uGMRT HI 21-cm absorption observations of two extremely inverted spectrum sources

We report the detection of HI 21-cm absorption in a member of the rare and recently discovered class of compact radio sources: extremely inverted spectrum extragalactic radio sources (EISERS). The EISERS conceivably form a special subclass of the inverted spectrum radio galaxies since the spectral index of the optically thick part of the spectrum for these sources crosses the synchrotron self-absorption limit of α = +2.5 (S(ν) ∝ να). We searched for HI absorption in two EISERS using the recently upgraded Giant Metrewave Radio Telescope (uGMRT) and detected an absorption feature in one of them. The strong associated HI absorption detected against the source J1209−2032 (z = 0.4040) implies an optical depth of 0.178 ± 0.02, corresponding to an HI column density of 34.8 ± 2.9 × 1020 cm−2, for an assumed HI spin temperature of 100 K and covering factor of 1. This is among the highest known optical depth and HI column densities found for compact radio sources of peaked spectrum type and supports the free-free absorption model for the steeply inverted radio spectrum of this source. For the other source, J1549+5038 (z = 2.171), no HI absorption was detected in our observations.

GMRT observations of a first sample of ‘Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS)’ candidates in the Northern sky

We present an extension of our search for ‘Extremely Inverted Spectrum Extragalactic Radio Sources’ (EISERS) to the northern celestial hemisphere. With an inverted radio spectrum of slope α > +2.5, these rare sources would either require a non-standard particle acceleration mechanism (in the framework of synchrotron self-absorption hypothesis), or a severe free-free absorption which attenuates practically all of their synchrotron radiation at metre wavelengths. By applying a sequence of selection filters, a list of 15 EISERS candidates is extracted out by comparing two large-sky radio surveys, WENSS (325 MHz) and TGSS-ADR1 (150 MHz), which overlap across 1.03π steradian of the sky. Here we report quasi-simultaneous GMRT observations of these 15 EISERS candidates at 150 MHz and 325 MHz, in an attempt to accurately define their spectra below the turnover frequency. Out of the 15 candidates observed, two are confirmed as EISERS, since the slope of the inverted spectrum between these two frequencies is found to be significantly larger than the critical value αc = +2.5: the theoretical limit for the standard case of synchrotron self-absorption (SSA). For another 3 sources, the spectral slope is close to, or just above the critical value αc. Nine of the sources have GPS type radio spectra. The parsec-scale radio structural information available for the sample is also summarised.

Zeeman splitting of electron spectrum in HgTe quantum wells near the Dirac point

The Zeeman splitting of the conduction band in the HgTe quantum wells both with normal and inverted spectrum has been studied experimentally in a wide electron density range. The simultaneous analysis of the Shubnikov–de Haas oscillations in low magnetic fields at different tilt angles and of the shape of the oscillations in moderate magnetic fields gives a possibility to find the ratio of the Zeeman splitting to the orbital one and anisotropy of g -factor. It is shown that the ratios of the Zeeman splitting to the orbital one are close to each other for both types of structures, with a normal and inverted spectrum and they are close enough to the values calculated within kP method. In contrast, the values of g -factor anisotropy in the structures with normal and inverted spectra are strongly different and for both cases differ significantly from the calculated ones. We assume that such disagreement with calculations is a result of the interface inversion asymmetry in the HgTe quantum well, which is not taken into account in the kP calculations.

Analysis and discussion of a sample of 25 gigahertz-peaked spectra pulsars

We identified gigahertz-peaked spectra behavior from our radio interferometric observations at low frequencies using the Giant Metrewave Radio Telescope. We modeled the turnover spectra based on thermal free-free absorption in the interstellar medium. The free-free absorption is believed to be responsible for the inverted spectrum. Using the model, we were able to put some observational constrains on the physical parameters of the absorbing matter, which allows us to distinguish between the possible sources of absorption.

Representationalism and the Transparency of Experience

This chapter discusses the twin theses of representationalism and the transparency of experience. In place of standard representationalism, the chapter argues for “modest representationalism”: the thesis that two people who are visually representing the same scene will have experiences that are, phenomenally, pretty similar. While this might sound like an untenable compromise position, neo-pragmatism provides it with a solid theoretical foundation. This chapter also criticizes an argument of Ned Block’s, which attempts to move from the relatively uncontroversial claim that we can imagine someone with an inverted spectrum, to the much more controversial claim that we must acknowledge that there could be a significant phenomenal difference between the way something looks to two different normal people. The chapter also argues that phenomenal content is in principle amenable to verbal description in a sufficiently rich public language.

Wittgenstein y la hipótesis del espectro visual invertido

Un caso de inversión peligrosa del espectro visual es aquél en el que dos personas tienen impresiones visuales distintas cuando ven el mismo objeto y sin embargo coinciden al afirmar que el objeto es, por ejemplo, rojo. De acuerdo con Ned Block, si los casos de inversión del espectro visual son concebibles, entonces puede construirse un argumento a favor de la existencia de los qualia: si dos sujetos tienen distintas impresiones visuales, entonces deben estar diciendo cosas distintas cuando profieren "esto es rojo" al describir su modo de ver el mismo objeto. Según Block, Wittgenstein se comprometió con la posibilidad de un tipo de inversión del espectro visual que permite defender la existencia de los qualia y, sin embargo, Wittgenstein parece oponerse a la idea de los qualia, luego hay una incoherencia en su pensamiento. Nuestra tesis en este trabajo es que el argumento de Block no es un problema para la posición de Wittgenstein con respecto al significado de los términos relativos a la percepción y que tal incoherencia en su pensamiento es solo aparente. TITLE: Wittgenstein and the Inverted Spectrum HypothesisABSTRACTA case of dangerous inverted spectrum is that one in which two subjects have different impressions when they see the same object and both claim, for example, that the object is red. According to Ned Block, if cases of dangerous inverted spectrum are conceivable, then we can offer an argument in favour of existence of qualia: two subjects with inverted spectrum don’t mean the same when they utter that the object is red. According to Block, Wittgenstein accepted the possibility of a kind of case of inverted spectrum that allows to defend the existence of qualia, and nevertheless Wittgenstein seems to be opposed to the idea of qualia. There is thus, it seems, an incoherence in the Wittgensteinian thought. Our thesis in this paper is that Wittgensteinian’ discussion about the inverted spectrum hypothesis is a particular discussion about the grammar of perceptual terms that pretends to show the irrelevance of our sensations in determining the meaning of perceptual terms. From this interpretation, the discussion of a case of inverted spectrum does not mean the commitment with the possibility of that case, and then such incoherence on Wittgenstein’ thought is only apparent.