CO Clouds Research Articles

Multiwavelength study of the HII region LHA 120-N11 in the Large Magellanic Cloud with eROSITA

Aims. We studied the diffuse X-ray emission around the HII region LHA 120-N11, which is one of the most active star-forming regions in the Large Magellanic Cloud. We want to determine the nature of the diffuse X-ray emission and improve our understanding of its origin including related interactions with the cold interstellar medium. Methods. We analyzed the diffuse X-ray emission observed with the extended Roentgen Survey with an Imaging Telescope Array (eROSITA) on the Spectrum-Roentgen-Gamma mission to determine the physical properties of the hot diffuse X-ray emission. Four spectral extraction regions were defined based on the morphology of the X-ray emission. We also studied HI and CO data, as well as Hα line emission in the optical, and compared them with the properties of the diffuse X-ray emission. Results. The X-ray emission in the four regions is well fitted with an absorbed model consisting of thermal plasma models (vapec) yielding temperatures of kT = ~0.2 keV and kT = 0.8–1.0 keV. The comparison of the X-ray absorption column density and the hydrogen column density shows that the X-ray dark lane located north of N11 is apparently caused by the absorption by HI and CO clouds. By estimating the energy budget of the thermal plasma, we also investigated the heating mechanism of the X-ray emitting plasma. The energy of the diffuse X-ray emission in the superbubble which is a star-forming bubble with a radius of ~120 pc including OB associations LH9, LH10, LH11, and LH13 can be explained by heating from high-mass stars. In the surrounding regions we find that the energy implied by the X-ray emission suggests that additional heating might have been caused by shocks generated by cloud–cloud collisions.

SuperCAM CO(3-2) APEX survey at a 6 pc resolution in the Small Magellanic Clouds

The Small Magellanic Cloud (SMC) is an ideal laboratory for studying the properties of star-forming regions thanks to its low metallicity, which has an impact on the molecular gas abundance. However, a small number of molecular gas surveys of the entire galaxy have been carried out in the last few years, limiting the measurements of interstellar medium (ISM) properties in a homogeneous manner. We present the CO($3-2$) APEX survey at a $6$ pc resolution of the bar of the SMC, observed with the SuperCAM receiver attached to the APEX telescope. This high-resolution survey has allowed us to study certain properties of the ISM and to identify CO clouds in the innermost parts of the H$_2$ envelopes. We black adopted the CO analysis in the SMC bar comparing the CO($3-2$) survey with that of the CO($2-1$) of a similar resolution. We studied the CO($3-2$)-to-CO($2-1$) ratio ($R_ $), which is very sensitive to the environment properties (e.g., star-forming regions). We analyzed the correlation of this ratio with observational quantities that trace the star formation such as the local CO emission, the Spitzer color $ $, and the total IR surface brightness measured from the Spitzer and Herschel bands. For the identification of the CO($3-2$) clouds, we used the CPROPS algorithm, which allowed us to measure the physical properties of the clouds. We analyzed the scaling relationships of such physical properties. black We obtained $R_ 0.02$ for the SW bar and a slightly higher ratio 0.1,$ for N66 in the SMC. We found that $R_ $ varies from region to region, depending on the star formation activity. In regions dominated by HII and photo-dissociated regions (e.g., N22, N66) $R_ $ tends to be higher than the median values. Meanwhile, lower values were found toward quiescent clouds. We also found that $R_ black is correlated with the IR color $ $ and the total IR surface brightness. This finding indicates that $R_ $ increases with environmental properties, such as the dust temperature, total gas density, and radiation field. We identified $225$ molecular clouds with sizes of $R > 1.5$ pc and signal-to-noise ratios (S/N) of $> 3$ black of which only $17$ black are well resolved CO($3-2$) clouds black with S/N $ 5$. These $17$ clouds follow consistent scaling relationships to the inner Milky Way clouds but with some departures. For instance, CO($3-2$) tends to be less turbulent and less luminous than the inner Milky Way clouds of similar sizes. Finally, we estimated a median virial-based CO($3-2$)-to-H$_2$ conversion factor of for the total sample.

