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We present a protocol for automated fitting of magnetic Moment Tensor Potential explicitly including magnetic moments in its functional form. For the fitting of this potential we use energies, forces, stresses, and magnetic forces (negative derivatives of energies with respect to magnetic moments) of configurations selected with an active learning algorithm. These selected configurations are computed using constrained density functional theory, which enables calculating energies and their derivatives for both equilibrium and non-equilibrium (excited) magnetic states. We test our protocol on the system of B1-CrN and demonstrate that the automatically trained magnetic Moment Tensor Potential reproduces mechanical, dynamical, and thermal properties, of B1-CrN in the paramagnetic state with respect to density functional theory and experiments.

Transition metal dichalcogenides (TMDCs) have emerged as highly promising materials for nanophotonics and optoelectronics due to their exceptionally high refractive indices, strong excitonic photoluminescence (PL) in monolayer configurations, and the versatility to engineer van der Waals (vdW) heterostructures. In this work, we exploit the intense excitonic PL of a MoSe$_2$ monolayer combined with the high refractive index of bulk WS$_2$ to fabricate microdisk cavities with tunable light emission characteristics. These microdisks are created from a 50-nm-thick WS$_2$/MoSe$_2$/WS$_2$ double heterostructure using frictional mechanical scanning probe lithography. The resulting cavities achieve a 4-10-fold enhancement in excitonic PL from the MoSe$_2$ monolayer at wavelengths near 800 nm. The excitonic PL peak is modulated by sharp spectral features, which correspond to whispering gallery modes (WGMs) supported by the cavity. A microdisk with a diameter of 2.35 $\mu$m demonstrates WGMs with a quality factor of up to 700, significantly surpassing theoretical predictions and suggesting strong potential for lasing applications. The spectral positions of the WGMs can be finely tuned by adjusting the microdisk's diameter and thickness, as confirmed by theoretical calculations. This approach offers a novel route for developing ultra-compact, all-TMDC double heterostructure light sources with record-small size.

Optical nonlinearities are crucial for advanced photonic technologies since they allow photons to be managed by photons. Exciton-polaritons resulting from strong light-matter coupling are hybrid in nature: they combine small mass and high coherence of photons with strong nonlinearity enabled by excitons, making them ideal for ultrafast all-optical manipulations. Among the most prospective polaritonic materials are halide perovskites since they require neither cryogenic temperatures nor expensive fabrication techniques. Here we study strikingly nonlinear self-action of ultrashort polaritonic pulses propagating in planar MAPbBr$_3$ perovskite slab waveguides. Tuning input pulse energy and central frequency, we experimentally observe various scenarios of its nonlinear evolution in the spectral domain, which include peak shifts, narrowing, or splitting driven by self-phase modulation, group velocity dispersion, and self-steepening. The theoretical model provides complementary temporal traces of pulse propagation and reveals the transition from the birth of a doublet of optical solitons to the formation of a shock wave, both supported by the system. Our results represent an important step in ultrafast nonlinear on-chip polaritonics in perovskite-based systems.

With the growing number of satellite-based Quantum Key Distribution (QKD) payload launches, it becomes essential to ensure compatibility across different platforms for satellite tracking and quantum signal acquisition. In this paper, the Technology Innovation Institute (TII) presents the development of the Abu Dhabi Quantum Optical Ground Station (ADQOGS) for secure free-space optical communications. With the know-how of GA-Synopta's experienced engineering team, we have developed a versatile multi-wavelength quantum acquisition and tracking system tailored to support various upcoming space-based QKD missions, crucial for the practical implementation of global quantum communication networks. This system is capable of handling multiple wavelengths, ranging from 600 nm to 1560 nm for downlink beacons and 1530 nm to 1610 nm for uplink beacons. It includes a free-space quantum module adequate to detect QKD signals at $780\pm10$ nm and $850\pm3$ nm and offers spatial and spectral filtering capabilities along with a motorized polarization correction system.

We present the analysis of optical data of a bright and extremely-rapidly evolving transient, AT2024wpp, whose properties are similar to the enigmatic AT2018cow (aka the Cow). AT2024wpp rose to a peak brightness of c=-21.9mag in 4.3d and remained above the half-maximum brightness for only 6.7d. The blackbody fits to the multi-band photometry show that the event remained persistently hot (T>20000K) with a rapidly receding photosphere (v~11500km/s) until the end of the photometric dataset at +16.1d post-discovery. This behaviour mimics that of AT2018cow, albeit with a several times larger photosphere. The spectra are consistent with blackbody emission throughout our spectral sequence ending at +21.9d, showing a tentative, very broad emission feature at 5500{\AA} -- implying that the optical photosphere is likely within a near-relativistic outflow. Furthermore, reports of strong X-ray and radio emission cement the nature of AT2024wpp as a likely Cow-like transient. AT2024wpp is only the second event of the class with optical polarimetry. Our BVRI observations obtained from +6.1 to +14.4d show a low polarisation of P<0.5% across all bands, similar to AT2018cow that was consistent with P~0% during the same outflow-driven phase. In the absence of evidence for a preferential viewing angle, it is unlikely that both events would have shown low polarisation in the case that their photospheres were aspherical. As such, we conclude that the near-relativistic outflows launched in these events are likely highly spherical, but polarimetric observations of further events are crucial to constrain their ejecta geometry and stratification in detail., Comment: 10 pages, 6 figures. Submitted to MNRAS

