arXiv subjects

J. Rho

Publications and source records attributed to J. Rho.

44 records · Page 3Linked to original sources

arXiv2006

X-Ray Observations of SN 1006 with Integral

The remnant of the supernova of 1006 AD, the remnant first showing evidence for the presence of X-ray synchrotron emission from shock-accelerated electrons, was observed for ~1000 ksec with INTEGRAL for the study of electron acceleration to very high energies. The aim of the observation was to characterize the synchrotron emission, and attempt to detect non-thermal bremsstrahlung, using the combination of IBIS and JEM-X spatial and spectral coverage. The source was detected with JEM-X between 2.4 and 8.4 keV bands, and not detected with either ISGRI or SPI above 20 keV. The ISGRI upper limit is about a factor of four above current model predictions, but confirms the presence of steepening in the power-law extrapolated from lower energies (< 4 keV).

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arXiv2006

Spectacular Spitzer images of the Trifid Nebula: Protostars in a young, massive-star-forming region

Spitzer IRAC and MIPS images of the Trifid Nebula (M20) reveal its spectacular appearance in infrared light, highlighting the nebula's special evolutionary stage. The images feature recently-formed massive protostars and numerous young stellar objects, and a single O star that illuminates the surrounding molecular cloud from which it formed, and unveil large-scale, filamentary dark clouds. The hot dust grains show contrasting infrared colors in shells, arcs, bow-shocks and dark cores. Multiple protostars are detected in the infrared, within the cold dust cores of TC3 and TC4, which were previously defined as Class 0. The cold dust continuum cores of TC1 and TC2 contain only one protostar each. The Spitzer color-color diagram allowed us to identify ~160 young stellar objects and classify them into different evolutionary stages. The diagram also revealed a unique group of YSOs which are bright at 24 micron but have the spectral energy distribution peaking at 5-8 micron. Despite expectation that Class 0 sources would be "starless" cores, the Spitzer images, with unprecedented sensitivity, uncover mid-infrared emission from these Class 0 protostars. The mid-infrared detections of Class 0 protostars show that the emission escapes the dense, cold envelope of young protostars. The mid-infrared emission of the protostars can be fit by two temperatures of 150 and 400 K; the hot core region is probably optically thin in the mid-infrared regime, and the size of hot core is much smaller than that of the cold envelope. The presence of multiple protostars within the cold cores of Class 0 objects implies that clustering occurs at this early stage of star formation. The TC3 cluster shows that the most massive star is located at the center of the cluster and at the bottom of the gravitational-potential well.

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arXiv2005

Chandra observation of the multiple merger cluster Abell 521

We present the Chandra analysis of the galaxy cluster A521 (z=0.247). The high resolution of the Chandra observation has allowed us to refine the original merging scenario proposed for A521, and to reveal new features in its X-ray emission. A521 has strongly substructured ICM density and temperature maps. Its X-ray diffuse emission is elongated along a NW/SE direction (SX2) and shows two major components, a main cluster and a northern group of galaxies. This latter is in turn substructured, showing a clump of cold and very dense gas centred on the BCG, and a northern tail aligned in the SX2 direction. A compression of the X-ray isophotes is also observed South of the BCG. We conclude that the northern group is infalling onto the main cluster along the NW/SE direction. This hypothesis is corroborated by the presence of a hot bar in the ICM temperature map located between the southern and northern regions, as the gas could be compressionally heated due to the subclusters' collision. The hot region corresponds to the eastern part of an over-dense ridge of galaxies, along which it was originally suggested that a merging of subclusters has recently occurred along the line of sight. An alternative hypothesis about the origin of the hot central bar is that we could observe in projection the shock fronts due to this older cluster-cluster collision. Two other structures possibly interacting with the main cluster are detected on the West and North-East sides of the BCG. We also uncover the presence of two northern edges in the ICM density. A521 is a spectacular example of a multiple merger cluster made up by several substructures converging at different epochs towards the centre of the system. The very pertubed dynamical state of this cluster is also confirmed by our discovery of a radio relic in its South-East region.

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arXiv2004

Structure and Colors of Diffuse Emission in the Spitzer Galactic First Look Survey

We investigate the density structure of the interstellar medium using new high-resolution maps of the 8 micron, 24 micron, and 70 micron surface brightness towards a molecular cloud in the Gum Nebula, made as part of the Spitzer Space Telescope Galactic First Look Survey. The maps are correlated with 100 micron images measured with IRAS. At 24 and 70 micron, the spatial power spectrum of surface brightness follows a power law with spectral index -3.5. At 24 micron, the power law behavior is remarkably consistent from the 0.2 degree size of our maps down to the 5 arcsecond spatial resolution. Thus, the structure of the 24 micron emission is self-similar even at milliparsec scales. The combined power spectrum produced from Spitzer 24 micron and IRAS 25 micron images is consistent with a change in the power law exponent from -2.6 to -3.5. The decrease may be due to the transition from a two-dimensional to three-dimensional structure. Under this hypothesis, we estimate the thickness of the emitting medium to be 0.3 pc.

