arXiv subjects

S. Rukdee

Publications and source records attributed to S. Rukdee.

9 recordsLinked to original sources

arXiv2026

The SRG/eROSITA All-Sky Survey DR2: Cumulative X-ray catalogues from the first three surveys and multi-wavelength counterparts in the western Galactic hemisphere

The eROSITA telescope array on board the Spektrum-Roentgen-Gamma (SRG) mission began its all-sky survey program in December 2019, scanning the sky at an approximately six-month cadence. Here, we present new catalogues of point-like and extended X-ray sources derived from the first three completed eROSITA all-sky surveys (eRASS:3), covering the western Galactic hemisphere, which Germany's eROSITA consortium holds proprietary rights for. We describe the observational strategy, data processing, and analysis pipelines. We also characterise the resulting X-ray source populations. The eRASS:3 main catalogue contains nearly two million sources, including 1,911,744 point-like and 63,796 extended sources, detected in the 0.2-2.3 keV energy band, eROSITA's most sensitive energy window. This volume has doubled the X-ray source content of eRASS1 and provides a comprehensive census of X-ray-emitting objects across diverse astrophysical classes. We also released a secondary hard catalogue of nearly 15,000 sources detected in the harder 2.3-5.0 keV energy band. In addition, we released six catalogues, three for the main sample and three for the hard sample, in which we identify and classify the optical and infrared counterparts of eRASS:3 point-like sources. This approach delivers a homogeneous, high-quality identification of mostly extragalactic X-ray emitters; for example, we estimate that approximately 88% of the 1.4 million counterparts identified within the footprints of the Legacy Survey Imaging for DESI are extragalactic sources. This work has enabled the generation of samples optimised for completeness and purity, while expanding the discovery space for rare populations. The second data release (DR2) of the SRG/eROSITA all-sky survey is a catalogue-only release, comprising the catalogues presented in this work together with an updated version of the eROSITA upper flux limit server.

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arXiv2026

Probing habitable regions with SRG/eROSITA

Stellar high-energy radiation is a key driver of atmospheric erosion and evolution in exoplanets, directly affecting their long-term habitability. We present a comprehensive study on stellar high-energy radiation and its impact on exoplanetary atmospheres, leveraging data from the \textit{SRG/eROSITA} all-sky survey. Our sample consists of 3750 main-sequence stars identified by cross-matching with \textit{Gaia} DR3. Utilizing X-ray spectral fits from the \textit{eROSITA} catalog, we computed X-ray ($L_X$) and combined extreme-ultraviolet (EUV) luminosities ($L_{\mathrm{EUV}}$), which we used to derive XUV fluxes at the habitable zone ($F_{\mathrm{XUV,HZ}}$). We find that the majority of stars in our sample are significantly more XUV-active than the Sun, with habitable zone fluxes ranging from $10^0$ to $10^5$ erg~cm$^{-2}$~s$^{-1}$. The ratio of $L_{\mathrm{XUV}}/L_{\mathrm{bol}}$ is found to be higher for cooler, magnetically active stars, highlighting their potentially hazardous nature for planetary atmospheres. Applying the energy-limited escape model, we computed atmospheric mass-loss rates for hypothetical earth-like planets located at the habitable zone of each star. We also present local maps for distances up to $500$~pc of the average XUV flux, revealing ``hazard zones'' where stellar radiation could significantly influence planetary atmospheric evolution. This work demonstrates the power of X-ray surveys in constraining the high-energy environments of exoplanets and underscores the critical role of stellar activity in planetary habitability.

astro-ph.EP↗

arXiv2025

Is the high-energy environment of K2-18b special?

K2-18b lies near the radius valley that separates super-Earths and sub-Neptunes, marking a key transitional regime in planetary and atmospheric composition. The system offers a valuable opportunity to study how M-dwarf high-energy stellar radiation influences atmospheric stability and the potential for sustaining volatile species, especially important in the context of the upcoming ELT and its ANDES spectrograph. This study characterizes the high-energy environment of K2-18 with X-ray observations from eROSITA, the soft X-ray instrument on the Spectrum-Roentgen-Gamma (SRG) mission, Chandra, and XMM-Newton. We derive a representative 0.2-2 keV X-ray flux with an APEC thermal plasma model fitted with the Bayesian X-ray Analysis (BXA). With the observed X-ray flux from the exoplanet host star, we estimate the photoevaporative mass loss of exoplanet K2-18b using the energy-limited model. In addition, we examine the thermal structure of the system based on a hydrodynamic model. In 100 ks XMM-Newton observation we identified K2-18 as a very faint X-ray source with $\mathrm{F_X = 10^{-15}\ erg\,s^{-1}\,cm^{-2}}$, with an activity level of (Lx/Lbol) $\sim 10^{-5}$. A small flare has been detected during the observation. The planet is irradiated by an X-ray flux of $\mathrm{F_{pl,X} = 12\pm3\ erg\,s^{-1}\,cm^{-2}}$. The X-ray flux measurement of K2-18 gives important limitations for atmospheric escape and photochemical modeling of its exoplanets. Despite its near orbit around an M-dwarf star, K2-18b's low activity level environment suggests that it can retain an atmosphere, supporting recent tentative detections of atmospheres.

