Search

Supernova remnants (SNRs) provide insights into cosmic-ray acceleration and magnetic field dynamics at shock fronts. Recent X-ray polarimetric measurements by the Imaging X-ray Polarimetry Explorer (IXPE) have revealed radial magnetic fields near particle acceleration sites in young SNRs, including Cassiopeia A, Tycho, and SN 1006. We present here the spatially resolved IXPE X-ray polarimetric observation of the northwestern rim of SNR RX J1713.7–3946. For the first time, our analysis shows that the magnetic field in the particle acceleration sites of this SNR is oriented tangentially with respect to the shock front. Because of the lack of precise Faraday rotation measurements in the radio band, this was not possible before. The average measured polarization degree (PD) of the synchrotron emission is 12.5% ± 3.3%, lower than the one measured by IXPE in SN 1006, comparable to the Tycho one, but notably higher than the one in Cassiopeia A. On subparsec scales, localized patches within RX J1713.7–3946 display a PD of up to 41.5% ± 9.5%. These results are compatible with a shock-compressed magnetic field. However, in order to explain the observed PD, either the presence of a radial net magnetic field upstream of the shock or partial reisotropization of the turbulence downstream by radial magnetohydrodynamical instabilities can be invoked. From comparison of PD and magnetic field distribution with γ -rays and ^12 CO data, our results provide new inputs in favor of a leptonic origin of the γ -ray emission.

Cosmic metals are believed to originate from stellar and supernovae (SNe) nucleosynthesis, dispersed into the interstellar medium (ISM) through stellar winds and supernova explosions. In this paper, we present the clear evidence of metal enrichment by a Type Ic SN 2011jm in the galaxy NGC 4809, utilizing high spatial resolution integral field unit observations obtained from the Very Large Telescope/Multi Unit Spectroscopic Explorer. Despite SN 2011jm being surrounded by metal-deficient ISM (∼0.25 Z _⊙ ) at a scale about 100 pc, we clearly detect enriched oxygen abundance (∼0.35 Z _⊙ ) and a noteworthy nitrogen-to-oxygen ratio at the SN site. Remarkably, the metal pollution is confined to a smaller scale (≲13 pc). We posit that the enhanced ionized metal stems from stellar winds emitted by massive stars or previous SN explosions. This observation may represent the first direct detection of chemical pollution by stellar feedback in star-forming galaxies beyond the Local Volume.

The problem of missing Galactic supernova remnants (SNRs) refers to the issue that the currently known Galactic SNRs are significantly incomplete compared to the theoretical prediction. To expand the sample of Galactic SNRs, we use GLEAM and THOR+VGPS data across four wave bands ranging from 118 to 1420 MHz to drive a spectral index map covering the region within 26.°6 < l < 30.°6, ∣ b ∣ ≤ 1.°25, where numerous SNR candidates were recently found. By using the spectral index map of the sky region and detailed analysis of the spectral indices of individual sources, we confirmed four SNR candidates, namely G26.75+0.73, G27.06+0.04, G28.36+0.21, and G28.78−0.44, as SNRs. Additionally, we discovered an expanding molecular superbubble located in this region, discussed pulsars associated with SNR candidates, and discovered a long H α filament that spatially overlaps with the candidate G29.38+0.10. We suggest that the problem of missing Galactic SNRs not only arises from observation limitations, but also could be due to the low-density environments of some SNRs, and the different supernova explosion properties.

Supernova remnants (SNRs) exert a strong influence on the physics and chemistry of the nearby molecular clouds through shock waves and the cosmic rays (CRs) they accelerate. To investigate the SNR–cloud interaction in the prototype interacting SNR W28 (G6.4−0.1), we present new observations of the HCO ^+ , $\mathrm{HCN}$ , and HNC J = 1–0 lines, supplemented by archival data of CO isotopes, N _2 H ^+ and H ^13 CO ^+ . We compare the spatial distribution and spectral line profiles of different molecular species. Using the local thermodynamic equilibrium assumption, we obtain an abundance ratio of N (HCO ^+ )/ N (CO) ∼10 ^−4 in the northeastern shocked cloud, which is higher by an order of magnitude than the values in unshocked clouds. This can be accounted for by the chemistry jointly induced by shock and CRs, with the physical parameters previously obtained from observations: preshock density n _H ∼ 2 × 10 ^5 cm ^−3 , CR ionization rate ζ = 2.5 × 10 ^−15 s ^−1 , and shock velocity V _s = 15–20 km s ^−1 . Toward a point outside the northeastern boundary of W28 with a known high CR ionization rate, we estimate the abundance ratio N (HCO ^+ )/ N (N _2 H ^+ ) ≈ 0.6–3.3, which can be reproduced by a chemical simulation if a high density n _H ∼ 2 × 10 ^5 cm ^−3 is adopted.

