Supernova Remnant CTB Research Articles

Implications of the pulsar wind nebula scenario for a TeV gamma-ray source VER J2016+371

We present multiwavelength studies of a TeV gamma-ray source VER J2016+371 suggested to be associated with a supernova remnant CTB 87 (G74.9+1.2) and based on X-ray and radio morphologies, CTB 87 is identified as an evolved pulsar wind nebula. A source in the vicinity of VER J2016+371 is also detected at GeV energies by Fermi Gamma Ray Space Telescope suggesting a likely counterpart at GeV energies. We find that a broken power-law (BPL) distribution of electrons can explain the observed data at radio, X-ray and TeV energies, however, is not sufficient to explain the data at MeV--GeV energies. A Maxwellian distribution of electrons along with the BPL distribution of electrons in low magnetic fields can explain the observed multiwavelength data spanned from radio to TeV energies suggesting this as the most likely scenario for this source. We also find that although the hadronic model can explain the observed GeV--TeV data for the ambient matter density of $\sim 20~ \rm cm^{-3}$, no observational support for such high ambient density makes this hadronic scenario unlikely for this source.

3D hydrodynamic simulations of the Galactic supernova remnant CTB 109

Using detailed 3D hydrodynamic simulations we study the nature of the Galactic supernova remnant (SNR) CTB 109 (G109.1-1.0), which is well-known for its semicircular shape and a bright diffuse X-ray emission feature inside the SNR. Our model has been designed to explain the observed morphology, with a special emphasis on the bright emission feature inside the SNR. Moreover, we determine the age of the remnant and compare our findings with X-ray observations. With CTB 109 we test a new method of detailed numerical simulations of diffuse young objects, using realistic initial conditions derived directly from observations. We performed numerical 3D simulations with the RAMSES code. The initial density structure has been directly taken from $^{12}$CO emission data, adding an additional dense cloud, which, when it is shocked, causes the bright emission feature. From parameter studies we obtained the position $(\ell , b)=(109.1545^\circ , -1.0078^\circ)$ for an elliptical cloud with $n_\text{cloud}=25~\text{cm}^{-3}$ based on the preshock density from Chandra data and a maximum diameter of 4.54 pc, whose encounter with the supernova (SN) shock wave generates the bright X-ray emission inside the SNR. The calculated age of the remnant is about 11,000 yr according to our simulations. In addition, we can also determine the most probable site of the SN explosion. Hydrodynamic simulations can reproduce the morphology and the observed size of the SNR CTB 109 remarkably well. Moreover, the simulations show that it is very plausible that the bright X-ray emission inside the SNR is the result of an elliptical dense cloud shocked by the SN explosion wave. We show that numerical simulations using observational data for an initial model can produce meaningful results.

Suzaku studies of the supernova remnant CTB 109 hosting the magnetar 1E 2259+586

Abstract Ages of the magnetar 1E 2259+586 and the associated supernova remnant CTB 109 were studied. Analyzing the Suzaku data of CTB 109, its age was estimated to be ∼ 14 kyr, which is much younger than the measured characteristic age of 1E 2259+586, 230 kyr. This reconfirms the previously reported age discrepancy of this magnetar/remnant association, and suggests that the characteristic ages of magnetars are generally over-estimated as compared to their true ages. This discrepancy is thought to arise because the former are calculated without considering decay of the magnetic fields. This novel view is supported independently by much stronger Galactic-plane concentration of magnetars than other pulsars. The process of magnetic field decay in magnetars is mathematically modeled. It is implied that magnetars are much younger objects than previously considered, and can dominate new-born neutron stars.

NuSTAROBSERVATIONS OF THE MAGNETAR 1E 2259+586

We report on new broad band spectral and temporal observations of the magnetar 1E 2259+586, which is located in the supernova remnant CTB 109. Our data were obtained simultaneously with the Nuclear Spectroscopic Telescope Array (NuSTAR) and Swift, and cover the energy range from 0.5-79 keV. We present pulse profiles in various energy bands and compare them to previous RXTE results. The NuSTAR data show pulsations above 20 keV for the first time and we report evidence that one of the pulses in the double-peaked pulse profile shifts position with energy. The pulsed fraction of the magnetar is shown to increase strongly with energy. Our spectral analysis reveals that the soft X-ray spectrum is well characterized by an absorbed double-blackbody or blackbody plus power-law model in agreement with previous reports. Our new hard X-ray data, however, suggests that an additional component, such as a power-law, is needed to describe the NuSTAR and Swift spectrum. We also fit the data with the recently developed coronal outflow model by Beloborodov for hard X-ray emission from magnetars. The outflow from a ring on the magnetar surface is statistically preferred over outflow from a polar cap.

