arXiv subjects

E. Sonbas

Publications and source records attributed to E. Sonbas.

At least 19 recordsLinked to original sources

arXiv2025

A Combined TESS and Ground-based Study of Transit Timing Variations in HAT-P-16, TOI-1516 and TOI-2046 Systems

We present new results for the hot Jupiters HAT-P-16b, TOI-1516b, and TOI-2046b, based on photometric observations collected using both space- and ground-based facilities. Ground-based data were collected in the 2020-2024 time span with the 0.6 m telescope (ADYU60) located at the Adiyaman University Application and Research Center (Adiyaman, T\"{u}rkiye) and the 1.0 m telescope at the T\"{u}rkiye National Observatory (TUG, T\"{u}rkiye). Through a combination of fits to our ground-based data, the mid-transit times data from TESS and additional data taken from the literature, we present an updated linear ephemeris for each system. Transit timing variations (TTVs) were analyzed using linear, orbital decay, and apsidal precession models. The resulting BIC($\Delta$BIC) values indicate that the orbital decay model is statistically favored for HAT-P-16b and TOI-1516b, while the constant period model is preferred for TOI-2046b. False alarm probabilities (FAPs) were computed to assess the significance of any periodic signals. TOI-1516b displays a strong TTV signal with a FAP (of 0.0001) well below the 0.01 threshold, suggesting a likely dynamical origin that warrants further investigation. The higher FAP value (0.0055) for HAT-P-16b suggests that the case of a possible presence of an additional body in the system is less convincing. In contrast, the much higher FAP value (0.0196) for TOI-2046b implies that there are no statistically significant TTVs.

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arXiv2025

A high geometric albedo and small size of the Haumea cluster member (24835) 1995 SM55 from a stellar occultation and photometric observations

Trans-Neptunian objects (TNOs) are among the most ancient bodies of the solar system. Understanding their physical properties is key to constraining their origin and the evolution of the outer regions beyond Neptune. Stellar occultations provide highly accurate size and shape information. (24835) 1995 SM55 is one of the few members of the Haumea cluster and thus of particular interest. We aimed to determine its projected size, absolute magnitude, and geometric albedo, and to compare these with Haumea. A stellar occultation on 25 February 2024 was observed from five sites, with seven positive detections and 33 negative chords. An elliptical fit to the occultation chords yields semi-axes of $(104.3 \pm 0.4) \times (83.5 \pm 0.5)$ km, giving an area-equivalent diameter of $186.7 \pm 1.8$ km, smaller than the 250 km upper limit from Herschel thermal data. Photometry provides an absolute magnitude $H_V = 4.55 \pm 0.03$, a phase slope of $0.04 \pm 0.02$ mag/deg, and a $V-R = 0.37 \pm 0.05$. The rotational variability has an amplitude $\Delta m = 0.05$ mag, but the period remains uncertain. Combining occultation and photometry, we derive a geometric albedo $p_V = 0.80 \pm 0.04$, one of the highest values measured for a TNO. This value is slightly higher than that of Haumea, consistent with the interpretation that 1995 SM55 belongs to the Haumea cluster.

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arXiv2025

Constraining Lens Masses in Moderately to Highly Magnified Microlensing Events from Gaia

Microlensing events provide a unique way to detect and measure the masses of isolated, non-luminous objects, particularly dark stellar remnants. Under certain conditions, it is possible to measure the mass of these objects using photometry alone, specifically when a microlensing light curve displays a finite source (FS) effect. This effect generally occurs in highly magnified light curves, i.e. when the source and the lens are very well aligned. In this study, we analyse Gaia Alerts and Gaia Data Release 3 datasets, identifying four moderate-to-high-magnification microlensing events without a discernible FS effect. The absence of this effect suggests a large Einstein radius, implying substantial lens masses. In each event, we constrained the FS effect, and therefore established lower limits for the angular Einstein radius and lens mass. Additionally, we used the DarkLensCode software to obtain the mass, distance, and brightness distribution for the lens based on the Galactic model. Our analysis established lower mass limits of $\sim 0.7$ $M_{\odot}$ for one lens and $\sim 0.3-0.5$ $M_{\odot}$ for two others. A DarkLensCode analysis supports these findings, estimating lens masses in the range of $\sim 0.42-1.70$ $M_{\odot}$ and dark lens probabilities exceeding 80%. These results strongly indicate that the lenses are stellar remnants, such as white dwarfs or neutron stars. While further investigations are required to confirm the nature of these lenses, we demonstrate a straightforward yet effective approach to identifying stellar remnant candidates.

