arXiv subjects

Sangwook Park

Publications and source records attributed to Sangwook Park.

At least 55 records · Page 3Linked to original sources

arXiv2013

A Super-Solar Metallicity for the Progenitor of Kepler's Supernova

We have performed deep X-ray observations of the remnant of Kepler's supernova (SN 1604) as a Key Project of the Suzaku Observatory. Our main goal is to detect secondary Fe-peak elements in the SN ejecta to gain insights into the Type Ia supernova explosion mechanism and the nature of the progenitor. Here we report our initial results. We made a conclusive detection of X-ray emission lines from highly ionized Mn, Cr, and Ni as well as Fe. The observed Mn-to-Cr line flux ratio is ~0.60, ~30% larger than that measured in Tycho's remnant. We estimate a Mn-to-Cr mass ratio of ~0.77, which is strongly suggestive of a large neutron excess in the progenitor star before the onset of the thermonuclear runaway. The observed Ni-to-Fe line flux ratio (~0.03) corresponds to a mass ratio of ~0.06, which is generally consistent with the products of explosive Si-burning regime in Type Ia explosion models, and rules out contamination from the products of neutron-rich nuclear statistical equilibrium in the shocked ejecta. Together with the previously suggested luminous nature of the explosion, these mass ratios provide strong evidence for a super-solar metallicity in the SN progenitor (~3 Z_sun). Kepler's supernova was likely the thermonuclear explosion of a white dwarf formed in the recent past that must have exploded through a relatively prompt channel.

astro-ph.HE↗

arXiv2012

The Origin of Kepler's Supernova Remnant

It is now well established that Kepler's supernova remnant is the result of a Type Ia explosion. With an age of 407 years, and an angular diameter of ~ 4', Kepler is estimated to be between 3.0 and 7.0 kpc distant. Unlike other Galactic Type Ia supernova remnants such as Tycho and SN 1006, and SNR 0509-67.5 in the Large Magellanic Cloud, Kepler shows evidence for a strong circumstellar interaction. A bowshock structure in the north is thought to originate from the motion of a mass-losing system through the interstellar medium prior to the supernova. We present results of hydrodynamical and spectral modeling aimed at constraining the circumstellar environment of the system and the amount of 56Ni produced in the explosion. Using models that contain either 0.3 M_sun (subenergetic) or 1 M_sun (energetic) of 56Ni, we simulate the interaction between supernova Ia ejecta and various circumstellar density models. Based on dynamical considerations alone, we find that the subenergetic models favor a distance to the SNR of < 6.4 kpc, while the model that produces 1 M_sun of 56Ni requires a distance to the SNR of > 7 kpc. The X-ray spectrum is consistent with an explosion that produced ~ 1 M_sun of 56Ni, ruling out the subenergetic models, and suggesting that Kepler's SNR was a SN 1991T-like event. Additionally, the X-ray spectrum rules out a pure 1/r^2 wind profile expected from isotropic mass loss up to the time of the supernova. Introducing a small cavity around the progenitor system results in modeled X-ray spectra that are consistent with the observed spectrum. If a wind shaped circumstellar environment is necessary to explain the dynamics and X-ray emission from the shocked ejecta in Kepler's SNR, then we require that the distance to the remnant be greater than 7 kpc.

astro-ph.HE↗

arXiv2012

Spitzer Imaging and Spectral Mapping of the Oxygen-Rich Supernova Remnant G292.0+1.8

We present mid-infrared continuum and emission line images of the Galactic oxygen-rich supernova remnant (SNR) G292.0+1.8, acquired using the MIPS and IRS instruments on the Spitzer Space Telescope. The MIPS 24 micron and 70 micron images of G292.0+1.8 are dominated by continuum emission from a network of filaments encircling the SNR. The morphology of the SNR, as seen in the mid-infrared, resembles that seen in X-rays with the Chandra X-ray Observatory. Most of the mid-infrared emission in the MIPS images is produced by circumstellar dust heated in the non-radiative shocks around G292.0+1.8, confirming the results of earlier mid-IR observations with AKARI. In addition to emission from hot dust, we have also mapped atomic line emission between 14 micron and 36 micron using IRS spectral maps. The line emission is primarily associated with the bright oxygen-rich optical knots, but is also detected from fast-moving knots of ejecta. We confirm our earlier detection of 15-25 micron emission characteristic of magnesium silicate dust in spectra of the radiatively shocked ejecta. We do not detect silicon line emission from any of the radiatively shocked ejecta in the southeast of the SNR, possibly because that the reverse shock has not yet penetrated most of the Si-rich ejecta in that region. This may indicate that G292.0+1.8 is less evolved in the southeast than the rest of the SNR, and may be further evidence in favor of an asymmetric SN explosion as proposed in recent X-ray studies of G292.0+1.8.