X-RAY OBSERVATIONS OF THE SUPERNOVA REMNANT CTB 87 (G74.9+1.2): AN EVOLVED PULSAR WIND NEBULA

Pulsar wind nebulae (PWNe) studies with the Chandra X-ray Observatory have opened a new window to address the physics of pulsar winds, zoom on their interaction with their hosting supernova remnant (SNR) and interstellar medium, and identify their powering engines. We here present a new 70 ks, plus an archived 18 ks, Chandra ACIS observation of the SNR CTB 87 (G74.9+1.2), classified as a PWN with unusual radio properties and poorly studied in X-rays. We find that the peak of the X-ray emission is clearly offset from the peak of the radio emission by ~100" and located at the southeastern edge of the radio nebula. We detect a point source - the putative pulsar - at the peak of the X-ray emission and study its spectrum separately from the PWN. This new point source, CXOU J201609.2+371110, is surrounded by a compact nebula displaying a torus-like structure and possibly a jet. A more extended diffuse nebula is offset from the radio nebula, extending from the point source to the northwest for ~250" The spectra of the point source, compact nebula and extended diffuse nebula are all well described by a power law model with a photon index of 1.1 (0.7-1.6), 1.2 (0.9-1.4) and 1.7 (1.5-1.8), respectively, for a column density N_H = 1.38 (1.21-1.57) x 10^22 cm^-2 (90% confidence). The total X-ray luminosity of the source is ~1.6 x 10^34 erg s^-1 at an assumed distance of 6.1 kpc, with ~2% and 6% contribution from the point source and compact nebula, respectively. The observed properties suggest that CTB 87 is an evolved (~5-28 kyr) PWN, with the extended radio emission likely a `relic' PWN, as in Vela-X and G327.1-1.1. To date, however, there is no evidence for thermal X-ray emission from this SNR, and the SNR shell is still missing, suggesting expansion into a low-density medium (n_0 < 0.2 cm^-3, assuming D = 6.1 kpc), likely caused by a stellar wind bubble blown by the progenitor star.

Interstellar gas towards CTB 37A and the TeV gamma-ray source HESS J1714-385

Observations of dense molecular gas towards the supernova remnants CTB 37A (G348.5+0.1) and G348.5-0.0 were carried out using the Mopra and Nanten2 radio telescopes. We present CO(2-1) and CS(1-0) emission maps of a region encompassing the CTB 37A TeV gamma-ray emission, HESS J1714-385, revealing regions of dense gas within associated molecular clouds. Some gas displays good overlap with gamma-ray emission, consistent with hadronic gamma-ray emission scenarios. Masses of gas towards the HESS J1714-385 TeV gamma-ray emission region were estimated, and were of the order of 10^3-10^4 solar masses. In the case of a purely hadronic origin for the gamma-ray emission, the cosmic ray flux enhancement is ~80-1100 times the local solar value. This enhancement factor and other considerations allow a discussion of the age of CTB 37A, which is consistent with ~10^4 yr.

PROPER MOTIONS AND ORIGINS OF AXP 1E 2259+586 AND AXP 4U 0142+61

Using high-resolution NIR images supported by laser guide star adaptive optics from the Keck II telescope from 2005 to 2012, we have measured the proper motions of two anomalous X-ray pulsars, AXP 1E 2259+586 and AXP 4U 0142+61. The proper motion of AXP 1E 2259+586 in the sky frame is (μ_α, μ_δ) = (– 6.4 ± 0.6, –2.3 ± 0.6) mas yr^(–1) and that of AXP 4U 0142+61 is (μ_α, μ_δ) = (– 4.1 ± 1, 1.9 ± 1) mas yr^(–1). After correcting for the velocity of the progenitors, we calculate the tangential ejection velocities of the magnetars to be 157 ± 17 km s^(–1) and 102 ± 26 km s^(–1) respectively. The proper motion vector of AXP 1E 2259+586 is directed away from the putative center of the supernova remnant CTB 109 that has long been proposed to be associated with AXP 1E 2259+586. This is significant evidence for linking the pulsar with CTB 109. We comment on the possible movement of CTB 109 after the explosion. We narrow the search cone for the birthsite or remnant of AXP 4U 0142+61 to an opening angle of 24°. However, we are unable to find any suitable association.

