On invisible plasma content in radio-loud AGNs: the case of TeV blazar Markarian 421

Invisible plasma content in blazar jets such as protons and/or thermal electron–positron ( e ^±) pairs is explored through combined arguments of dynamical and radiative processes. By comparing physical quantities required by the internal shock model with those obtained through the observed broad-band spectra for Mrk 421, we obtain that the ratio of the Lorentz factors of a pair of cold shells resides in about 2 ∼ 20, which implies that the shocks are at most mildly relativistic. Using the obtained Lorentz factors, the total mass density ρ in the shocked shells is investigated. The upper limit of ρ is obtained from the condition that thermal bremsstrahlung emission should not exceed the observed γ-ray luminosity, whilst the lower limit is constrained from the condition that the energy density of non-thermal electrons is smaller than that of the total plasma. Then, we find ρ is 10²–10³ times heavier than that of non-thermal electrons for pure   e ^±  pairs, while 10²–10⁶ times heavier for pure electron–proton ( e / p ) content, implying the existence of a large amount of invisible plasma. The origin of the continuous blazar sequence is briefly discussed and we speculate that the total mass density and/or the blending ratio of e ^± pairs and e / p plasma could be new key quantities for the origin of the sequence.     

Wisps and knots in the central Crab nebula

The fast-spinning Crab pulsar (∼30 turn s⁻¹), which powers the massive expansion and synchrotron emission of the entire Crab nebula, is surrounded by quasi-stationary features such as fibrous arc-like wisps and bright polar knots in the radial range of 2×10¹⁶≲ r ≲2×10¹⁷ cm, as revealed by high-resolution (∼0.1 arcsec) images from the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope ( HST ). The spin-down energy flux (∼5×10³⁸ erg s⁻¹) from the pulsar to the luminous outer nebula, which occupies the radial range 0.1≲ r ≲2 pc, is generally believed to be transported by a magnetized relativistic outflow of an electron–positron e^± pair plasma. It is then puzzling that mysterious structures like wisps and knots, although intrinsically dynamic in synchrotron emission, remain quasi-stationary on time-scales of a few days to a week in the relativistic pulsar wind. Here we demonstrate that, as a result of slightly inhomogeneous wind streams emanating from the rotating pulsar, fast magnetohydrodynamic (MHD) shock waves are expected to appear in the pulsar wind at relevant radial distances in the forms of wisps and knots. While forward fast MHD shocks move outward with a speed close to the speed of light c , reverse fast MHD shocks may appear quasi-stationary in space under appropriate conditions. In addition, Alfvénic fluctuations in the shocked magnetized pulsar wind can effectively scatter synchrotron beams from gyrating relativistic electrons and positrons.     

The multi-band non-thermal emission from the supernova remnant RX J1713.7−3946

Non-thermal X-rays and very high energy (VHE) γ-rays have been detected from the supernova remnant (SNR) RX J1713.7−3946, and the recent observations with the Suzaku satellite clearly reveal a spectral cut-off in the X-ray spectrum, which directly relates to the cut-off of the energy spectrum of the parent electrons. However, whether the origin of the VHE γ-rays from the SNR is hadronic or leptonic is still in debate. We studied the multi-band non-thermal emission from RX J1713.7−3946 based on a semi-analytical approach towards the non-linear shock acceleration process by including the contribution of the accelerated electrons to the non-thermal radiation. The results show that the multi-band observations on RX J1713.7−3946 can be well explained in the model with appropriate parameters, and the TeV γ-rays have hadronic origin, i.e. they are produced via proton–proton interactions as the relativistic protons accelerated by the shock collide with the ambient matter.     