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arXiv2012

An X-Ray Study of Supernova Remnant N49 and Soft Gamma-Ray Repeater 0526-66 in the Large Magellanic Cloud

We report on the results from our deep Chandra observation (120 ks) of the supernova remnant (SNR) N49 and soft Gamma-ray repeater (SGR) 0526-66 in the Large Magellanic Cloud. We firmly establish the detection of an ejecta "bullet" beyond the southwestern boundary of N49. The X-ray spectrum of the bullet is distinguished from that of the main SNR shell, showing significantly enhanced Si and S abundances. We also detect an ejecta feature in the eastern shell, which shows metal overabundances similar to those of the bullet. If N49 was produced by a core-collapse explosion of a massive star, the detected Si-rich ejecta may represent explosive O-burning or incomplete Si-burning products from deep interior of the supernova. On the other hand, the observed Si/S abundance ratio in the ejecta may favor Type Ia origin for N49. We refine the Sedov age of N49, tau_Sed ~ 4800 yr, with the explosion energy E_0 ~ 1.8 x 10^51 erg. Our blackbody (BB) + power law (PL) model for the quiescent X-ray emission from SGR 0526-66 indicates that the PL photon index (Gamma ~ 2.5) is identical to that of PSR 1E1048.1-5937, the well-known candidate transition object between anomalous X-ray pulsars and SGRs. Alternatively, the two-component BB model implies X-ray emission from a small (R ~ 1 km) hot spot(s) (kT ~ 1 keV) in addition to emission from the neutron star's cooler surface (R ~ 10 km, kT ~ 0.4 keV). There is a considerable discrepancy in the estimated column toward 0526-66 between BB+PL and BB+BB model fits. Discriminating these spectral models would be crucial to test the long-debated physical association between N49 and 0526-66.

astro-ph.HE↗

arXiv2011

A New Evolutionary Phase of Supernova Remnant 1987A

We have been monitoring the supernova remnant (SNR) 1987A with {\it Chandra} observations since 1999. Here we report on the latest change in the soft X-ray light curve of SNR 1987A. For the last $\sim$1.5 yr (since day $\sim$8000), the soft X-ray flux has significantly flattened, staying (within uncertainties) at $f_{\rm X}$ $\sim$ 5.7 $\times$ 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$ (corresponding to $L_{\rm X}$ $\sim$ 3.6 $\times$ 10$^{36}$ erg s$^{-1}$) in the 0.5--2 keV band. This remarkable change in the recent soft X-ray light curve suggests that the forward shock is now interacting with a decreasing density structure, after interacting with an increasing density gradient over $\sim$10 yr prior to day $\sim$8000. Possibilities may include the case that the shock is now propagating beyond a density peak of the inner ring. We briefly discuss some possible implications on the nature of the progenitor and the future prospects of our {\it Chandra} monitoring observations.

astro-ph.HE↗

arXiv2011

The Chandra Carina Complex Project: Deciphering the Enigma of Carina's Diffuse X-ray Emission

We present a 1.42 square degree mosaic of diffuse X-ray emission in the Great Nebula in Carina from the Chandra X-ray Observatory Advanced CCD Imaging Spectrometer camera. After removing >14,000 X-ray point sources from the field, we smooth the remaining unresolved emission, tessellate it into segments of similar apparent surface brightness, and perform X-ray spectral fitting on those tessellates to infer the intrinsic properties of the X-ray-emitting plasma. By modeling faint resolved point sources, we estimate the contribution to the extended X-ray emission from unresolved point sources and show that the vast majority of Carina's unresolved X-ray emission is truly diffuse. Line-like correlated residuals in the X-ray spectral fits suggest that substantial X-ray emission is generated by charge exchange at the interfaces between Carina's hot, rarefied plasma and its many cold neutral pillars, ridges, and clumps.

astro-ph.SR↗

arXiv2011

SNR 0104-72.3: A remnant of Type Ia Supernova in a Star-forming region?

