Evidence for a Black Hole and Accretion Disk in the LINER NGC 4203

We examine the supernova remnant SNR 0540-697 in the Large Magellanic Cloud (LMC) using data from the Chandra Advanced CCD Imaging Spectrometer. The X-ray emission from this supernova remnant (SNR) had previously been hidden in the bright emission of the nearby X-ray binary LMC X-1; however, new observations with Chandra can finally reveal the SNR's structure and spectrum. We find the SNR to be a thick-shelled structure about 19 pc in diameter, with a brightened northeast region. Spectral results suggest a temperature of 0.31 keV and an X-ray luminosity (0.3-3.0 keV) of 8.4x1033 ergs s(-1). We estimate an age of 12,000-20,000 yr for this SNR, but note that this estimate does not take into account the possibility of cavity expansion or other environmental effects.     

Nucleosynthesis and Mixing in Cassiopeia A

We present results from the first light observations of the Cassiopeia A supernova remnant (SNR) by the Chandra X-Ray Observatory. Based on representative spectra from four selected regions, we investigate the processes of nucleosynthesis and mixing in Cas A. We make the first unequivocal identification of iron-rich ejecta produced by explosive silicon burning in a young Galactic SNR. Elsewhere in the remnant, we see silicon-rich ejecta from explosive oxygen burning. The Fe-rich ejecta lie outside the Si-rich material, indicating that bulk motions were extensive and energetic enough in Cas A to cause a spatial inversion of a significant portion of the supernova core. It is likely that this inversion was caused by "Fe"-rich ejecta emerging in plumes from the rising bubbles in the neutrino-driven convection layer during the supernova explosion. In addition, the radioactive decay energy from 56Ni may have contributed to the subsequent evolution of the material. We have also discovered faint, well-defined filaments with featureless X-ray spectra that are possibly sites of cosmic-ray acceleration in Cas A.     

X-ray detection of the monoceros supernova remnant

The Monoceros nebula is seen in the optical and in radio as a 3°.5 degree diameter ring. It is believed to be an old supernova remnant. Here is reported the detection of X-rays from the Monoceros nebula, confirming its supernova remnant nature.Einstein imaging proportional counter observations of 0.2 to 5 keV X-rays were analyzed to produce a surface brightness map. Preliminary modeling of the Monoceros supernova remnant yields an age of 50 000 years. A large age is expected for such a large remnant. However, the remnant is found to still be in the adiabatic blast wave stage of evolution.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Limits on the X-ray and optical luminosity of the progenitor of the Type Ia supernova 2007sr

We present Hubble Space Telescope ( HST )/Wide Field Planetary Camera 2 (WFPC2), Galaxy Evolution Explorer ( GALEX ) and Chandra observations of the position of the Type Ia supernova 2007sr in the Antennae galaxy, taken before the explosion. No source is found in any of the observations, allowing us to put interesting constraints on the progenitor luminosity. In total there is about 450 ks of Chandra data, spread over seven different observations. Limiting magnitudes of far-ultraviolet (FUV) (23.7 AB mag), near-ultraviolet (NUV) (23.8 AB mag), F555W (26.5 Vega mag) and F814W (24.5–25 Vega mag) are derived. The distance to the Antennae galaxy is surprisingly poorly known, with almost a factor of 2 difference between the latest distance based on the tip of the red giant branch (13.3 Mpc) and the distance derived from the 2007sr light curve (25 Mpc). Using these distances we derive limits on absolute optical and UV magnitudes of any progenitor but these are still above the brightest (symbiotic) proposed progenitors. From the Chandra data a 3σ upper limit to the X-ray luminosity of  0.5–8.0 × 10³⁷ erg s⁻¹  in the 0.3–1 keV range is found. This is below the X-ray luminosity of the potential progenitor of the Type Ia supernova 2007on that we recently discovered and for which we report a corrected X-ray luminosity. If that progenitor is confirmed it suggests the two supernovae have different progenitors. The X-ray limit is comparable to the brightest supersoft X-ray sources in the Galaxy, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) and significantly below the luminosities of the brightest supersoft and quasi-soft X-ray sources found in nearby galaxies, ruling out such sources as progenitors of this Type Ia supernova.     

