arXiv subjects

A. Udalski

Publications and source records attributed to A. Udalski.

At least 91 records · Page 5Linked to original sources

arXiv2020

The newly discovered Be/X-ray binary Swift J004516.6-734703 in the SMC: witnessing the emergence of a circumstellar disc

We report on the discovery of Swift J004516.6-734703, a Be/X-ray binary system by the Swift SMC Survey, S-CUBED. Swift J004516.6-734703, or SXP 146.6, was found to be exhibiting a bright (~10^37 erg/s) X-ray outburst in 2020 June 18. The historical UV and IR light-curves from OGLE and Swift/UVOT showed that after a long period of steady brightness, it experienced a significant brightening beginning around 2019 March. This IR/UV rise is likely the signature of the formation of a circumstellar disc, confirmed by the presence of strong a Hα line in SALT spectroscopy, that was not previously present. Periodicity analysis of the OGLE data reveals a plausible 426 day binary period, and in the X-ray a pulsation period of 146.6s period is detected. The onset of X-ray emission from Swift J004516.6-734703 is likely the signature of a Type-I outburst from the first periastron passage of the neutron star companion through the newly formed circumstellar disc. We note that the formation of the circumstellar disc began at the predicted time of the previous periastron passage, suggesting its formation was spurred by tidal interaction with the neutron star.

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arXiv2020

Additional Galactic Cepheids from the OGLE Survey

We report on the results of a comprehensive search for Cepheid variable stars in the fields photometricallymonitored by the OGLE Galaxy Variability Survey. We identify 742 Cepheids not included in the first release of the OGLE Collection of Galactic Cepheids and reclassify several dozen previously published variables. The upgraded collection comprises 1974 classical, 1625 type II, and 119 anomalous Cepheids located in the area of about 3000 square degrees covering the disk and bulge of the Milky Way. We present the most interesting objects in our sample: an isolated group of long-period doublemode classical Cepheids pulsating in the fundamental mode and first overtone, the first known Galactic double-mode Cepheid with the second and third overtone excited, double-mode type II Cepheids, candidates for single-mode first-overtone BL Her stars, and Cepheids showing simultaneous eclipsing variability. We also discuss type II and anomalous Cepheids that are potential members of globular clusters.

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arXiv2020

Properties of the Milky Way's Old Populations Based on Photometric Metallicities of the OGLE RR Lyrae Stars

We have used photometric data on almost 91 000 fundamental-mode RR Lyrae stars (type RRab) detected by the OGLE survey to investigate properties of old populations in the Milky Way. Based on their metallicity distributions, we demonstrate that the Galaxy is built from three distinct old components: halo, bulge, and disk. The distributions reach their maxima at approximately [Fe/H]_J95 = -1.2, -1.0, and -0.6 dex on the Jurcsik's metallicity scale, respectively. We find that, very likely, the entire halo is formed from infalling dwarf galaxies. It is evident that halo stars penetrate the inner regions of the Galactic bulge. We estimate that about one-third of all RR Lyr stars within the bulge area belong in fact to the halo population. The whole old bulge is dominated by two populations, A and B, represented by a double sequence in the period-amplitude (Bailey) diagram. The boundary in iron abundance between the halo and the disk population is at about [Fe/H]_J95 = -0.8 dex. Using Gaia DR2 for RRab stars in the disk area, we show that the observed dispersion of proper motions along the Galactic latitude decreases smoothly with the increasing metal content excluding a bump around [Fe/H]_J95 = -1.0 dex.

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arXiv2020

Swift J004427.3-734801- a probable Be/white dwarf system in the Small Magellanic Cloud

Swift J004427.3-734801 is an X-ray source in the Small Magellanic Cloud (SMC) that was first discovered as part of the Swift S-CUBED programme in January 2020. It was not detected in any of the previous 3 years worth of observations. The accurate positional determination from the X-ray data have permitted an optical counterpart to be identified which has the characteristics of an O9V-B2III star. Evidence for the presence of an IR excess and significant I-band variability strongly suggest that this is an OBe type star. Over 17 years worth of optical monitoring by the OGLE project reveal periods of time in which quasi-periodic optical flares occur at intervals of ~21.5d. The X-ray data obtained from the S-CUBED project reveal a very soft spectrum, too soft to be that from accretion on to a neutron star or black hole. It is suggested here that this a rarely identified Be star-white dwarf binary in the SMC.

