arXiv subjects

M. F. Bietenholz

Publications and source records attributed to M. F. Bietenholz.

At least 55 records · Page 3Linked to original sources

arXiv2008

Shell revealed in SN 1979C

VLBI observations at 5 GHz have revealed that supernova 1979C, in the galaxy M100 in the Virgo cluster, has shell structure. The shell is approximately circular with the 50% contour deviating from a circle by an rms of no more than 8% of the radius. The brightness distribution along the ridge may vary. The position of the center of the shell is at R.A.=12h 22m 58.66758s and decl.=15^o 47' 51.9695" (J2000), with a standard error of 0.8 mas in each of the coordinates. No isolated central component is visible above a flux density limit of ~0.15 mJy which corresponds to an upper limit of ~15 times the corresponding spectral luminosity of the Crab Nebula. The radio lightcurve is clearly falling again and the radio spectrum is now flatter than at earlier times. SN 1979C is only the fourth, and oldest (optically identified) supernova of which a detailed image could be obtained.

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arXiv2008

Supernova 1996cr: SN 1987A's Wild Cousin?

We report on new VLT optical spectroscopic and multi-wavelength archival observations of SN1996cr, a previously identified ULX known as Circinus Galaxy X-2. Our optical spectrum confirms SN1996cr as a bona fide type IIn SN, while archival imaging isolates its explosion date to between 1995-02-28 and 1996-03-16. SN1996cr is one of the closest SNe (~3.8 Mpc) in the last several decades and in terms of flux ranks among the brightest radio and X-ray SNe ever detected. The wealth of optical, X-ray, and radio observations that exist for this source provide relatively detailed constraints on its post-explosion expansion and progenitor history, including an preliminary angular size constaint from VLBI. The archival X-ray and radio data imply that the progenitor of SN1996cr evacuated a large cavity just prior to exploding: the blast wave likely expanded for ~1-2 yrs before eventually striking the dense circumstellar material which surrounds SN1996cr. The X-ray and radio emission, which trace the progenitor mass-loss rate, have respectively risen by a factor of ~2 and remained roughly constant over the past 7 yr. This behavior is reminiscent of the late rise of SN1987A, but 1000 times more luminous and much more rapid to onset. Complex Oxygen line emission in the optical spectrum further hints at a possible concentric shell or ring-like structure. The discovery of SN1996cr suggests that a substantial fraction of the closest SNe observed in the last several decades have occurred in wind-blown bubbles. An Interplanetary Network position allows us to reject a tentative GRB association with BATSE 4B960202. [Abridged]

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arXiv2008

An extremely luminous X-ray outburst at the birth of a supernova

Massive stars end their short lives in spectacular explosions, supernovae, that synthesize new elements and drive galaxy evolution. Throughout history supernovae were discovered chiefly through their delayed optical light, preventing observations in the first moments (hours to days) following the explosion. As a result, the progenitors of some supernovae and the events leading up to their violent demise remain intensely debated. Here we report the serendipitous discovery of a supernova at the time of explosion, marked by an extremely luminous X-ray outburst. We attribute the outburst to the break-out of the supernova shock-wave from the progenitor, and show that the inferred rate of such events agrees with that of all core-collapse supernovae. We forecast that future wide-field X-ray surveys will catch hundreds of supernovae each year in the act of explosion, and thereby enable crucial neutrino and gravitational wave detections that may ultimately unravel the explosion mechanism.

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arXiv2008

Supernova VLBI in the present and with the SKA

VLBI is the only technology that will allow sub-milliarcsecond resolution imaging in the near future. As such, it is the only way to image expanding supernovae in nearby galaxies. Such images potentially allow us to study the early evolution of neutron stars or black holes left behind by core-collapse supernovae, the circumstellar wind history of the supernova progenitor stars, the shock acceleration of cosmic-ray particles in supernovae as well as the evolutionary process by which supernova shells merge into, and enrich, the ISM. I will discuss the results of the on-going VLBI imaging campaigns on supernovae 1986J and 1993J. I will also discuss the impact on supernova VLBI of the proposed South-African Karoo Array Telescope and Australian ASKAP arrays, as well as the SKA itself, as these telescopes will greatly increase the sensitivity of the global VLBI network.

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arXiv2008

The Expansion and Radio Spectral Index of G21.5-0.9: Is PSR J1833-1034 the Youngest Pulsar?

