arXiv subjects

Sangwook Park

Publications and source records attributed to Sangwook Park.

At least 73 records · Page 4Linked to original sources

arXiv2008

High-Resolution X-ray Spectroscopy of SNR 1987A: Chandra LETG and HETG Observations in 2007

We present an extended analysis of the deep Chandra LETG and HETG observations of the supernova remnant 1987A (SNR 1987A) carried out in 2007. The global fits to the grating spectra show that the temperature of the X-ray emitting plasma in the slower shocks in this system has remained stable for the last three years, while that in the faster shocks has decreased. This temperature evolution is confirmed by the first light curves of strong X-ray emission lines and their ratios. On the other hand, bulk gas velocities inferred from the X-ray line profiles are too low to account for the post-shock plasma temperatures inferred from spectral fits. This suggests that the X-ray emission comes from gas that has been shocked twice, first by the blast wave and again by shocks reflected from the inner ring of SNR 1987A. A new model that takes these considerations into account gives support to this physical picture.

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arXiv2008

Nonthermal X-Rays from Supernova Remnant G330.2+1.0 and the Characteristics of its Central Compact Object

We present results from our X-ray data analysis of the SNR G330.2+1.0 and its CCO, CXOU J160103.1--513353 (J1601). Using our XMM-Newton and Chandra observations, we find that the X-ray spectrum of J1601 can be described by neutron star atmosphere models (T ~ 2.5--3.7 MK). Assuming the distance of d ~ 5 kpc for J1601 as estimated for SNR G330.2+1.0, a small emission region of R ~ 1--2 km is implied. X-ray pulsations previously suggested by Chandra are not confirmed by the XMM-Newton data, and are likely not real. However, our timing analysis of the XMM-Newton data is limited by poor photon statistics, and thus pulsations with a relatively low amplitude (i.e., an intrinsic pulsed-fraction < 40%) cannot be ruled out. Our results indicate that J1601 is a CCO similar to that in the Cassiopeia A SNR.X-ray emission from SNR G330.2+1.0 is dominated by power law continuum (Gamma ~ 2.1--2.5) which primarily originates from thin filaments along the boundary shell. This X-ray spectrum implies synchrotron radiation from shock-accelerated electrons with an exponential roll-off frequency ~ 2--3 x 10^17 Hz. For the measured widths of the X-ray filaments (D ~ 0.3 pc) and the estimated shock velocity (v_s ~ a few x 10^3 km s^-1), a downstream magnetic field B ~ 10--50 $μ$G is derived. The estimated maximum electron energy E_max ~ 27--38 TeV suggests that G330.2+1.0 is a candidate TeV gamma-ray source. We detect faint thermal X-ray emission in G330.2+1.0. We estimate a low preshock density n_0 ~ 0.1 cm^-3, which suggests a dominant contribution from an inverse Compton mechanism (than the proton-proton collision) to the prospective gamma-ray emission. Follow-up deep radio, X-ray, and gamma-ray observations will be essential to reveal the details of the shock parameters and the nature of particle accelerations in this SNR.

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arXiv2007

A Half-Megasecond Chandra Observation of the Oxygen-Rich Supernova Remnant G292.0+1.8

We report on our initial analysis of a deep 510 ks observation of the Galactic oxygen-rich supernova remnant (SNR) G292.0+1.8 with the {\it Chandra X-ray Observatory}. Our new {\it Chandra} ACIS-I observation has a larger field of view and an order of magnitude deeper exposure than the previous {\it Chandra} observation, which allows us to cover the entire SNR and to detect new metal-rich ejecta features. We find a highly non-uniform distribution of thermodynamic conditions of the X-ray emitting hot gas that correlates well with the optical [O {\small III}] emission, suggesting the possibility that the originating supernova explosion of G292.0+1.8 was itself asymmetric. We also reveal spectacular substructures of a torus, a jet, and an extended central compact nebula all associated with the embedded pulsar J1124$-$5916.