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arXiv2020

A free-floating or wide-orbit planet in the microlensing event OGLE-2019-BLG-0551

High-cadence observations of the Galactic bulge by the microlensing surveys led to the discovery of a handful of extremely short-timescale microlensing events that can be attributed to free-floating or wide-orbit planets. Here, we report the discovery of another strong free-floating planet candidate, which was found from the analysis of the gravitational microlensing event OGLE-2019-BLG-0551. The light curve of the event is characterized by a very short duration (<3 d) and a very small amplitude (< 0.1 mag). From modeling of the light curve, we find that the Einstein timescale, tE = 0.381 +/- 0.017 d, is much shorter, and the angular Einstein radius, thetaE = 4.35 +/- 0.34 uas, is much smaller than those of typical lensing events produced by stellar-mass lenses (tE ~ 20 d, thetaE ~ 0.3 mas), indicating that the lens is very likely to be a planetary-mass object. We conduct an extensive search for possible signatures of a companion star in the light curve of the event, finding no significant evidence for the putative host star. For the first time, we also demonstrate that the angular Einstein radius of the lens does not depend on blending in the low-magnification events with strong finite source effects.

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arXiv2020

Ogle-2018-blg-0677lb: A super earth near the galactic bulge

We report the analysis of the microlensing event OGLE-2018-BLG-0677. A small feature in the light curve of the event leads to the discovery that the lens is a star-planet system. Although there are two degenerate solutions that could not be distinguished for this event, both lead to a similar planet-host mass ratio. We perform a Bayesian analysis based on a Galactic model to obtain the properties of the system and find that the planet corresponds to a super-Earth/sub-Neptune with a mass $M_{\mathrm{planet}} = {3.96}^{+5.88}_{-2.66}\mathrm{M_\oplus}$. The host star has a mass $ M_{\mathrm{host}} = {0.12}^{+0.14}_{-0.08}\mathrm{M_\odot}$. The projected separation for the inner and outer solutions are ${0.63}^{+0.20}_{-0.17}$~AU and ${0.72}^{+0.23}_{-0.19}$~AU respectively. At $Δχ^2=χ^2({\rm 1L1S})-χ^2({\rm 2L1S})=46$, this is by far the lowest $Δχ^2$ for any securely-detected microlensing planet to date, a feature that is closely connected to the fact that it is detected primarily via a "dip" rather than a "bump".

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arXiv2020

A plethora of new R Coronae Borealis stars discovered from a dedicated spectroscopic follow-up survey

It is increasingly suspected that the rare R Coronae Borealis (RCB) stars - hydrogen-deficient and carbon-rich supergiant stars - are the products of mergers of CO/He white-dwarf binary systems in the intermediate mass regime ($0.6 15000$ K). Forty of these belong to the Milky Way and five are located in the Magellanic Clouds. We also confirmed that the long lasting candidate KDM 5651 is indeed a new Magellanic RCB star, increasing the total number of Magellanic Cloud RCB stars to 30. We have increased by $\sim$50\% the total number of RCB stars known, now reaching 147. We also include a list of 14 strong RCB candidates, most certainly observed during a dust obscuration phase. From the detection efficiency and success rate so far, we estimate that there should be no more than 500 RCB stars/HdC stars in the Milky Way.

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arXiv2020

Over 78 000 RR Lyrae Stars in the Galactic Bulge and Disk from the OGLE Survey

We present an upgrade of the OGLE Collection of RR Lyrae stars in the Galactic bulge and disk. The size of our sample has been doubled and reached 78 350 RR Lyr variables, of which 56 508 are fundamental-mode pulsators (RRab stars), 21 321 pulsate solely in the first-overtone (RRc stars), 458 are classical double-mode pulsators (RRd stars), and 63 are anomalous RRd variables (including five triple-mode pulsators). For all the newly identified RR Lyr stars, we publish time-series photometry obtained during the OGLE Galaxy Variability Survey. We present the spatial distribution of RR Lyr stars on the sky, provide a list of globular clusters hosting RR Lyr variables, and discuss the Petersen diagram for multimode pulsators. We find new RRd stars belonging to a compact group in the Petersen diagram (with period ratios P_1O/P_F = 0.74 and fundamental-mode periods P_F = 0.44 d) and we show that their spatial distribution is roughly spherically symmetrical around the Milky Way center.