Chandra High-Resolution Camera observations of the luminous X-ray source in the starburst galaxy M82

We analyse Chandra High Resolution Camera observations of the starburst galaxy M82, concentrating on the most luminous X-ray source. We find a position for the source of         (J2000) with a 1 σ radial error of 0.7 arcsec. The accurate X-ray position shows that the luminous source is neither at the dynamical centre of M82 nor coincident with any suggested radio AGN candidate. The source is highly variable between observations, which suggests that it is a compact object and not a supernova or remnant. There is no significant short-term variability within the observations. Dynamical friction and the off-centre position place an upper bound of 10⁵–10⁶ M_⊙ on the mass of the object, depending on its age. The X-ray luminosity suggests a compact object mass of at least 500 M_⊙. Thus the luminous source in M82 may represent a new class of compact object with a mass intermediate between those of stellar-mass black hole candidates and supermassive black holes.     

GLITP optical monitoring of QSO 0957+561: VR light curves and variability

The Gravitational Lenses International Time Project (GLITP) collaboration observed the first gravitational lens system (QSO 0957+561) from 2000 February 3 to March 31. The daily VR observations were made with the 2.56-m Nordic Optical Telescope at Roque de los Muchachos Observatory, La Palma, Spain. We have derived detailed and robust VR light curves of the two components Q0957+561A and Q0957+561B. In spite of the excellent sampling rate, we have not found evidence in favour of true daily variability. With respect to variability on time-scales of several weeks, we measure VR gradients of about −0.8 mmag d⁻¹ in Q0957+561A and +0.3 mmag d⁻¹ in Q0957+561B. The gradients are very probably originated in the far source. Thus, adopting this reasonable hypothesis (intrinsic variability), we compare them to the expected gradients during the evolution of a compact supernova remnant at the redshift of the source quasar. The starburst scenario is roughly consistent with some former events, but the new gradients do not seem to be caused by supernova remnant activity.     

Cassiopeia A: dust factory revealed via submillimetre polarimetry

If Type II supernovae – the evolutionary end points of short-lived, massive stars – produce a significant quantity of dust  (>0.1 M_⊙)  then they can explain the rest-frame far-infrared emission seen in galaxies and quasars in the first Gyr of the Universe. Submillimetre (submm) observations of the Galactic supernova remnant, Cas A, provided the first observational evidence for the formation of significant quantities of dust in Type II supernovae. In this paper, we present new data which show that the submm emission from Cas A is polarized at a level significantly higher than that of its synchrotron emission. The orientation is consistent with that of the magnetic field in Cas A, implying that the polarized submm emission is associated with the remnant. No known mechanism would vary the synchrotron polarization in this way and so we attribute the excess polarized submm flux to cold dust within the remnant, providing fresh evidence that cosmic dust can form rapidly. This is supported by the presence of both polarized and unpolarized dust emission in the north of the remnant where there is no contamination from foreground molecular clouds. The inferred dust polarization fraction is unprecedented  ( f _pol∼ 30 per cent)  which, coupled with the brief time-scale available for grain alignment (<300 yr), suggests that supernova dust differs from that seen in other Galactic sources (where   f _pol= 2−7  per cent) or that a highly efficient grain alignment process must operate in the environment of a supernova remnant.     

Torus formation in neutron star mergers and well-localized short gamma-ray bursts

Merging neutron stars (NSs) are hot candidates for the still enigmatic sources of short gamma-ray bursts (GRBs). If the central engines of the huge energy release are accreting relic black holes (BHs) of such mergers, it is important to understand how the properties of the BH–torus systems, in particular disc masses and mass and rotation rate of the compact remnant, are linked to the characterizing parameters of the NS binaries. For this purpose, we present relativistic smoothed particle hydrodynamic simulations with conformally flat approximation of the Einstein field equations and a physical, non-zero temperature equation of state. Thick disc formation is highlighted as a dynamical process caused by angular momentum transfer through tidal torques during the merging process of asymmetric systems or in the rapidly spinning triaxial post-merger object. Our simulations support the possibility that the first well-localized short and hard GRBs 050509b, 050709, 050724, 050813 have originated from NS merger events and are powered by neutrino-antineutrino annihilation around a relic BH–torus system. Using model parameters based on this assumption, we show that the measured GRB energies and durations lead to estimates for the accreted masses and BH mass accretion rates which are compatible with theoretical expectations. In particular, the low-energy output and short duration of GRB 050509b set a very strict upper limit of less than 100 ms for the time interval after the merging until the merger remnant has collapsed to a BH, leaving an accretion torus with a small mass of only  ∼0.01 M_⊙  . This favours a (nearly) symmetric NS+NS binary with a typical mass as progenitor system.     

