arXiv subjects

M. A. Perez-Torres

Publications and source records attributed to M. A. Perez-Torres.

69 records · Page 4Linked to original sources

arXiv2002

A distorted radio shell in the young supernova SN1986J

We report here on 5 GHz global very-long-baseline interferometry (VLBI) observations of SN 1986J, 16 yr after its explosion. We obtained a high-resolution image of the supernova, which shows a distorted shell of radio emission, indicative of a deformation of the shock front. The angular size of the shell is $\sim4.7 {\rm mas}$, corresponding to a linear size of $\sim6.8 \times 10^{17} {\rm cm}$ for a distance of 9.6 Mpc to NGC 891. The average speed of the shell has decreased from $\sim$7400 \kms in 1988.74 down to about $6300 {\rm km s^{-1}}$ in 1999.14, indicative of a mild deceleration in the expansion of SN 1986J. Assuming a standard density profile for the progenitor wind ($ρ_{\rm cs} \propto r^{-s}, s=2$), the swept-up mass by the shock front is $\sim$2.2\Ms. This large swept-up mass, coupled with the mild deceleration suffered by the supernova, suggests that the mass of the hydrogen-rich envelope ejected at explosion was $\ga12$ \Ms. Thus, the supernova progenitor must have kept intact most of its hydrogen-rich envelope by the time of explosion, which favours a single, massive star progenitor scenario. We find a flux density for SN 1986J of $\sim$7.2 mJy at the observing frequency of 5 GHz, which results in a radio luminosity of $\sim1.4 \times 10^{37} {\rm erg s^{-1}}$ for the frequency range $10^7$--$10^{10}$ Hz ($α=-0.69; S_ν\propto ν^α)$. We detect four bright knots that delineate the shell structure, and an absolute minimum of emission, which we tentatively identify with the centre of the supernova explosion. If this is the case, SN 1986J has then suffered an asymmetric expansion. We suggest that this asymmetry is due to the collision of the supernova ejecta with an anisotropic, clumpy (or filamentary) medium.

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arXiv2002

How is really decelerating the expansion of SN1993J?

SN1993J is to date the radio supernova whose evolution has been monitored in greatest detail and the one which holds best promise for a comprehensive theoretical-observational analysis. The shell-like radio structure of SN1993J has expanded in general accord with models of shock excited emission, showing almost circular symmetry for over 8 years, except for a bright feature at the south-eastern region of the shell that has been observed at every epoch. The spectrum of SN1993J has flattened from alpha =-1 to alpha =-0.67 (S_(ν) propto nu**(alpha)). The decelerated expansion can be modeled well with a single slope but apparently better with two slopes. There are also intriguing hints of structure in the expansion curve. The results by the two VLBI groups carrying out this research show general agreement, but also some differences. A comparison of the optical and VLBI results about the details of the deceleration show some discrepancies.

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arXiv2002

Strongly decelerated expansion of SN1979C

We observed SN1979C in M100 on 4 June 1999, about twenty years after explosion, with a very sensitive four-antenna VLBI array at the wavelength of 18cm. The distance to M100 and the expansion velocities are such that the supernova cannot be fully resolved by our Earth-wide array. Model-dependent sizes for the source have been determined and compared with previous results. We conclude that the supernova shock was initially in free expansion for 6 +/- 2 yrs and then experienced a very strong deceleration. The onset of deceleration took place a few years before the abrupt trend change in the integrated radio flux density curves. We estimate the shocked swept-up mass to be about 1.6 solar masses, assuming a standard density profile for the CSM. Such a swept-up mass for SN1979C suggests a mass of the hydrogen-rich envelope ejected at explosion no larger than $M_{\rm env} \sim$ 0.9 solar masses. If SN1979C originated in a binary star the low value of M_env suggests that the companion of the progenitor star stripped off most of the hydrogen-rich envelope mass of the presupernova star prior to the explosion.