The innermost portions of the Milky Way's stellar halo have avoided scrutiny until recently. The lack of wide-area survey data, made it difficult to reconstruct an uninterrupted view of the density distribution of the metal-poor stars inside the Solar radius. In this study, we utilize red giant branch (RGB) stars from Gaia, with metallicities estimated using spectro-photometry from Gaia Data Release 3. Accounting for Gaia's selection function, we examine the spatial distribution of metal-poor ([M/H]<-1.3) RGB stars, from the Galactic centre (r~1 kpc) out to beyond the Solar radius (r~18 kpc). Our best-fitting single-component cored power-law model shows a vertical flattening of ~0.5 and a slope -3.4, consistent with previous studies. Motivated by the mounting evidence for two distinct stellar populations in the inner halo, we additionally test a range of two-component models. One of the components models the tidal debris from the Gaia Sausage/Enceladus merger, while the other captures the Aurora population -- stars that predate the Galactic disk formation. Our best-fit two-component model suggests that both populations contribute equally around the Solar radius, but Aurora dominates the inner halo with a steeper power-law index of -4.5, in agreement with the nitrogen-rich star distribution measured by Horta et al. (2021)., Comment: Submitted to MNRAS

The stellar halo of the Milky Way comprises an abundance of chemical signatures from accretion events and \textit{in-situ} evolution, that form an interweaving tapestry in kinematic space. To untangle this, we consider the mixtures of chemical information, in a given region of integral of motion space, as a variant of the blind source separation problem and utilise non-negative matrix factorisation (NMF). Specifically, we examine the variation in [Fe/H], [Mg/Fe], and [Al/Fe] distributions of APOGEE DR17 stars across the $(E,L_z)$ plane of the halo. When 2 components are prescribed, the NMF algorithm splits stellar halo into low- and high-energy components in the $(E,L_z)$ plane which approximately correspond to the accreted and \textit{in-situ} halo respectively. We use these two components to define a new boundary between the \textit{in-situ} and the accreted stellar halo. Moreover, we calculate the components fractional contribution to the stellar halo as a function of energy, galactocentric spherical radius, height, and galactocentric cylindrical radius. Using a stellar halo defined by kinematic cuts, we find that the halo transitions from \textit{in-situ} dominated to accretion dominated at $E \approx -1.67 \times 10^5$ (km/s)$^2$ (using the potential in McMillan 2017), and at $(r,z,R) \approx (8.7, 3.0, 8.1)$ kpc. The low-energy component is found to span a range of [Al/Fe] that falls beyond the typically accepted \textit{in-situ} floor of [Al/Fe] $=0$. Upon prescribing more components to the NMF model, we find hints of the existence of overlapping chemical evolution sequences that other techniques struggle to find. We also examine features within these components that resemble known substructures in the halo, such as \textit{Eos} and \textit{Aurora}. This work provides insight into their origin and the part they play in the Milky Way's formation., Comment: 17 pages, 15 figures, submitted to MNRAS. Comments welcome!

X-ray polarization is a unique new probe of the particle acceleration in astrophysical jets made possible through the Imaging X-ray Polarimetry Explorer. Here we report on the first dense X-ray polarization monitoring campaign on the blazar Mrk 421. Our observations were accompanied by an even denser radio and optical polarization campaign. We find significant short-timescale variability in both X-ray polarization degree and angle, including a $\sim90^\circ$ angle rotation about the jet axis. We attribute this to random variations of the magnetic field, consistent with the presence of turbulence but also unlikely to be explained by turbulence alone. At the same time, the degree of lower-energy polarization is significantly lower and shows no more than mild variability. Our campaign provides further evidence for a scenario in which energy-stratified shock-acceleration of relativistic electrons, combined with a turbulent magnetic field, is responsible for optical to X-ray synchrotron emission in blazar jets., Comment: 23 pages, including 8 pages of appendices. 12 figures, 3 tables. Submitted to ApJ

Thanks to their record high refractive index and giant optical anisotropy, van der Waals (vdW) materials have accelerated the development of nanophotonics. However, traditional high refractive index materials, such as titanium dioxide (TiO2), still dominate in the most important visible range. This is due to the current lack of transparent vdW materials across the entire visible spectrum. In this context, we propose that germanium disulfide (GeS2) could offer a significant breakthrough. With its high refractive index, negligible losses, and biaxial optical anisotropy across the whole visible range, GeS2 has the potential to complement TiO2 and close the application gap of vdW materials in the visible spectrum. The addition of GeS2 could have a profound impact on the design of van der Waals nanophotonic circuits for any operation wavelength from ultraviolet to infrared, emphasizing the significance of the potential impact of GeS2 on the field of nanophotonics., Comment: 15 pages, 4 figures

SN 2023tsz is a Type Ibn supernova (SNe Ibn) discovered in an extremely low-mass host. SNe Ibn are an uncommon subtype of stripped-envelope core-collapse SNe. They are characterised by narrow helium emission lines in their spectra and are believed to originate from the collapse of massive Wolf-Rayet (WR) stars, though their progenitor systems still remain poorly understood. In terms of energetics and spectrophotometric evolution, SN 2023tsz is largely a typical example of the class, although line profile asymmetries in the nebular phase are seen, which may indicate the presence of dust formation or unshocked circumstellar material. Intriguingly, SN 2023tsz is located in an extraordinarily low-mass host galaxy that is in the 2nd percentile for SESN host masses and star formation rates (SFR). The host has a radius of 1.0 kpc, a $g$-band absolute magnitude of $-12.73$, and an estimated metallicity of $\log(Z_{*}/Z_{\odot}$) = $-1.56$. The SFR and metallicity of the host galaxy raise questions about the progenitor of SN 2023tsz. The low SFR suggests that a star with sufficient mass to evolve into a WR would be uncommon in this galaxy. Further, the very low-metallicity is a challenge for single stellar evolution to enable H and He stripping of the progenitor and produce a SN Ibn explosion. The host galaxy of SN 2023tsz adds another piece to the ongoing puzzle of SNe Ibn progenitors, and demonstrates that they can occur in hosts too faint to be observed in contemporary sky surveys at a more typical SN Ibn redshift., Comment: 14 pages, 10 figures, submitted to MNRAS

A search for the $K^{+} \to \pi^{0}\pi^{0}\pi^{0}e^+\nu$ decay is performed by the OKA collaboration. The search is based on $3.65 \times 10^9 ~ K^+$ decays. No signal is observed. The upper limit set is $BR(K^{+} \to \pi^{0}\pi^{0}\pi^{0}e^+\nu) < 5.4\times 10^{-8} ~ 90\%$ CL, 65 times lower than the one currently listed by PDG.