An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. II. CO Cloud Catalog

The nature of molecular clouds and their statistical behavior in subsolar metallicity environments are not fully explored yet. We analyzed data from an unbiased CO (J = 2–1) survey at the spatial resolution of ∼2 pc in the northern region of the Small Magellanic Cloud with the Atacama Compact Array to characterize the CO cloud properties. A cloud-decomposition analysis identified 426 spatially/velocity-independent CO clouds and their substructures. Based on the cross-matching with known infrared catalogs by Spitzer and Herschel, more than 90% CO clouds show spatial correlations with point sources. We investigated the basic properties of the CO clouds and found that the radius–velocity linewidth (R–σ v ) relation follows the Milky Way-like power-law exponent, but the intercept is ∼1.5 times lower than that in the Milky Way. The mass functions (dN/dM) of the CO luminosity and virial mass are characterized by an exponent of ∼1.7, which is consistent with previously reported values in the Large Magellanic Cloud and in the Milky Way.

Synthetic Next Generation Very Large Array line observations of a massive star-forming cloud

Context. Studies of the interstellar medium and the pre-stellar cloud evolution require spectral line observations that have a high sensitivity and high angular and velocity resolution. Regions of high-mass star formation are particularly challenging because of line-of-sight confusion, inhomogeneous physical conditions, and potentially very high optical depths. Aims. We wish to quantify to what accuracy the physical conditions within a massive star-forming cloud can be determined from observations. We are particularly interested in the possibilities offered by the Next Generation Very Large Array (ngVLA) interferometer. Methods. We used data from a magnetohydrodynamic simulation of star formation in a high-density environment. We concentrated on the study of a filamentary structure that has physical properties similar to a small infrared-dark cloud. We produced synthetic observations for spectral lines observable with the ngVLA and analysed these to measure column density, gas temperature, and kinematics. Results were compared to ideal line observations and the actual 3D model. Results. For a nominal cloud distance of 4kpc, ngVLA provides a resolution of ~0.01 pc even in its most compact configuration. For abundant molecules, such as HCO+, NH3, N2H+, and CO isotopomers, cloud kinematics and structure can be mapped down to subarcsecond scales in just a few hours. For NH3, a reliable column density map could be obtained for the entire 15″ × 40″ cloud, even without the help of additional single-dish data, and kinetic temperatures are recovered to a precision of ~1 K. At higher frequencies, the loss of large-scale emission becomes noticeable. The line observations are seen to accurately trace the cloud kinematics, except for the largest scales, where some artefacts appear due to the filtering of low spatial frequencies. The line-of-sight confusion complicates the interpretation of the kinematics, and the usefulness of collapse indicators based on the expected blue asymmetry of optically thick lines is limited. Conclusions. The ngVLA will be able to provide accurate data on the small-scale structure and the physical and chemical state of star-forming clouds, even in high-mass star-forming regions at kiloparsec distances. Complementary single-dish data are still essential for estimates of the total column density and the large-scale kinematics.

Confirmation of dust scattering echo around MAXI J1421–613 by Swift observation

MAXIJ1421–613 is an X-ray burster discovered by Monitor of All-sky X-ray Image (MAXI) on 9 January 2014 and is considered to be a low-mass X-ray binary. A previous study analyzing follow-up observation data obtained by Suzaku on 31 January to 3 February 2014 reported that an annular emission of ∼3′–9′ radius was found around the transient source. The most plausible origin of the annular emission is a dust scattering echo by the outburst of MAXIJ1421–613. In this paper, we confirm the annular emission by analyzing the data of the Swift follow-up observation which was conducted by the photon counting mode on 18 January 2014. In a radial profile, we found an annular emission at ∼2′.5–4′.5. Its spectrum was well explained by an absorbed power law, and the photon index was higher than that of MAXIJ1421–613 itself by ΔΓ∼2. The flux and radius of the annular emission observed by Swift are explained by dust scattering of the same outburst as is responsible for the annular emission observed by Suzaku. Assuming that the dust layer causing the annular emission found by Swift is located at the same position as the CO cloud in front of MAXIJ1421–613, the distance to the transient source was estimated to be ∼3 kpc, which is consistent with the value estimated by the previous study of Suzaku.

