arXiv subjects

Sangwook Park

Publications and source records attributed to Sangwook Park.

81 records · Page 5Linked to original sources

arXiv2003

0103-72.6: A New Oxygen-Rich Supernova Remnant in the Small Magellanic Cloud

0103$-$72.6, the second brightest X-ray supernova remnant (SNR) in the Small Magellanic Cloud (SMC), has been observed with the {\it Chandra X-Ray Observatory}. Our {\it Chandra} observation unambiguously resolves the X-ray emission into a nearly complete, remarkably circular shell surrounding bright clumpy emission in the center of the remnant. The observed X-ray spectrum for the central region is evidently dominated by emission from reverse shock-heated metal-rich ejecta. Elemental abundances in this ejecta material are particularly enhanced in oxygen and neon, while less prominent in the heavier elements Si, S, and Fe. We thus propose that 0103$-$72.6 is a new ``oxygen-rich'' SNR, making it only the second member of the class in the SMC. The outer shell is the limb-brightened, soft X-ray emission from the swept-up SMC interstellar medium. The presence of O-rich ejecta and the SNR's location within an H{\small II} region attest to a massive star core-collapse origin for 0103$-$72.6. The elemental abundance ratios derived from the ejecta suggest an $\sim$18 M$_{\odot}$ progenitor star.

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arXiv2003

Detection of Magnesium-Rich Ejecta in the Middle-Aged Supernova Remnant N49B

The middle-aged supernova remnant (SNR) N49B in the Large Magellanic Cloud has been observed with the {\it Chandra X-Ray Observatory}. The superb angular resolution of {\it Chandra} resolves the complex structure of X-ray emitting filaments across the SNR. All observed features are soft ($E <$ 3 keV) and we find no evidence for either point-like or extended hard emission within the SNR. Spectral lines from O, Ne, Mg, Si, S, and Fe are present. Equivalent width images for the detected elemental species and spatially-resolved spectral analysis reveal the presence of Mg-rich ejecta within the SNR. We find no such enrichment in O or Ne, which may reflect details of the nucleosynthesis process or the heating and cooling of the ejecta as it evolved. The bright circumferential filaments are emission from the shocked dense interstellar medium (ISM). We detect faint diffuse X-ray emission that extends beyond the X-ray bright filaments toward the west and southeast. These features appear to be the blast wave shock front expanding into lower density portions of the ISM seen in projection. We set an upper limit of $\sim$$2\times 10^{33}$ ergs s$^{-1}$ on the 0.5 $-$ 5 keV band X-ray luminosity of any embedded compact object.

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arXiv2003

An X-Ray Pulsar in the Oxygen-Rich 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 supernova remnant (SNR) G292.0+1.8 using the High Resolution Camera on the Chandra X-ray Observatory. The X-ray period (P=0.13530915 s) is consistent with extrapolation of the radio pulse period of PSR J1124-5916 for a spindown rate of dP/dt=7.6E-13 s/s. The X-ray pulse is single peaked and broad with a FWHM width of 0.23P (83 degrees). The pulse-averaged X-ray spectral properties of the pulsar are well described by a featureless power law model with an absorbing column density, N_H= 3.1E21 atoms/cm^2; photon index, gamma = 1.6; and unabsorbed 0.3-10 keV band luminosity, L_X = 7.2E32 erg/s. We plausibly identify the location of the pulsar's termination shock. Pressure balance between the pulsar wind and the larger synchrotron nebula, as well as lifetime issues for the X-ray-emitting electrons, argues for a particle- dominated PWN that is far from the minimum energy condition. Upper limits on the surface temperature of the neutron star are at, or slightly below, values expected from ``standard'' cooling curves. There is no optical counterpart to the new pulsar; its optical luminosity is at least a factor of 5 below that of the Crab pulsar.