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arXiv2024

X-ray and gamma-ray timing of GRB 180720B, GRB 181222B, GRB 211211A and GRB 220910A observed with Fermi and ASIM

We present a timing study of the gamma and X-ray observations and analysis of a sample of bright gamma-ray bursts (GRBs; i.e. GRB 180720B, GRB 181222B, GRB 211211A and GRB 220910A), including the very bright and long GRB 211211A (a.k.a. kilonova candidate). They have been detected and observed by the Atmosphere-Space Interactions Monitor (ASIM) installed on the International Space Station (ISS) and the Gamma-ray Burst Monitor (GBM) on-board the Fermi mission. The early (T-T0=s) and high-energy (0.3-20 MeV) ASIM High Energy Detector (HED) and (150 keV-30 MeV) Fermi (BGO) light curves show well-defined peaks with a low quasi-periodic oscillation (QPO) frequency between 2.5-3.5 Hz that could be identified with the spin of the neutron star in the binary mergers (coinciding with the orbital frequency of the binary merger) originating these GRBs. These QPOs consist on the first detection of low-frequency QPOs (<10 Hz) detected in magnetars so far. We also detect a strong QPO at 21.8-22 Hz in GRB 181222B together with its (less significant) harmonics. The low-frequency QPO would correspond to the signal of the orbiting neutron star (NS) previous to the final coalescence giving rise to the gravitational-wave (GW) signal.

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arXiv2024

The Morphology of Asteroidal Dust Around White Dwarf Stars: Optical and Near-infrared Pulsations in G29-38

More than 36 years have passed since the discovery of the infrared excess from circumstellar dust orbiting the white dwarf G29-38, which at 17.5 pc it is the nearest and brightest of its class. The precise morphology of the orbiting dust remains only marginally constrained by existing data, subject to model-dependent inferences, and thus fundamental questions of its dynamical origin and evolution persist. This study presents a means to constrain the geometric distribution of the emitting dust using stellar pulsations measured at optical wavelengths as a variable illumination source of the dust, which re-radiates primarily in the infrared. By combining optical photometry from the Whole Earth Telescope with 0.7-2.5 micron spectroscopy obtained with SpeX at NASA's Infrared Telescope Facility, we detect luminosity variations at all observed wavelengths, with variations at most wavelengths corresponding to the behavior of the pulsating stellar photosphere, but towards the longest wavelengths the light curves probe the corresponding time-variability of the circumstellar dust. In addition to developing methodology, we find pulsation amplitudes decrease with increasing wavelength for principal pulsation modes, yet increase beyond approximately 2 microns for nonlinear combination frequencies. We interpret these results as combination modes deriving from principal modes of identical l values and discuss the implications for the morphology of the warm dust. We also draw attention to some discrepancies between our findings and theoretical expectations for the results of the non-linearity imposed by the surface convection zone on mode--mode interactions and on the behavior of the first harmonic of the highest-amplitude pulsation mode.

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arXiv2023

Lens mass estimate in the Galactic disk extreme parallax microlensing event Gaia19dke

We present the results of our analysis of Gaia19dke, an extraordinary microlensing event in the Cygnus constellation that was first spotted by the {\gaia} satellite. This event featured a strong microlensing parallax effect, which resulted in multiple peaks in the light curve. We conducted extensive photometric, spectroscopic, and high-resolution imaging follow-up observations to determine the mass and the nature of the invisible lensing object. Using the Milky Way priors on density and velocity of lenses, we found that the dark lens is likely to be located at a distance of $D_L =(3.05^{+4.10}_{-2.42})$kpc, and has a mass of $M_L =(0.51^{+3.07}_{-0.40}) M_\odot$. Based on its low luminosity and mass, we propose that the lens in Gaia19dke event is an isolated white dwarf.