H i Narrow-line Self-absorptions toward the High-mass Star-forming Region G176.51+00.20

Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) 19-beam tracking observational mode, high-sensitivity and high-velocity-resolution H i spectral lines have been observed toward the high-mass star-forming region G176.51+00.20. This is a pilot study searching for H i narrow-line self-absorption (HINSA) toward high-mass star-forming regions where bipolar molecular outflows have been detected. This work is confined to the central seven beams of FAST. Two HINSA components are detected in all seven beams, which correspond to a strong CO emission region (SCER; with a velocity of ∼−18 km s−1) and a weak CO emission region (WCER; with a velocity of ∼−3 km s−1). The SCER detected in Beam 3 is probably more suitably classified as a WCER. In the SCER, the HINSA is probably associated with the molecular material traced by the CO. The fractional abundance of HINSA ranges from ∼1.1 × 10−3 to ∼2.6 × 10−2. Moreover, the abundance of HINSA in Beam 1 is lower than that in the surrounding beams (i.e., Beams 2 and 4–7). This possible ring could be caused by the ionization of H i or the relatively rapid conversion from H i to H2 in the higher-density inner region. In the WCER (including Beam 3 in the SCER), the HINSA is probably not associated with CO clouds, but with CO-dark or CO-faint gas.

Chemical Evolution During the Formation of Molecular Clouds

To study the chemical evolution during the formation of molecular clouds, we model three types of clouds with different density structures: collapsing spherical, collapsing ellipsoidal, and static spherical profiles. The collapsing models are better than the static models in matching the observational characteristics in typical molecular clouds. This is mainly because the gravity can speed up the formation of some important molecules (e.g., H2, CO, OH) by increasing the number density during collapse. The different morphologies of prolate, oblate, and spherical clouds lead to differences in chemical evolution, which are mainly due to their different evolution of number density. We also study the effect of initial chemical compositions on chemical evolution, and find that H atoms can accelerate OH formation by two major reactions: O + H → OH in gas phase and on dust grain surfaces, leading to the models in which hydrogen is mainly atomic initially better match observations than the models in which hydrogen is mainly molecular initially. Namely, to match observations, initially hydrogen must be mostly atomic. The CO molecules are able to form even without the pre-existence of H2. We also study the influence of gas temperature, dust temperature, intensity of interstellar radiation field and cosmic-ray ionization rate on chemical evolution in static clouds. The static CO clouds with high dust temperature, strong radiation field, and intensive cosmic rays are transient due to rapid CO destruction.

SPLASH: the Southern Parkes Large-Area Survey in Hydroxyl – data description and release

ABSTRACT We present the full data release for the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH), a sensitive, unbiased single-dish survey of the Southern Galactic Plane in all four ground-state transitions of the OH radical at 1612, 1665, 1667, and 1720 MHz. The survey covers the inner Galactic Plane, Central Molecular Zone, and Galactic Centre over the range |b| < 2°, 332$^{\circ }\, \lt l \lt $ 10°, with a small extension between 2$^{\circ }\, \lt b \lt $ 6°, 358$^{\circ }\, \lt l \lt $ 4°. SPLASH is the most sensitive large-scale survey of OH to-date, reaching a characteristic root-mean-square sensitivity of ∼15 mK for an effective velocity resolution of ∼0.9 km s−1. The spectral line datacubes are optimized for the analysis of extended, quasi-thermal OH, but also contain numerous maser sources, which have been confirmed interferometrically and published elsewhere. We also present radio continuum images at 1612, 1666, and 1720 MHz. Based on initial comparisons with 12CO(J = 1–0), we find that OH rarely extends outside CO cloud boundaries in our data, but suggest that large variations in CO-to-OH brightness temperature ratios may reflect differences in the total gas column density traced by each. Column density estimation in the complex, continuum-bright Inner Galaxy is a challenge, and we demonstrate how failure to appropriately model sub-beam structure and the line-of-sight source distribution can lead to order-of-magnitude errors. Anomalous excitation of the 1612 and 1720 MHz satellite lines is ubiquitous in the inner Galaxy, but is disabled by line overlap in and around the Central Molecular Zone.