We propose and numerically investigate a novel compact planar microcavity design based on a high-index dielectric slab waveguide with embedded monolayer semiconductor. In comparison to more traditional vertical Bragg microcavities, our design relies on the transmission of guided optical modes and achieves strong exciton-photon coupling in a chip-compatible and compact geometry with sub-100 nm thickness. We show that Rabi splitting values of more than 70 meV can be obtained in planar microcavities with the total length below 5 um. Further, we reveal the dependence of Rabi splitting on the dimensions of the structure and explain it with a simple theoretical model. Our results contribute towards the development of novel compact 2D semiconductor-based components for integrated photonic circuits.

By using the Efetov's super-symmetric formalism we computed analytically the mean spectral density $\rho(E)$ for the L\'evy and the L\'evy -Rosenzweig-Porter random matrices which off-diagonal elements are strongly non-Gaussian with power-law tails. This makes the standard Hubbard-Stratonovich transformation inapplicable to such problems. We used, instead, the functional Hubbard-Stratonovich transformation which allowed to solve the problem analytically for large sizes of matrices. We show that $\rho(E)$ depends crucially on the control parameter that drives the system through the transition between the ergodic and the fractal phases and it can be used as an order parameter., Comment: 16 pages, 10 figures

Polarization mode dispersion (PMD) in optical fibers poses a major challenge for maintaining the fidelity of quantum states for quantum communications. In this work, a comprehensive model linking the probability of quantum measurement errors (infidelity) to PMD is developed and validated by experimental measurements of differential group delay and quantum bit error rate (QBER). Our research proposes effective methods to mitigate PMD effects for broadband entangled photons and evaluates the impact of higher-order PMD effects. The model provides an experimentally verified framework for the optimization of commercial quantum key distribution systems in deployed fiber optic lines., Comment: 11 pages, 8 figures, research paper

X-ray pulsars experiencing extreme mass accretion rates can produce neutrino emission in the MeV energy band. Neutrinos in these systems are emitted in close proximity to the stellar surface and subsequently undergo gravitational bending in the space curved by a neutron star. This process results in the formation of a distinct beam pattern of neutrino emission and gives rise to the phenomenon of neutrino pulsars. The energy flux of neutrinos, when averaged over the neutron star's pulsation period, can differ from the isotropic neutrino energy flux, which impacts the detectability of bright pulsars in neutrinos. We investigate the process of neutrino beam pattern formation, accounting for neutron star transparency to neutrinos and gravitational bending. Based on simulated neutrino beam patterns, we estimate the potential difference between the actual and apparent neutrino luminosity. We show that the apparent luminosity can greatly exceed the actual luminosity, albeit only in a small fraction of cases, depending on the specific equation of state and the mass of the star. For example, the amplification can exceed a factor of ten for $\approx0.05\%$ of typical neutron stars with mass of $1.4\,M_\odot$. Strong amplification is less probable for neutron stars of higher mass. In the case of strange stars, a fraction of high energy neutrinos can be absorbed and the beam pattern, as well as the amplification of apparent neutrino luminosity, depend on neutrino energy., Comment: submitted to MNRAS, 13 pages, 9 figures

We present multiwavelength polarization measurements of the luminous blazar Mrk~501 over a 14-month period. The 2--8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100-ks observations spanning from 2022 March to 2023 April. Each IXPE observation was accompanied by simultaneous X-ray data from NuSTAR, Swift/XRT, and/or XMM-Newton. Complementary optical-infrared polarization measurements were also available in the B, V, R, I, and J bands, as were radio polarization measurements from 4.85 GHz to 225.5 GHz. Among the first five IXPE observations, we did not find significant variability in the X-ray polarization degree and angle with IXPE. However, the most recent sixth observation found an elevated polarization degree at $>3\sigma$ above the average of the other five observations. The optical and radio measurements show no apparent correlations with the X-ray polarization properties. Throughout the six IXPE observations, the X-ray polarization degree remained higher than, or similar to, the R-band optical polarization degree, which remained higher than the radio value. This is consistent with the energy-stratified shock scenario proposed to explain the first two IXPE observations, in which the polarized X-ray, optical, and radio emission arises from different regions., Comment: Accepted for publication in ApJ

We analyze high-resolution hydrodynamics simulations of an isolated disk dwarf galaxy with an explicit model for unresolved turbulence and turbulence-based star formation prescription. We examine the characteristic values of the star formation efficiency per free-fall time, $\epsilon_\mathrm{ff}$, and its variations with local environment properties, such as metallicity, UV flux, and surface density. We show that the star formation efficiency per free-fall time in $\approx 10$ pc star-forming regions of the simulated disks has values in the range $\epsilon_\mathrm{ff}\approx 0.01-0.1$, similar to observational estimates, with no trend with metallicity and only a weak trend with the UV flux. Likewise, $\epsilon_{\rm ff}$ estimated using projected patches of 500 pc size does not vary with metallicity and shows only a weak trend with average UV flux and gas surface density. The characteristic values of $\epsilon_\mathrm{ff}\approx 0.01-0.1$ arise naturally in the simulations via the combined effect of dynamical gas compression and ensuing stellar feedback that injects thermal and turbulent energy. The compression and feedback regulate the virial parameter, $\alpha_\mathrm{vir}$, in star-forming regions, limiting it to $\alpha_\mathrm{vir}\approx 3-10$. Turbulence plays an important role in the universality of $\epsilon_\mathrm{ff}$ because turbulent energy and its dissipation are not sensitive to metallicity and UV flux that affect thermal energy. Our results indicate that the universality of observational estimates of $\epsilon_\mathrm{ff}$ can be plausibly explained by the turbulence-driven and feedback-regulated properties of star-forming regions., Comment: Version accepted for publication in the Open Journal of Astrophysics