Chandraobservation of the Galactic supernova remnant CTB 109 (G109.1–1.0)

Context: We study the X-ray emission of the Galactic supernova remnant (SNR) CTB 109 (G109.1-1.0), which is well-known for its enigmatic half-shell morphology both in radio and in X-rays and is associated with the anomalous X-ray pulsar (AXP) 1E2259+586. Aims: We want to understand the origin of the X-ray bright feature inside the SNR called the Lobe and the details of the interaction of the SNR shock wave with the ambient interstellar medium (ISM). Methods: The Lobe and the northeastern part of the SNR were observed with Chandra ACIS-I. We analysed the spectrum of the X-ray emission by dividing the entire observed emission into small regions. The X-ray emission is best reproduced with one-component or two-component non-equilibrium ionisation models depending on the position. In the two-component model one emission component represents the shocked ISM and the other the shocked ejecta. Results: We detect enhanced element abundances, in particular for Si and Fe, in and around the Lobe. There is one particular region next to the Lobe with a high Si abundance of 3.3 (2.6 - 4.0) times the solar value. This is the first, unequivocal detection of ejecta in CTB 109. Conclusions: The new Chandra data confirm that the Lobe was created by the interaction of the SNR shock and the supernova ejecta with dense and inhomogeneous medium in the environment of SNR CTB 109. The newly calculated age of the SNR is t ~ 1.4 x 10^4 yr.

A view of supernova remnant CTB 37A with the Fermi Gamma-ray Space Telescope

Supernovae and their remnants have long been favored as cosmic ray accelerators. Recent data from the Fermi Gamma-ray Space Telescope has given us an improved window into such sources, including the remnant CTB 37A. Using the Fermi Large Area Telescope, we found significant gamma-ray emission coincident with the remnant, which also emits in radio, X-ray, and very high energy gamma-rays. We modeled the multiwavelength spectrum using a combination of hadronic and leptonic emission with reasonable parameter values and determined that CTB 37A is a potential cosmic ray accelerator commensurate with direct observations. By assembling statistically significant populations of such objects, we will be able to more fully illuminate the mystery of cosmic ray origins.

The supernova remnant CTB 37B and its associated magnetar CXOU J171405.7-381031: evidence for a magnetar-driven remnant

We discuss the association between the candidate magnetar CXOU J171405.7-381031 and the supernova remnant CTB 37B. The recent detection of the period derivative of the object allowed an estimation of a young characteristic age of only ∼ 1000 yr. This value is too small to be compatible even with the minimum radius of the remnant being ≥ 10 pc, the value corresponding to the lower limit of the estimated distance of 10.2 ± 3.5 kpc, unless the true distance happens to be even smaller than the lower limit. We argue that a consistent scenario for the remnant's origin, in which the latter is powered by the energy injected by a young magnetar, is indeed more accurate to explain the young age, and demonstrates its non-standard (i.e. magnetar-driven) nature.

AN ENERGETIC MAGNETAR IN HESS J1713–381/CTB 37B

We obtained a second Chandra timing measurement of the 3.82 s pulsar CXOU J171405.7-381031 in the supernova remnant (SNR) CTB 37B, which shows that it is spinning down rapidly. The average period derivative of (5.88+/-0.08)E-11 over the 1 year time span corresponds to a dipole magnetic field strength B = 4.8E14 G, well into the magnetar range. The spin-down power E-dot = 4.2E34 erg/s is among the largest for magnetars, and the corresponding characteristic age Tau = P/2P-dot = 1030 years is comparable to estimates of the age of the SNR. The period derivative enables us to recover probable pulsations in an ASCA observation taken in 1996, which yields a mean characteristic age of 860 years over the longer 13 year time span. The source is well detected up to 10 keV, and its composite spectrum is typical of a magnetar. CTB 37B hosts HESS J1713-381, the first TeV source that is coincident with a magnetar. While the TeV emission has been attributed to the SNR shell, it is possibly centrally peaked, and we hypothesize that this particularly young, energetic magnetar may contribute to the HESS source. We also searched for pulsations from another source in a HESS SNR, XMMU J173203.3-344518 in HESS J1731-347/G353.6-0.7 but could not confirm pulsations or long-term flux variability, making it more likely that this source is a weakly magnetized central compact object.