We report our 110 ks Chandra observations of the supernova remnant (SNR) 0104-72.3 in the Small Magellanic Cloud (SMC). The X-ray morphology shows two prominent lobes along the northwest-southeast direction and a soft faint arc in the east. Previous low resolution X-ray images attributed the unresolved emission from the southeastern lobe to a Be/X-ray star. Our high resolution Chandra data clearly shows that this emission is diffuse, shock-heated plasma, with negligible X-ray emission from the Be star. The eastern arc is positionally coincident with a filament seen in optical and infrared observations. Its X-ray spectrum is well fit by plasma of normal SMC abundances, suggesting that it is from shocked ambient gas. The X-ray spectra of the lobes show overabundant Fe, which is interpreted as emission from the reverse-shocked Fe-rich ejecta. The overall spectral characteristics of the lobes and the arc are similar to those of Type Ia SNRs, and we propose that SNR 0104-72.3 is the first case for a robust candidate Type Ia SNR in the SMC. On the other hand, the remnant appears to be interacting with dense clouds toward the east and to be associated with a nearby star-forming region. These features are unusual for a standard Type Ia SNR. Our results suggest an intriguing possibility that the progenitor of SNR 0104-72.3 might have been a white dwarf of a relatively young population.

astro-ph.GA↗

arXiv2010

Suzaku Observations of the Prototype Wind-Blown Bubble NGC 6888

We present an analysis of the Suzaku observations of the prototype wind-blown bubble NGC 6888 which is based both on use of standard spectral models and on a direct comparison of theoretical models with observations. The X-ray spectra of NGC 6888 are soft and most of the X-rays are in the (0.3 - 1.5 keV) energy range. But, hard X-rays (1.5 - 4.0 keV) are also detected (~10% of the observed flux). The corresponding spectral fits require a relatively cool plasma with kT < 0.5 keV but much hotter plasma with temperature kT > 2.0 keV is needed to match the observed hard X-ray emission. We find no appreciable temperature variations within the hot bubble in NGC 6888. The derived abundances (N, O, Ne) are consistent with those of the optical nebula. This indicates a common origin of the X-ray emitting gas and the outer cold shell: most of the X-ray plasma (having non-uniform spatial distribution: clumps) has flown into the hot bubble from the optical nebula. If the electron thermal conduction is efficient, this can naturally explain the relatively low plasma temperature of most of the X-ray emitting plasma. Alternatively, the hot bubble in NGC 6888 will be adiabatic and the cold clumps are heated up to X-ray temperatures likely by energy exchange between the heavy particles (hot ions diffusing into the cold clumps).

astro-ph.HE↗

arXiv2010

Five Years of Mid-Infrared Evolution of the Remnant of SN 1987A: The Encounter Between the Blast Wave and the Dusty Equatorial Ring

We have used the Spitzer satellite to monitor the mid-IR evolution of SN 1987A over a 5 year period spanning the epochs between days 6000 and 8000 since the explosion. The supernova (SN) has evolved into a supernova remnant (SNR) and its radiative output is dominated by the interaction of the SN blast wave with the pre-existing equatorial ring (ER). The mid-IR spectrum is dominated by emission from ~180 K silicate dust, collisionally-heated by the hot X-ray emitting gas with a temperature and density of ~5x10^6 K and 3x10^4 cm-3, respectively. The mass of the radiating dust is ~1.2x10^(-6) Msun on day 7554, and scales linearly with IR flux. The infrared to soft-X-ray flux ratio is roughly constant with a value of 2.5. Gas-grain collisions therefore dominate the cooling of the shocked gas. The constancy of of this ratio suggests that very little grain processing or gas cooling have occurred throughout this epoch. The shape of the dust spectrum remained unchanged during the observations while the total flux increased with a time dependence of t^(0.87), t being the time since the first encounter between the blast wave and the ER. These observations are consistent with the transitioning of the blast wave from free expansion to a Sedov phase as it propagates into the main body of the ER.

astro-ph.CO↗

arXiv2010

Chandra HETG Observations of the Colliding Stellar Wind System WR 147

We present an extended analysis of deep Chandra HETG observations of the WR+OB binary system WR 147 that was resolved into a double X-ray source (Zhekov & Park, 2010, ApJ, 709, L119). Our analysis of the profiles of strong emission lines shows that their centroids are blue-shifted in the spectrum of the northern X-ray source. We find no suppressed forbidden line in the He-like triplets which indicates that the X-ray emitting region is not located near enough to the stars in the binary system to be significantly affected by their UV radiation. The most likely physical picture that emerges from the entire set of HETG data suggests that the northern X-ray source can be associated with the colliding stellar wind region in the wide WR+OB binary system, while the X-rays of its southern counterpart, the WN8 star, are result from stellar wind shocking onto a close companion (a hypothesized third star in the system).

astro-ph.SR↗

arXiv2010

Resolved shock structure of the Balmer-dominated filaments in Tycho's supernova remnant: Cosmic-ray precursor?