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arXiv2020

The Cluster Ages Experiment (CASE). VIII. Age and Distance of the Globular Cluster 47 Tuc from the Analysis of Two Detached Eclipsing Binaries

We use photometric and spectroscopic observations of the eclipsing binary E32 in the globular cluster 47 Tuc to derive the masses, radii, and luminosities of the component stars. The system has an orbital period of 40.9 d, a markedly eccentric orbit with e = 0.24, and is shown to be a member of or a recent escaper from the cluster. We obtain Mp = 0.862(5) Msun , Rp = 1.183(3) Rsun , Lp = 1.65(5) Lsun for the primary and Ms = 0.827(5) Msun , Rs = 1.004(4) Rsun , Ls = 1.14(4) Lsun for the secondary. Based on these data and on an earlier analysis of the binary V69 in 47 Tuc we measure the distance to the cluster from the distance moduli of the component stars, and, independently, from a color - surface brightness calibration. We obtain 4.55(3) and 4.50(7) kpc, respectively - values compatible within 1 sigma with recent estimates based on Gaia DR2 parallaxes. By comparing the M - R diagram of the two binaries and the color-magnitude diagram of 47 Tuc to Dartmouth model isochrones we estimate the age of the cluster to be 12.0 pm 0.5 Gyr, and the helium abundance of the cluster to be Y approx 0.25.

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arXiv2020

Mapping the Northern Galactic Disk Warp with Classical Cepheids

We present an updated three dimensional map of the Milky Way based on a sample of 2431 classical Cepheid variable stars, supplemented with about 200 newly detected classical Cepheids from the OGLE survey. The new objects were discovered as a result of a dedicated observing campaign of the ~280 square degree extension of the OGLE footprint of the Galactic disk during 2018-2019 observing seasons. These regions cover the main part of the northern Galactic warp that has been deficient in Cepheids so far. We use direct distances to the sample of over 2390 classical Cepheids to model the distribution of the young stellar population in the Milky Way and recalculate the parameters of the Galactic disk warp. Our data show that its northern part is very prominent and its amplitude is ~10% larger than that of the southern part. By combining Gaia astrometric data with the Galactic rotation curve and distances to Cepheids from our sample, we construct a map of the vertical component of the velocity vector for all Cepheids in the Milky Way disk. We find large-scale vertical motions with amplitudes of 10-20 km/s, such that Cepheids located in the northern warp exhibit large positive vertical velocity (toward the north Galactic pole), whereas those in the southern warp - negative vertical velocity (toward the south Galactic pole).

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arXiv2019

Candidates for non-pulsating stars located in the Cepheid instability strip in the Large Magellanic Cloud based on Strömgren photometry

We present candidates for non-pulsating stars lying in the classical Cepheid instability strip based on OGLE photometric maps combined with Strömgren photometry obtained with the 4.1-m SOAR telescope, and Gaia DR2 data in four fields in the Large Magellanic Cloud. We selected 19 candidates in total. After analysis of their light curves from OGLE surveys we found that all these stars appear to be photometrically stable at the level of a few mmag. Our results show that non-pulsating stars might constitute to about 21%-30% of the whole sample of giant stars located in the classical instability strip. Furthermore, we identified potential candidates for classical Cepheids with hot companions based on their Strömgren colours.