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arXiv2023

Optical Counterparts of Two Candidate Ultraluminous X-ray Sources in NGC 4536

Archival {\it XMM-Newton}, {\it Chandra} and {\it Hubble Space Telescope (HST)} data have been used to study the X-ray and optical properties of two candidate ultraluminous X-ray sources (ULXs) in NGC\,4536. In order to search for potential optical counterparts, relative astrometry between {\it Chandra} and {\it HST} was improved, and as a result, optical counterparts were detected for both X-ray sources. To complement our findings (based on the archival data), ground-based optical spectra of the counterparts were obtained with the 6m BTA Telescope located at the Special Astrophysical Observatory (SAO). The calculated redshift (z = 0.4391$\pm$0.0010) for one of the sources (X-3) indicates that the source is, in fact, a background active galactic nucleus (AGN). Two possible optical counterparts (s1 and s2) were found for X-2. Whether s1 is point-like or an extended source is unclear: If it is point-like and the emission is dominated by the donor its spectral type indicates O-B star. The second source (s2) is point-like and is consistent with the colors and absolute magnitudes of a red supergiant.

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arXiv2023

GRB 221009A: Discovery of an Exceptionally Rare Nearby and Energetic Gamma-Ray Burst

We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image (MAXI), and Neutron Star Interior Composition Explorer Mission (NICER). This energetic GRB was located relatively nearby (z = 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (b = 4.3 degrees) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (~> 10kpc). We present analysis of the light curves and spectra at X-ray and UV/optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at T0 + 4.5 ks than any previous GRB observed by Swift. In its rest frame GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10^4 long GRBs were as energetic as GRB 221009A; such a large E_gamma,iso implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ~<1 per 1000 yr - making this a truly remarkable opportunity unlikely to be repeated in our lifetime.

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arXiv2022

Possible optical counterparts of ULXs in NGC 1672

In this study, we use archival data from HST, Chandra, XMM-Newton, and Swift-XRT, to probe the nature of 9 (X1-X9) candidate ULXs in NGC 1672. Our study focuses on using the precise source positions obtained via improved astrometry based on {\it Chandra} and HST observations to search for and identify optical counterparts for these ULXs.Unique optical counterparts are identified for X2 an{d X6; two potential counterparts were determined for X1, X5 and X7 within the respective error radii while no optical counterparts were found for the remaining four sources. Based on spectral energy distributions (SEDs), X-ray and optical temporal analyses, some evidences about the nature of X1 and X2 were obtained.

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arXiv2022

Optical Counterparts of ULXs in NGC 1672

In this work, we deploy archival data from {\it HST}, {\it Chandra}, {\it XMM-Newton}, and {\it Swift-XRT}, to probe the nature of 9 candidate ULXs in NGC 1672. Specifically, our study focuses on using the precise source positions obtained via improved astrometry based on {\it Chandra} and {\it HST} observations to search for and identify potential optical counterparts for these ULXs. Unique optical counterparts are identified for two of the ULX candidates i.e., X2 and X6; for three of the candidates i.e., X1, X5 and X7, we found two potential counterparts for each source within the respective error radii. No optical counterparts were found for the remaining four sources. The spectral energy distribution of X2 is fitted to a blackbody spectrum with a temperature of $\sim$ 10$^{4}$K and the spectral class of the source is determined to be B7$-$A3, a supergiant donor star. We used colour magnitude diagrams (CMDs) to investigate ages of the counterparts. Of all the sources studied, X9 exhibits the most variability whereby the X-ray flux varies by a factor of $\sim$ 50 over a time period spanning 2004 to 2019, and also traces a partial q-curve-like feature in the hardness-intensity diagram, hinting at possible spectral transitions.

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arXiv2022

Host galaxy magnitude of OJ 287 from its colours at minimum light

OJ 287 is a BL Lacertae type quasar in which the active galactic nucleus (AGN) outshines the host galaxy by an order of magnitude. The only exception to this may be at minimum light when the AGN activity is so low that the host galaxy may make quite a considerable contribution to the photometric intensity of the source. Such a dip or a fade in the intensity of OJ 287 occurred in November 2017, when its brightness was about 1.75 magnitudes lower than the recent mean level. We compare the observations of this fade with similar fades in OJ 287 observed earlier in 1989, 1999, and 2010. It appears that there is a relatively strong reddening of the B$-$V colours of OJ 287 when its V-band brightness drops below magnitude 17. Similar changes are also seen V$-$R, V$-$I, and R$-$I colours during these deep fades. These data support the conclusion that the total magnitude of the host galaxy is $V=18.0 \pm 0.3$, corresponding to $M_{K}=-26.5 \pm 0.3$ in the K-band. This is in agreement with the results, obtained using the integrated surface brightness method, from recent surface photometry of the host. These results should encourage us to use the colour separation method also in other host galaxies with strongly variable AGN nuclei. In the case of OJ 287, both the host galaxy and its central black hole are among the biggest known, and its position in the black hole mass-galaxy mass diagram lies close to the mean correlation.