We report on the results from Hα imaging observations of the eastern limb of Tycho's supernova remnant (SN1572) using the Wide Field Planetary Camera 2 on the Hubble Space Telescope. We resolve the detailed structure of the fast, collisionless shock wave into a delicate structure of nearly edge-on filaments. We find a gradual increase of Hα intensity just ahead of the shock front, which we interpret as emission from the thin (~1") shock precursor. We find that a significant amount of the Hα emission comes from the precursor and that this could affect the amount of temperature equilibration derived from the observed flux ratio of the broad and narrow Hα components. The observed Hα emission profiles are fit using simple precursor models, and we discuss the relevant parameters. We suggest that the precursor is likely due to cosmic rays and discuss the efficiency of cosmic-ray acceleration at this position.

astro-ph.HE↗

arXiv2010

Evolution of the Chandra CCD Spectra of SNR 1987A: Probing the Reflected-Shock Picture

We continue to explore the validity of the reflected shock structure (RSS) picture in SNR 1987A that was proposed in our previous analyses of the X-ray emission from this object. We used an improved version of our RSS model in a global analysis of 14 CCD spectra from the monitoring program with Chandra. In the framework of the RSS picture, we are able to match both the expansion velocity curve deduced from the analysis of the X-ray images and light curve. Using a simplified analysis, we also show that the X-rays and the non-thermal radio emission may originate from the same shock structure (the blast wave). We believe that using the RSS model in the analysis of grating data from the Chandra monitoring program of SNR 1987A that cover a long enough time interval, will allow us to build a more realistic physical picture and model of SNR 1987A.

astro-ph.HE↗

arXiv2010

Discovery of X-Ray-Emitting O-Ne-Mg-Rich Ejecta in the Galactic Supernova Remnant Puppis A

We report on the discovery of X-ray--emitting O-Ne-Mg-rich ejecta in the middle-aged Galactic O-rich supernova remnant Puppis A with Chandra and XMM-Newton. We use line ratios to identify a low-ionization filament running parallel to the northeastern edge of the remnant that requires supersolar abundances, particularly for O, Ne, and Mg, which we interpret to be from O-Ne-Mg-rich ejecta. Abundance ratios of Ne/O, Mg/O, and Fe/O are measured to be ~2, ~2, and <0.3 times the solar values. Our spatially-resolved spectral analysis from the northeastern rim to the western rim otherwise reveals sub-solar abundances consistent with those in the interstellar medium. The filament is coincident with several optically emitting O-rich knots with high velocities. If these are physically related, the filament would be a peculiar fragment of ejecta. On the other hand, the morphology of the filament suggests that it may trace ejecta heated by a shock reflected strongly off the dense ambient clouds near the northeastern rim.

astro-ph.HE↗

arXiv2010

The Outer Shock of the Oxygen-Rich Supernova Remnant G292.0+1.8: Evidence for the Interaction with the Stellar Winds from its Massive Progenitor

We study the outer-shock structure of the oxygen-rich supernova remnant G292.0+1.8, using a deep observation with the Chandra X-ray Observatory. We measure radial variations of the electron temperature and emission measure that we identify as the outer shock propagating into a medium with a radially decreasing density profile. The inferred ambient density structure is consistent with models for the circumstellar wind of a massive progenitor star rather than for a uniform interstellar medium. The estimated wind density n_H = 0.1 ~ 0.3 cm^-3) at the current outer radius (~7.7 pc) of the remnant is consistent with a slow wind from a red supergiant (RSG) star. The total mass of the wind is estimated to be ~ 15 - 40 solar mass (depending on the estimated density range), assuming that the wind extended down to near the surface of the progenitor. The overall kinematics of G292.0+1.8 are consistent with the remnant expanding through the RSG wind.