We report on new 5-GHz VLA radio observations of the pulsar-powered supernova remnant G21.5-0.9. These observations have allowed us to make a high-quality radio image of this remnant with a resolution of ~0.7". It has a filamentary structure similar to that seen in the Crab Nebula. Radio structure suggestive of the torus seen around the Crab pulsar is tentatively identified. We also compared the new image with one taken ~15 yr earlier at 1.5 GHz, both to find the expansion speed of the remnant and to make a spectral index image. Between 1991 and 2006, we find that the average expansion rate of the remnant is 0.11 +/- 0.02 %/year, corresponding, for a distance of 5 kpc, to a speed of 910 +/- 160 km/s wrt. the centre of the nebula. Assuming undecelerated expansion, this expansion speed implies that the age of G21.5-0.9 is 870 (+200,-150) yr, which makes PSR J1833-1034 one of the youngest, if not the youngest, known pulsars in the Galaxy.

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arXiv2007

SN 1993J VLBI (IV): A Geometric Determination of the Distance to M81 with the Expanding Shock Front Method

We compare the angular expansion velocities, determined with VLBI, with the linear expansion velocities measured from optical spectra for supernova 1993J in the galaxy M81, over the period from 7 d to ~9 yr after shock breakout. We estimate the distance to SN 1993J using the Expanding Shock Front Method (ESM). We find the best distance estimate is obtained by fitting the angular velocity of a point halfway between the contact surface and outer shock front to the maximum observed hydrogen gas velocity. We obtain a direct, geometric, distance estimate for M81 of D=3.96+-0.05+-0.29 Mpc with statistical and systematic error contributions, respectively, corresponding to a total standard error of $+-0.29 Mpc. The upper limit of 4.25 Mpc corresponds to the hydrogen gas with the highest observed velocity reaching no farther out than the contact surface a few days after shock breakout. The lower limit of 3.67 Mpc corresponds to this hydrogen gas reaching as far out as the forward shock for the whole period, which would mean that Rayleigh-Taylor fingers have grown to the forward shock already a few days after shock breakout. Our distance estimate is 9+-13 % larger than that of 3.63+-0.34 Mpc from the HST Key Project, which is near our lower limit but within the errors.

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arXiv2007

SN 2001em: No Jet-Driven Gamma Ray Burst Event

We report on our second-epoch VLBI and VLA observations of the Type Ib/c supernova 2001em, five years after the explosion. It was suggested that SN 2001em might be a jet-driven gamma ray burst (GRB), with the jet oriented near the plane of the sky, which would entail relativistic expansion or motion. Our VLBI image shows that SN 2001em is still unresolved five years after the explosion. For a distance of 83 Mpc (H_0 = 70 km/s/Mpc), the nominal expansion velocity is 5800 +/- 10,000 km/s, and the proper motion is 33,000 +/- 34,000 km/s. Our values are inconsistent with either relativistic expansion or motion, but are consistent with the non-relativistic expansion speeds and small proper motions seen in other supernovae. In particular these values are consistent with radio emission from SN 2001em being due to normal, non-relativistic supernova ejecta interacting with the circumstellar medium. Our VLA observations show a power-law decay in flux density since the time of the peak in the 8.4 GHz radio lightcurve in ~2003.

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arXiv2007

Diffusive Synchrotron Radiation from Pulsar Wind Nebulae

Diffusive Synchrotron Radiation (DSR) is produced by charged particles as they random walk in a stochastic magnetic field. The spectrum of the radiation produced by particles in such fields differs substantially from those of standard synchrotron emission because the corresponding particle trajectories deviate significantly from gyration in a regular field. The Larmor radius, therefore, is no longer a good measure of the particle trajectory. In this paper we analyze a special DSR regime which arises as highly relativistic electrons move through magnetic fields which have only random structure on a wide range of spatial scales. Such stochastic fields arise in turbulent processes, and are likely present in pulsar wind nebulae (PWNe). We show that DSR generated by a single population of electrons can reproduce the observed broad-band spectra of PWNe from the radio to the X-ray, in particular producing relatively flat spectrum radio emission as is usually observed in PWNe. DSR can explain the existence of several break frequencies in the broad-band emission spectrum without recourse to breaks in the energy spectrum of the relativistic particles. The shape of the radiation spectrum depends on the spatial spectrum of the stochastic magnetic field. The implications of the presented DSR regime for PWN physics are discussed.

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arXiv2006

Radio Images of 3C 58: Expansion and Motion of its Wisp

New 1.4 GHz VLA observations of the pulsar-powered supernova remnant 3C 58 have resulted in the highest-quality radio images of this object to date. The images show filamentary structure over the body of the nebula. The present observations were combined with earlier ones from 1984 and 1991 to investigate the variability of the radio emission on a variety of time-scales. No significant changes are seen over a 110 day interval. In particular, the upper limit on the apparent projected velocity of the wisp is 0.05c. The expansion rate of the radio nebula was determined between 1984 and 2004, and is 0.014+/-0.003%/year, corresponding to a velocity of 630+/-70 km/s along the major axis. If 3C 58 is the remnant of SN 1181, it must have been strongly decelerated, which is unlikely given the absence of emission from the supernova shell. Alternatively, the low expansion speed and a number of other arguments suggest that 3C 58 may be several thousand years old and not be the remnant of SN 1181.