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arXiv2007

Chandra X-Ray Study of Galactic Supernova Remnant G299.2-2.9

We report on observations of the Galactic supernova remnant (SNR) G299.2$-$2.9 with the {\it Chandra X-Ray Observatory}. The high resolution images with {\it Chandra} resolve the X-ray-bright knots, shell, and diffuse emission extending beyond the bright shell. Interior to the X-ray shell is faint diffuse emission occupying the central regions of the SNR. Spatially-resolved spectroscopy indicates a large foreground absorption ($N_{\rm H}$ $\sim$ 3.5 $\times$ 10$^{21}$ cm$^{-2}$), which supports a relatively distant location ($d$ $\sim$ 5 kpc) for the SNR. The blast wave is encountering a highly inhomogeneous ambient medium with the densities ranging over more than an order of magnitude ($n_0$ $\sim$ 0.1 $-$ 4 cm$^{-3}$). Assuming the distance of $d$ $\sim$ 5 kpc, we derive a Sedov age of $τ$ $\sim$ 4500 yr and an explosion energy of $E_0$ $\sim$ 1.6 $\times$ 10$^{50}$ ergs. The ambient density structure and the overall morphology suggest that G299.2$-$2.9 may be a limb-brightened partial shell extending to $\sim$7 pc radius surrounded by fainter emission extending beyond that to a radius of $\sim$9 pc. This suggests the SNR exploded in a region of space where there is a density gradient whose direction lies roughly along the line of sight. The faint central region shows strong line emission from heavy elements of Si and Fe, which is caused by the presence of the overabundant stellar ejecta there. We find no evidence for stellar ejecta enriched in light elements of O and Ne. The observed abundance structure of the metal-rich ejecta supports a Type Ia origin for G299.2$-$2.9.

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arXiv2007

Chandra Observations of Supernova 1987A

We have been monitoring Supernova (SN) 1987A with {\it Chandra X-Ray Observatory} since 1999. We present a review of previous results from our {\it Chandra} observations, and some preliminary results from new {\it Chandra} data obtained in 2006 and 2007. High resolution imaging and spectroscopic studies of SN 1987A with {\it Chandra} reveal that X-ray emission of SN 1987A originates from the hot gas heated by interaction of the blast wave with the ring-like dense circumstellar medium (CSM) that was produced by the massive progenitor's equatorial stellar winds before the SN explosion. The blast wave is now sweeping through dense CSM all around the inner ring, and thus SN 1987A is rapidly brightening in soft X-rays. At the age of 20 yr (as of 2007 January), X-ray luminosity of SN 1987A is $L_{\rm X}$ $\sim$ 2.4 $\times$ 10$^{36}$ ergs s$^{-1}$ in the 0.5$-$10 keV band. X-ray emission is described by two-component plane shock model with electron temperatures of $kT$ $\sim$ 0.3 and 2 keV. As the shock front interacts with dense CSM all around the inner ring, the X-ray remnant is now expanding at a much slower rate of $v$ $\sim$ 1400 km s$^{-1}$ than it was until 2004 ($v$ $\sim$ 6000 km s$^{-1}$).

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arXiv2006

Discovery of a Candidate Central Compact Object in the Galactic Nonthermal SNR G330.2+1.0

We report on the discovery of a pointlike source (CXOU J160103.1$-$513353) at the center of a Galactic supernova remnant (SNR) G330.2+1.0 with {\it Chandra X-Ray Observatory}. The X-ray spectrum fits a black-body (BB) model with $kT$ $\sim$ 0.49 keV, implying a small emission region of $R$ $\sim$ 0.4 km at the distance of 5 kpc. The estimated X-ray luminosity is $L_X$ $\sim$ 1 $\times$ 10$^{33}$ ergs s$^{-1}$ in the 1 $-$ 10 keV band. A power law model may also fit the observed spectrum, but the fit results in a very large photon index, $Γ$ $\sim$ 5. We find no counterparts at other wavelengths. The X-ray emission was steady over the $\sim$13 hr observation period, showing no variability. While we find marginal evidence for X-ray pulsations ($P$ $\approx$ 7.5 s), the presence of a pulsar at the position of this object is not conclusive with the current data, requiring an independent confirmation. These results are generally consistent with an interpretation of this object as a Central Compact Object associated with SNR G330.2+1.0.

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arXiv2006

SN 1987A After 18 Years: Mid-Infrared GEMINI and SPITZER Observations of the Remnant