Magnetars and central compact objects (CCOs) are subgroups of neutron stars that show a number of properties distinguished from canonical radio pulsars. We performed radio observations of three magnetars, SGR 0418+5729, 1E 2259+586, and 4U 0142+61, and a CCO, PSR J1852+0040, with the Five-hundred-meter Aperture Spherical radio Telescope at 1.25 GHz, aiming to search for radio pulsations in their quiescent states. During two observation epochs, no radio pulses have been detected toward any target above a significance of signal-to-noise ratio = 7 from either the direct folding or blind search. We provided the most stringent upper limit of radio flux (≲2–4 μ Jy) for the magnetars and the CCO. For the magnetars with long periods, the real upper limits are likely 1 order of magnitude larger due to the red noise. The deep radio observations suggest that these magnetars and the CCO are indeed radio-quiet sources or unfavorably beamed.

Young supernova remnants strongly modify the surrounding magnetic fields, which in turn play an essential role in accelerating cosmic rays (CRs). The X-ray polarization measurements probe magnetic field morphology and turbulence at the immediate acceleration site. We report the X-ray polarization distribution in the northeastern shell of SN 1006 from a 1 Ms observation with the Imaging X-ray Polarimetry Explorer. We found an average polarization degree of 22.4% ± 3.5% and an average polarization angle of −45.°4 ± 4.°5 (measured on the plane of the sky from north to east). The X-ray polarization angle distribution reveals that the magnetic fields immediately behind the shock in the northeastern shell of SN 1006 are nearly parallel to the shock normal or radially distributed, similar to that in the radio observations, and consistent with the quasi-parallel CR acceleration scenario. The X-ray emission is marginally more polarized than that in the radio band. The X-ray polarization degree of SN 1006 is much larger than that in Cas A and Tycho, together with the relatively tenuous and smooth ambient medium of the remnant, favoring that CR-induced instabilities set the magnetic turbulence in SN 1006, and CR acceleration is environment-dependent.

Galactic supernova remnants (SNRs) play an important role in our understanding of supernovae and their feedback on the interstellar environment. SNR G352.7-0.1 is special for its thermal composite morphology and double-ring structure. We have performed spectroscopic mapping of the ^12 CO and ^13 CO J = 2–1 lines toward G352.7-0.1 with the Atacama Pathfinder Experiment telescope. Broad ^12 CO lines are found in the northeastern ring at a local-standard-of-rest velocity range of ∼−50 to −30 km s ^−1 , suggesting that the remnant is interacting with molecular clouds at ∼−51 km s ^−1 . Thus, we adopt a distance of ∼10.5 kpc for this SNR. The momentum and kinetic energy of the shocked gas along the line of sight are estimated to be ∼10 ^2 M _☉ km s ^−1 and ∼10 ^46 erg, respectively. We also find an expanding structure around the remnant, which is possibly related to the wind-blown bubble of the progenitor star. From Fermi-LAT data in the energy range 0.1–500 GeV, we find no gamma-ray counterparts to G352.7-0.1.

Ions (e.g., ${{\rm{H}}}_{3}^{+}$ , H _2 O ^+ ) have been used extensively to quantify the cosmic-ray ionization rate (CRIR) in diffuse sightlines. However, measurements of the CRIR in low-to-intermediate density gas environments are rare, especially when background stars are absent. In this work, we combine molecular line observations of CO, OH, CH, and HCO ^+ in the star-forming cloud IC 348, and chemical models to constrain the value of the CRIR and study the response of the distribution of chemical abundances. The cloud boundary is found to have an A _V of approximately 4 mag. From the interior to the exterior of the cloud, the observed ^13 CO line intensities drop by an order of magnitude. The calculated average abundance of ^12 CO (assuming ^12 C/ ^13 C=65) is (1.2 ± 0.9) × 10 ^−4 , which decreases by a factor of 6 from the interior to the outside regions. The average abundance of CH (3.3 ± 0.7 × 10 ^−8 ) is in good agreement with previous findings in diffuse and translucent clouds ( A _V < 5 mag). However, we did not find a decline in CH abundance in regions of high extinction ( A _V ≃ 8 mag) as previously reported in Taurus. By comparing the observed molecular abundances and chemical models, we find a decreasing trend of the CRIR as A _V increases. The inferred CRIR of ζ _cr = (4.7 ± 1.5) × 10 ^−16 s ^−1 at low A _V is consistent with ${{\rm{H}}}_{3}^{+}$ measurements toward two nearby massive stars.