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arXiv2002

X-Ray Emission from Multi-Phase Shock in the Large Magellanic Cloud Supernova Remnant N49

The supernova remnant (SNR) N49 in the Large Magellanic Cloud has been observed with the Advanced CCD Imaging Spectrometer (ACIS) on board the {\it Chandra X-Ray Observatory}. The superb angular resolution of the {\it Chandra}/ACIS images resolves a point source, the likely X-ray counterpart of soft gamma-ray repeater SGR 0526$-$66, and the diffuse filaments and knots across the SNR. These filamentary features represent the blast wave sweeping through the ambient interstellar medium and nearby dense molecular clouds. We detect metal-rich ejecta beyond the main blast wave shock boundary in the southwest of the SNR, which appear to be explosion fragments or ``bullets'' ejected from the progenitor star. The detection of strong H-like Si line emission in the eastern side of the SNR requires multi-phase shocks in order to describe the observed X-ray spectrum, whereas such a multi-phase plasma is not evident in the western side. This complex spectral structure of N49 suggests that the postshock regions toward the east of the SNR might have been reheated by the reverse shock off the dense molecular clouds while the blast wave shock front has decelerated as it propagates into the dense clouds. The X-ray spectrum of the detected point-like source is continuum dominated and can be described with a power law of $Γ$ $\sim$ 3. This provides a confirmation that this point-like X-ray source is the counterpart of SGR 0526$-$66 in the quiescent state.

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arXiv2002

The X-ray Spectrum of Supernova Remnant 1987A

We discuss the X-ray emission observed from Supernova Remnant 1987A with the Chandra X-ray Observatory. We analyze a high resolution spectrum obtained in 1999 October with the high energy transmission grating (HETG). From this spectrum we measure the strengths and an average profile of the observed X-ray lines. We also analyze a high signal-to-noise ratio CCD spectrum obtained in 2000 December. The good statistics (~ 9250 counts) of this spectrum and the high spatial resolution provided by the telescope allow us to perform spectroscopic analyses of different regions of the remnant. We discuss the relevant shock physics that can explain the observed X-ray emission. The X-ray spectra are well fit by plane parallel shock models with post-shock electron temperatures of ~ 2.6 keV and ionization ages of ~ 6 x 10^10 cm^3/s. The combined X-ray line profile has a FWHM of ~ 5000 km/s, indicating a blast wave speed of ~ 3500 km/s. At this speed, plasma with a mean post-shock temperature of ~ 17 keV is produced. This is direct evidence for incomplete electron-ion temperature equilibration behind the shock. Assuming this shock temperature, we constrain the amount of collisionless electron heating at the shock front at T_e0 / T_s = 0.11 (+0.02,-0.01). We find that the plasma has low metallicity (abundances are ~ 0.1 -- 0.4 solar) and is nitrogen enriched (N/O ~ 0.8 by number), similar to that found for the equatorial ring. Analysis of the spectra from different regions of the remnant reveals slight differences in the parameters of the emitting plasma. The plasma is cooler near the optical Spot 1 (at position angle ~ 30 degrees) and in the eastern half of the remnant, where the bright optical spots are found, than in the western half, consistent with the presence of slower (~ 500 km/s) shocks entering denser ring material.

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arXiv2001

The Structure of the Oxygen-rich Supernova Remnant G292.0+1.8 from Chandra X-ray Images: Shocked Ejecta and Circumstellar Medium

We present results from the observation of the young Galactic supernova remnant (SNR) G292.0+1.8 with the Advanced CCD Imaging Spectrometer (ACIS) on board the {\it Chandra X-ray Observatory}. In the 0.3 $-$ 8 keV band, the high resolution ACIS images reveal a complex morphology consisting of knots and filaments, as well as the blast wave around the periphery of the SNR. We present equivalent width (EW) maps for the elemental species O, Ne, Mg, and Si, which allow us to identify regions of enhanced metallicity in the SNR. G292.0+1.8 is bright in O, Ne, and Si; weaker in S and Ar; with little Fe. The EW and broad-band images indicate that the metal-rich ejecta are distributed primarily around the periphery of the SNR. The central belt-like structure has normal solar-type composition, strongly suggesting that it is primarily emission from shocked circumstellar medium rather than metal-rich ejecta. We propose that the belt traces its origin to enhanced mass loss in the star's equatorial plane during the slow, red supergiant phase. We also identify thin filaments with normal composition, centered on and extending nearly continuously around the outer boundary of the SNR. These may originate in a shell caused by the stellar winds from the massive progenitor in the red/blue supergiant phases, over-run by the blast wave.