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arXiv2005

Imaging of Radio Supernovae

As the shock and expanding shell of a supernova plow out through the circumstellar material at thousands of km/s, radio emission is generated. VLBI observations of this radio emission are presently the only means to directly image the expanding shell of any supernova much farther away than SN 1987A. The last decade has seen great progress in VLBI imaging of radio supernovae. In particular, SN 1993J in the galaxy M81 provided a rare opportunity to closely study an expanding supernova, and has been intensively observed. I summarize some of the results on SN 1993J and other radio supernovae, and compare the different observed radio supernovae. I briefly discuss the future prospects of radio supernovae imaging.

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arXiv2005

An Upper Limit on the Expansion Velocity of GRB Candidate SN 2001em

We report on VLBI observations of the Type I b/c supernova 2001em, three years after the explosion. It had been suggested that SN 2001em might be a jet-driven gamma ray burst (GRB), with the jet oriented far from the line of sight so that the GRB would not be visible from earth. To test this conjecture, we determined the size of SN 2001em. It is only marginally resolved at our resolution of ~0.9 mas. The 3-sigma upper limit on the major axis angular size of the radio source was 0.59 mas (FWHM of an elliptical Gaussian), corresponding to a one-sided apparent expansion velocity of 70,000 km/s for a distance of 80 Mpc. No low-brightness jet was seen in our image to a level of 4% of the peak brightness. If we assume instead a spherical shell geometry typical of a supernova, we find the angular radius of SN 2001em was 0.17 (-0.10,+0.06) mas, implying an isotropic expansion velocity of 20,000 (-12,000, +7,000) km/s, which is comparable to the expansion velocities of supernova shells. Our observations, therefore, are not consistent with a relativistically expanding radio source in SN 2001em, but are instead consistent with a supernova shell origin of SN 2001's radio emission.

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arXiv2004

The Crab Nebula's Wisps in Radio and Optical

We present four new, high-resolution VLA radio images of the Crab nebula, taken between 2001 Feb. and Apr. The radio images show systematic variability in the Crab's radio emission throughout the region near the pulsar. The principal geometry of the variable features is that of elliptical ripples very similar to the optical wisps. The radio wisps are seen to move systematically outward with projected speeds of up to 0.3c. Comparing the new radio images to our earlier ones from 1998 and 2000, we show there are also more slowly moving features somewhat farther away from the pulsar. In particular, there is a prominent moving feature to the northwest of the pulsar which has a projected speed of order 10,000 km/s. Striation is seen throughout the nebula, suggesting the presence of wave-like disturbances propagating through the synchrotron bubble. The radio images were taken simultaneously with HST optical observations. Comparing the radio to the optical images, we find that the radio wisps are sometimes displaced from the optical ones or have no optical counterparts. We also find that some optical wisps in particular, the brightest optical wisps near the pulsar, do not seem to have radio counterparts. In the exterior of the nebula, by contrast, there is generally a good correspondence between the radio and optical features.

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arXiv2004

The Location of the Core in M81

We report on VLBI observations of M81*, the northwest-southeast oriented nuclear core-jet source of the spiral galaxy M81, at five different frequencies between 1.7 and 14.8 GHz. By phase referencing to supernova 1993J we can accurately locate the emission region of M81* in the galaxy's reference frame. Although the emission region's size decreases with increasing frequency while the brightness peak moves to the southwest, the emission region seems sharply bounded to the southwest at all frequencies. We argue that the core must be located between the brightness peak at our highest frequency (14.8 GHz) and the sharp bound to the southwest. This narrowly constrains the location of the core, or the purported black hole in the center of the galaxy, to be within a region of +/-0.2 mas or +/-800 AU (at a distance of ~4 Mpc). This range includes the core position that we determined earlier by finding the most stationary point in the brightness distribution of M81* at only a single frequency. This independent constraint therefore strongly confirms our earlier core position. Our observations also confirm that M81* is a core-jet source, with a one-sided jet that extends to the northeast from the core, on average curved somewhat to the east, with a radio spectrum that is flat or inverted near the core and steep at the distant end. The brightness peak is unambiguously identified with the variable jet rather than the core, which indicates limitations in determining the proper motion of nearby galaxies and in refining the extragalactic reference frame.