We present high resolution 11.7 and 18.3um mid-IR images of SN 1987A obtained on day 6526 with T-ReCS attached to the Gemini telescope. The 11.7um flux has increased significantly since our last observations on day 6067. The images clearly show that all the emission arises from the equatorial ring (ER). Spectra obtained with Spitzer, on day 6184 with MIPS at 24um, on day 6130 with IRAC in the 3.6-8um region, and on day 6190 with IRS in the 12-37um region show that the emission consists of thermal emission from silicate dust that condensed out in the red giant wind of the progenitor star. The dust temperature is ~166K, and the emitting dust mass is ~2.6 x 10-6 Msun. Lines of [Ne II]12.82um and [Ne III]15.56um are clearly present, as well as a weak [Si II]34.8um line. We also detect two lines near 26um which we tentatively ascribe to [Fe II]25.99um and [O IV]25.91um. Comparison of the Gemini 11.7um image with X-ray images from Chandra, UV-optical images from HST, and radio synchrotron images obtained by the ATCA show generally good correlation of the images across all wavelengths. Because of the limited resolution of the mid-IR images we cannot uniquely determine the location or heating mechanism of the dust giving rise to the emission. The dust could be collisionally heated by the X-ray emitting plasma, providing a unique diagnostic of plasma conditions. Alternatively, the dust could be radiatively heated in the dense UV-optical knots that are overrun by the advancing supernova blast wave. In either case the dust-to-gas mass ratio in the circumstellar medium around the supernova is significantly lower than that in the general ISM of the LMC, suggesting either a low condensation efficiency in the wind of the progenitor star, or the efficient destruction of the dust by the SN blast wave.

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arXiv2006

Evolutionary Status of SNR 1987A at the Age of Eighteen

$\sim$18 yr after the supernova explosion, the blast wave of SNR 1987A is entering the main body of the equatorial circumstellar material, which is causing a dramatic brightening of the remnant. We recently reported the observational evidence for this event from our {\it Chandra} data (Park et al. 2005b; P05 hereafter). We present here the temporal evolution of the X-ray emitting shock parameters and the detailed description of the spectral and image analysis of SNR 1987A, on which P05 was based. While the remnant becomes brighter, the softening of the overall X-ray spectrum continues and is enhanced on around day $\sim$6200 (since the explosion). The two-component shock model indicates that the electron temperatures have been changing for the last $\sim$6 yr. The X-ray spectrum is now described by $kT$ $\sim$ 0.3 keV and 2.3 keV thermal plasmas which are believed to characteristically represent the shock-heated density gradient along the boundary between the H{\small II} region and the dense inner ring. As the blast wave sweeps through the inner circumstellar ring shining in X-rays, we expect that the shock parameters continue to change, revealing the density and abundance structure of the inner ring. Follow-up {\it Chandra} observations will thus uncover the past history of the progenitor's stellar evolution. The origin of the relatively faint hard X-ray emission ($E$ $>$ 3 keV) from SNR 1987A is still unclear (thermal vs. nonthermal). Considering the continuous brightening of the hard band intensity, as well as the soft band flux, follow-up monitoring observations will also be essential to reveal the origin of the hard X-ray emission of SNR 1987A.

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arXiv2006

Chandra LETG Observations of Supernova Remnant 1987A

We discuss the results from deep Chandra LETG observations of the supernova remnant 1987A (SNR 1987A). We find that a distribution of shocks, spanning the same range of velocities (from 300 to 1700 km/s) as deduced in the first part of our analysis (Zhekov et al. 2005, ApJL, 628, L127), can account for the entire X-ray spectrum of this object. The post-shock temperature distribution is bimodal, peaking at kT 0.5 and 3 keV. Abundances inferred from the X-ray spectrum have values similar to those for the inner circumstellar ring, except that the abundances of nitrogen and oxygen are approximately a factor of two lower than those inferred from the optical/UV spectrum. The velocity of the X-ray emitting plasma has decreased since 1999, apparently because the blast wave has entered the main body of the inner circumstellar ring.

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arXiv2005

Supernova Remnant 1987A: High Resolution Images and Spectrum from Chandra Observations

We report on the morphological and spectral evolution of SNR 1987A from the monitoring observations with the Chandra/ACIS. As of 2005, the X-ray-bright lobes are continuously brightening and expanding all around the ring. The softening of the overall X-ray spectrum also continues. The X-ray lightcurve is particularly remarkable: i.e., the recent soft X-ray flux increase rate is significantly deviating from the model which successfully fits the earlier data, indicating even faster flux increase rate since early 2004 (day ~6200). We also report results from high resolution spectral analysis with deep Chandra/LETG observations. The high resolution X-ray line emission features unambiguously reveal that the X-ray emission of SNR 1987A is originating primarily from a "disk" along the inner ring rather than from a spherical shell. We present the ionization structures, elemental abundances, and the shock velocities of the X-ray emitting plasma.

