AstroSat observed transient neutron star low-mass X-ray binary XTE J1701-462 for a total duration of ∼ 135 ks during its 2022 outburst. We report the results of a detailed spectral and timing analysis carried out using this data. The source traced a complete ‘Z’ shaped structure in the hardness intensity diagram (HID). The source exhibited an extended horizontal branch and a short-dipping flaring branch in the HID. The spectra of the source were fitted with different approaches. We find that most suitable spectral model comprises emission from a standard multi-color accretion disk (diskbb in XSPEC) and Comptonized radiation from a hot central corona, described by Comptb model of XSPEC. The observed disk component is cool, having a temperature in the range of ∼ 0.28 – 0.42 keV and truncated far (∼ 250 - 1600 km) from the compact object. The Compton corona has an optical depth in the range of ∼ 3.4 – 5.1 and a temperature in the range of 3.3 – 4.5 keV. The disk and corona flux as well as truncation radius vary significantly along the HID. The temperature kTin depends on both luminosity and inner disk radius and hence shows marginal variation as compared to the truncation radius. We discuss possible scenarios to explain the relationship between the spectral evolution and motion of the source along the HID. The timing analysis revealed horizontal branch oscillations (HBOs) in the frequency range ∼ 34 – 40 Hz. The frequency and rms strength of HBO vary systematically as the source moves along the horizontal branch (HB). The observed correlation of the HBO properties with the position on the HB is similar to that previously reported in this source using RXTE data during the 2006 outburst of the source. The source also showed normal branch oscillations (NBOs) with frequency ∼ 6.7 Hz in the middle and the lower normal branch. The energy-dependent study of the HBO properties suggests that the HBO is stronger in the higher energy band. We also observed very-low frequency noise (VLFN) and band-limited noise (BLN) components in the power density spectra. The break frequency of BLN component was found to be tightly correlated with the HBO frequency. We discuss possible models to explain the origin and nature of the observed features in the PDS.

The cosmic 21 cm signal serves as a crucial probe for studying the evolutionary history of the Universe. However, detecting the 21 cm signal poses significant challenges due to its extremely faint nature. To mitigate the interference from the Earth’s radio frequency interference (RFI), the ground and the ionospheric effects, the Discovering the Sky at the Longest Wavelength (DSL) project will deploy a constellation of satellites in Lunar orbit, with its high-frequency daughter satellite tasked with detecting the global 21 cm signal from cosmic dawn and reionization era (CD/EoR).We intend to employ the Vari-Zeroth-Order Polynomial (VZOP) for foreground fitting and subtracting. We have studied the effect of thermal noise, thermal radiation from the Moon, the Lunar reflection, anisotropic frequency-dependent beam, inaccurate antenna beam pattern, and RFI contamination. We discovered that the RFI contamination can significantly affect the fitting process and thus prevent us from detecting the signal. Therefore, experimenting on the far side of the moon is crucial. We also discovered that using VZOP together with DSL, after 1080 orbits around the Moon, which takes about 103 days, we can successfully detect the CD/EoR 21 cm signal.

This work presents visual morphological and dynamical classifications for 637 spatially resolved galaxies, most of which are at intermediate redshift (z ∼ 0.3), in the Middle-Ages Galaxy Properties with Integral field spectroscopy (MAGPI) Survey. For each galaxy, we obtain a minimum of 11 independent visual classifications by knowledgeable classifiers. We use an extension of the standard Dawid-Skene Bayesian model introducing classifier-specific confidence parameters and galaxy-specific difficulty parameters to quantify classifier confidence and infer reliable statistical confidence estimates. Selecting sub-samples of 86 bright (r < 20 mag) high-confidence (> 0.98) morphological classifications at redshifts (0.2 ≤ z ≤ 0.4), we confirm the full range of morphological types is represented in MAGPI as intended in the survey design. Similarly, with a sub-sample of 82 bright high-confidence stellar kinematic classifications, we find that the rotating and non-rotating galaxies seen at low redshift are already in place at intermediate redshifts. We do not find evidence that the kinematic morphology-density relation seen at z ∼ 0 is established at z ∼ 0.3. We suggest that galaxies without obvious stellar rotation are dynamically pre-processed sometime before z ∼ 0.3 within lower mass groups before joining denser environments.

Current and future surveys rely on machine learning classification to obtain large and complete samples of transients. Many of these algorithms are restricted by training samples that contain a limited number of spectroscopically confirmed events. Here, we present the first real-time application of Active Learning to optimise spectroscopic follow-up with the goal of improving training sets of early type Ia supernovae (SNe Ia) classifiers.