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arXiv2001

The role of synchrotron self-absorption in the late radio emission of SN1993J

The standard model for radio supernovae considers that the observed synchrotron radio emission arises from the high-energy shell that results from the strong interaction between the expanding supernova ejecta and the circumstellar medium. This emission is considered to be only partially absorbed by ionized thermal electrons in the circumstellar wind of the progenitor star. Based on a study of the radio light curves of the type II supernova SN1993J, we present evidence of synchrotron self-absorption. Our modeling of the radio light curves requires a large initial magnetic field, of about 30 Gauss, and the existence of an (initially) highly-relativistic population of electrons. We also show that while at early epochs the dominant absorption mechanism is external absorption by thermal electrons, at late epochs and long wavelengths the dominant absorption mechanism is synchrotron self-absorption. Consequently, the spectral turnover takes place at much shorter wavelengths than expected in the standard model, and at long wavelengths (greater than about 90cm at current epochs) the flux predictions depart substantially from those of the standard model.

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arXiv2001

Absolute kinematics of radio source components in the complete S5 polar cap sample I. First and second epoch maps at 8.4 GHz

We observed the thirteen extragalactic radio sources of the S5 polar cap sample at 8.4 GHz with the Very Long Baseline Array, on 1997.93 and 1999.41. We present the maps from those two epochs and briefly discuss the morphological changes experimented by some of the radio sources in the 1.4 yr elapsed. These results correspond to the first two epochs at 8.4 GHz of a program directed to study the absolute kinematics of the radio source components of the members of the sample by means of phase delay astrometry at 8.4, 15 and 43 GHz.

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arXiv2001

Space-VLBI phase-reference mapping and astrometry

We present 5 GHz space-VLBI observations of the quasar pair B1342+662/B1342+663 that demonstrate the feasibility of phase-reference techniques using an antenna in space. The space-based data were taken by the satellite HALCA, of the space-VLBI mission VSOP. From residual (referenced) phases we derive an upper bound of 10 meters to the uncertainty of the spacecraft orbit reconstruction. An analysis of the phase-reference maps of the sources additionally suggests that the above mentioned uncertaintyis likely not larger than 3 meters. With errors of this magnitude, HALCA is a useful tool for astrometric studies of close pairs of radio sources.

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arXiv2000

VLBI imaging of the gravitational lenses B1422+231 and MGJ0414+0534

We present wide-field images of the quadruple gravitational lenses B1422+231 and MGJ0414+0534 obtained from global Very Long Baseline Interferometry (VLBI) observations at 8.4 GHz on 23 November 1997. We present also a lens model for MGJ0414+0534, which reproduces the core positions and flux densities of the VLBI images, combining a singular isothermal ellipsoid with external shear, and a singular isothermal sphere to represent, respectively, the main lens galaxy and its neighbor, a faint galaxy near one of the images.

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arXiv2000

On the way to global phase-delay astrometry

The use of the phase-delay improves substantially the accuracy obtained in VLBI astrometry with respect to the group-delay observable. Recently, Ros et al. (1999) have extended the related phase-connection technique to a triangle of radio sources with relative separations up to 6.8deg (the S5 sources BL1803+784/ QSO1928+738/ BL2007+777). This technique has also been extended for separations up to 15deg in the studies of the pair of S5 radio sources QSO1150+812/ BL1803+784 (Perez-Torres et al. 2000). We are carrying out a long-term astrometric programme at 8.4, 15, and 43 GHz to determine the absolute kinematics of radio source components in the 13 members of the complete S5 polar cap sample. For each epoch, all the radio sources are observed at the same frequency for 24 hours in total. Bootstrapping techniques are used to phase-connect the data jointly for the 13 objects throughout the observations. An accurate registration of the maps of the radio sources at different frequencies will allow a study of jet components with unprecedented precision and provide spectral information. This observing scheme could be extended in the future to other regions of the sky and eventually lead to global phase-delay astrometry.

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arXiv2000

VLBI imaging of the gravitational lens MGJ0414+0534

We observed the quadruple gravitationally lensed image of MGJ0414+0534 on 23 November 1997 with a global VLBI array at 8.4 GHz. We report wide-field imaging results of its four components at submilliarcsecond resolution, displaying complex core-like and jet-like extended structures. A simple model combining a singular isothermal ellipsoid to represent the main lens galaxy, external shear, and a singular isothermal sphere to represent an additional, nearby object accounts well for the core positions and flux densities of the VLBI images. This model predicts delays between the different lensed images of several weeks.