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arXiv2005

Supernova Remnant 1987A: Opening the Future by Reaching the Past

We report an up-turn in the soft X-ray light curve of supernova remnant (SNR) 1987A in late 2003 (~6200 days after the explosion), as observed with the Chandra X-ray Observatory. Since early 2004, the rapid increase of the 0.5-2 keV band X-ray light curve can no longer be described by the exponential density distribution model with which we successfully fitted the data between 1990 and 2003. Around day ~6200, we also find that the fractional contribution to the observed soft X-ray flux from the decelerated shock begins to exceed that of the fast shock and that the X-ray brightening becomes "global" rather than "spotty". We interpret these results as evidence that the blast wave has reached the main body of the dense circumstellar material all around the inner ring. This interpretation is supported by other recent observations, including a deceleration of the radial expansion of the X-ray remnant, a significant up-turn in the mid-IR intensities, and the prevalence of the optical hot spots around the entire inner ring, all of which occur at around day 6000. In contrast to the soft X-ray light curve, the hard band (3-10 keV) X-ray light curve increases at a much lower rate which is rather similar to the radio light curve. The hard X-ray emission may thus originate from the reverse shock where the radio emission is likely produced. Alternatively, the low increase rate of the hard X-rays may simply be a result of the continuous softening of the overall X-ray spectrum.

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arXiv2005

A Candidate Neutron Star Associated with Galactic Center Supernova Remnant Sagittarius A East

We present imaging and spectral studies of the supernova remnant (SNR) Sagittarius (Sgr) A East from deep observations with the {\it Chandra X-Ray Observatory}. The spatially-resolved spectral analysis of Sgr A East reveals the presence of a two-temperature thermal plasma ($kT$ $\sim$ 1 keV and 5 keV) near the center of the SNR. The central region is dominated by emission from highly-ionized Fe-rich ejecta. We estimate a conservative upper limit on the total Fe ejecta mass of the SNR, M$_{Fe}$ $<$ 0.27 M$_{\odot}$. Comparisons with standard SN nucleosynthesis models suggest that this Fe mass limit is consistent with a Type II SN explosion for the origin of Sgr A East. On the other hand, the soft X-ray emission extending toward the north of the SNR can be described by a single-temperature ($kT$ $\sim$ 1.3 keV) thermal plasma with normal chemical composition. This portion of the SNR is thus X-ray emission from the heated interstellar medium rather than the metal-rich stellar ejecta. We point out that a hard pointlike source CXOGC J174545.5$-$285829 (the so-called ``cannonball'') at the northern edge of the SNR shows unusual X-ray characteristics among other Galactic center sources. The morphological, spectral, and temporal characteristics of this source suggest an identification as a high-velocity neutron star. Based on the suggested Type II origin for the SNR Sgr A East and the proximity between the two, we propose that CXOGC J174545.5$-$285829 is a high-velocity neutron star candidate, born from the core-collapse SN which also created the SNR Sgr A East.

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arXiv2005

Supernova Remnant 1987A: The Latest Report from the Chandra X-Ray Observatory

We continue monitoring supernova remnant (SNR) 1987A with the {\it Chandra X-ray Observatory}. As of 2004 January, bright X-ray spots in the northwest and the southwest are now evident in addition to the bright eastern ring. The overall X-ray spectrum, Since 2002 December, can be described by a planar shock with an electron temperature of $\sim$2.1 keV. The soft X-ray flux is now 8 $\times$ 10$^{-13}$ ergs cm$^{-2}$ s$^{-1}$, which is about five times higher than four years ago. This flux increase rate is consistent with our prediction based on an exponential density distribution along the radius of the SNR between the H{\small II} region and the inner ring. We still have no direct evidence of a central point source, and place an upper limit of $L_X$ = 1.3 $\times$ 10$^{34}$ ergs s$^{-1}$ on the 3$-$10 keV band X-ray luminosity.

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arXiv2004

A Chandra View of The Morphological And Spectral Evolution of Supernova Remnant 1987A

We present an update on the results of our monitoring observations of the X-ray remnant of supernova (SN) 1987A with the {\it Chandra X-Ray Observatory}. As of 2002 December, we have performed a total of seven observations of SN 1987A. The high angular resolution images from the latest data reveal developments of new X-ray bright spots in the northwestern and the southwestern portions of the remnant as well as changes on the eastern side. The latest 0.5-2 keV band flux ($f_X$ $\sim$ 6 $\times$ 10$^{-13}$ ergs cm$^{-2}$ s$^{-1}$) is four times brighter than three years earlier. The overall X-ray emission is primarily from the blast wave shock with $kT$ $\sim$ 2.4 keV. As the blast wave approaches the dense circumstellar material, the contribution from the decelerated slow shock ($kT$ $\sim$ 0.22 keV) to the observed X-ray emission is becoming significant. The increase of this slow shock contribution over the last two years is particularly noticeable in the western half of the remnant. These results indicate that the shock front is now reaching the main body of the inner circumstellar ring. Based on the best-fit two-shock spectral model, we derive approximate densities of the X-ray-emitting regions ($n_e$ $\sim$ 235 cm$^{-3}$ for the fast shock and $n_e$ $\sim$ 7500 cm$^{-3}$ for the slow shock). We obtain an upper limit on the observed X-ray luminosity of any embedded point source ($L_X$ $\le$ 1.5 $\times$ 10$^{34}$ ergs s$^{-1}$) in the 2$-$10 keV band. The X-ray remnant continues to expand linearly at a rate of 4167 km s$^{-1}$.