Using a photometric classifier for early SN Ia, we apply an Active Learning strategy for follow-up optimisation using the real-time FINK broker processing of the ZTF public stream. We perform follow-up observations at the ANU 2.3m telescope in Australia and obtain 92 spectroscopic classified events that are incorporated in our training set.

We show that our follow-up strategy yields a training set that, with 25% less spectra, improves classification metrics when compared to publicly reported spectra. Our strategy selects in average fainter events and, not only supernovae types, but also microlensing events and flaring stars which are usually not incorporated on training sets.

Our results confirm the effectiveness of active learning strategies to construct optimal training samples for astronomical classifiers. With the Rubin Observatory LSST soon online, we propose improvements to obtain earlier candidates and optimise follow-up. This work paves the way to the deployment of real-time AL follow-up strategies in the era of large surveys.

We present a chemo-dynamical study conducted with 2dF$$ + $$AAOmega of $$ \sim 6000$$ Gaia DR3 non-variable candidate metal-poor stars that lie in the direction of the Galactic plane. Our spectral analysis reveals 15 new extremely metal-poor (EMP) stars, with the lowest metallicity at $$\left[ {{\rm{Fe/H}}} \right] = - 4.0 \pm 0.2$$ dex. Two of the EMP stars are also carbon enhanced, with the largest enhancement of $$\left[ {{\rm{C/Fe}}} \right] = 1.3 \pm 0.1$$ occurring in a dwarf. Using our $$\left[ {{\rm{C/Fe}}} \right]$$ results, we demonstrate that the number of carbon-depleted stars decreases with lower metallicities, and the fraction of carbon-enhanced stars increases, in agreement with previous studies.

Our dynamical analysis reveals that the fraction of prograde and retrograde disk stars, defined as $${z_{{\rm{max}}}} \lt 3$$ kpc, with $${J_\phi }/{J_{{\rm{tot}}}} \gt 0.75$$ and $${J_\phi }/{J_{{\rm{tot}}}} \lt - 0.75$$ respectively, changes as metallicities decrease. Disk stars on retrograde orbits make up $$ \sim 10$$% of all the stars in our sample with metallicities below $$ - 2.1$$ dex. Interestingly, the portion of retrograde disk stars compared with the number of kinematically classified halo stars is approximately constant at $$4.6$$ % for all metallicities below $$ - 1.5$$ dex. We also see that $${J_\phi }$$ increases from $$380 \pm 50$$ to $$1320 \pm 90$$ km s$${^{ - 1}}$$ kpc across metallicity range $$ - 1.5$$ to $$ - 1.1$$, consistent with the spin-up of the Galactic disk. Over the metallicity range $$ - 3.0 \lt \left[ {{\rm{Fe/H}}} \right] \lt - 2.0$$, the slopes of the metallicity distribution functions for the prograde and retrograde disk stars are similar and comparable to that for the halo population. However, detailed chemical analyses based on high resolution spectra are needed to distinguish the accreted versus in-situ contributions. Finally, we show that our spectroscopic parameters reveal serious systematics in the metallicities published in recent studies that apply various machine learning techniques to Gaia XP spectra.

We carry out timing and spectral studies of the Be/X-ray binary pulsar GX 304-1 using NuStar and XMM-Newton observations. We construct the long-term spin period evolution of the pulsar which changes from a long-term spin-up (∼ 1.3 × 10−13 Hz s−1) to a long-term spin-down (∼ −3.4 × 10−14 Hz s−1) trend during a low luminosity state (∼ 1034–35 erg s−1). A prolonged low luminosity regime (LX ∼ 1034–35 erg s−1) was detected during 2005-2010 and spanning nearly five years since 2018 December. The XMM-Newton and NuStar spectra can be described with a power law plus blackbody model having an estimated luminosity of ∼ 2.5 × 1033 erg s−1 and ∼ 3.6 × 1033 erg s−1 respectively. The inferred radius of the blackbody emission is about 100-110 m which suggests a polar-cap origin of this component. From long-term ultraviolet observations of the companion star, an increase in the ultraviolet signatures is detected preceding the X-ray outbursts. The spectral energy distribution of the companion star is constructed which provides a clue of possible UV excess when X-ray outbursts were detected from the neutron star compared to the quiescent phase. We explore plausible mechanisms to explain the long-term spin-down and extended low luminosity manifestation in this pulsar. We find that sustained accretion from a cold disc may explain the prolonged low luminosity state of the pulsar since December 2018 but the pulsar was undergoing normal accretion during the low luminosity period spanning 2005-2010.