Although the standard $\Lambda$+Cold Dark Matter ($\Lambda$CDM) model is well tested on large scales, the primordial power spectrum may deviate from the $\Lambda$CDM spectrum on small scales due to specific dark matter properties or alternative inflationary models. These deviations affect the formation of dark matter structure, which subsequently leads to different observable properties of galaxies. In this work, we study the impact of a blue and red tilted power spectrum on the central density of dwarf galaxies. To do this, we model densities of dwarf galaxies using a combination of high-resolution numerical simulations and galaxy formation model. The model galaxies in $\Lambda$CDM are consistent with observations of 41 faint dwarf satellite galaxies of the Milky Way. The deviations from the $\Lambda$CDM power spectrum are constrained by the central matter densities of dwarf galaxies, which set stringent constraints on the possible small-scale tilt of the primordial power spectrum, improving on the current limits. Moreover, similar analysis can be applied to test any feature in the power spectrum at small scales between $k\sim 10-100$~Mpc$^{-1}$., Comment: 12 pages, 7 figures; Comments are welcome

We present the first X-ray spectropolarimetric results for Cygnus X-1 in its soft state from a campaign of five IXPE observations conducted during 2023 May-June. Companion multiwavelength data during the campaign are likewise shown. The 2-8 keV X-rays exhibit a net polarization degree PD=1.99%+/-0.13% (68% confidence). The polarization signal is found to increase with energy across IXPE's 2-8 keV bandpass. The polarized X-rays exhibit an energy-independent polarization angle of PA=-25.7+/-1.8 deg. East of North (68% confidence). This is consistent with being aligned to Cyg X-1's AU-scale compact radio jet and its pc-scale radio lobes. In comparison to earlier hard-state observations, the soft state exhibits a factor of 2 lower polarization degree, but a similar trend with energy and a similar (also energy-independent) position angle. When scaling by the natural unit of the disk temperature, we find the appearance of a consistent trendline in the polarization degree between soft and hard states. Our favored polarimetric model indicates Cyg X-1's spin is likely high (a* above ~0.96). The substantial X-ray polarization in Cyg X-1's soft state is most readily explained as resulting from a large portion of X-rays emitted from the disk returning and reflecting off the disk surface, generating a high polarization degree and a polarization direction parallel to the black hole spin axis and radio jet. In IXPE's bandpass, the polarization signal is dominated by the returning reflection emission. This constitutes polarimetric evidence for strong gravitational lensing of X-rays close to the black hole., Comment: 20 pages, accepted for publication in ApJL

Quantum key distribution (QKD) via satellite links is the only currently viable solution to create quantum-backed secure communication at a global scale. To achieve intercontinental coverage with available technology one must adopt a "flying trusted node" paradigm, in which users fully trust the satellite platform. Here, inspired by the concept of distributed secret sharing and the imminent projected launch of several QKD-equipped satellites, we propose a parallel trusted node approach, in which key distribution is mediated by several satellites in parallel. This has the effect of distributing the trust, removing single points of failure and reducing the necessary assumptions. In addition, we discuss the versatility that an optical ground station should provide to execute such a protocol and, in general, to be fully integrated into a multi-party global quantum network., Comment: 6 pages, 2 figures

We present results from a five-month-long observing campaign of the unusual transient AT 2022fnm, which displays properties common to both luminous red novae (LRNe) and intermediate-luminosity red transients (ILRTs). Although its photometric evolution is broadly consistent with that of LRNe, no second peak is apparent in its light curve, and its spectral properties are more reminiscent of ILRTs. It has a fairly rapid rise time of 5.3$\pm$1.5 d, reaching a peak absolute magnitude of $-12.7\pm$0.1 (in the ATLAS $o$ band). We find some evidence for circumstellar interaction, and a near-infrared excess becomes apparent at approximately +100 d after discovery. We attribute this to a dust echo. Finally, from an analytical diffusion toy model, we attempted to reproduce the pseudo-bolometric light curve and find that a mass of $\sim$4 M$_\odot$ is needed. Overall, the characteristics of AT 2022fnm are consistent with a weak stellar eruption or an explosion reminiscent of low-energy type IIP supernovae, which is compatible with expectations for ILRTs., Comment: Accepted to A&A

Discovery of pulsations from a number of ULXs proved that accretion onto neutron stars can produce luminosities exceeding the Eddington limit by several orders of magnitude. The conditions necessary to achieve such high luminosities as well as the exact geometry of the accretion flow in the neutron star vicinity are, however, a matter of debate. The pulse phase-resolved polarization measurements that became possible with the launch of the Imaging X-ray Polarimetry Explorer (IXPE) can be used to determine the pulsar geometry and its orientation relative to the orbital plane. They provide an avenue to test different theoretical models of ULX pulsars. In this paper we present the results of three IXPE observations of the first Galactic ULX pulsar Swift J0243.6+6124 during its 2023 outburst. We find strong variations in the polarization characteristics with the pulsar phase. The average polarization degree increases from about 5% to 15% as the flux dropped by a factor of three in the course of the outburst. The polarization angle (PA) as a function of the pulsar phase shows two peaks in the first two observations, but changes to a characteristic sawtooth pattern in the remaining data set. This is not consistent with a simple rotating vector model. Assuming the existence of an additional constant polarized component, we were able to fit the three observations with a common rotating vector model and obtain constraints on the pulsar geometry. In particular, we find the pulsar angular momentum inclination with respect to the line of sight of 15-40 deg, the magnetic obliquity of 60-80 deg, and the pulsar spin position angle of -50 deg, which significantly differs from the constant component PA of about 10 deg. Combining these X-ray measurements with the optical PA, we find evidence for at least a 30 deg misalignment between the pulsar angular momentum and the binary orbital axis., Comment: 13 pages, 10 figures, version published in A&A