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arXiv2004

An X-Ray Pulsar, Metal-rich Ejecta, and Shocked Ambient Medium in the Supernova Remnant G292.0+1.8

We report the discovery of pulsed X-ray emission from the compact object CXOU J112439.1-591620 within the Galactic supernova remnant G292.0+1.8 using the High Resolution Camera on the Chandra X-Ray Observatory. The X-ray period is consistent with the extrapolation of the radio period and spindown rate of PSR J1124-5916. The X-ray pulse is single peaked and broad. There is no optical counterpart to a limit of M_V ~ 26. The pressure in the pulsar wind nebula is considerably less than that in the reverse-shock-heated ejecta and circumstellar medium, indicating that the reverse shock has not yet begun to interact with the nebula.

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arXiv2003

Nucleosynthesis in The Oxygen-Rich Supernova Remnant G292.0+1.8 from Chandra X-Ray Spectroscopy

We continue our analysis of the Galactic oxygen-rich supernova remnant (SNR) G292.0+1.8, which was observed with the {\it Chandra X-ray Observatory}. The high angular resolution {\it Chandra} data resolve metal-rich ejecta knots as well as the shocked circumstellar medium. X-ray emission from the ejecta material in G292.0+1.8 is dominated by highly ionized O, Ne and Mg. Measured abundance ratios suggest that this material was produced during the hydrostatic evolution of the massive progenitor star. In contrast to Cassiopeia A, there is little evidence for X-ray-emitting ejecta from explosive nucleosynthesis, i.e., material enriched in Si, S, and particularly, Fe. This limits the amount of mixing or overturning of deep ejecta material in G292.0+1.8 and suggests that the ejecta are strongly stratified by composition and that the reverse shock has not propagated to the Si/S or Fe-rich zones. On the other hand, the bright equatorial belt is dominated by X-ray emission with normal chemical composition, which supports shocked dense circumstellar material for its origin. We find that the thermal pressure in the SNR is much higher than the pressure in the pulsar wind nebula (PWN), indicating that the reverse shock has not yet begun to interact with the PWN.

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arXiv2003

Characteristics of Diffuse X-Ray Line Emission within 20 pc of the Galactic Center

Over the last 3 yrs, the Galactic center (GC) region has been monitored with the Chandra X-Ray Observatory. With 11 Chandra observations through 2002 June, the total effective exposure reaches ~590 ks, providing significant photon statistics on the faint, filamentary, diffuse X-ray emission. The true-color X-ray image and the equivalent width (EW) images for the detected elemental species demonstrate that the diffuse X-ray features have a broad range of spatio-spectral properties. Enhancements of the low-ionization-state, or ``neutral'' Fe line emission (E~6.4 keV) to the northeast of Sgr A* can be interpreted as fluorescence within the dense ISM resulting from irradiation by hard, external X-ray sources. They may also be explained by emission induced by the bombardments by high energy particles on the ISM, such as unresolved supernova (SN) ejecta intruding into dense ISM. The detection of molecular cloud counterparts to the 6.4 keV Fe line features indicates that these Fe line features are associated with dense GC clouds and/or active star-forming regions, which supports the X-ray reflection and/or SN ejecta origins for the Fe line emission. We detect highly ionized S and Si lines which are generally coincident with the neutral Fe line emission and the dense molecular clouds in the northeast of Sgr A*. These hot plasmas are likely produced by massive star-forming activities and/or SNRs. In contrast, we find that highly ionized He-like Fe line emission (E~6.7 keV) is primarily distributed along the plane instead of being concentrated in the northeast of Sgr A*. The implied high temperature and the alignment along the plane are consistent with the magnetic confinement model.

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arXiv2003

Monitoring The Evolution of SNR 1987A with Chandra

We report on the results of our monitoring program of SNR 1987A with the Chandra X-Ray Observatory. The high resolution images and the spectra from the latest Chandra data suggest that the blast wave has reached the dense inner ring in the western side of the SNR, as well as in the east. The observed soft X-ray flux is increasing more rapidly than ever, and the latest flux is four times brighter than three years ago.

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