A deep search for pulsar wind nebulae using pulsar gating

Using the Australia Telescope Compact Array (ATCA) we have imaged the fields around five promising pulsar candidates to search for radio pulsar wind nebulae (PWNe). We have used the ATCA in its pulsar-gating mode; this enables an image to be formed containing only off-pulse visibilities, thereby dramatically improving the sensitivity to any underlying PWN. Data from the Molonglo Observatory Synthesis Telescope were also used to provide sensitivity on larger spatial scales. This survey found a faint new PWN around PSR B0906−49; here we report on non-detections of PWNe towards PSRs B1046−58, B1055−52, B1610−50 and J1105−6107. Our radio observations of the field around PSR B1055−52 argue against previous claims of an extended X-ray and radio PWN associated with the pulsar. If these pulsars power unseen, compact radio PWNe, upper limits on the radio flux indicate that a fraction of less than 10⁻⁶ of their spin-down energy is used to power this emission. Alternatively, PSRs B1046−58 and B1610−50 may have relativistic winds similar to other young pulsars and the unseen PWN may be resolved and fainter than our surface brightness sensitivity threshold. We can then determine upper limits on the local interstellar medium (ISM) density of 2.2×10⁻³ and 1×10⁻² cm⁻³, respectively. Furthermore, we derive the spatial velocities of these pulsars to be ∼450 km s⁻¹ and thus rule out the association of PSR B1610−50 with supernova remnant (SNR) G332.4+00.1 (Kes 32). Strong limits on the ratio of unpulsed to pulsed emission are also determined for three pulsars.     

Re-identification of G35.6−0.4 as a supernova remnant

G35.6−0.4 is an extended radio source in the Galactic plane which has previously been identified as either a supernova remnant or an H  ii region. Observations from the Very Large Array Galactic Plane Survey at 1.4 GHz with a resolution of 1 arcmin allow the extent of G35.6−0.4 to be defined for the first time. Comparison with other radio survey observations show that this source has a non-thermal spectral index, with   S ∝ν^{−0.47±0.07}  . G35.6−0.4 does not have obvious associated infrared emission, so it is identified as a Galactic supernova remnant, not an H  ii region. It is  ≈15 × 11 arcmin²  in extent, showing partial limb brightening.     

Characteristics of the supernova remnant G351.7+0.8 and a distance argument against its association with PSR J1721−3532

New images of the supernova remnant (SNR) G351.7+0.8 are presented based on 21-cm H  i -line emission and continuum emission data from the Southern Galactic Plane Survey. SNR G351.7+0.8 has a flux density of 8.4 ± 0.7 Jy at 1420 MHz. Its spectral index is 0.52 ± 0.25 ( S = v ^−α) between 1420 and 843 MHz, typical of adiabatically expanding shell-like remnants. H  i observations show structures possibly associated with the SNR in the radial velocity range of −10 to −18 km s⁻¹, and suggest a distance of 13.2 kpc and a radius of 30.7 pc. The estimated Sedov age for G351.7+0.8 is less than  6.8×10⁴ yr  . A young radio pulsar PSR J1721−3532 lies close to SNR G351.7+0.8 on the sky. The new distance and age of G351.7+0.8 and recent proper motion measurements of the pulsar strongly argue against an association between SNR G351.7+0.8 and PSR J1721−3532. There is an unidentified, faint X-ray point source 1RXS J172055.3−353937 which is close to G351.7+0.8. This may be a neutron star potentially associated with G351.7+0.8.     

GMRT observations of four suspected supernova remnants near the Galactic Centre

We have observed two fields – Field I     ,     and Field II     ,     – with the Giant Metrewave Radio Telescope (GMRT) at 330 MHz. In the first field, we have studied the candidate supernova remnant (SNR) G3.1−0.6 and, based on its observed morphology, spectral index and polarization, confirmed it to be an SNR. We find this supernova to have a double ring appearance with a strip of emission on its western side passing through its centre.
We have discovered two extended curved objects in the second field, which appears to be part of a large shell-like structure. It is possibly the remains of an old supernova in the region. Three suspected SNRs, G356.3−0.3, G356.6+0.1 and G357.1−0.2, detected in the MOST 843-MHz survey of the GC region appear to be located on this shell-like structure. While both G356.3−0.3 and G356.6+0.1 seem to be parts of this shell, G357.1−0.2, which has a steeper spectrum above 1 GHz, could be a background SNR seen through the region. Our H  i absorption observation towards the candidate SNR G357.1−0.2 indicates that it is at a distance of more than 6 kpc from us.     