We present estimates of the ultraviolet (UV) and Lyman continuum flux density contributed by galaxies of luminosities from $M_{\rm UV}\approx -25$ to $M_{\rm UV}=-4$ at redshifts $5\leq z\leq 10$ using a galaxy formation model that reproduces properties of local dwarf galaxies down to the luminosities of the ultra-faint satellites. We characterize the UV luminosity function (LF) of galaxies and their abundance as a function of the ionizing photon emission rate predicted by our model and present accurate fitting functions describing them. Although the slope of the LF becomes gradually shallower with decreasing luminosity due to feedback-driven outflows, the UV LF predicted by the model remains quite steep at the luminosities $M_{\rm UV}\lesssim -14$. After reionization, the UV LF flattens at $M_{\rm UV}\gtrsim -12$ due to UV heating of intergalactic gas. However, before reionization, the slope of the LF remains steep and approximately constant from $M_{\rm UV}\approx -14$ to $M_{\rm UV}=-4$. We show that for a constant ionizing photon escape fraction the contribution of faint galaxies with $M_{\rm UV}>-14$ to the UV flux and ionizing photon budget is $\approx 40-60\%$ at $z>7$ and decreases to $\approx 20\%$ at $z=6$. Before reionization, even ultra-faint galaxies of $M_{\rm UV}>-10$ contribute $\approx 10-25\%$ of ionizing photons. If the escape fraction increases strongly for fainter galaxies, the contribution of $M_{\rm UV}>-14$ galaxies before reionization increases to $\approx 60-75\%$. Our results imply that dwarf galaxies fainter than $M_{\rm UV}=-14$, beyond the James Webb Space Telescope limit, contribute significantly to the UV flux density and ionizing photon budget before reionization alleviating requirements on the escape fraction of Lyman continuum photons., Comment: 13 pages, 9 figures, revised in response to referee, typo in eq 4 fixed. Published in the Open Journal of Astrophysics

Star formation estimates based on the counting of YSOs is commonly applied to nearby star-forming regions in the Galaxy. With this method, the SFRs are measured using the counts of YSOs in a particular protostellar Class, a typical protostellar mass, and the lifetime associated with this Class. However, the assumptions underlying the validity of the method such as that of a constant star formation history (SFH) and whether the method is valid for all protostellar Classes has never been fully tested. In this work, we use Monte Carlo models to test the validity of the method. We build synthetic clusters in which stars form at times that are randomly drawn from a specified SFH. The latter is either constant or time-dependent with a burst like behavior. The masses of the protostars are randomly drawn from an IMF which can be either similar to that of the Milky Way field or be variable . For each star in every cluster, the lifetimes associated with the different protostellar classes are also randomly drawn from Gaussian distribution functions centered around their most likely value as suggested by the observations. We find that only the SFR derived using the Class 0 population can reproduce the true SFR at all epochs, and this is true irrespective of the shape of the SFH. For a constant SFH, the SFR derived using the more evolved populations of protostars (Classes I, F, II, and III) reproduce the real SFR only at later epochs which correspond to epochs at which their numbers have reached a steady state. For a time-dependent burst-like SFH, all SFR estimates based on the number counts of the evolved populations fail to reproduce the true SFR. We also show how the offsets between Class I and Class II based SFRs and the true SFR plotted as a function of the number ratios of Class I and Class II versus Class III YSOs can be used in order to constrain the SFH of observed molecular clouds., Comment: Accepted for publication in Astronomy & Astrophysics. Missing references included. Matches the published version

We report on the detection of X-ray polarisation in the black-hole X-ray binary Swift J1727.8$-$1613 during its dim hard spectral state by the Imaging X-ray Polarimetry Explorer (IXPE). This is the first detection of X-ray polarisation at the transition from the soft to the hard state in an X-ray binary. We find an averaged 2$-$8 keV polarisation degree of (3.3 ${\pm}$ 0.4) % and a corresponding polarisation angle of 3{\deg} ${\pm}$ 4{\deg}, which matches the polarisation detected during the rising stage of the outburst, in September$-$October 2023, within 1${\sigma}$ uncertainty. The observational campaign complements previous studies of this source and enables comparison of the X-ray polarisation properties of a single transient across the X-ray hardness-intensity diagram. The complete recovery of the X-ray polarisation properties, including the energy dependence, came after a dramatic drop in the X-ray polarisation during the soft state. The new IXPE observations in the dim hard state at the reverse transition indicate that the accretion properties, including the geometry of the corona, appear to be strikingly similar to the bright hard state during the outburst rise despite the X-ray luminosities differing by two orders of magnitude., Comment: 8 pages, 6 figures. Accepted for publication in A&A Letters