Spatially resolved X-ray study of the mixed morphology SNR G359.1-0.5

ABSTRACT We present a detailed spatially resolved X-ray study of Chandra observations towards the central region of the supernova remnant (SNR) G359.1-0.5. We removed 168 point-like sources and performed an adaptive smoothing on the entire field-of-view of the Chandra ACIS-I observation. The images obtained show an extended X-ray emission below $4.0\, {\rm keV}$ that partially fills the interior of G359.1-0.5. In order to characterize the X-ray emission, we divide the emitting area into 10 different regions inside the remnant. We identify the presence of at least four emission lines that confirm the thin thermal plasma origin. All spectral regions are well fitted by a recombining plasma model (VRNEI) with an absorbing column density range from 1.5 to $2.1\times 10^{22}\, {\rm cm}^{-2}$, and electron temperatures from 0.14 to $0.19\, {\rm keV}$. We also calculated an average electron density of $0.7\, {\rm cm}^{-3}$, estimated an age of 2.8 × 104 yr for the remnant, which can be lower if the filling factor is considered, and an X-ray luminosity of $1.6 \times 10^{35} \, {\rm erg}\, {\rm s}^{-1}$. Finally, we show that the X-ray emission is morphologically anti-correlated with adjacent CO clouds interacting with the SNR. Our results favour that the recombining plasma inside G359.1-0.5 could be explained via adiabatic cooling.

Burton’s Curse: The Impact of Bulk Flows on the Galactic Longitude–Velocity Diagram and the Illusion of a Continuous Perseus Arm

Abstract In this work, we demonstrate that the Perseus Arm is not a continuous structure of molecular gas in the second quadrant. We first show that the observed, distanced-resolved velocity structure of the Galaxy in the outer disk is capable of creating illusory spiral arms, as was first proposed by Burton. Second, we measure the distances to a collection of CO clouds at velocities consistent with the Perseus arm with 135° < l < 160°. We find these distances using 3D dust maps from Green et al. We determine that these molecular clouds do not preferentially lie at the distance of a purported Perseus arm but rather extend over 3 kpc in distance, with some evidence for a closer, high pitch angle structure between 1 and 1.5 kpc away. Finally, we demonstrate that velocity perturbations of the amplitude found near the Perseus arm can wreak havoc on our interpretation of the longitude–velocity diagram for more than half of the Milky Way disk.

An Unbiased CO Survey toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. I. Overview: CO Cloud Distributions

Abstract We have analyzed the data from a large-scale CO survey toward the northern region of the Small Magellanic Cloud (SMC) obtained with the Atacama Compact Array (ACA) stand-alone mode of ALMA. The primary aim of this study is to comprehensively understand the behavior of CO as an H2 tracer in a low-metallicity environment (Z ∼ 0.2 Z ⊙). The total number of mosaic fields is ∼8000, which results in a field coverage of 0.26 deg2 (∼2.9 ×105 pc2), corresponding to ∼10% of the area of the galaxy. The sensitive ∼2 pc resolution observations reveal the detailed structure of the molecular clouds previously detected in the single-dish NANTEN survey. We have detected a number of compact CO clouds within lower H2 column density (∼1020 cm−2) regions whose angular scale is similar to the ACA beam size. Most of the clouds in this survey also show peak brightness temperature as low as <1 K, which for optically thick CO emission implies an emission size much smaller than the beam size, leading to beam dilution. The comparison between an available estimation of the total molecular material traced by thermal dust emission and the present CO survey demonstrates that more than ∼90% of H2 gas cannot be traced by the low-J CO emission. Our processed data cubes and 2D images are publicly available.