Non-thermal emission from Vela X and PWN G0.9＋0.1

We study the multi-waveband non-thermal emission from the pulsar wind neb- ulae (PWNe) Vela X and G0.9 + 0.1 in the frame of a time-dependent model describing non-thermal radiation from the PWNe. In such a model, the relativistic wind of parti- cles driven by a central pulsar blows into the ambient medium and creates a termination shock that accelerates the particles to very high energy in a PWN. The non-thermal pho-tons in the PWN are produced both by synchrotron radiation and the inverse Compton process, with electrons coming directly from the pulsar magnetosphere and electrons be- ing accelerated at the termination shock. We apply this model to reproduce the observed multi-waveband photon spectra of Vela X and the G0.9+0.1, both of which have been detected emitting very high energy photons. Our results indicate that TeV photons are produced by the inverse Compton scattering of the high-energy electrons in the infrared photon field in both Vela X and PWN G0.9+0.1. The TeV photons from these two PWNe may have leptonic origins.     

Aspect angle for interstellar magnetic field in SN 1006

A number of important processes taking place around strong shocks in supernova remnants (SNRs) depend on the shock obliquity. The measured synchrotron flux is a function of the aspect angle between interstellar magnetic field (ISMF) and the line of sight. Thus, a model of non-thermal emission from SNRs should account for the orientation of the ambient magnetic field. We develop a new method for the estimation of the aspect angle, based on the comparison between observed and synthesized radio maps of SNRs, making different assumptions about the dependence of electron injection efficiency on the shock obliquity. The method uses the azimuthal profile of radio surface brightness as a probe for orientation of ambient magnetic field because it is almost insensitive to the downstream distribution of magnetic field and emitting electrons. We apply our method to a new radio image of SN 1006 produced on the basis of archival Very Large Array and Parkes data. The image recovers emission from all spatial structures with angular scales from a few arcsec to 15 arcmin. We explore different models of injection efficiency and find the following best-fitting values for the aspect angle of SN 1006:  φ_o= 70^o± 4.2^o  if the injection is isotropic,  φ_o= 64^o± 2.8^o  for quasi-perpendicular injection (SNR has an equatorial belt in both cases) and  φ_o= 11^o± 0.8^o  for quasi-parallel injection (polar-cap model of SNR). In the last case, SN 1006 is expected to have a centrally peaked morphology contrary to what is observed. Therefore, our analysis provides some indication against the quasi-parallel injection model.     

Active galactic nuclei and pulsars as cosmic ray sources

Relativistic e^± particles and cosmic rays are accelerated in the magnetospheres of supermassive black holes and neutron stars. The possibility of synchrotron radiation with extremely high intensity inside the deepest regions of magnetospheres is investigated. Very high brightness temperatures are expected for such radiation by relativistic protons, which can be made even higher in the presence of non-stationary conditions, Doppler boosting and coherent processes. The main parameters for models of such high-brightness-temperature radiation are determined. Two types of active galactic nuclei (AGNs) are expected. One type is associated with the acceleration and ejection of relativistic e^± particles only (probably non-IDV sources and FR-I radio galaxies). The second type of AGN is also associated with e^± acceleration, but is dominated by the contribution of relativistic protons (probably IDV sources and FR-II radio galaxies). Analogous objects for pulsars are plerion and shell supernova remnants with neutron stars or pulsars without synchrotron nebulae, respectively.     