astro-ph.EP↗

arXiv2024

The eROSITA DR1 variability catalogue

With its first All-Sky Survey (eRASS1), the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) mission has offered an unprecedented, comprehensive view of the variable X-ray sky. Featuring enhanced sensitivity, broader energy coverage, and improved resolution compared to prior surveys, the eRASS1 Data Release 1 (DR1) catalogue underwent a variability analysis, and in this paper, we performed an advanced variability analysis focusing on a substantial subset of 128,669 sources, all exhibiting a net count exceeding ten. We performed multiple variability tests, utilising conventional normalised excess variance (NEV), maximum amplitude variability (AMP), and Bayesian excess variance methods (bexvar). Within the 128,669 DR1 sources with light curves, our research pinpointed 808 light curves that show hints of variability according to the AMP test, and 298 according to the NEV test. However, after applying suitable thresholds, 90 (123) sources were found to be significantly variable according to the AMP (NEV) tests. In addition, 1342 sources are considered variable according to the Bayesian test bexvar. The total number of unique sources is 1709, and they form the catalogue of variable sources released with this paper. We cross-matched with existing X-ray catalogues and identified 258, 318, 598, and 120 sources in 4XMM DR13, 2SXPS, 2RXS, and CSC2.1, respectively. In this paper, we analyse the variability of eRASS1 sources on a timescale of only a few days. To study the physics of variable sources, we need more deeply pointed observations with other X-ray missions or at least the final depth of the eRASS:8 observations. A substantial 52 % of the eRASS1 variable sources were first discovered with eROSITA. The DR1 variability catalogue is excellent for follow-up observations with telescopes such as XMM-Newton, Chandra, or Swift.

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arXiv2024

X-ray variability of the triplet star system LTT1445 and evaporation history of the exoplanets around its A component

The high-energy environment of the host stars could be deleterious for their planets. It is crucial to ascertain this contextual information to fully characterize the atmospheres of terrestrial exoplanets. We aim to fully characterize a unique triple system, LTT1445, with three known rocky exoplanets around LTT 1445A. The X-ray irradiation and flaring of this system are studied through a new 50 ks Chandra observation, which is divided into 10 ks, 10 ks, and 30 ks segments conducted two days apart, and two months apart, respectively. This is complemented by an archival Chandra observation approximately one year earlier and repeated observations with eROSITA (extended ROentgen Survey with an Imaging Telescope Array), the soft X-ray instrument on the Spectrum-Roentgen-Gamma (SRG) mission, enabling the investigation of X-ray flux behavior across multiple time scales. With the observed X-ray flux from the exoplanet host star A, we estimate the photo-evaporation mass loss of each exoplanet. With the planet modeling package, VPLanet, we predict the evolution and anticipated current atmospheric conditions. Our Chandra observations indicate LTT 1445C as the dominant X-ray source, with additional contribution from LTT 1445B. LTT 1445A, a slowly-rotating star, exhibits no significant flare activity in the new Chandra dataset. Comparing the flux incident on the exoplanets, LTT 1445BC components do not pose a greater threat to the planets orbiting LTT 1445A than the emission from A itself. According to the results from the simulation, LTT 1445Ad might have the capacity to retain its water surface.

astro-ph.EP↗

arXiv2022

First wide field-of-view X-ray observations by a lobster eye focusing telescope in orbit