The reflection of X-ray radiation produced near a compact object from its stellar companion contributes to the orbital variability of polarization in X-ray binaries. The X-rays are reflected mainly via Thomson scattering resulting in a high polarization. The orbital variability of the polarization strongly depends on the inclination and the orbital parameters allowing us to constrain them. To explore this phenomenon, we present analytical single-scattering models for the polarized reflection. We find that while diluted by the direct emission, the reflection can produce a polarization degree of about 1$\%$ in the case of a large reflection albedo. We fitted the orbital variations of the X-ray polarization observed by the Imaging X-ray Polarimetry Explorer from an accreting weakly magnetized neutron star `clocked burster' GS 1826$-$238 and found that the amplitude of the variations is too large to be primarily caused by the companion star. The polarized reflection is more significant if the compact object is obscured from the observer, and thus it should be more easily observable in certain high-inclination targets., Comment: 8 pages, 10 figures, submitted to A&A

We discuss the dependence of the critical properties of the Anderson model on the dimension $d$ in the language of $\beta$-function and renormalization group recently introduced in Ref.[arXiv:2306.14965] in the context of Anderson transition on random regular graphs. We show how in the delocalized region, including the transition point, the one-parameter scaling part of the $\beta$-function for the fractal dimension $D_{1}$ evolves smoothly from its $d=2$ form, in which $\beta_2\leq 0$, to its $\beta_\infty\geq 0$ form, which is represented by the regular random graph (RRG) result. We show how the $\epsilon=d-2$ expansion and the $1/d$ expansion around the RRG result can be reconciled and how the initial part of a renormalization group trajectory governed by the irrelevant exponent $y$ depends on dimensionality. We also show how the irrelevant exponent emerges out of the high-gradient terms of expansion in the nonlinear sigma-model and put forward a conjecture about a lower bound for the fractal dimension. The framework introduced here may serve as a basis for investigations of disordered many-body systems and of more general non-equilibrium quantum systems., Comment: 13 pages, 12 figures. Comments are welcome!

Integration of 2D semiconductors with photonic crystal slabs provides an attractive approach to achieve strong light--matter coupling and exciton-polariton formation in a planar chip-compatible geometry. However, for the development of practical devices, it is crucial that the polariton excitations in the structure are easily tunable and exhibit strong nonlinear response. Here we study neutral and charged exciton-polaritons in an electrostatically gated planar photonic crystal slab with an embedded monolayer semiconductor MoSe$_2$ and experimentally demonstrate strong polariton nonlinearity, which can be tuned via gate voltage. We find that modulation of dielectric environment within the photonic crystal results in the formation of two distinct resonances with significantly different nonlinear response, which enables optical switching with ultrashort laser pulses. Our results open new avenues towards development of active polaritonic devices in a compact chip-compatible implementation.

We present optical and near-infrared photometry and spectroscopy of SN 2023aew and our findings on its remarkable properties. This event, initially resembling a Type IIb supernova (SN), rebrightens dramatically $\sim$90 d after the first peak, at which time its spectrum transforms into that of a SN Ic. The slowly evolving spectrum specifically resembles a post-peak SN~Ic with relatively low line velocities even during the second rise. The second peak, reached 119 d after the first peak, is both more luminous ($M_r = -18.75\pm0.04$ mag) and much broader than those of typical SNe Ic. Blackbody fits to SN 2023aew indicate that the photosphere shrinks almost throughout its observed evolution, and the second peak is caused by an increasing temperature. Bumps in the light curve after the second peak suggest interaction with circumstellar matter (CSM) or possibly accretion. We consider several scenarios for producing the unprecedented behavior of SN 2023aew. Two separate SNe, either unrelated or from the same binary system, require either an incredible coincidence or extreme fine-tuning. A pre-SN eruption followed by a SN requires an extremely powerful, SN-like eruption (consistent with $\sim$10$^{51}$ erg) and is also disfavored. We therefore consider only the first peak a true stellar explosion. The observed evolution is difficult to reproduce if the second peak is dominated by interaction with a distant CSM shell. A delayed internal heating mechanism is more likely, but emerging embedded interaction with a CSM disk should be accompanied by CSM lines in the spectrum, which are not observed, and is difficult to hide long enough. A magnetar central engine requires a delayed onset to explain the long time between the peaks. Delayed fallback accretion onto a black hole may present the most promising scenario, but we cannot definitively establish the power source., Comment: 21 pages + 14 appendix pages, 16 figures + 6 appendix figures. Accepted by A&A

We present an extensive analysis of the optical and UV properties of AT2023clx, the closest TDE to date, that occurred in the nucleus of the interacting LINER galaxy, NGC3799 (z=0.01107). After correcting for the host reddening (E(B-V) = 0.179 mag), we find its peak absolute g-band magnitude to be -18.03{+/-}0.07 mag, and its peak bolometric luminosity to be L=(1.57{+/-}0.19)x10^43 erg/s. AT2023clx displays several distinctive features: first, it rose to peak within 10.4{+/-}2.5 days, making it the fastest rising TDE to date. Our SMBH mass estimate of M_BH ~ 10^6 Msol rules out the possibility of an intermediate-mass BH as the reason for the fast rise. Dense spectral follow-up reveals a blue continuum that cools slowly and broad Balmer and HeII lines as well as weak HeI 5876,6678 emission features that are typically seen in TDEs. The early, broad (width ~ 15000 km/s) profile of Ha matches theoretical expectations from an optically thick outflow. A flat Balmer decrement (~ 1.58) suggests that the lines are collisionally excited rather than being produced via photoionisation, in contrast to typical active galactic nuclei. A second distinctive feature, seen for the first time in TDE spectra, is a sharp, narrow emission peak at a rest wavelength of ~ 6353 A. This feature is clearly visible up to 10d post-peak; we attribute it to clumpy material preceding the bulk outflow, which manifests as a high-velocity component of Ha (-9584 km/s). Its third distinctive feature is the rapid cooling during the first ~ 20 days after peak, reflected as a break in the temperature evolution. Combining these findings, we propose a scenario for AT2023clx involving the disruption of a very low-mass star (<=0.1 Msol) with an outflow launched in our line of sight and with disruption properties that led to efficient circularisation and prompt accretion disc formation, observed through a low-density photosphere., Comment: Accepted for publication in Astronomy and Astrophysics (in production; official acceptance date: 28/06/2024)