ALMA CO Observations of the Mixed-morphology Supernova Remnant W49B: Efficient Production of Recombining Plasma and Hadronic Gamma Rays via Shock–Cloud Interactions

Abstract We carried out new CO(J = 2–1) observations toward the mixed-morphology supernova remnant (SNR) W49B with the Atacama Large Millimeter/submillimeter Array. We found that CO clouds at ∼10 km s−1 show a good spatial correspondence to the synchrotron radio continuum as well as to an X-ray deformed shell. The bulk mass of molecular clouds accounts for the western part of the shell, not the eastern shell, where near-infrared H2 emission is detected. The molecular clouds at ∼10 km s−1 show higher kinetic temperatures of ∼20–60 K, suggesting that modest shock heating occurred. The expanding motion of the clouds with ΔV ∼ 6 km s−1 was formed by strong winds from the progenitor system. We argue that the barrel-like structure of Fe-rich ejecta was possibly formed not only by an asymmetric explosion, but also by interactions with dense molecular clouds. We also found a negative correlation between the CO intensity and the electron temperature of recombining plasma, implying that the origin of the high-temperature recombining plasma in W49B can be understood to be the thermal conduction model. The total energy of accelerated cosmic-ray protons W p is estimated to be ∼2 × 1049 erg by adopting an averaged gas density of ∼650 ± 200 cm−3. The SNR age–W p diagram indicates that W49B shows one of the highest in situ values of W p among gamma-ray-bright SNRs.

The peculiar nebula Simeis 57

Simeis 57 (HS 191) is an optically bright nebula in the Cygnus X region with a peculiar appearance that suggests an outflow from a rotating source. Newly obtained observations and archival data reveal Simeis 57 as a low-density (ne ~ 100 cm−3) nebula with an east-to-west excitation gradient. The extinction of the nebula is AV ≤ 2 mag. The nebula is recognizable but not prominent in mid- and far-infrared images. In its direction, half a dozen small CO clouds have been identified at VLSR = +5 km s−1. One of these coincides with both the optical nebula and a second CO cloud at the nebular velocity VLSR ≈−10 km−1. No luminous stars are embedded in these molecular clouds, nor are any obscured by them and no sufficiently luminous stars are found in the immediate vicinity of the nebula. Instead, all available data points to the evolved star HD 193793 = WR 140 (an O4-5 supergiant and WC7 Wolf–Rayet binary) as the source of excitation, notwithstanding its large separation of 50′, about 25 pc at the stellar distance of 1.7 kpc. Simeis 57 appears to be a part of a larger structure surrounding the HI void centered on HD 193793.

A new search for star forming regions in the southern outer Galaxy

Context. Star formation in the outer Galaxy is thought to be different from that in the inner Galaxy, as it is subject to different environmental parameters such as metallicity, interstellar radiation field, or mass surface density, which all change with galactocentric radius. Extending our star formation knowledge, from the inner to the outer Galaxy, helps us to understand the influences of the change of the environment on star formation throughout the Milky Way. Aims. We aim to obtain a more detailed view on the structure of the outer Galaxy, determining physical properties for a large number of star forming clumps and understanding star formation outside the solar circle. As one of the largest expanding Galactic super-shells is present in the observed region, a unique opportunity is taken here to investigate the influence of such an expanding structure on star formation as well. Methods. We used pointed 12CO(2–1) observations conducted with the APEX telescope to determine the velocity components towards 830 dust clumps identified from 250 μm Herschel/Hi-GAL SPIRE emission maps in the outer Galaxy between 225° < ℓ < 260°. We determined kinematic distances from the velocity components, in order to analyze the structure of the outer Galaxy and to estimate physical properties such as dust temperatures, bolometric luminosities, clump masses, and H2 column densities for 611 clumps. For this, we determined the dust spectral energy density distributions from archival mid-infrared to submillimeter (submm) emission maps. Results. We find the identified CO clouds to be strongly correlated with the highest column density parts of the H I emission distribution, spanning a web of bridges, spurs, and blobs of star forming regions between the larger complexes, unveiling the complex three-dimensional structure of the outer Galaxy in unprecedented detail. Using the physical properties of the clumps, we find an upper limit of 6% (40 sources) capable of forming high-mass stars. This is supported by the fact that only two methanol Class II masers, or 34 known or candidate H II regions, are found in the whole survey area, indicating an even lower fraction that are able to form high-mass stars in the outer Galaxy. We fail to find any correlation of the physical parameters of the identified (potential) star forming regions with the expanding supershell, indicating that although the shell organizes the interstellar material into clumps, the properties of the latter are unaffected. Conclusions. Using the APEX telescope in combination with publicly available Hi-GAL, MSX, and Wise continuum emission maps, we were able to investigate the structure and properties of a region of the Milky Way in unprecedented detail.