As a novel X-ray focusing technology, lobster eye micro-pore optics (MPO) feature both a wide observing field of view and true imaging capability, promising sky monitoring with significantly improved sensitivity and spatial resolution in soft X-rays. Since first proposed by Angel (1979), the optics have been extensively studied, developed and trialed over the past decades. In this Letter, we report on the first-light results from a flight experiment of the Lobster Eye Imager for Astronomy ($LEIA$), a pathfinder of the wide-field X-ray telescope of the Einstein Probe mission. The piggyback imager, launched in July 2022, has a mostly un-vignetted field of view of $18.6^\circ \times 18.6^\circ $. Its spatial resolution is in the range of 4$-$7 arcmin in FWHM and the focal spot effective area is 2$-$3 cm$^2$, both showing only mild fluctuations across the field of view. We present images of the Galactic center region, Sco X-1 and the diffuse Cygnus Loop nebular taken in snapshot observations over 0.5$-$4 keV. These are truly wide-field X-ray images of celestial bodies observed, for the first time, by a focusing imaging telescope. Initial analyses of the in-flight data show excellent agreement between the observed images and the on-ground calibration and simulations. The instrument and its characterization are briefly described, as well as the flight experiment. The results provide a solid basis for the development of the present and proposed wide-field X-ray missions using lobster eye MPO.

astro-ph.HE↗

arXiv2022

First light of BEaTriX, the new testing facility for the modular X-ray optics of the ATHENA mission

The Beam Expander Testing X-ray facility (BEaTriX) is a unique X-ray apparatus now operated at the Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Brera (OAB), in Merate, Italy. It has been specifically designed to measure the point spread function (PSF) and the effective area (EA) of the X-ray mirror modules (MMs) of the Advanced Telescope for High-ENergy Astrophysics (ATHENA), based on silicon pore optics (SPO) technology, for verification before integration into the mirror assembly. To this end, BEaTriX generates a broad, uniform, monochromatic, and collimated X-ray beam at 4.51 keV. [...] In BEaTriX, a micro-focus X-ray source with a titanium anode is placed in the focus of a paraboloidal mirror, which generates a parallel beam. A crystal monochromator selects the 4.51 keV line, which is expanded to the final size by a crystal asymmetrically cut with respect to the crystalline planes. [...] After characterization, the BEaTriX beam has the nominal dimensions of 170 mm x 60 mm, with a vertical divergence of 1.65 arcsec and a horizontal divergence varying between 2.7 and 3.45 arcsec, depending on the monochromator setting: either high collimation or high intensity. The flux per area unit varies from 10 to 50 photons/s/cm2 from one configuration to the other. The BEaTriX beam performance was tested using an SPO MM, whose entrance pupil was fully illuminated by the expanded beam, and its focus was directly imaged onto the camera. The first light test returned a PSF and an EA in full agreement with expectations. As of today, the 4.51 keV beamline of BEaTriX is operational and can characterize modular X-ray optics, measuring their PSF and EA with a typical exposure of 30 minutes. [...] We expect BEaTriX to be a crucial facility for the functional test of modular X-ray optics, such as the SPO MMs for ATHENA.

astro-ph.IM↗

arXiv2020

TARdYS: Design and Prototype of an Exoplanet Hunter for TAO using a R6 Echelle Grating

One limitation in characterizing exoplanet candidates is the availability of infrared, high-resolution spectrographs. An important factor in the scarcity of high precision IR spectrographs is the high cost of these instruments. We present a new optical design, which leads to a cost-effective solution. Our instrument is a high-resolution (R=60,000) infrared spectrograph with a R6 Echelle grating and an image slicer. We compare the best possible performance of quasi-Littrow and White Pupil setups, and prefer the latter because it achieves higher image quality. The instrument is proposed for the University of Tokyo Atacama Observatory (TAO) 6.5 m telescope in Chile. The Tao Aiuc high Resolution (d) Y band Spectrograph (TARdYS) covers 0.843-1.117 um. To reduce the cost, we squeeze 42 spectral orders onto a 1K detector with a semi-cryogenic solution. We obtain excellent resolution even when taking realistic manufacturing and alignment tolerances as well as thermal variations into account. In this paper, we present early results from the prototype of this spectrograph at ambient temperature.

astro-ph.IM↗

arXiv2018

Precision stellar radial velocity measurements with FIDEOS at the ESO 1-m telescope of La Silla

We present results from the commissioning and early science programs of FIDEOS, the new high-resolution echelle spectrograph developed at the Centre of Astro Engineering of Pontificia Universidad Catolica de Chile, and recently installed at the ESO 1m telescope of La Silla. The instrument provides spectral resolution R = 43,000 in the visible spectral range 420-800 nm, reaching a limiting magnitude of 11 in V band. Precision in the measurement of radial velocity is guaranteed by light feeding with an octagonal optical fibre, suitable mechanical isolation, thermal stabilisation, and simultaneous wavelength calibration. Currently the instrument reaches radial velocity stability of = 8 m/s over several consecutive nights of observation.

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