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arXiv2001

Monitoring the Evolution of the X-ray Remnant of SN 1987A

We report on the results of our monitoring program of the remnant of SN 1987A with the Advanced CCD Imaging Spectrometer (ACIS) on board the {\it Chandra X-ray Observatory}. Two new observations have been performed in AO2, bringing the total to four monitoring observations over the past two years. Over this time period, new techniques for correction of ``Charge Transfer Inefficiency (CTI)'' and for use of charge spreading to provide angular resolution somewhat better than the pixel size of the CCD detector have become available at Penn State. We have processed all four observations using sub-pixel resolution to obtain the highest possible angular resolution, and using our CTI correction software to provide more reliable spectral analysis and flux estimations. The high angular resolution images indicate that the X-ray bright knots are convincingly correlated with the optical spots, primarily at $\la$1 keV, while higher energy photons are very well correlated with radio images. Our data also provide marginal evidence for radial expansion of the X-ray remnant at a rate of 5200 $\pm$ 2100 km s$^{-1}$. The X-ray flux appears to linearly increase by $\sim$60% over the 18 month period of these observations. The spectrum is dominated by broad complexes of atomic emission lines and can be fit with a simple model of a plane-parallel shock with electron temperatures of $kT$ $\sim$ 2 $-$ 4 keV and a postshock electron density of $n_{e}$ $\sim$ 210 $-$ 420 cm$^{-3}$. The implied 0.5 $-$ 10 keV band luminosity in 2001 April is $\sim$1.3 $\times$ 10$^{35}$ ergs s$^{-1}$; as of that date, we still observe no direct evidence for the central point source, with an upper limit on the {\it observed} luminosity of $L_{X}$ $\sim$ 5.5 $\times$ 10$^{33}$ ergs s$^{-1}$ in the 2 $-$ 10 keV band.

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arXiv2000

ASCA Observation of an "X-ray Shadow" in the Galactic Plane

The diffuse X-ray background (DXB) emission near the Galactic plane ($l,b \sim 25.6^{\circ},0.78^{\circ}$) has been observed with $ASCA$. The observed region is toward a Galactic molecular cloud which was recently reported to cast a deep X-ray shadow in the 0.5 $-$ 2.0 keV band DXB. The selection of this particular region is intended to provide a constraint on the spatial distribution of the DXB emission along the line of sight: i.e., the molecular cloud is optically thick at $<$2 keV and so the bulk of the observed soft X-rays {\it must} originate in the foreground of the cloud, which is at $\sim$3 kpc from the Sun. In the 0.8 $-$ 9.0 keV band, the observed spectrum is primarily from multiple components of thermal plasmas. We here report a detection of soft X-ray (0.5 $-$ 2 keV) emission from an $\sim10^{7}$ K thermal plasma. Comparisons with the {\it ROSAT} data suggest that this soft X-ray emission is absorbed by $N_H$ = 1 $-$ 3 $\times$ 10$^{21}$ cm$^{-2}$, which implies a path-length through the soft X-ray emitting regions of $\la$1 kpc from the Sun.

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arXiv1996

Evidence for an X-ray Emitting Galactic Bulge: Shadows Cast by Distant Molecular Gas

A mosaic of 7 ROSAT PSPC pointed observations in the direction of (l,b ~ 10,0 deg) reveals deep X-ray shadows in the 0.5-2.0 keV band cast by dense molecular gas. The comparison between the observed on-cloud and off-cloud X-ray fluxes indicates that ~43% of the diffuse X-ray background in this direction in both the 3/4 keV and 1.5 keV bands originates behind the molecular gas, which is located at 2-4 kpc from the Sun. Given the short mean free path of X-rays in the 3/4 keV band in the Galactic plane (~1 kpc assuming an average space density of 1 cm^-3), this large percentage of the observed flux which originates beyond the molecular gas most likely indicates a strong enhancement in the distribution of X-ray emitting gas in the Galactic center region, possibly associated with a Galactic X-ray bulge.

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