Shocked molecular gas towards the supernova remnant G359.1–0.5 and the Snake

We have found a bar of shocked molecular hydrogen (H₂) towards the OH(1720 MHz) maser located at the projected intersection of supernova remnant (SNR)  G359.1–0.5  and the non-thermal radio filament known as the Snake. The H₂ bar is well aligned with the SNR shell and almost perpendicular to the Snake. The OH(1720 MHz) maser is located inside the sharp western edge of the H₂ emission, which is consistent with the scenario in which the SNR drives a shock into a molecular cloud at that location. The spectral line profiles of ¹²CO, HCO⁺ and CS towards the maser show broad-line absorption, which is absent in the ¹³CO spectra and most probably originates from the pre-shock gas. A density gradient is present across the region and is consistent with the passage of the SNR shock, while the H₂ filament is located at the boundary between the pre-shock and post-shock regions.     

Interaction between the north-eastern boundary of Sgr A East and giant molecular clouds

We have detected the   v = 1 → 0 S(1) (λ= 2.1218 μm)  and   v = 2 → 1 S(1) (λ= 2.2477 μm)  lines of H₂ in the Galactic Centre, in a  90 × 27 arcsec²  region between the north-eastern boundary of the non-thermal source Sgr A East, and the giant molecular cloud (GMC)  M−0.02 − 0.07  . The detected  H₂ v = 1 → 0  S(1) emission has an intensity of  1.6–21 × 10⁻¹⁸ W m⁻² arcsec⁻²  and is present over most of the region. Along with the high intensity, the large linewidths  (FWHM = 40–70 km s⁻¹)  and the  H₂ v = 2 → 1 S(1)  to   v = 1 → 0 S(1)  line ratios (0.3–0.5) can be best explained by a combination of C-type shocks and fluorescence. The detection of shocked H₂ is clear evidence that Sgr A East is driving material into the surrounding adjacent cool molecular gas. The H₂ emission lines have two velocity components at ∼+50 and  ∼0 km s⁻¹  , which are also present in the NH₃(3, 3) emission mapped by McGary, Coil & Ho. This two-velocity structure can be explained if Sgr A East is driving C-type shocks into both the  GMC M−0.02 − 0.07  and the northern ridge of McGary et al.     

Self-consistent computation of γ-ray spectra due to proton–proton interactions in black hole systems

In the inner regions of an accretion disc around a black hole, relativistic protons can interact with ambient matter to produce electrons, positrons and γ-rays. The resultant steady-state electron and positron particle distributions are self-consistently computed taking into account Coulomb and Compton cooling,  e⁻ e⁺  pair production (due to γ–γ annihilation) and pair annihilation. While earlier works used the diffusion approximation to obtain the particle distributions, here we solve a more general integro-differential equation that correctly takes into account the large change in particle energy that occurs when the leptons Compton scatter off hard X-rays. Thus this formalism can also be applied to the hard state of black hole systems, where the dominant ambient photons are hard X-rays. The corresponding photon energy spectrum is calculated and compared with broad-band data of black hole binaries in different spectral states. The results indicate that the γ-ray spectra  ( E > 0.8 MeV)  of both the soft and hard spectral states and the entire hard X-ray/γ-ray spectrum of the ultrasoft state could be due to p–p interactions. These results are consistent with the hypothesis that there always exists in these systems a γ-ray spectral component due to p–p interactions that can contribute between 0.5 and 10 per cent of the total bolometric luminosity. The model predicts that GLAST would be able to detect black hole binaries and provide evidence for the presence of non-thermal protons, which in turn would give insight into the energy dissipation process and jet formation in these systems.     

TeV γ-rays from old supernova remnants

We study the emission from an old supernova remnant (SNR) with an age of around 10⁵ yr and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 10⁵ yr, proton acceleration is efficient enough to emit TeV γ-rays both at the shock of the SNR and that in the GMC. The maximum energy of primarily accelerated electrons is so small that TeV γ-rays and X-rays are dominated by hadronic processes,  π⁰  -decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 10⁵ yr, there are few high-energy particles emitting TeV γ-rays because of the energy-loss effect and/or the wave-damping effect occurring at low-velocity isothermal shocks. For old SNRs or SNR–GMC interacting systems capable of generating TeV γ-ray emitting particles, we calculated the ratio of TeV γ-ray (1–10 TeV) to X-ray (2–10 keV) energy flux and found that it can be more than  ∼10²  . Such a source showing large flux ratio may be a possible origin of recently discovered unidentified TeV sources.     

