اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدVLBI observations of SN 2008iz: I. Expansion velocity and limits on anisotropic expansion
168
0
0.0
(
0
)
تأليف
A. Brunthaler
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We present observations of the recently discovered supernova 2008iz in M82 with the VLBI High Sensitivity Array at 22 GHz, the Very Large Array at frequencies of 1.4, 4.8, 8.4, 22 and 43 GHz, and the Chandra X-ray observatory. The supernova was clearly detected on two VLBI images, separated by 11 months. The source shows a ring-like morphology and expands with a velocity of ~23000 km/s. The most likely explosion date is in mid February 2008. The measured expansion speed is a factor of ~2 higher than expected under the assumption that synchrotron self-absorption dominates the light curve at the peak, indicating that this absorption mechanism may not be important for the radio emission. We find no evidence for an asymmetric explosion. The VLA spectrum shows a broken power law, indicating that the source was still optically thick at 1.4 GHz in April 2009. Finally, we report upper limits on the X-ray emission from SN 2008iz and a second radio transient recently discovered by MERLIN observations.
قيم البحث
اقرأ أيضاً
We present first results from the ongoing radio monitoring of SN 2008iz in M82. The VLBI images reveal a shell-like structure with circular symmetry, which expands in a self-similar way. There is strong evidence of a compact component with a steep sp
ectrum at the center of the shell. The expansion curve obtained from our VLBI observations is marginally decelerated (m = 0.89) and can be modeled simultaneously with the available radio light curves. While the results of this simultaneous fitting are not conclusive (i.e. different combinations of values of the magnetic field, CSM density profile, and electron energy distribution, provide fits to the available data with similar quality), additional observations should allow a more robust and detailed modeling.
We report on new Very Long Baseline Interferometry radio measurements of supernova 2014C in the spiral galaxy NGC 7331, made with the European VLBI Network ~5 yr after the explosion, as well as on flux density measurements made with the Jansky Very L
arge Array (VLA). SN 2014C was an unusual supernova, initially of Type Ib, but over the course of ~1 yr it developed strong H$alpha$ lines, implying the onset of strong interaction with some H-rich circumstellar medium (CSM). The expanding shock-front interacted with a dense shell of circumstellar material during the first year, but has now emerged from the dense shell and is expanding into the lower density CSM beyond. Our new VLBI observations show a relatively clear shell structure and continued expansion with some deceleration, with a suggestion that the deceleration is increasing at the latest times. Our multi-frequency VLA observations show a relatively flat powerlaw spectrum with $S_ u propto u^{-0.56 pm 0.03}$, and show no decline in the radio luminosity since $tsim1$ yr.
We report on a VLA survey for late-time radio emission from 59 supernovae (SNe) of Type I b/c, which have been associated with long-duration gamma-ray bursts (GRBs). An off-axis GRB burst (i.e. whose relativistic jet points away from us) is expected
to have late-time radio emission even in the absence of significant prompt gamma-ray emission. From our sample, we detected only SN 2003gk with an 8.4-GHz flux density of $2260 pm 130 ,mu$Jy. Our subsequent VLBI observations of SN 2003gk, at an age of $sim$8 yr, allowed us to determine its radius to be $(2.4 pm 0.4) times 10^{17}$ cm, or $94 pm 15$ light days. This radius rules out relativistic expansion as expected for an off-axis GRB jet, and instead suggests an expansion speed of $sim 10:000$ km s$^{-1}$ typical for non-relativistic core-collapse supernovae. We attribute the late-onset radio emission to interaction of the ejecta with a dense shell caused by episodic mass-loss from the progenitor. In addition, we present new calculations for the expected radio lightcurves from GRB jets at various angles to the line of sight, and compare these to our observed limits on the flux densities of the remainder of our SN sample. From this comparison we can say that only a fraction of broadlined Type I b/c SNe have a radio-bright jet similar to those seen for GRB afterglows at cosmological distances. However, we also find that for a reasonable range of parameters, as might be representative of the actual population of GRB events rather than the detected bright ones, the radio emission from the GRB jets can be quite faint, and that at present, radio observations do not place strong constraints on off-axis GRB jets.
We re-examine the contentious question of constraints on anisotropic expansion from Type Ia supernovae (SNIa) in the light of a novel determination of peculiar velocities, which are crucial to test isotropy with supernovae out to distances $lesssim 2
00/h$ Mpc. We re-analyze the Joint Light-Curve Analysis (JLA) Supernovae (SNe) data, improving on previous treatments of peculiar velocity corrections and their uncertainties (both statistical and systematic) by adopting state-of-the-art flow models constrained independently via the 2M$++$ galaxy redshift compilation. We also introduce a novel procedure to account for colour-based selection effects, and adjust the redshift of low-$z$ SNe self-consistently in the light of our improved peculiar velocity model. We adopt the Bayesian hierarchical model texttt{BAHAMAS} to constrain a dipole in the distance modulus in the context of the $Lambda$CDM model and the deceleration parameter in a phenomenological Cosmographic expansion. We do not find any evidence for anisotropic expansion, and place a tight upper bound on the amplitude of a dipole, $|D_mu| < 5.93 times 10^{-4}$ (95% credible interval) in a $Lambda$CDM setting, and $|D_{q_0}| < 6.29 times 10^{-2}$ in the Cosmographic expansion approach. Using Bayesian model comparison, we obtain posterior odds in excess of 900:1 (640:1) against a constant-in-redshift dipole for $Lambda$CDM (the Cosmographic expansion). In the isotropic case, an accelerating universe is favoured with odds of $sim 1100:1$ with respect to a decelerating one.
Cosmic parallax is the change of angular separation between pair of sources at cosmological distances induced by an anisotropic expansion. An accurate astrometric experiment like Gaia could observe or put constraints on cosmic parallax. Examples of a
nisotropic cosmological models are Lemaitre-Tolman-Bondi void models for off-center observers (introduced to explain the observed acceleration without the need for dark energy) and Bianchi metrics. If dark energy has an anisotropic equation of state, as suggested recently, then a substantial anisotropy could arise at $z lesssim 1$ and escape the stringent constraints from the cosmic microwave background. In this paper we show that such models could be constrained by the Gaia satellite or by an upgraded future mission.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