MIXED-MORPHOLOGY SUPERNOVA REMNANTS IN X-RAYS: ISOTHERMAL PLASMA IN HB21 AND PROBABLE OXYGEN-RICH EJECTA IN CTB 1

(Abridged) We present an analysis of X-ray observations made of the Galactic supernova remnants (SNRs) HB21 (G89.0+4.7) and CTB 1 (G116.9+0.2), two well-known mixed-morphology (MM) SNRs. We find a marked contrast between the X-ray properties of these SNRs: for HB21, the extracted ASCA spectra of the northwest and southeast regions of the X-ray emitting plasma can be fit with a single thermal model with marginally enhanced silicon and sulfur abundances. For both of these regions, the derived column density and temperature are N_H~0.3x10^22 cm^-2 and kT~0.7 keV, respectively. No significant spatial differences in temperature or elemental abundances between the two regions are detected and the X-ray-emitting plasma in both regions is close to ionization equilibrium. Our Chandra spectral analysis of CTB 1 reveals that this source is likely an oxygen-rich SNR with enhanced abundances of oxygen and neon. The extracted ASCA spectra for the southwestern and northeastern regions of CTB 1 cannot be fit with a single thermal component. Based on our fits to these spectra, we derive a column density N_H~0.6x10^22 cm^-2 and a temperature for the soft thermal component of kT_soft~0.28 keV. The hard emission from the southwest may be modeled with either a thermal component (kT_hard~3 keV) or by a power law component (Gamma~2-3) while the hard emission from the northeast may be modeled with a power law component (Gamma~1.4). We have also extracted ASCA GIS spectra of the discrete X-ray source 1WGA J0001.4+6229 which is seen in projection toward CTB 1. These spectra are best fit using a power-law model with a photon index Gamma=2.2^{+0.5}_{-1.2} which is typical for featureless power-law continua produced by rotation-powered pulsars. This source may be a neutron star associated with CTB 1.

Structure and kinematics of the interstellar medium near WR 137 and supernova remnants CTB 87 and G73.9+0.9

Using the results of our Hα interferometric observations and observational data on the 21 cm and CO lines, we have analyzed the structure and kinematics of the interstellar medium in the extended vicinity of the star WR 137 and the supernova remnants CTB 87 and G73.9+0.9. A shell structure with a radius of up to 40′ observable in optical lines has been discovered around WR 137. The high-velocity motions of ionized hydrogen inside this shell can be interpreted as expansion of the gas swept out by the wind of WR 137 at velocities of up to 60 km/s. The ionized hydrogen near WR 137 emits at the systematic velocity VLSR ∼ 6–18 km/s. The expansion ofG73.9+0.9 at a velocity of up to 55 km/s has been confirmed. The systematic velocities of the ionized hydrogen toward this supernova remnant are VLSR ≃ −14…+14 km/s. An HI shell around G73.9+0.9 has been detected at velocities VLSR≃−14…−8 km/s. A very faint optical shell of CTB 87 with a size of about 20′ has also been detected. Evidence that CTB 87 is located in the Cygnus Arm is presented.

The Nature of a Cosmic-Ray Accelerator, CTB 37 B, Observed with Suzaku and Chandra