TeV gamma-rays from accreting magnetars in massive binaries

This paper focuses on neutron stars (NS) of the magnetar type inside massive binary systems. We determine the conditions under which the matter from the stellar wind can penetrate the inner magnetosphere of the magnetar. At a certain distance from the NS surface, the magnetic pressure can balance the gravitational pressure of the accreting matter, creating a very turbulent, magnetized transition region. It is suggested that this region provides good conditions for the acceleration of electrons to relativistic energies. These electrons lose energy due to the synchrotron process and inverse Compton (IC) scattering of the radiation from the nearby massive stellar companion, producing high-energy radiation from X-rays up to ∼TeV γ-rays. The primary γ-rays can be further absorbed in the stellar radiation field, developing an IC  e^±  pair cascade. We calculate the synchrotron X-ray emission from primary electrons and secondary  e^±  pairs and the IC γ-ray emission from the cascade process. It is shown that quasi-simultaneous observations of the TeV γ-ray binary system LSI +61 303 in the X-ray and TeV γ-ray energy ranges can be explained with such an accreting magnetar model.     

Near-infrared [Fe ii] emission from supernova remnants and the supernova rate of starburst galaxies

In an effort better to calibrate the supernova rate of starburst galaxies as determined from near-infrared [Fe  ii ] features, we report on a [Fe  ii ] λ 1.644 μm line-imaging survey of a sample of 42 optically selected supernova remnants (SNRs) in M33. A wide range of [Fe  ii ] luminosities are observed within our sample (from less than 6 to 695 L_⊙). Our data suggest that the bright [Fe  ii ] SNRs are entering the radiative phase and that the density of the local interstellar medium (ISM) largely controls the amount of [Fe  ii ] emission. We derive the following relation between the [Fe  ii ] λ 1.644 μm line luminosity of radiative SNRs and the electronic density of the post-shock gas, n _e: L _{[Fe  ii ]}     (cm⁻³). We also find a correlation in our data between L _{[Fe  ii ]} and the metallicity of the shock-heated gas, but the physical interpretation of this result remains inconclusive, as our data also show a correlation between the metallicity and n _e. The dramatically higher level of [Fe  ii ] emission from SNRs in the central regions of starburst galaxies is most likely due to their dense environments, although metallicity effects might also be important. The typical [Fe  ii ]-emitting lifetime of a SNR in the central regions of starburst galaxies is found to be of the order of 10⁴ yr. On the basis of these results, we provide a new empirical relation allowing the determination of the current supernova rate of starburst galaxies from their integrated near-infrared [Fe  ii ] luminosity.     

The neutral carbon distribution in a bipolar outflow/molecular disc source configuration: G35.2−0.74N

Maps are presented of ³ P ₁→³ P ₀[C  i ] and J =2→1 C¹⁸O line emission from the interstellar molecular cloud G35.2−0.74N. The maps are interpreted with reference to a previous model for the structure of the cloud in which opposing jets from a central object, embedded in a rotating interstellar disc, precess and drive a bipolar molecular outflow. The C¹⁸O emission traces the rotating interstellar disc, but the [C  i ] emission shows several features. An unresolved component is observed which probably results from dissociation of CO in the centre of the disc by UV radiation from the central source. Background [C  i ] emission is also observed which shares the rotation of the disc on larger scales. The C  i /CO ratio in these components is typically a few per cent. High-velocity [C  i ] emission, where C  i /CO is high (>0.1–0.4), is observed between the CO molecular outflow and the cavity exacavated by the jet. This material has probably been accelerated by the jet but dissociated by far-UV radiation propagating through the cavity. The C  i /CO ratio falls as the shocked outflow later sweeps up CO.     