We have conducted a widefield, wideband, snapshot survey using the Australian SKA Pathfinder (ASKAP) referred to as the Rapid ASKAP Continuum Survey (RACS). RACS covers ≈ 90% of the sky, with multiple observing epochs in three frequency bands sampling the ASKAP frequency range of 700–1 800 MHz. This paper describes the third major epoch at 1 655.5 MHz, RACS-high, and the subsequent imaging and catalogue data release. The RACS-high observations at 1 655.5MHz are otherwise similar to the previously released RACS-mid (at 1367.5 MHz), and were calibrated and imaged with minimal changes. From the 1 493 images covering the sky up to declination ≈ +48°, we present a catalogue of 2 677 509 radio sources. The catalogue is constructed from images with a median root-mean-square noise of ≈ 195 μJy PSF−1 (point-spread function) and a median angular resolution of 11″. 8 × 8″. 1. The overall reliability of the catalogue is estimated to be 99.18 %, and we find a decrease in reliability as angular resolution improves. We estimate the brightness scale to be accurate to 10 %, and the astrometric accuracy to be within ≈ 0″. 6 in right ascension and ≈ 0″. 7 in declination after correction of a systematic declination-dependent offset. All data products from RACS-high, including calibrated visibility datasets, images from individual observations, full-sensitivity mosaics, and the all-sky catalogue are available at the CSIRO ASKAP Science Data Archive.

With wide-field phased array feed technology, the Australian Square Kilometre Array Pathfinder (ASKAP) is ideally suited to search for seemingly rare radio transient sources that are difficult to discover previous-generation narrow-field telescopes. The Commensal Real-time ASKAP Fast Transient (CRAFT) Survey Science Project has developed instrumentation to continuously search for fast radio transients (duration ≲ 1 second) with ASKAP, with a particular focus on finding and localising Fast Radio Bursts (FRBs). Since 2018, the CRAFT survey has been searching for FRBs and other fast transients by incoherently adding the intensities received by individual ASKAP antennas, and then correcting for the impact of frequency dispersion on these short-duration signals in the resultant incoherent sum (ICS) in real-time. This low-latency detection enables the triggering of voltage buffers, which facilitates the localisation of the transient source and the study of spectro-polarimetric properties at high time resolution. Here we report the sample of 43 FRBs discovered in this CRAFT/ICS survey to date. This includes 22 FRBs that had not previously been reported: 16 FRBs localised by ASKAP to. ≲ 1 arcsec and 6 FRBs localised to ∼ 10 arcmin. Of the new arcsecond-localised FRBs, we have identified and characterised host galaxies (and measured redshifts) for 11. The median of all 30 measured host redshifts from the survey to date is z = 0.23. We summarise results from the searches, in particular those contributing to our understanding of the burst progenitors and emission mechanisms, and on the use of bursts as probes of intervening media. We conclude by foreshadowing future FRB surveys with ASKAP using a coherent detection system that is currently being commissioned. This will increase the burst detection rate by a factor of approximately ten and also the distance to which ASKAP can localise FRBs.

Since August 2014, a monitoring survey at a frequency of 111 MHz has been conducted on the Large Phased Array (LPA) radio telescope of the P.N. Lebedev Physical Institute (LPI). We report the discovery of a bright pulse having a dispersion measure (DM) equal to 134.4 ± 2 pc cm–3, a peak flux density (Sp) equal to 20 ± 4 Jy and a half-width (We) equal to 211 ± 6 ms. The excessive DM of the pulse, after taking into account the MilkyWay contribution, is 114 pc cm–3 that indicates its extragalactic origin. Such value of DM corresponds to the luminosity distance 713 Mpc. The above parameters make the pulse to be a reliable candidate to the fast radio burst (FRB) event, and then it is the second FRB detected at such a large λ ∼ 2.7 m wavelength and the first one among non-repeating FRBs. The normalized luminosity Lν of the event, which we have designated as FRB 20190203, estimated under assumption that the whole excessive DM is determined by the intergalactic environment toward the host galaxy, is equal to ≃ 1034 erg s–1Hz–1. In addition to the study of radio data we analyzed data from the quasi-simultaneous observations of the sky in the high energy (≥ 80 keV) band by the omnidirectional detector SPI/ACS aboard the INTEGRAL orbital observatory (in order to look for a possible gamma-ray counterpart of FRB 20190203). We did not detect any transient events exceeding the background at a statistically significant level. In the INTEGRAL archive, the FRB 20190203 localization region has been observed many times with with a total exposure of ∼ 73.2 days. We have analyzed the data but were unable to find any reliable short gamma-ray bursts from the FRB 20190203 position. Finally we note that the observed properties of FRB 20190203 can be reproduced well in the framework of a maser synchrotron model operating in the far reverse shock (at a distance of ∼ 1015 cm) of a magnetar. However, triggering the burst requires a high conversion efficiency (at the level of 1%) of the shock wave energy into the radio emission.