X-ray observations of the edge-on star-forming galaxy NGC 891 and its supernova SN1986J

We present XMM–Newton observations of NGC 891, a nearby edge-on spiral galaxy. We analyse the extent of the diffuse emission emitted from the disc of the galaxy, and find that it has a single-temperature profile with best-fitting temperature of 0.26 keV, though the fit of a dual-temperature plasma with temperatures of 0.08 and 0.30 keV is also acceptable. There is a considerable amount of diffuse X-ray emission protruding from the disc in the north-west direction out to approximately 6 kpc. We analyse the point-source population using a Chandra observation, using a maximum-likelihood method to find that the slope of the cumulative luminosity function of point sources in the galaxy is  −0.77^+0.13_−0.1  . Using a sample of other local galaxies, we compare the X-ray and infrared properties of NGC 891 with those of 'normal' and starburst spiral galaxies, and conclude that NGC 891 is most likely a starburst galaxy in a quiescent state. We establish that the diffuse X-ray luminosity of spirals scales with the far-infrared luminosity as   L _X∝ L ^0.87±0.07_FIR  , except for extreme starbursts, and NGC 891 does not fall in the latter category. We study the supernova SN1986J in both XMM–Newton and Chandra observations, and find that the X-ray luminosity has been declining with time more steeply than expected  ( L _X∝ t ⁻³)  .     

A Narrow Band Chandra X-ray Analysis of SNR 3C 391

We present narrow-band and equivalent width (EW) images of the thermal composite supernova remnant (SNR) 3C 391 in the X-ray emission lines of Mg, Si and S using the Chandra ACIS Observational data. The EW images reveal the spatial distribution of the emission of the metal species Mg, Si and S in the remnant. They have a clumpy structure similar to that seen in the broadband diffuse emission, suggesting that they are largely of interstellar origin. We find an interesting finger-like feature protruding outside the southwestern radio border of the remnant, somewhat similar to the jet-like Si structure found in the famous SNR Cas A. This feature may possibly be the debris of the jet of ejecta from an asymmetrical supernova explosion of a massive progenitor star.     

High Spatial Resolution X-Ray Spectroscopy of Cas A with Chandra

We present high spatial resolution X-ray spectroscopy of the supernova remnant Cassiopeia A with the Chandra observations. The X-ray emitting region of this remnant was divided into 38 × 34 pixels of 10″× 10″ each. Spectra of 960 pixels were created and fitted with an absorbed two component non-equilibrium ionization model. From the results of the spectral analysis we obtained maps of absorbing column density, temperatures, ionization ages, and the abundances of Ne, Mg, Si, S, Ca and Fe. The Si, S and possibly Ca abundance maps show obvious jet structures, while Fe does not follow the jet but seems to be distributed perpendicular to it. The abundances of Si, S and Ca show tight correlations between one another over a range of about two dex. This suggests that they are ejecta from explosive Oburning and incomplete Si-buming. Meanwhile, the Ne abundance is well correlated with that of Mg, indicating them to be the ashes of explosive C/Ne burning. The Fe abundance is positively correlated with that of Si when the latter is lower than 3 times the solar value, and is negatively correlated when higher. We suggest that such a two phase correlation is due to the different ways in which Fe was synthesized.     

Unifying neutron star sub-populations in the supernova fallback accretion model

We employ the supernova fallback disk model to simulate the spin evolution of isolated young neutron stars(NSs). We consider the submergence of the NS magnetic fields during the supercritical accretion stage and its succeeding reemergence. It is shown that the evolution of the spin periods and the magnetic fields in this model is able to account for the relatively weak magnetic fields of central compact objects and the measured braking indices of young pulsars. For a range of initial parameters, evolutionary links can be established among various kinds of NS sub-populations including magnetars, central compact objects and young pulsars. Thus, the diversity of young NSs could be unified in the framework of the supernova fallback accretion model.     

Nature of the X-ray Source in Supernova Remnant RCW103

I discuss the nature of the compact X-ray source in the center of the supernova remnant RCW 103. Several models, based on the accretion onto a compact object are briefly discussed. I show that it is more likely that the central X-ray source is an accreting neutron star than an accreting black hole. I also argue that models of a disrupted binary system consisting of an old accreting neutron star and a new one observed as a 69-ms pulsar are most favored. This revised version was published online in July 2006 with corrections to the Cover Date.     