X-ray binary systems consist of a companion star and a compact object in close orbit. Thanks to their copious X-ray emission, these objects have been studied in detail using X-ray spectroscopy and timing. The inclination of these systems is a major uncertainty in the determination of the mass of the compact object using optical spectroscopic methods. In this paper, we present a new method to constrain the inclination of X-ray binaries, which is based on the modeling of the polarization of X-rays photons produced by a compact source and scattered off the companion star. We describe our method and explore the potential of this technique in the specific case of the low mass X-ray binary GS 1826-238 observed by the Imaging X-ray Polarimetry Explorer (IXPE) observatory., Comment: 9 pages, 8 figures, accepted for publication in ApJ

We report on an observational campaign on the bright black hole X-ray binary Swift J1727.8$-$1613 centered around five observations by the Imaging X-ray Polarimetry Explorer (IXPE). These observations track for the first time the evolution of the X-ray polarization of a black hole X-ray binary across a hard to soft state transition. The 2--8 keV polarization degree decreased from $\sim$4\% to $\sim$3\% across the five observations, but the polarization angle remained oriented in the North-South direction throughout. Based on observations with the Australia Telescope Compact Array (ATCA), we find that the intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization. Assuming the radio polarization aligns with the jet direction (which can be tested in the future with higher spatial resolution images of the jet), our results imply that the X-ray corona is extended in the disk plane, rather than along the jet axis, for the entire hard intermediate state. This in turn implies that the long ($\gtrsim$10 ms) soft lags that we measure with the Neutron star Interior Composition ExploreR (NICER) are dominated by processes other than pure light-crossing delays. Moreover, we find that the evolution of the soft lag amplitude with spectral state does not follow the trend seen for other sources, implying that Swift J1727.8$-$1613 is a member of a hitherto under-sampled sub-population., Comment: Accepted for publication in ApJ. 21 pages, 8 figures

Cir X-1 is a neutron star X-ray binary characterized by strong variations in flux during its eccentric $\sim$16.6 days orbit. There are also strong variations in the spectral state, and historically it has shown both atoll and Z state properties. We observed the source with the Imaging X-ray Polarimetry Explorer during two orbital segments, 6 days apart, for a total of 263~ks. We find an X-ray polarization degree in these segments of $1.6\%\pm0.3\%$ and $1.4\%\pm0.3\%$ at polarization angles of $37^\circ\pm5^\circ$ and $-12^\circ\pm7^\circ$, respectively. Thus we observed a rotation of the polarization angle by $49^\circ\pm8^\circ$ along the orbit. Because variations of accretion flow, and then of the hardness ratio, are expected during the orbit, we also studied the polarization binned in hardness ratio, and found the polarization angle differing by $67^\circ\pm11^\circ$ between the lowest and highest values of the hardness ratio. We discuss possible interpretations of this result that could indicate a possible misalignment between the symmetry axes of the accretion disk and the Comptonizing region caused by the misalignment of the neutron star's angular momentum with respect to the orbital one., Comment: 15 pages, 6 figures, Accepted for publication in ApJL

Twist-controlled moire superlattices (MS) have emerged as a versatile platform in which to realize artificial systems with complex electronic spectra. Bernal-stacked bilayer graphene (BLG) and hexagonal boron nitride (hBN) form an interesting example of the MS that has recently featured a set of unexpected behaviors, such as unconventional ferroelectricity and electronic ratchet effect. Yet, the understanding of the BLG/hBN MS electronic properties has, at present, remained fairly limited. Here we develop a multi-messenger approach that combines standard magnetotransport techniques with low-energy sub-THz excitation to get insights into the properties of this MS. We show that BLG/hBN lattice alignment results in the emergence of compensated semimetals at some integer fillings of the moire bands separated by van Hove singularities where Lifshitz transition occurs. A particularly pronounced semimetal develops when 8 electrons reside in the moire unit cell, where coexisting high-mobility electron and hole systems feature a strong magnetoresistance reaching 2350 % already at B=0.25 T. Next, by measuring the THz-driven Nernst effect in remote bands, we observe valley splitting, pointing to an orbital magnetization characterized by a strongly enhanced effective g-factor of 340. Last, using THz photoresistance measurements, we show that the high-temperature conductivity of the BLG/hBN MS is limited by electron-electron umklapp processes. Our multi-facet analysis introduces THz-driven magnetotransport as a convenient tool to probe the band structure and interaction effects in vdW materials and provides a comprehension of the BLG/hBN MS., Comment: 9 pages, 4 figures

The $K^{+} \to \pi^{+}\pi^{0}\pi^{0}\gamma$ decay is observed by the OKA collaboration. The branching ratio is measured to be $(4.1 \pm 0.9(stat) \pm 0.4(syst))\times 10^{-6}$. The branching ratio and $\gamma$ energy spectrum are consistent with ChPT prediction., Comment: arXiv admin note: text overlap with arXiv:2310.16429

A high statistics data sample of the $K^{+}$ decays is recorded by the OKA collaboration. A missing mass analysis is performed to search for a light invisible pseudoscalar axion-like particle (ALP) $a$ in the decay $K^{+} \to \pi^{+} \pi^{0} a$. No signal is observed, the upper limits for the branching ratio of the decay are calculated. The $90\%$ confidence level upper limit is changing from $2.5\cdot10^{-6}$ to $2\cdot10^{-7}$ for the ALP mass from 0 to 200 MeV/$c^{2}$, except for the region of $\pi^{0}$ mass, where the upper limit is $4.4\cdot10^{-6}$., Comment: 6 pages, 6 figures