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arXiv2022

Refinement of the convex shape model and tumbling spin state of (99942) Apophis using the 2020-2021 apparition data

Context. The close approach of the near-Earth asteroid (99942) Apophis to Earth in 2029 will provide a unique opportunity to examine how the physical properties of the asteroid could be changed due to the Earth's gravitational perturbation. As a result, the Republic of Korea is planning a rendezvous mission to Apophis. Aims. Our aim was to use photometric data from the apparitions in 2020-2021 to refine the shape model and spin state of Apophis. Methods. Using thirty-six 1 to 2-m class ground-based telescopes and the Transiting Exoplanet Survey Satellite, we performed a photometric observation campaign throughout the 2020-2021 apparition. The convex shape model and spin state were refined using the light-curve inversion method. Results. According to our best-fit model, Apophis is rotating in a short axis mode with rotation and precession periods of 264.178 hours and 27.38547 hours, respectively. The angular momentum vector orientation of Apophis was found as (275$^\circ$, -85$^\circ$) in the ecliptic coordinate system. The ratio of the dynamic moments of inertia of this asteroid was fitted to $I_a:I_b:I_c=0.64:0.97:1$, which corresponds to an elongated prolate ellipsoid. These findings regarding the spin state and shape model could be used to not only design the space mission scenario but also investigate the impact of the Earth's tidal force during close encounters.

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arXiv2021

The Transient Ultra-luminous X-ray Source, ULX-4, in M51

We present the results of a temporal and spectral analysis of the transient source ULX-4 in the galaxy M51. The data used were drawn from Chandra, XMM-Newton and Swift-XRT archives, spanning the years 2000-2019. The X-ray flux of the source is seen to vary by two orders of magnitudes within a month but a short-term variability was not observed over the time intervals of 100-2000 second in the 0.3-10 keV energy band. We find some evidence for the existence of bi-modality feature in the flux distribution of ULX-4. We identified two optical sources as possible counterparts within an error radius of 0."18 at 95% confidence level for ULX-4 based on the archival HST/ACS and HST/WFC3 data. Blackbody fits of the spectral energy distributions (SEDs) indicate the spectral type to be B-type stars. One of these counterparts exhibits a low-amplitude optical periodicity of 264 days in the F606W filter; if we assume this apparent periodicity is associated with the orbital motion of the donor, then it is more likely that the donor is a red supergiant satisfying the long periodicity and accretion via Roche-lobe overflow. Consequently, the SED would then have to be interpreted as a superposition of emissions from a cold donor and a hot flow component, most likely from an accretion disk. If, on the other hand, the periodicity is super orbital in nature i.e., due to possible interactions of the compact object with a circumstellar disk, the donor could then be a Be/X star hosting a neutron star.

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arXiv2021

A Temporal Scale to Track the Spectral Transitions in Low-Mass X-ray Binaries

The results of a temporal analysis of observations for a sample of nine low-mass X-ray binaries (LMXBs) are presented. Of these sources, five host a neutron star (NS) primary (4U1608-52, Aql X-1, 4U1705-44, GX17+2 and Cyg X-2), and four host a black hole (BH) (GX339-4, XTE J1859+226, H1743-322 and MAXI J1659-152). The NS group includes three Atolls and two Z-type sources. We utilized archival Proportional Counter Array (PCA)/RXTE data to construct high-resolution lightcurves. A wavelet transform of the lightcurves is deployed to extract a minimal time scale (MTS) associated with the spectral state of the sources. The MTS, together with the fractional root-mean-square (RMS) and hardness ratios, is used to construct RMS-MTS and hardness-MTS diagrams that enable a direct comparison of the evolution of spectral transitions in the target sources. Observations with high fractional RMS and high hardness cluster in a broad region occupied jointly by BH and NS sources. For low fractional RMS observations the Atolls exhibit large MTS whereas Z-type sources exhibit small MTS. This new feature raises the possibility of discriminating between these two types of sources. Moreover, in the hardness-MTS plane, BH sources are the sole occupiers of the low-hardness and small-MTS domain thus potentially signaling a unique property for distinguishing BH and NS hosts in LMXBs.