Supernova remnants are commonly considered to produce most of the Galactic cosmic rays via diffusive shock acceleration. However, many questions regarding the physical conditions at shock fronts, such as the magnetic-field morphology close to the particle acceleration sites, remain open. Here we report the detection of a localized polarization signal from some synchrotron X-ray emitting regions of Tycho’s supernova remnant made by the Imaging X-ray Polarimetry Explorer. The derived degree of polarization of the X-ray synchrotron emission is 9% ± 2% averaged over the whole remnant, and 12% ± 2% at the rim, higher than the value of polarization of 7%–8% observed in the radio band. In the west region, the degree of polarization is 23% ± 4%. The degree of X-ray polarization in Tycho is higher than for Cassiopeia A, suggesting a more ordered magnetic field or a larger maximum turbulence scale. The measured tangential direction of polarization corresponds to the radial magnetic field, and is consistent with that observed in the radio band. These results are compatible with the expectation of turbulence produced by an anisotropic cascade of a radial magnetic field near the shock, where we derive a magnetic-field amplification factor of 3.4 ± 0.3. The fact that this value is significantly smaller than those expected from acceleration models is indicative of highly anisotropic magnetic-field turbulence, or that the emitting electrons either favor regions of lower turbulence, or accumulate close to where the orientation of the magnetic field is preferentially radially oriented due to hydrodynamical instabilities.

The cosmic-ray ionization rate (CRIR, ζ _2 ) is one of the key parameters controlling the formation and destruction of various molecules in molecular clouds. However, the current most commonly used CRIR tracers, such as ${{\rm{H}}}_{3}^{+}$ , OH ^+ , and H _2 O ^+ , are hard to detect and require the presence of background massive stars for absorption measurements. In this work, we propose an alternative method to infer the CRIR in diffuse clouds using the abundance ratios of OH/CO and HCO ^+ /CO. We have analyzed the response of chemical abundances of CO, OH, and HCO ^+ on various environmental parameters of the interstellar medium in diffuse clouds and found that their abundances are proportional to ζ _2 . Our analytic expressions give an excellent calculation of the abundance of OH for ζ _2 ≤ 10 ^−15 s ^−1 , which are potentially useful for modeling chemistry in hydrodynamical simulations. The abundances of OH and HCO ^+ were found to monotonically decrease with increasing density, while the CO abundance shows the opposite trend. With high-sensitivity absorption transitions of both CO (1–0) and (2–1) lines from Atacama Large Millimeter/submillimeter Array, we have derived the H _2 number densities ( ${n}_{{{\rm{H}}}_{2}}$ ) toward 4 lines of sight; assuming a kinetic temperature of T _k = 50 K, we find a range of (0.14 ± 0.03–1.2 ± 0.1) × 10 ^2 cm ^−3 . By comparing the observed and modeled HCO ^+ /CO ratios, we find that ζ _2 in our diffuse gas sample is in the range of ${1.0}_{-1.0}^{+14.8}$ × 10 ${}^{-16}\mbox{--}{2.5}_{-2.4}^{+1.4}$ × 10 ^−15 s ^−1 . This is ∼2 times higher than the average value measured at higher extinction, supporting an attenuation of CRs as suggested by theoretical models.

We present a deep optical imaging campaign on the starburst galaxy NGC 4631 with CFHT/MegaCam. By supplementing the HST/ACS and Chandra/ACIS archival data, we search for the optical counterpart candidates of the five brightest X-ray sources in this galaxy, four of which are identified as ultraluminous X-ray sources (ULXs). The stellar environments of the X-ray sources are analyzed using the extinction-corrected color–magnitude diagrams and the isochrone models. We discover a highly asymmetric bubble nebula around X4 that exhibits different morphology in the H α and [O iii ] images. The [O iii ]/H α ratio map shows that the H α -bright bubble may be formed mainly via the shock ionization by the one-sided jet/outflow, while the more compact [O iii ] structure is photoionized by the ULX. We constrain the bubble expansion velocity and interstellar medium density with the MAPPINGS V code and hence estimate the mechanical power injected into the bubble as P _w ∼ 5 × 10 ^40 erg s ^−1 and the corresponding bubble age as ∼7 × 10 ^5 yr. Relativistic jets are needed to provide such a level of mechanical power with a mass-loss rate of ∼10 ^−7 M _⊙ yr ^−1 . Besides the accretion, the black hole spin is likely an additional energy source for the super-Eddington jet power.