Abstract We report on Suzaku and Chandra observations of the young supernova remnant CTB 37 B, from which TeV $\gamma$-rays were detected by the H.E.S.S. Cherenkov telescope. The 80 ks Suzaku observation provided us with a clear image of diffuse emission and high-quality spectra. The spectra revealed that the diffuse emission is comprised of thermal and non-thermal components. The thermal component can be represented by an NEI model with a temperature, a pre-shock electron density and an age of 0.9$\pm$0.2 keV, 0.4$\pm$0.1 cm$^{-3}$, and 650$^{+2500}_{-300}$yr, respectively. This suggests that the explosion of CTB 37 B occurred in a low-density space. A non-thermal power-law component was found from the southern region of CTB 37 B. Its photon index of $\sim$1.5 and a high roll-off energy ($&amp;gt;rsim$15 keV) indicate efficient cosmic-ray acceleration. A comparison of this X-ray spectrum with the TeV $\gamma$-ray spectrum leads us to conclude that the TeV $\gamma$-ray emission seems to be powered by either multi-zone Inverse Compton scattering or the decay of neutral pions. The point source resolved by Chandra near the shell is probably associated with CTB 37 B, because of the common hydrogen column density with the diffuse thermal emission. Spectral and temporal characteristics suggest that this source is a new anomalous X-ray pulsar.

Discovery of a VHE gamma-ray source coincident with the supernova remnant CTB 37A

Aims. The supernova remnant (SNR) complex CTB 37 is an interesting candidate for observations with very high energy (VHE) γ-ray telescopes such as HESS. In this region, three SNRs are seen. One of them is potentially associated with several molecular clouds, a circumstance that can be used to probe the acceleration of hadronic cosmic rays.

Chandra and HESS observations of the supernova remnant CTB 37B

The >100 GeV gamma-ray source, HESS J1713-381, apparently associated with the shell-type supernova remnant (SNR) CTB 37B, was discovered using H.E.S.S. in 2006. X-ray follow-up observations with Chandra were performed in 2007 with the aim of identifying a synchrotron counterpart to the TeV source and/or thermal emission from the SNR shell. These new Chandra data, together with additional TeV data, allow us to investigate the nature of this object in much greater detail than was previously possible. The new X-ray data reveal thermal emission from a ~4' region in close proximity to the radio shell of CTB 37B. The temperature of this emission implies an age for the remnant of ~5000 years and an ambient gas density of ~0.5 cm-3. Both these estimates are considerably uncertain due to the asymmetry of the SNR and possible modifications of the kinematics due to efficient cosmic ray (CR) acceleration. A bright (~7x10-13erg cm-2s-1) and unresolved (<1'') source (CXOU J171405.7-381031) with a soft (\Gamma ~3.3) non-thermal spectrum is also detected in coincidence with the radio shell. Absorption indicates a column density consistent with the thermal emission from the shell suggesting a genuine association rather than a chance alignment. The observed TeV morphology is consistent with an origin in the complete shell of CTB 37B. The lack of diffuse non-thermal X-ray emission suggests an origin of the gamma-ray emission via the decay of neutral pions produced in interactions of protons and nuclei, rather than inverse Compton (IC) emission from relativistic electrons.

Evidence for Shocked Molecular Gas in the Galactic Supernova Remnant CTB 109 (G109.1-1.0)

We report the detection of molecular clouds around the X-ray-bright interior feature in the Galactic supernova remnant (SNR) CTB 109 (G109.1-1.0). This feature, called the lobe, has been previously suggested to be the result of an interaction of the SNR shock wave with a molecular cloud complex. We present new high-resolution X-ray data from the Chandra X-Ray Observatory and new high-resolution CO data from the Five College Radio Astronomy Observatory that show the region of interaction with the cloud complex in greater detail. The CO data reveal three clouds around the lobe in the velocity interval -57 km s-1 < v < -52 km s-1. The velocity profiles of 12CO in various parts of the east cloud are well fit with a Gaussian; however, at the position at which the CO cloud and the lobe overlap, the velocity profile has an additional component toward higher negative velocities. The molecular hydrogen density in this part of the cloud is relatively high (N ≈ 1.9 × 1020 cm-2), whereas the foreground absorption in X-rays (NH ≈ 4.5 × 1021 cm-2), obtained from Chandra data, is lower than in other parts of the cloud and in the north and south clouds. These results indicate that the east cloud has been hit by the SNR blast wave on the western side, forming the bright X-ray lobe.

