Combination Of Thermal Emission Research Articles

Detection of significant X-ray polarization from transient NS-LMXB XTE J1701−462 with IXPE and its implication on the coronal geometry

ABSTRACT In this paper, we performed a spectro-polarimetric analysis of the transient NS-LMXB XTE J1701−462 using IXPE, NICER, and NuSTAR data during its 2022 outburst. We report significant detection of energy-dependent polarization in the X-ray signal from the source on 2022 September 29 in the 2–4, 4–8, and 2–8 keV energy bands with a polarization degree of 3.9 ± 0.3, 5.5 ± 0.6, and 4.5 ± 0.4 per cent, respectively. The polarization angle in the overall 2–8 keV band was ∼143° ± 2°. The spectra were modelled using a combination of thermal emission from an accretion disc, Comptonized emission from a hot electron plasma (or corona) and a Gaussian line. From spectro-polarimetric analysis, the polarization degree due to the disc emission had an upper limit of ∼11.5 per cent, and that of the Comptonized emission was constrained at 7.7 ± 2.5 per cent (at the 3σ level). The results suggest that the Comptonized component probably originates from a spreading-layer/boundary-layer above the neutron star surface. IXPE observation of the source on 2022 October 8 does not show significant polarization which can be attributed to a weakening of the coronal Comptonized emission. The implication of these results are discussed.

Probing spectral and temporal evolution of the neutron star low-mass X-ray binary 4U 1724–30 with AstroSat

ABSTRACT We report the broad-band spectro-temporal study of the poorly studied accreting neutron star (NS) low mass X-ray binary (LMXB) 4U 1724–30 using data from Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counters (LAXPC) instruments on board AstroSat. The dim persistent LMXB source was observed with AstroSat over 4 epochs in 2017, all of which corresponded to a low-luminosity non-thermal emission dominated (hard/island) emission state with modest spectral evolution. All the X-ray broad-band spectra can be modelled by a combination of thermal emission from the NS boundary layer (BL) or NS surface and a non-thermal emission component possibly originating from the inverse Comptonization of the disc seed photons. We investigate the presence of frequency and energy-dependent variabilities to probe the origin of the disc/coronal fluctuations. We also report the detection of a Type-I X-ray burst displaying a photospheric radius expansion (PRE). During the burst, a hard X-ray shortage in the 30–80 keV energy band and the enhancement of the persistent emission reveal the burst feedback on the overall accretion process. Using the touch-down burst flux ∼4.25 × 10−8 erg s−1 cm−2, the distance of the source is estimated as ∼8.4 kpc.

Radio observations of evaporating objects in the Cygnus OB2 region

We present observations of the Cygnus OB2 region obtained with the Giant Metrewave Radio Telescope (GMRT) at frequencies of 325 and 610 MHz. In this contribution we focus on the study of proplyd-like objects (also known as free-floating evaporating gas globules or frEGGs) that typically show an extended cometary morphology. We identify eight objects previously studied at other wavelengths and derive their physical properties by obtaining their optical depth at radio-wavelengths. Using their geometry and the photoionization rate needed to produce their radio-continuum emission, we find that these sources are possibly ionized by a contribution of the stars Cyg OB2 #9 and Cyg OB2 #22. Spectral index maps of the eight frEGGs were constructed, showing a flat spectrum in radio frequencies in general. We interpret these as produced by optically thin ionized gas, although it is possible that a combination of thermal emission, not necessarily optically thin, produced by a diffuse gas component and the instrument response (which detects more diffuse emission at low frequencies) can artificially generate negative spectral indices. In particular, for the case of the Tadpole we suggest that the observed emission is not of non-thermal origin despite the presence of regions with negative spectral indices in our maps.

Terahertz emission from biased AlGaN/GaN high-electron-mobility transistors

We report on the results of a comprehensive study of THz emission from a set of dedicated AlGaN/GaN high-electron-mobility transistors. We find that voltage-biased transistors indeed emit in the THz frequency range, as reported in the literature; however, our data let us conclude that this radiation cannot be directly attributed to plasmonic instability phenomena. Instead, two other distinct mechanisms are identified. One is based on high-frequency self-oscillations originating from positive feedback within the frequency range where the transistor provides gain. Such oscillations are especially facilitated by the integration of antenna structures and cease to exist after taking specific measures for circuit stabilization. Another mechanism is identified for the case of broadband emission from multifinger transistors fabricated without any specific antenna. In contrast to the predictions of the plasmonic instability picture, the spectra of this emission depend on the gate and drain biasing conditions and on the bias modulation frequency. This emission can be understood as a combination of thermal emission from the heated material and from thermally excited plasmons and trap states.

Carrier emission from the electronic states of self-assembled indium arsenide quantum dots

We have used the new technique of high resolution (Laplace) transient spectroscopy to examine the electronic states of ensembles of self-assembled quantum dots of InAs in a GaAs matrix. These have been produced by solid source MBE. We have monitored the s and p state occupancies as a function of time under thermal excitation over a range of temperatures after electrons have been captured by the quantum dots with different Fermi level positions. This can provide more information about the interaction of the dots with the host matrix than is possible with optical techniques and gives new fundamental insights into how such dots may operate in electronic devices such as memory and sensors. The increase in resolution of Laplace transient spectroscopy over conventional experiments reveals quite specific rates of carrier loss which we attribute to tunnelling at low temperatures and a combination of thermal emission and tunnelling as the temperature is increased.

Comparison of Star Clusters With and Without Wolf‐Rayet Stars in Wolf‐Rayet Galaxies

We compare the properties of young star clusters with and without Wolf-Rayet (W-R) stars in W-R galaxies using optical, near-infrared imagery and optical spectroscopy. Our work identifies the clusters with W-R stars in these galaxies for the first time. With this information, comparisons of clusters with and without W-R stars are now possible, enabling us to understand the chemical and morphological impact of massive stars on their environment and to constrain the parameters for modeling these systems. We find that clusters with W-R stars (W-R clusters) are systematically younger, bluer clusters. Knowing this age difference between the two cluster sets, we use an evolutionary scenario to interpret their other properties. Young clusters, typically W-R clusters, have a Str?mgren sphere-like gas configuration. They also tend to have H - K colors redder than those of theoretical models. We interpret the H - K excess as a combination of thermal emission from hot dust, nebular emission, and molecular emission. Older clusters, typically clusters without W-R stars, have ionized gas in a superbubble configuration caused by the prolonged influence of stellar winds and supernovae. The H - K excess is generally absent for these clusters. The nitrogen-to-oxygen abundance ratio (N/O) does not appear to increase as a function of age over the first 10 Myr. Systems without W-R stars do appear to have a significant, elevated N/O over systems with W-R stars in the metallicity range 12 + log(O/H) = 7.7-7.9. For the entire metallicity range in our sample, this finding is only marginally significant. We concur with previous studies, which find no correlation between the sulfur-to-oxygen abundance ratio and metallicity.

UV time-dependent emission in SY Muscae

Ultraviolet spectra acquired with the International Ultraviolet Explorer (IUE) of SY Mus = HD 10036 on 20 September 1980 and 11 June 1981 indicate a substantial enhancement of UV emission over a nine month period. The general UV flux level appears to have increased by approximately one order of magnitude between the first and second observing epochs. The strong ultraviolet continuum evident throughout the entire IUE spectral range λλ1200-3200 A on 11 June 1981 is closely approximated by a star with Teff = 40,000 K, where previously on 20 September 1980 the continuum distribution presented a more complex structure that is possibly explained by a combination of thermal emission from an early type main sequence star, and nebular recombination emission (Michalitsianos et al. 1981).