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arXiv2000

Towards global-phase-delay VLBI astrometry: observations of QSO 1150+812 and BL 1803+784

On 18 November 1993, we observed the QSO 1150+812 and the BL Lac object 1803+784, nearly 15°apart on the sky, with a VLBI array, simultaneously recording at 8.4 and 2.3 GHz. Using difference astrometry techniques, we determined the coordinates of 1803+784 relative to those of 1150+812 to be Δα= 6h 7m 33.18469 arcsec +- 0.00020 arcsec Δδ= -2deg 30' 25''.13557 +- 0''.00075 These standard errors contain estimated contributions from the propagation medium, the effects of source opacity, and the possible misidentification of the reference point in some of the maps. We obtained comparably accurate and consistent relative positions of the radio sources with GPS-based ionosphere corrections, thus demonstrating that dual-frequency observations are not required for state-of-the-art accuracy in VLBI astrometry. These results also demonstrate the feasibility of using single-frequency measurements for sources separated by 15$^\circ$ on the sky, and open the avenue for the application of this technique on a full-sky scale.

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arXiv2000

The complete S5 polar cap sample: en route to phase-delay global astrometry

We report on the present status of our S5 polar cap phase-connected astrometry program. We observe 13 radio sources in the northernmost 20deg of the sky at the wavelengths of 3.6cm and 2cm, and we plan to extend the program to 0.7cm. We phase-connect jointly all our data successfully. We image the radio sources and some of them show morphological changes, in which astrometric registration is needed to determine the kinematics of the source components. We aim at unprecedented astrometric accuracy and at a check of the jet standard model at the 5-10 microarcsec/yr level.

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arXiv2000

VLBI difference astrometry at 43GHz

From 43GHz VLBA observations of the pair of radio sources 1928+738 and 2007+777 we have demonstrated the feasibility of precision phase-delay differential astrometric techniques at millimeter wavelengths. For a pair of sources with 5 degrees separation and high antenna elevations, we have shown that present astrometric models and millimeter arrays are advanced enough to model the differential phase-delay to within 2 picoseconds, less than one tenth of a phase-cycle at 43GHz. The root-mean-square of the differential phase-delay residuals is dominated by the fluctuations of the atmospheric water vapor. We have determined the relative position of the observed sources with a precision twofold better than previous determinations at lower frequencies and, more importantly, largely free from ambiguous definitions of the reference point on the structure of the radio sources. Our result makes 43GHz VLBI phase-delay differential astrometry an ideal tool to study the absolute kinematics of the highly variable structures of regions near the core of extragalactic radio sources.

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arXiv1999

High Precision Astrometry with VLBI: from the Triangle 1803+784/1928+738/2007+777 to the Complete S5 Polar Cap Sample

The VLBI technique can image compact radio sources with resolutions of the order of the milliarcsecond and can determine astrometrically relative positions with submilliarcsecond precisions. We determined the angular separations among the radio sources in the triangle 1803+784/1928+738/2007+777 with submilliarcsecond accuracy from a weighted least squares analysis of the differential phases. The comparison of the estimated separation of the pair 1928+738/2007+777 with previous such estimates obtained from data at different epochs allowed us to align the maps of 1928+738 at diferent epochs relative to those of 2007+777, showing that the jet components of 1928+738 display proper motions, and suggesting an identification for the position of the core of this radio source. We have also observed astrometrically the complete S5 polar cap sample, that among the 13 sources within 20 degrees to the celestial North Pole includes the above 3. These observations are carried out in the framework of a long-term astrometric program to determine the absolute kinematics of radio sources in the S5 complete sample.

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arXiv1998

Differential Astrometry over 15 degrees

We observed the pair of radio sources 1150+812 and 1803+784 in November 1993 with a VLBI array, simultaneously recording at 8.4 and 2.3 GHz. We determined the angular separation between the two sources with submilliarcsecond accuracy by using differential techniques. This result demonstrates the feasibility of high precision differential astrometry for radio sources separated in the sky by almost 15 degrees, and opens the avenue to its application to larger samples of radio sources.

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arXiv1997

Deceleration in the Expansion of SN1993J

A rarity among supernova, SN 1993J in M81 can be studied with high spatial resolution. Its radio power and distance permit VLBI observations to monitor the expansion of its angular structure. This radio structure was previously revealed to be shell-like and to be undergoing a self-similar expansion at a constant rate. From VLBI observations at the wavelengths of 3.6 and 6 cm in the period 6 to 42 months after explosion, we have discovered that the expansion is decelerating. Our measurement of this deceleration yields estimates of the density profiles of the supernova ejecta and circumstellar material in standard supernova explosion models.

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