively, the transient behavior of the X-ray source could indicate that the X-rays arose as a consequence of an interaction between V838 Mon's ejecta and its early-type (B3 V) companion.

astro-ph.SR↗

arXiv2010

MHD Accretion-Disk Winds as X-ray Absorbers in AGNs

We present the 2D ionization structure of self-similar MHD winds off accretion disks around irradiated by a central X-ray source. Based on earlier observational clues and theoretical arguments, we focus our attention on a subset of these winds, namely those with radial density dependence n(r)~1/r. We employ the photoionization code XSTAR to compute the ionic abundances of a large number of ions of different elements and then compile their line-of-sight (LOS) absorption columns. Particular attention is paid to the absorption measure distribution (AMD), namely their Hydrogen-equivalent column per logarithmic ionization parameter ξinterval, d N_H/(d \log ξ), which provides a measure of the winds' radial density profiles. For n(r)~1/r the AMD is found to be independent of ξ, in good agreement with its behavior inferred from the X-ray spectra of several AGNs. For the specific wind structure and X-ray spectrum we also compute detailed absorption line profiles for a number of ions to obtain their LOS velocities, v~100-300 km/sec (at \log ξ~2-3) for Fe XVII and v~1,000-4,000 km/sec (at \log ξ~4-5) for Fe XXV, in good agreement with the observation. Our models describe the X-ray absorption properties of these winds with only two parameters, namely the mass-accretion rate \dot{m} and LOS angle θ. The probability of obscuration of the X-ray ionizing source in these winds decreases with increasing \dot{m} and increases steeply with θ. As such, we concur with previous authors that these wind configurations, viewed globally, incorporate all the requisite properties of the parsec scale "torii" invoked in AGN unification schemes. We indicate that a combination of the AMD and absorption line profile observations can uniquely determine these model parameters and their bearing on AGN population demographics.

astro-ph.HE↗

arXiv2009

Remarkable Spectral Variability of PDS 456

We report on the highest to date signal-to-noise-ratio X-ray spectrum of the luminous quasar PDS 456, as obtained during two XMM-Newton orbits in September 2007. The present spectrum is considerably different from several previous X-ray spectra recorded for PDS 456 since 1998. The ultra-high-velocity outflow seen as recently as February 2007 is not detected in absorption. Conversely, a significant reflection component is detected. The reflection model suggests the reflecting medium may be outflowing at a velocity v/c = -0.06 +/- 0.02. The present spectrum is analyzed in the context of the previous ones in an attempt to understand all spectra within the framework of a single model. We examine whether an outflow with variable partial covering of the X-ray source along the line of sight that also reflects the source from other lines of sight can explain the dramatic variations in the broad-band spectral curvature of PDS 456. It is established that absorption plays a major role in shaping the spectrum of other epochs, while the 2007 XMM-Newton spectrum is dominated by reflection, and the coverage of the source by the putative outflow is small (< 20%).

astro-ph.CO↗

arXiv2009

X-Ray Absorption Analysis of MCG-6-30-15: Discerning Three Kinematic Systems

By analyzing the X-ray spectrum of MCG-6-30-15 obtained with the HETG spectrometer on board the Chandra Observatory, we identify three kinematically distinct absorption systems; two outflow components intrinsic to MCG-6-30-15, and one local at z = 0. The slow outflow at -100 +/- 50 km s^-1 has a large range of ionization manifested by absorption from 24 different charge states of Fe, which enables a detailed reconstruction of the absorption measure distribution (AMD). This AMD spans five orders of magnitude in ionization parameter: -1.5 < log xi < 3.5 (cgs units), with a total column density of N_H = (5.3 +/- 0.7) x 10^21 cm^-2. The fast outflow at -1900 +/- 150 km s^-1 has a well defined ionization parameter with log xi = 3.82 +/- 0.03 (cgs units) and column density N_H = 8.1 +/- 0.7 x 10^22 cm^-2. Assuming this component is a thin, uniform, spherical shell, it can be estimated to lie within 11 light days of the \agn center. The third component, most clearly detected in the lower oxygen charge states O^+1 - O^+6, has been confused in the past with the fast outflow, but is identified here with local gas z = 0 and a total column density N_H of a few 10^20 cm^-2. Finally, we exploit the excellent spectral resolution of the HETGS and use the present spectrum to determine the rest-frame wavelengths of oxygen inner-shell lines that were previously uncertain.

astro-ph.GA↗