G309.2–00.6 and jets in supernova remnants

We present Australia Telescope Compact Array observations of the supernova remnant (SNR) G309.2–00.6. In a 1.3-GHz continuum image the remnant appears as a near-circular shell, but with two brightened and distorted arcs of emission on opposite sides. H  i absorption against the SNR yields a distance in the range 5.4 to 14.1 kpc, corresponding to an age (1−20) × 10³ yr. On the basis of the morphology of the SNR we argue that it is a younger analogue of the W 50/SS 433 system, and that its unusual appearance is a result of opposed jets or outflows from a central source. A jet-like feature and breaks in the shell can both be seen along the axis of proposed outflow, providing further support for this interpretation; the central source itself is not detected. The SNR may be interacting with the adjacent H  ii region RCW 80 through an extension of the proposed outflow beyond its shell. This would put the SNR at the lower limit of its distance range and would imply an age 4000 yr. We consider other SNRs similar to G309.2–00.6, and propose remnants whose shells are affected by jets as one of several classes of SNR from which the presence of a central source can be inferred.     

The high-velocity molecular stream in W51B: a ring structure with compact H ii regions

We present ¹³ CO J  = 1 − 0 line observations of the H  ii region complex W51B located in the high-velocity (HV) stream. These observations reveal a filamentary and clumpy structure in the molecular gas. The mean local standard of rest (LSR) velocity ∼ + 65 km s⁻¹ of the molecular gas in this region is greater than the maximum velocities allowed by kinematic Galactic rotation curves. The size and mass of the molecular clouds are ∼ 48 × 17 pc² and ∼ 2.4 × 10⁵ M⊙ respectively. In a position–velocity diagram, molecular gas in the southern part comprises a redshifted ring structure with v _LSR=+ 60 to +73 km s⁻¹. The velocity gradient of this ring is ∼ 0.5 km s⁻¹ pc⁻¹, and the mass is ∼ 6.2 × 10⁴ M⊙. If we assume that the ring is expanding with a uniform velocity, the expansion velocity, radius and kinetic energy are ∼ 7 km s⁻¹, ∼ 13 pc and ∼ 3.0 × 10 ⁴⁹ erg respectively. The kinetic energy and mass spectrum of the ring could be explained by an expanding cylindrical cloud with a centrally condensed mass distribution. The locations of two compact H  ii regions, G49.0−0.3 and G48.9−0.3, coincide with the two molecular clumps in this ring. We discuss star formation, and the mechanism that produced the ring structure.     

A large-scale survey of X-ray filaments in the Galactic Centre

We present a catalogue of 17 filamentary X-ray features located within a  68 × 34  arcmin²  view centred on the Galactic Centre region from images taken by Chandra . These features are described by their morphological and spectral properties. Many of the X-ray features have non-thermal spectra that are well fitted by an absorbed power law. Of the 17 features, we find six that have not been previously detected, four of which are outside the immediate  20 × 20  arcmin²  area centred on the Galactic Centre. Seven of the 17 identified filaments have morphological and spectral properties expected for pulsar wind nebulae (PWNe) with X-ray luminosities of  5 × 10³²  to 10³⁴ erg s⁻¹ in the 2.0–10.0 keV band and photon indices in the range of  Γ= 1.1  to 1.9. In one feature, we suggest the strong neutral Fe Kα emission line to be a possible indicator for past activity of Sgr A*. For G359.942−0.03, a particular filament of interest, we propose the model of a ram pressure confined stellar wind bubble from a massive star to account for the morphology, spectral shape and 6.7 keV He-like Fe emission detected. We also present a piecewise spectral analysis on two features of interest, G0.13−0.11 and G359.89−0.08, to further examine their physical interpretations. This analysis favours the PWN scenario for these features.