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arXiv2019

V346 Nor: the post-outburst life of a peculiar young eruptive star

FU Orionis-type objects (FUors) are young low-mass stars undergoing powerful accretion outbursts. The increased accretion is often accompanied by collimated jets and energetic, large-scale molecular outflows. The extra heating during the outburst may also induce detectable geometrical, chemical, and mineralogical changes in the circumstellar material, affecting possible planet formation around these objects. V346 Nor is a southern FUor with peculiar spectral characteristics. Decades after the beginning of its outburst, it unexpectedly underwent a fading event around 2010 due to a decrease in the mass accretion rate onto the star by at least two orders of magnitude. Here we present optical and near-infrared photometry and spectroscopy obtained after the minimum. Our light curves show a gradual re-brightening of V346 Nor, with its Ks-band brightness only 1.5 mag below the outburst brightness level. Our VLT/XSHOOTER spectroscopic observations display several strong forbidden emission lines towards the source from various metals and molecular hydrogen, suggesting the launch of a new jet. Our N-band spectrum obtained with VLT/VISIR outlines a deeper silicate absorption feature than before, indicating that the geometry of the circumstellar medium has changed in the post-outburst period compared to peak brightness.

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arXiv2019

Infrared spectroscopy of the recent outburst in V1047 Cen (Nova Centauri 2005)

Fourteen years after its eruption as a classical nova (CN), V1047 Cen (Nova Cen 2005) began an unusual re-brightening in 2019 April. The amplitude of the brightening suggests that this is a dwarf nova (DN) eruption in a CN system. Very few CNe have had DN eruptions within decades of the main CN outburst. The 14 years separating the CN and DN eruptions of V1047 Cen is the shortest of all instances recorded thus far. Explaining this rapid succession of CN and DN outbursts in V1047 Cen may be challenging within the framework of standard theories for DN outbursts. Following a CN eruption, the mass accretion rate is believed to remain high $(\dot{M}\sim10^{-8}$M$_\odot$yr$^{-1})$ for a few centuries, due to the irradiation of the secondary star by the still-hot surface of the white dwarf. Thus a DN eruption is not expected to occur during this high mass accretion phase as DN outbursts, which result from thermal instabilities in the accretion disk, and arise during a regime of low mass accretion rate $(\dot{M}\sim10^{-10}$M$_\odot$yr$^{-1})$. Here we present near-infrared spectroscopy to show that the present outburst is most likely a DN eruption, and discuss the possible reasons for its early occurrence. Even if the present re-brightening is later shown to be due to a cause other than a DN outburst, the present study provides invaluable documentation of this unusual event.

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arXiv2019

The January 2016 eruption of recurrent nova LMC 1968

We present a comprehensive review of all observations of the eclipsing recurrent Nova LMC 1968 in the Large Magellanic Cloud which was previously observed in eruption in 1968, 1990, 2002, 2010, and most recently in 2016. We derive a probable recurrence time of $6.2 \pm 1.2$ years and provide the ephemerides of the eclipse. In the ultraviolet-optical-IR photometry the light curve shows high variability right from the first observation around two days after eruption. Therefore no colour changes can be substantiated. Outburst spectra from 2016 and 1990 are very similar and are dominated by H and He lines longward of 2000 Angstrom. Interstellar reddening is found to be E(B-V) = $0.07\pm0.01$. The super soft X-ray luminosity is lower than the Eddington luminosity and the X-ray spectra suggest the mass of the WD is larger than 1.3 M$_\odot$. Eclipses in the light curve suggest that the system is at high orbital inclination. On day four after the eruption a recombination wave was observed in Fe II ultraviolet absorption lines. Narrow line components are seen after day 6 and explained as being due to reionisation of ejecta from a previous eruption. The UV spectrum varies with orbital phase, in particular a component of the He II 1640 Angstrom emission line, which leads us to propose that early-on the inner WD Roche lobe might be filled with a bound opaque medium prior to the re-formation of an accretion disk. Both this medium and the ejecta can cause the delay in the appearance of the soft X-ray source.

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arXiv2019

Discovery of a very young high-mass X-ray binary associated with the supernova remnant MCSNRJ0513-6724 in the LMC

We report the discovery of a very young high-mass X-ray binary (HMXB) system associated with the supernova remnant (SNR) MCSNRJ0513-6724 in the Large Magellanic Cloud (LMC), using XMM-Newton X-ray observations. The HMXB is located at the geometrical centre of extended soft X-ray emission, which we confirm as an SNR. The HMXB spectrum is consistent with an absorbed power law with spectral index ~1.6 and a luminosity of 7x10^{33} ergs/s (0.2--12 keV). