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Register a new userAn Ejecta Kinematics Study of Keplers Supernova Remnant with High-Resolution $Chandra$ HETG Spectroscopy
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We report our measurements of the bulk radial velocity from a sample of small, metal-rich ejecta knots in Keplers Supernova Remnant (SNR). We measure the Doppler shift of the He-like Si K$alpha$ line center energy in the spectra of these knots based on our $Chandra$ High-Energy Transmission Grating Spectrometer (HETGS) observation to estimate their radial velocities. We estimate high radial velocities of up to $sim$ 8,000 km s$^{-1}$ for some of these ejecta knots. We also measure proper motions for our sample based on the archival $Chandra$ Advanced CCD Imaging Spectrometer (ACIS) data taken in 2000, 2006, and 2014. Our measured radial velocities and proper motions indicate that some of these ejecta knots are almost freely-expanding after $sim$ 400 years since the explosion. The fastest moving knots show proper motions up to $sim$ 0.2 arcseconds per year. Assuming that these high velocity ejecta knots are traveling ahead of the forward shock of the SNR, we estimate the distance to Keplers SNR $d$ $sim$ 4.4 to 7.5 kpc. We find that the ejecta knots in our sample have an average space velocity of $ v_{s} sim$ 4,600 km s$^{-1}$ (at a distance of 6 kpc). We note that 8 out of the 15 ejecta knots from our sample show a statistically significant (at the 90$%$ confidence level) redshifted spectrum, compared to only two with a blueshifted spectrum. This may suggest an asymmetry in the ejecta distribution in Keplers SNR along the line of sight, however a larger sample size is required to confirm this result.
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As one of the best-characterized stellar-mass black holes, with good measurements of its mass, distance and inclination, V404 Cyg is the ideal candidate to study Eddington-limited accretion episodes. After a long quiescent period, V404 Cyg underwent a new outburst in June 2015. We obtained two Chandra HETG exposures of 20 ksec and 25 ksec. Many strong emission lines are observed; the ratio of Si He-like triplet lines gives an estimate for the formation region distance of $4times10^{11}$ cm, while the higher ionization Fe XXV He-like triplet gives an estimate of $7times10^9$ cm. A narrow Fe K$alpha$ line is detected with an equivalent width greater than 1 keV in many epochs, signaling that we do not directly observe the central engine. Obscuration of the central engine and strong narrow emission lines signal that the outer disk may be illuminated, and its structure may help to drive the strong variability observed in V404 Cyg. In the highest flux phases, strong P-Cygni profiles consistent with a strong disk wind are observed. The kinetic power of this wind may be extremely high.
Based on observations with the $Chandra$ X-ray Observatory, we present the latest spectral evolution of the X-ray remnant of SN 1987A (SNR 1987A). We present a high-resolution spectroscopic analysis using our new deep ($sim$312 ks) $Chandra$ HETG observation taken in March 2018, as well as archival $Chandra$ gratings spectroscopic data taken in 2004, 2007, and 2011 with similarly deep exposures ($sim$170 - 350 ks). We perform detailed spectral model fits to quantify changing plasma conditions over the last 14 years. Recent changes in electron temperatures and volume emission measures suggest that the shocks moving through the inner ring have started interacting with less dense circumstellar material, probably beyond the inner ring. We find significant changes in the X-ray line flux ratios (among H- and He-like Si and Mg ions) in 2018, consistent with changes in the thermal conditions of the X-ray emitting plasma that we infer based on the broadband spectral analysis. Post-shock electron temperatures suggested by line flux ratios are in the range $sim$0.8 - 2.5 keV as of 2018. We do not yet observe any evidence of substantial abundance enhancement, suggesting that the X-ray emission component from the reverse-shocked metal-rich ejecta is not yet significant in the observed X-ray spectrum.
We report measurements of proper motion, radial velocity, and elemental composition for 14 X-ray knots in Keplers supernova remnant (SNR) using Chandra data. The highest speed knots show both large proper motions (0.11-0.14 /yr) and high radial velocities (v ~ 8,700--10,020 km/s) with estimated space velocities comparable to the typical Si velocity (~10,000 km/s) seen in SN Ia near maximum light. High speed ejecta knots appear only in specific locations and are morphologically and kinematically distinct from the rest of the ejecta. The proper motions of five knots extrapolate back over the age of Keplers SNR to a consistent central position that agrees well with previous determinations, but is less subject to systematic errors. These five knots are expanding at close to the free expansion rate (expansion indices of 0.75 <~ m <~ 1.0), which we argue indicates either that they were formed in the explosion with a high density contrast (more than 100 times the ambient density) or that they have propagated through relatively low density (n_H < 0.1 cm^-3) regions in the ambient medium. X-ray spectral analysis shows that the undecelerated knots have high Si and S abundances, a lower Fe abundance and very low O abundance, pointing to an origin in the partial Si-burning zone, which occurs in the outer layer of the exploding white dwarf for SN Ia models. Other knots show slower speeds and expansion indices consistent with decelerated ejecta knots or features in the ambient medium overrun by the forward shock. Our new accurate location for the explosion site has well-defined positional uncertainties allowing for a great reduction in the area to be searched for faint surviving donor stars under non-traditional single-degenerate SN Ia scenarios; because of the lack of bright stars in the search area the traditional scenario remains ruled out.
Supernova 1604 is the last Galactic supernova for which historical records exist. Johannes Keplers name is attached to it, as he published a detailed account of the observations made by himself and European colleagues. Supernova 1604 was very likely a Type Ia supernova, which exploded 350 pc to 750 pc above the Galactic plane. Its supernova remnant, known as Keplers supernova remnant, shows clear evidence for interaction with nitrogen-rich material in the north/northwest part of the remnant, which, given the height above the Galactic plane, must find its origin in mass loss from the supernova progenitor system. The combination of a Type Ia supernova and the presence of circumstellar material makes Keplers supernova remnant a unique object to study the origin of Type Ia supernovae. The evidence suggests that the progenitor binary system of supernova 1604 consisted of a carbon- oxygen white dwarf and an evolved companion star, which most likely was in the (post) asymptotic giant branch of its evolution. A problem with this scenario is that the companion star must have survived the explosion, but no trace of its existence has yet been found, despite a deep search. 1 Introduction; 2 The supernova remnant, its distance and multiwavelength properties; 2.1 Position, distance estimates and SN1604 as a runaway system; 2.2 X-ray imaging spectroscopy and SN1604 as a Type Ia supernova 2.3 The circumstellar medium as studied in the optical and infrared; 3 The dynamics of Keplers SNR; 3.1 Velocity measurements; 3.2 Hydrodynamical simulations; 4 The progenitor system of SN 1604; 4.1 Elevated circumstellar nitrogen abundances, silicates and a single degenerate scenario for SN1604; 4.2 Problems with a single degenerate Type Ia scenario for SN 1604; 4.3 Was SN 1604 a core-degenerate Type Ia explosion?; 4.4 What can we learn from the historical light curve of SN 1604? ; 5 Conclusions
The HETG can be used to obtain spatially resolved spectra of moderately extended sources. We present preliminary results for two well studied, oxygen rich supernova remnants in the Magellanic clouds, E0102-72 and N132D. The dispersed spectrum of E0102-72 shows images of the remnant in the light of individual emission lines from H-like and He-like ions of O, Mg, Ne and He-like Si with no evidence of Fe. The diameters of the images for various ions, measured in the cross-dispersion direction, increase monotonically with the ionization age for the given ion. This shows in detail the progression of the reverse shock through the expanding stellar ejecta. We see clear evidence for asymmetric Doppler shifts across E0102-72 of ~2000 km/s. These can be modelled approximately by a partially-filled, expanding shell inclined to the line of sight. The dispersed spectrum of N132D is more affected by spatial/spectral overlap but also shows monochromatic images in several strong lines. Preliminary spectra have been extracted for several bright knots. Some regions of oxygen-rich material, presumably stellar ejecta, are clearly identified. Additional details on E0102-72 are presented by Flanagan et al. and Davis et al. in these proceedings, and further analysis is in progress.
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