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arXiv2021

Probing Transit Timing Variations of Three Hot-Jupiters: HATP-36b, HATP-56b, and WASP-52b

We report the results of new transit observations for the three hot Jupiter-like planets HATP-36b, HATP-56b and WASP-52b respectively. Transit timing variations (TTVs) are presented for these systems based on observations that span the period 2016 - 2020. The data were collected with the 0.6 m telescope at Adiyaman University (ADYU60, Turkey) and the 1.0 m telescope at TÜBİTAK National Observatory (TUG, Turkey). Global fits were performed to the combined light curves for each system along with the corresponding radial velocity (RV) data taken from the literature. The extracted parameters (for all three systems) are found to be consistent with the values from previous studies. Through fits to the combined mid-transit times data from our observations and the data available in the literature, an updated linear ephemeris is obtained for each system. Although a number of potential outliers are noted in the respective O-C diagrams, the majority of the data are consistent within the 3$σ$ confidence level implying a lack of convincing evidence for the existence of additional objects in the systems studied.

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arXiv2021

Properties of slowly rotating asteroids from the Convex Inversion Thermophysical Model

Results from the TESS mission showed that previous studies strngly underestimated the number of slow rotators, revealing the importance of studying those asteroids. For most slowly rotating asteroids (P > 12), no spin and shape model is available because of observation selection effects. This hampers determination of their thermal parameters and accurate sizes. We continue our campaign in minimising selection effects among main belt asteroids. Our targets are slow rotators with low light-curve amplitudes. The goal is to provide their scaled spin and shape models together with thermal inertia, albedo, and surface roughness to complete the statistics. Rich multi-apparition datasets of dense light curves are supplemented with data from Kepler and TESS. In addition to data in the visible range, we also use thermal data from infrared space observatories (IRAS, Akari and WISE) in a combined optimisation process using the Convex Inversion Thermophysical Model (CITPM). This novel method has so far been applied to only a few targets, and in this work we further validate the method. We present the models of 16 slow rotators. All provide good fits to both thermal and visible data. The obtained sizes are on average accurate at the 5% precision, with diameters in the range from 25 to 145 km. The rotation periods of our targets range from 11 to 59 hours, and the thermal inertia covers a wide range of values, from 2 to <400 SI units, not showing any correlation with the period. With this work we increase the sample of slow rotators with reliable spin and shape models and known thermal inertia by 40%. The thermal inertia values of our sample do not display a previously suggested increasing trend with rotation period, which might be due to their small skin depth.

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arXiv2021

GRB 140102A: Insight into Prompt Spectral Evolution and Early Optical Afterglow Emission

We present and perform a detailed analysis of multi-wavelength observations of \thisgrb, an optical bright GRB with an observed reverse shock (RS) signature. Observations of this GRB were acquired with the BOOTES-4 robotic telescope, the \fermi, and the \swift missions. Time-resolved spectroscopy of the prompt emission shows that changes to the peak energy (\Ep) tracks intensity and the low-energy spectral index seems to follow the intensity for the first episode, whereas this tracking behavior is less clear during the second episode. The fit to the afterglow light curves shows that the early optical afterglow can be described with RS emission and is consistent with the thin shell scenario of the constant ambient medium. The late time afterglow decay is also consistent with the prediction of the external forward shock (FS) model. We determine the properties of the shocks, Lorentz factor, magnetization parameters, and ambient density of \thisgrb, and compare these parameters with another 12 GRBs, consistent with having RS produced by thin shells in an ISM-like medium. The value of the magnetization parameter ($R_{\rm B} \approx 18$) indicates a moderately magnetized baryonic dominant jet composition for \thisgrb. We also report the host galaxy photometric observations of \thisgrb obtained with 10.4m GTC, 3.5m CAHA, and 3.6m DOT telescopes and find the host (photo $z$ = $2.8^{+0.7}_{-0.9}$) to be a high mass, star-forming galaxy with a star formation rate of $20 \pm 10 \msun$ $\rm yr^{-1}$.

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arXiv2021

A Minimal Timescale for the Continuum in 4U 1608-52 and Aql X-1

Similar to black-hole X-ray binary (BHXRB) transients, hysteresis-like state transitions are also seen in some neutron-star X-ray binaries (NSXRBs). Using a method based on wavelets and lightcurves constructed from archival RXTE observations, we extract a minimal timescale over the complete range of transitions for 4U 1608-52 during the 2002 and 2007 outbursts and the 1999 and 2000 outbursts for Aql X-1. We present evidence for a strong positive correlation between this minimal timescale and a similar timescale extracted from the corresponding power spectra of these sources.

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