CO Outflow Candidates toward the W3/4/5 Complex. II. Feedback from Candidate Outflows

Abstract To date, few studies have focused on protostellar outflow feedback at scales larger than several parsecs. To address this paucity of research, we investigate the effects of feedback from CO outflow candidates on their parent clouds over ∼110 deg2 toward the W3/4/5 complex and its surroundings. Our search identified 265 13CO clouds with radii being ∼0.04–17.12 pc. We estimate the turbulent support and potential disruptive effect of the outflow activities through analyzing physical properties of outflow candidates and their host clouds in terms of turbulence and gravitational binding energy. We find that (1) clouds of larger size might be less affected by feedback; (2) the possible scale break is ≳4.7 pc for both turbulent support and potential disruptive effect; (3) if outflows couple to dense gas where stars are forming, for clouds in the Perseus arm, a scale ≲0.2–0.4 pc is sufficient for the energy and momentum injected by outflow activity to maintain turbulence, while for clouds in the Local arm, the scale is ≲0.1–0.2 pc; and (4) for clouds in the Perseus arm, a scale ≲0.3–1.0 pc is needed for outflow activity to potentially disperse material away from the natal clouds, while for clouds in the Local arm, the scale is ≲0.2–0.6 pc. The strength of outflow activity may affect the values in points 3 and 4. Finally, we find that outflow feedback probably possesses the power to alter the line width–size relation.

Cloud structures in M 17 SWex : Possible cloud–cloud collision

Abstract Using wide-field 13CO (J = 1−0) data taken with the Nobeyama 45 m telescope, we investigate cloud structures of the infrared dark cloud complex in M 17 with Spectral Clustering for Interstellar Molecular Emission Segmentation. In total, we identified 118 clouds that include 11 large clouds with radii larger than 1 pc. The clouds are mainly distributed in the two representative velocity ranges of 10–20 km s−1 and 30–40 km s−1. By comparing this with the ATLASGAL catalog, we found that the majority of the 13CO clouds with 10–20 km s−1 and 30–40 km s−1 are likely located at distances of 2 kpc (Sagittarius arm) and 3 kpc (Scutum arm), respectively. Analyzing the spatial configuration of the identified clouds and their velocity structures, we attempt to reveal the origin of the cloud structure in this region. Here we discuss three possibilities: (1) overlapping with different velocities, (2) cloud oscillation, and (3) cloud–cloud collision. In the position–velocity diagrams, we found spatially extended faint emission between ∼20 km s−1 and ∼35 km s−1, which is mainly distributed in the spatially overlapped areas of the clouds. Additionally, the cloud complex system is unlikely to be gravitationally bound. We also found that in some areas where clouds with different velocities overlapped, the magnetic field orientation changes abruptly. The distribution of the diffuse emission in the position–position–velocity space and the bending magnetic fields appear to favor the cloud–cloud collision scenario compared to other scenarios. In the cloud–cloud collision scenario, we propose that two ∼35 km s−1 foreground clouds are colliding with clouds at ∼20 km s−1 with a relative velocity of 15 km s−1. These clouds may be substructures of two larger clouds having velocities of ∼35 km s−1 (≳103 M⊙) and ∼20 km s−1 (≳104 M⊙), respectively.

ALMA Observations of the Molecular Clouds in NGC 625

Abstract We present the highest-resolution (1″) 12CO observations of molecular gas in the dwarf starburst galaxy NGC 625 to date, obtained with Atacama Large Millimeter/submillimeter Array. The molecular gas, which is distributed in discrete clouds within an area of 0.4 kpc2, does not have well-ordered large-scale motions. We measure a molecular mass in NGC 625 of 5.3 × 106 , assuming a Milky Way CO-to- conversion factor. We use the CPROPS package to identify molecular clouds and measure their properties. The 19 resolved CO clouds have a median radius of 20 pc, a median linewidth 2.5 , and a median surface density of 169 . Larson scaling relations suggest that molecular clouds in NGC 625 are mostly in virial equilibrium. Comparison of our high-resolution CO observations with a star formation rate map, inferred from ancillary optical observations, suggests that about 40% of the molecular clouds coincide with the brightest H ii regions. These bright H ii regions have a range of molecular gas depletion timescales, all within a factor of ∼3 of the global depletion time in NGC 625 of 106–134 Myr. The highest surface density molecular clouds toward the southwest of the galaxy, in a region we call the Butterfly, do not show strong star formation activity and suggest a depletion timescale longer than 5 Gyr.