A MERLIN Observation of PSR B1951+32 and Its Associated Plerion

In an investigative 16 hr L-band observation using the MERLIN radio interferometric array, we have resolved both the pulsar PSR B1951+32 and the structure within the flat spectral radio continuum region, believed to be the synchrotron nebula associated with the interaction of the pulsar and its "host" supernova remnant CTB 80. The extended structure we see, significant at ~4.5 σ, is of dimensions 25 × 075 and suggests a sharp bow-shaped arc of shocked emission, which is correlated with a similar structure observed in lower resolution radio maps and X-ray images. Using this MERLIN data as a new astrometric reference for other multiwavelength data, we can place the pulsar at one edge of the HST reported optical synchrotron knot, ruling out previously suggested optical counterparts and allowing an elementary analysis of the optical synchrotron emission that appears to trail the pulsar. The latter is possibly a consequence of pulsar wind replenishment, and we suggest that the knot is a result of magnetohydrodynamic (MHD) instabilities. These being so, it suggests a dynamical nature to the optical knot, which will require high-resolution optical observations to confirm.

X‐Ray Spectroscopy of PSR B1951+32 and Its Pulsar Wind Nebula

We present spatially resolved X-ray spectroscopy of PSR B1951+32 and its pulsar wind nebula (PWN) in supernova remnant (SNR) CTB 80 using a {\sl Chandra} observation. The {\sl Chandra} X-ray map reveals clearly various components of a ram-pressure confined PWN embedded in the SNR ejecta: a point source representing the pulsar, X-ray emission from the bow shock, a luminous X-ray tail, a 30$\arcsec$ diameter plateau whose northwestern part is absent, and the outside more diffuse X-ray emission. The plateau is just surrounded by the radio, [O III], [S II], and [N II] shells, and the outside diffuse emission is mostly within the H${\alpha}$ shells. While the spectra of all the features are well fitted with power law models, a power law plus blackbody model can fit the spectrum of the pulsar significantly better than using a power law model alone. Generally the spectra of these components obey the trend of steepening from the inside to the outside. However, the edge of the plateau probably has a harder spectrum than that of the central region of the plateau. The cause of the apparent hard spectrum of the plateau edge is unclear, and we speculate that it might be due to a shock between the PWN and the SNR ejecta. The possible blackbody radiation component from the pulsar has a temperature of 0.13$\pm0.02$ keV and an equivalent emitting radius of 2.2$^{+1.4}_{-0.8}$ (d/2 kpc) km, and is thus probably from the hot spots on the pulsar. We also show in this paper that the blackbody temperature of the entire surface of PSR B1951+32 is much lower than those predicted by the standard neutron star cooling models.

XMM‐NewtonObservations of the Galactic Supernova Remnant CTB 109 (G109.1−1.0)

We present the analysis of the X-ray Multi-Mirror Mission (XMM-Newton) European Photon Imaging Camera (EPIC) data of the Galactic supernova remnant (SNR) CTB 109 (G109.1-1.0). CTB 109 is associated with the anomalous X-ray pulsar (AXP) 1E 2259+586 and has an unusual semi-circular morphology in both the X-ray and the radio, and an extended X-ray bright interior region known as the `Lobe'. The deep EPIC mosaic image of the remnant shows no emission towards the west where a giant molecular cloud complex is located. No morphological connection between the Lobe and the AXP is found. We find remarkably little spectral variation across the remnant given the large intensity variations. All spectra of the shell and the Lobe are well fitted by a single-temperature non-equilibrium ionization model for a collisional plasma with solar abundances (kT = 0.5 - 0.7 keV, tau = n_e t = 1 - 4 x 10^11 s cm^-3, N_H = 5 - 7 x 10^21 cm^-2). There is no indication of nonthermal emission in the Lobe or the shell. We conclude that the Lobe originated from an interaction of the SNR shock wave with an interstellar cloud. Applying the Sedov solution for the undisturbed eastern part of the SNR, and assuming full equilibration between the electrons and ions behind the shock front, the SNR shock velocity is derived as v_s = 720 +/- 60 km s^-1, the remnant age as t = (8.8 +/- 0.9) x 10^3 d_3 yr, the initial energy as E_0 = (7.4 +/- 2.9) x 10^50 d_3^2.5 ergs, and the pre-shock density of the nuclei in the ambient medium as n_0 = (0.16 +/- 0.02) d_3^-0.5 cm^-3, at an assumed distance of D = 3.0 d_3 kpc. Assuming CTB 109 and 1E 2259+586 are associated, these values constrain the age and the environment of the progenitor of the SNR and the pulsar.