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arXiv2004

Far Infrared Imaging of NGC 55

We present images of the galaxy NGC 55 at 24, 70, and 160 micron obtained with the Multiband Imaging Photometer for Spitzer (MIPS) instrument aboard the Spitzer Space Telescope. The new images display the far infrared emission in unprecedented detail and demonstrate that the infrared morphology differs dramatically from that at shorter wavelengths. The most luminous emission region in the galaxy is marginally resolved at 24 micron and has a projected separation of nearly 520 pc from the peak emission in the optical and near infrared. This region is responsible for ~9% of the total emission at 24 micron and is likely a young star formation region. We show that this and other compact sources account for more than 1/3 of the total 24 micron emission. We compute a total infrared luminosity for NGC 55 of 1.2*10^9 L_sun. The star formation rate implied by our measurements is 0.22 M_sun/yr. We demonstrate that the cold dust is more extended than the warm dust in NGC 55--the minor-axis scale heights are 0.32, 0.43, and 0.49 kpc at 24, 70 and 160 micron, respectively. The dust temperature map shows a range of temperatures that are well-correlated with the 24 micron surface brightness, from 20 K in low-surface-brightness regions to 26 K in high-surface-brightness regions.

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arXiv2004

Chandra Observation of the Trifid Nebula: X-ray emission from the O star complex and actively forming pre-main sequence stars

The Trifid Nebula, a young star-forming HII region, was observed for 16 hours by the Chandra X-ray Observatory. We detected 304 X-ray sources, thirty percent of which are hard sources and seventy percent of of which have near-infrared counterparts. Chandra resolved the HD164492 multiple system into a number of discrete X-ray sources. X-ray emission is detected from components HD164492A (an O7.5III star which ionizes the nebula), B and C (a B6V star), and possibly D (a Be star). HD164492A has a soft spectrum (kT ~ 0.5 keV) while the component C blend shows much harder emission (kT ~ 6 keV). The soft spectrum of the O star is similar to emission seen from other single O stars and is probably produced by shocks within its massive stellar wind. Lack of hard emission suggests that neither a magnetically confined wind shock nor colliding wind emission is important in HD164492A. A dozen stars are found to have flares in the field and most of them are pre-main sequence stars (PMS). Six sources with flares have both optical and 2MASS counterparts. These counterparts are not embedded and thus it is likely that these sources are in a later stage of PMS evolution, possibly Class II or III. Two flare sources did not have any near-IR, optical, or radio counterparts. We suggest these X-ray flare stars are in an early pre-main sequence stage (Class I or earlier). We also detected X-ray sources apparently associated with two massive star forming cores, TC1 and TC4. The spectra of these sources show high extinction and X-ray luminosities of 2 - 5 x 10^{31} erg/s. If these sources are Class 0 objects, it is unclear if their X-ray emission is due to solar-type magnetic activities as in Class I objects, or some other mechanism.

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arXiv2002

OH (1720 MHz) Masers and Mixed-Morphology Supernova Remnants

Radio surveys of supernova remnants (SNRs) in the Galaxy have uncovered 19 SNRs accompanied by OH maser emission at 1720 MHz. This unusual class of maser sources is suggested to be produced behind a shock front from the expansion of a supernova remnant running into a molecular cloud. An important ingredient of this model is that X-ray emission from the remnant enhances the production of OH molecule. The role of X-ray emission from maser emitting (ME) SNRs is investigated by comparing the X-ray induced ionization rate with theory. One aspect of this model is verified: there is a strong association between maser emitting and mixed-morphology (MM) or thermal composite SNRs --center-filled thermal X-ray emission surrounded by shell-like radio morphology. We also present ROSAT and ASCA observations of two maser emitting SNRs: G21.8--0.6 (Kes 69) and G357.7--0.1 (Tornado).

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arXiv2000

Thermal and Nonthermal X-Ray Emission in SNR RCW 86

Supernova remnants may exhibit both thermal and nonthermal X-ray emission. Such remnants can be distinguished by the weakness of their X-ray lines, because of the presence of a strong nonthermal X-ray continuum. RCW 86 is a remnant with weak lines, resulting in low and peculiar abundances when thermal models alone are used to interpret its X-ray spectrum. This indicates the presence of a strong nonthermal synchrotron continuum. We analyze ASCA X-ray spectra of RCW 86 with the help of both nonequilibrium ionization thermal models and nonthermal synchrotron models. A two-temperature thermal model and a simple nonthermal model with an exponential cutoff (plus interstellar absorption) give reasonable results. We obtain blast wave velocity of 800 km/s, the shock ionization age of 1-3x10^11 s/cm^3, and the break in nonthermal spectra at 2-4x10^16 Hz. The strength of nonthermal continuum correlates well with the radio brightness in the bright SW section of the remnant. This is convincing evidence for X-ray synchrotron emission in RCW 86.

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