Uniform Distribution of the Extremely Overionized Plasma Associated with the Supernova Remnant G359.1-0.5

Abstract We report on the results of our detailed analyses on the peculiar recombining plasma of the supernova remnant (SNR) G359.1−0.5, and the interacting CO clouds. Combining Chandra and Suzaku data, we estimated the ionization state of the plasma with a careful treatment of the background spectrum. The average spectrum showed a remarkably large deviation of the electron temperature (∼0.17 keV) from the initial temperature (>16 keV), indicating that the plasma is in a highly recombination-dominant state. On the other hand, the recombination timescale (n e t) is comparable to those of the other recombining SNRs (∼4.2 × 1011 cm−3 s). We also searched for spatial variation of the plasma parameters, but found no significant differences. Using 12CO(J = 2–1) data obtained with NANTEN2, we found a new, plausible candidate for the interacting CO cloud, which has a line-of-sight velocity of ∼ −20 km s−1. This indicates that the SNR is located at a distance of ∼4 kpc, which is the foreground of the Galactic center, as previously reported. The associated CO cloud does not show clear spatial coincidence with the nearby GeV/TeV emission, indicating that the origins of the GeV/TeV emission are likely unrelated to G359.1−0.5.

Discovery of annular X-ray emission centered on MAXI J1421-613: Dust-scattering X-rays?

Abstract We report the discovery of an annular emission of $\sim\!\! {3^{\prime }}\!-\!{9^{\prime }}$ radius around the center of a transient source, the X-ray burster MAXI J1421-613, in the Suzaku follow-up analysis. The spectrum of the annular emission shows no significant emission-line structure, and is well explained by an absorbed power-law model with a photon index of $\sim\!\! 4.2$. These features exclude the possibility that the annular emission is a shell-like component of a supernova remnant. The spectral shape, the time history, and the X-ray flux of the annular emission agree with the scenario that the emission is due to a dust-scattering echo. The annular emission is made under a rare condition of the dust-scattering echo, where the central X-ray source, MAXI J1421-613, exhibits a short time outburst with three X-ray bursts and immediately re-enters a long quiescent period. The distribution of the hydrogen column density along the annular emission follows that of the CO intensity, which means that MAXI J1421-613 is located behind the CO cloud. We estimate the distance to MAXI J1421-613 to be $\sim\!\! 3\:$kpc assuming that the dust layer responsible for the annular emission is located at the same position as the CO cloud.

Molecular environments of the supernova remnant G359.1−0.5

ABSTRACT We report new CO observations and a detailed molecular-line study of the mixed morphology supernova remnant G359.1−0.5, which contains six OH (1720 MHz) masers along the radio shell, indicative of shock-cloud interaction. Observations of 12CO and 13CO J:1–0 lines were performed in a ∼38 × 38 arcmin area with the on-the-fly technique using the Kit Peak 12 Meter telescope. The molecular study has revealed the existence of a few clumps with densities ∼103 cm−3 compatible in velocity and position with the OH (1720 MHz) masers. These clumps, in turn, appear to be part of a larger, elongated molecular structure ∼34 arcmin long extending between −12.48 and +1.83 km s−1, adjacent to the western edge of the radio shell. According to the densities and relative position with respect to the masers, we conclude that the CO clouds depict unshocked gas, as observed in other remnants with OH (1720 MHz) masers. In addition, we investigated the distribution of the molecular gas towards the adjacent γ-ray source HESS J1745-303 (Aharonian et al. 2006) but could not find any morphological correlation between the γ-rays and the CO emission at any velocity in this region.