The spin evolution of nascent neutron stars

The loss of angular momentum owing to unstable r-modes in hot young neutron stars has been proposed as a mechanism for achieving the spin rates inferred for young pulsars. One factor that could have a significant effect on the action of the r-mode instability is fallback of supernova remnant material. The associated accretion torque could potentially counteract any gravitational-wave-induced spin-down, and accretion heating could affect the viscous damping rates and hence the instability. We discuss the effects of various external agents on the r-mode instability scenario within a simple model of supernova fallback on to a hot young magnetized neutron star. We find that the outcome depends strongly on the strength of the magnetic field of the star. Our model is capable of generating spin rates for young neutron stars that accord well with initial spin rates inferred from pulsar observations. The combined action of r-mode instability and fallback appears to cause the spin rates of neutron stars born with very different spin rates to converge, on a time-scale of approximately 1 year. The results suggest that stars with magnetic fields ≤10¹³ G could emit a detectable gravitational wave signal for perhaps several years after the supernova event. Stars with higher fields (magnetars) are unlikely to emit a detectable gravitational wave signal via the r-mode instability. The model also suggests that the r-mode instability could be extremely effective in preventing young neutron stars from going dynamically unstable to the bar-mode.     

Numerically investigating the peculiar periphery of a supernova remnant in the medium with a density gradient: the case of RCW 103

The young shell-type supernova remnant RCW 103 has peculiar properties in the X-ray morphology obtained with Chandra.The southeastern shell is brighter in the X-rays,and the curved border of the shell in this region is flatter than the other part.We investigate the formation of the peculiar periphery of the supernova remnant RCW 103 using 3D hydrodynamical simulation.Assuming that the supernova ejecta has been evolved in the medium with a density gradient,the detected shape of the periphery can be generally reproduced.For RCW 103,with the ejecta mass of 3.0 M,the density of the background material of 2.0 cm^-3,and a gradient of 3.3-4.0 cm^-3pc^-1,the X-ray periphery can be generally reproduced.The simulation turned out that the asymmetry of the SNR RCW 103 is mainly due to the inhomogeneous medium with a density gradient.     

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.     

Synchrotron and thermal X-ray emission from supernova remnants. Low radiation losses of electrons

A model is proposed for the nonthermal synchrotron emission from supernova remnants in the uniform interstellar medium. Some characteristics of nonthermal and thermal emission (luminosity and surface brightness distribution) are compared. The conditions when the nonthermal component can be prominent in the X-ray spectrum are specified. We point out some observational tests which will allow a number of parameters characterizing the cosmic ray injection on supernova remnant shocks to be estimated. The cases when electron radiation losses may be neglected are considered.     

Global very long-baseline interferometry observations of compact radio sources in M82

Observations of the starburst galaxy, M82, have been made with a 20-station global very long-baseline interferometry (VLBI) array at λ 18 cm. Maps are presented of the brightest young supernova remnants (SNR) in M82 and the wide-field mapping techniques used in making images over a field of view of ∼1 arcmin with 3-milliarcsecond resolution are discussed. A limit has been placed on the power-law deceleration of the young supernova remnant (SNR) 43.31+592 with an index greater than 0.73±0.11 from observations with the European VLBI Network. Using the global array we have resolved compact knots of radio emission in the source which, with future global observations, will enable better constraints to be placed on the expansion parameters of this SNR.
The latest global observations have also provided high-resolution images of the most compact radio source in M82, 41.95+575. We determine an upper limit to the radial expansion rate along the major axis of 2000 km s⁻¹. However, the new images also show structure resembling that of collimated ejection which brings into question the previous explanation of the source as being a result of the confinement of a supernova by a high-density circumstellar medium.
It is apparent that we are now able to image the brightest supernova remnants in M82 with a linear scale which allows direct comparison with galactic SNR such as Cassiopeia A.     

On the morphology of supernova remnants with pulsars

One of the intriguing aspects of supernova remnants is their morphology. While the majority of them look like hollow shells, a few, called plerions, are centrally filled like the Crab nebula, and some have a shell-plerion combination morphology. The centrally-filled component in these remnants is believed to be powered by a central pulsar. In this paper we present results of model calculations of the evolution of surface brightness and morphology of supernova remnants containing pulsars. We discuss how the morphology of a supernova remnant will depend on the velocity of expansion, the density of the ambient medium into which it is expanding, and the initial period and magnetic field strength of the central pulsar     

Accretion of gas on to nearby spiral galaxies

We present evidence for cosmological gas accretion on to spiral galaxies in the local universe. The accretion is seen through its effects on the dynamics of the extraplanar neutral gas. The accretion rates that we estimate for two nearby spiral galaxies are of the order of their star formation rates. Our model shows that most of the extraplanar gas is produced by supernova feedback (galactic fountain) and only 10–20 per cent comes from accretion. The accreting material must have low specific angular momentum about the disc's spin axis, although the magnitude of the specific angular momentum vector can be higher. We also explore the effects of a hot corona on the dynamics of the extraplanar gas and find that it is unlikely to be responsible for the observed kinematical pattern and the source of accreted gas. However, the interaction with the fountain flow should profoundly affect the hydrodynamics of the corona.