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arXiv2003

SN 1993J VLBI (III): The Evolution of the Radio Shell

We present two sequences of VLBI images of SN1993J in M81, with 24 images at 8.4 GHz and 19 images at 5.0 GHz, These sequences, from 50 d to ~9 yr after shock breakout, show the evolution of the expanding radio shell of an exploded star in detail. The images are all phase-referenced to the stable reference point of the core of M81, allowing us to display them relative to the supernova explosion center. At 50 d, SN1993J is almost unresolved with a radius of 520 AU. The shell structure becomes discernible at 175 d. The brightness of the ridge of the projected shell is not uniform, but rather varies by a factor of two, having a distinct maximum to the south-east and a minimum to the west. Over the next ~350 d, this pattern appears to rotate counter-clockwise. After two years, the structure becomes more complex with hot spots developing in the east, south, and west. The pattern of modulation continues to change, and after five years the three hot spots have shifted somewhat. After nine years, the radio shell has expanded to a radius of 19,000 AU. The brightness in the center of the images is lower than expected for an optically thin, spherical shell. Absorption in the center is favored over a thinner shell in the back and/or front. Allowing for absorption, we find that the thickness of the shell is (25+/-3)% of its outer radius. We find no compact source in the central region and conclude that any pulsar nebula in the center of SN 1993J is either much fainter than the Crab or affected by remaining significant internal radio absorption.

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arXiv2003

VLBI imaging of the RS CVn binary star system HR 5110

We present VLBI images of the RS CVn binary star HR 5110 (=BH CVn; HD 118216), obtained from observations made at 8.4 GHz on 1994 May 29/30 in support of the NASA/Stanford Gravity Probe B project. Our images show an emission region with a core-halo morphology. The core was 0.39 +/- 0.09 mas (FWHM) in size, or 66% +/- 20% of the 0.6 +/- 0.1 mas diameter of the chromospherically active K subgiant star in the binary system. The halo was 1.95 +/- 0.22 mas (FWHM) in size, or 1.8 +/- 0.2 times the 1.1 +/- 0.1 mas separation of the centers of the K and F stars. The core increased significantly in brightness over the course of the observations and seems to have been the site of flare activity that generated an increase in the total flux density of ~200% in 12 hours. The fractional circular polarization simultaneously decreased from ~10% to 2.5%.

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arXiv2002

SN 1986J VLBI. The Evolution and Deceleration of the Complex Source and a Search for a Pulsar Nebula

We report on VLBI observations of supernova 1986J in the spiral galaxy NGC 891 at two new epochs, 1990 July and 1999 February, t=7.4 and 15.9 yr after the explosion, and on a comprehensive analysis of these and earlier observations from t~4 yr after the explosion date, which we estimate to be 1983.2 +/- 1.1. The source is a shell or composite, and continues to show a complex morphology with large brightness modulations along the ridge and with protrusions. The supernova is moderately to strongly decelerated. The average outer radius expands as t^(0.71 +/- 0.11), and the expansion velocity has slowed to 6000 km/s at t=15.9 yr from an extrapolated 20,000 km/s at t=0.25 yr. The structure changes significantly with time, showing that the evolution is not self-similar. The shell structure is best visible at the latest epoch, when the protrusions have diminished somewhat in prominence and a new, compact component has appeared. The radio spectrum shows a clear inversion above 10 GHz. This might be related to a pulsar nebula becoming visible through the debris of the explosion. The radio flux density between 1.5 and 23 GHz decreases strongly with time, with the flux density proportional to t^(-2.94 +/- 0.24) between t~15 to 19 yr. This decrease is much more rapid than that found in earlier measurements up to t~6 yr.

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arXiv2001

The Crab Nebula's Moving Wisps in Radio

We present three high resolution radio images of the Crab nebula, taken in 1998.6, 1998.8 and 2000.1 with the VLA. These are the best radio images of the Crab to date. We show that, near the pulsar, there are significant changes between our three observing epochs. These changes have an elliptical geometry very similar to that of the optical wisps. One radio wisp in particular can be unambiguously identified between two of our observing epochs, and moves outward with an apparent velocity of ~0.24. The similarity in both morphology and behavior of the present radio wisps to the optical wisps suggests that they are associated. This implies that the radio wisps, like the optical ones, are likely manifestations of the shock in the Crab pulsar's wind. This suggests that the radio emitting electrons are accelerated in the same region as the ones responsible for the optical to X-ray emission, contrary to most current models.

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arXiv2001

Radio Spectral Index and Expansion of 3C58

We present new observations of the plerionic supernova remnant 3C58 with the VLA at 74 and 327 MHz. In addition, we re-reduced earlier observations at 1.4 and 4.9 GHz taken in 1973 and 1984. Comparing these various images, we find that: 1. the remnant has a flat and relatively uniform spectral index distribution, 2. any expansion of the remnant with time is significantly less than that expected for uniform, undecelerated expansion since the generally accepted explosion date in 1181 A.D., and 3. there is no evidence for a non-thermal synchrotron emission shell generated by a supernova shock wave, with any such emission having a surface brightness of <1 x 10^(-21) W / (m^2 Hz sr) at 327 MHz.

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