astro-ph.GA↗

arXiv2009

A Deep Chandra Observation of the Oxygen-Rich Supernova Remnant 0540-69.3 in the Large Magellanic Cloud

Using our deep ~120 ks Chandra observation, we report on the results from our spatially-resolved X-ray spectral analysis of the "oxygen-rich" supernova remnant (SNR) 0540-69.3 in the Large Magellanic Cloud. We conclusively establish the nonthermal nature of the "arcs" in the east and west boundaries of the SNR, which confirms the cosmic-ray electron acceleration in the supernova shock (B ~ 20-140 microG). We report tentative evidence for Fe overabundance in the southern region close to the outer boundary of the SNR. While such a detection would be intriguing, the existence of Fe ejecta is not conclusive with the current data because of poor photon statistics and limited plasma models. If it is verified using deeper X-ray observations and improved plasma models, the presence of Fe ejecta, which was produced in the core of the supernova, near the SNR's outer boundary would provide an intriguing opportunity to study the explosive nucleosynthesis and the ejecta mixing in this young core-collapse SNR. There is no evidence of X-ray counterparts for the optical O-rich ejecta in the central regions of the SNR.

astro-ph.HE↗

arXiv2009

Chandra Observations of WR147 Reveal a Double X-ray Source

We report the first results from deep X-ray observations of the Wolf-Rayet binary system WR147 with the Chandra HETG. Analysis of the zeroth order data reveals that WR147 is a double X-ray source. The northern counterpart is likely associated with the colliding wind region, while the southern component is certainly identified with the WN star in this massive binary. The latter is the source of high energy X-rays (including the Fe K_alpha complex at 6.67 keV) whose production mechanism is yet unclear. For the first time, X-rays are observed directly from a WR star in a binary system.

astro-ph.HE↗

arXiv2009

X-ray Evolution of SNR 1987A: The Radial Expansion

We present the evolution of the radial expansion of SNR 1987A as measured using Chandra X-ray observations taken over the last 10 years. To characterize the complex structure of the remnant and isolate the expansion measurement, we fit the images to several empirical models including: a simple circular torus, a torus with bilateral lobes, and a torus with four tangentially extended lobes. We discuss the results of this measure in the context of the overall evolution of the supernova remnant, for which we believe we have measured the end of the free expansion phase and its transition to the adiabatic phase (at least along the equatorial ring). The timing of this event is in agreement with early predictions of the remnant evolution.

astro-ph.HE↗

arXiv2009

Spitzer Spectroscopy of the Galactic Supernova Remnant G292.0+1.8: Structure and Composition of the Oxygen-Rich Ejecta

We present mid-infrared (5-40 micron) spectra of shocked ejecta in the Galactic oxygen-rich supernova remnant G292.0+1.8, acquired with the IRS spectrograph on board the Spitzer Space Telescope. The observations targeted two positions within the brightest oxygen-rich feature in G292.0+1.8. Emission lines of [Ne II] 12.8, [Ne III] 15.5, 36.0, [Ne V] 24.3 and [O IV] 25.9 are detected from the shocked ejecta. No discernible mid-IR emission from heavier species such as Mg, Si, S, Ar or Fe is detected in G292.0+1.8. We also detect a broad emission bump between 15 and 28 microns in spectra of the radiatively shocked O-rich ejecta in G292.0+1.8. We suggest that this feature arises from either shock-heated Mg2SiO4 (forsterite) dust in the radiatively shocked O-rich ejecta, or collisional excitation of PAHs in the blast wave of the SNR. If the former interpretation is correct, this would be the first mid-IR detection of ejecta dust in G292.0+1.8. A featureless dust continuum is also detected from non-radiative shocks in the circumstellar medium around G292.0+1.8. The mid-IR continuum from these structures is well described by a two-component dust model. The temperature of the hot dust component (mass ~ 0.002 Solar masses) is ~ 115 K, while that of the cold component (> = 3 Solar masses) is ~ 35 K. We attribute the hot component to collisionally heated dust in the circumstellar shocks in G292.0+1.8, and attribute the cold component to dust heated by the hard FUV radiation from the circumstellar shocks. Our models yield mid-IR line strengths consistent with M(O)/M(Ne) ~ 3, M(O)/M(Si) >~ 61 and M(O)/M(S) ~ 50. These ratios are difficult to reproduce with standard nucleosynthesis models of well-mixed SN ejecta (abridged).

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