For the search of the chiral magnetic effect (CME), STAR previously presented the results from isobar collisions (${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$, ${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) obtained through a blind analysis. The ratio of results in Ru+Ru to Zr+Zr collisions for the CME-sensitive charge-dependent azimuthal correlator ($\Delta\gamma$), normalized by elliptic anisotropy ($v_{2}$), was observed to be close to but systematically larger than the inverse multiplicity ratio. The background baseline for the isobar ratio, $Y = \frac{(\Delta\gamma/v_{2})^{\text{Ru}}}{(\Delta\gamma/v_{2})^{\text{Zr}}}$, is naively expected to be $\frac{(1/N)^{\text{Ru}}}{(1/N)^{\text{Zr}}}$; however, genuine two- and three-particle correlations are expected to alter it. We estimate the contributions to $Y$ from those correlations, utilizing both the isobar data and HIJING simulations. After including those contributions, we arrive at a final background baseline for $Y$, which is consistent with the isobar data. We extract an upper limit for the CME fraction in the $\Delta\gamma$ measurement of approximately $10\%$ at a $95\%$ confidence level on in isobar collisions at $\sqrt{s_{\text{NN}}} = 200$ GeV, with an expected $15\%$ difference in their squared magnetic fields., Comment: 19 pages, 14 figures

We present X-ray polarimetry observations from the Imaging X-ray Polarimetry Explorer (IXPE) of three low spectral peak and one intermediate spectral peak blazars, namely 3C 273, 3C 279, 3C 454.3, and S5 0716+714. For none of these objects was IXPE able to detect X-ray polarization at the 3$\sigma$ level. However, we placed upper limits on the polarization degree at $\sim$10-30\%. The undetected polarizations favor models where the X-ray band is dominated by unpolarized photons upscattered by relativistic electrons in the jets of blazars, although hadronic models are not completely eliminated. We discuss the X-ray polarization upper limits in the context of our contemporaneous multiwavelength polarization campaigns., Comment: 19 pages, 10 figures, 6 tables; submitted to the Astrophysical Journal

We use a suite of hydrodynamics simulations of the interstellar medium (ISM) within a galactic disk, which include radiative transfer, a non-equilibrium model of molecular hydrogen, and a realistic model for star formation and feedback, to study the structure of the ISM and H$_2$ abundance as a function of local ISM properties. We show that the star formation rate and structure of the ISM are sensitive to the metallicity of the gas with a progressively smoother density distribution with decreasing metallicity. In addition to the well-known trend of the HI-H$_2$ transition shifting to higher densities with decreasing metallicity, the maximum achieved molecular fraction in the interstellar medium drops drastically at $Z \lesssim 0.2 \, Z_\odot$ as the formation time of H$_2$ becomes much longer than a typical lifetime of dense regions of the ISM. We present accurate fitting formulae for both volumetric and projected $f_\mathrm{H_2}$ measured on different scales as a function of gas metallicity, UV radiation field, and gas density. We show that when the formulae are applied to the patches in the simulated galaxy the overall molecular gas mass is reproduced to better than a factor of $\lesssim 1.5$ across the entire range of metallicities and scales. We also show that the presented fit is considerably more accurate than any of the previous $f_\mathrm{H_2}$ models and fitting formulae in the low-metallicity regime. The fit can thus be used for modeling molecular gas in low-resolution simulations and semi-analytic models of galaxy formation in the dwarf and high-redshift regimes., Comment: 17 pages, 12 figures; submitted to ApJ

We investigate the impact of bursty star formation on several galaxy scaling relations of dwarf galaxies using the $\texttt{GRUMPY}$ galaxy formation model. While this model reproduces the star formation rate (SFR)-stellar mass, stellar mass-gas mass, and stellar mass-metallicity relations, the scatter of these relations in the original model is smaller than observed. We explore the effects of additional stochasticity of SFR on the scaling relations using a model that reproduces the level of SFR burstiness in high-resolution zoom-in simulations. The additional SFR stochasticity increases the scatter in the SFR-stellar mass relation to a level similar to that exhibited by most nearby dwarf galaxies. The most extreme observed starbursting dwarfs, however, require higher levels of SFR stochasticity. We find that bursty star formation increases the scatter in the colour-magnitude distribution (CMD) for brighter dwarf galaxies $(M_V < -12)$ to the observed level, but not for fainter ones for which scatter remains significantly smaller than observed. This is due to the predominant old stellar populations in these faint model galaxies and their generally declining SFR over the past 10 Gyrs, rather than quenching caused by reionization. We examine the possibility that the colour scatter is due to scatter in metallicity, but show that the level of scatter required leads to an overestimation of scatter in the metallicity-mass relation. This illustrates that the scatter of observed scaling relations in the dwarf galaxy regime represents a powerful constraint on the properties of their star formation., Comment: 14 pages, 8 figures, submitted to MNRAS

We report the first detection of the X-ray polarization of the bright transient Swift J1727.8-1613 with the Imaging X-ray Polarimetry Explorer. The observation was performed at the beginning of the 2023 discovery outburst, when the source resided in the bright hard state. We find a time- and energy-averaged polarization degree of 4.1%+/-0.2% and a polarization angle of 2.2+/-1.3 degrees (errors at 68% confidence level; this translates to about 20-sigma significance of the polarization detection). This finding suggests that the hot corona emitting the bulk of the detected X-rays is elongated, rather than spherical. The X-ray polarization angle is consistent with that found in sub-mm wavelengths. Since the sub-mm polarization was found to be aligned with the jet direction in other X-ray binaries, this indicates that the corona is elongated orthogonal to the jet., Comment: 10 pages, 3 figures, submitted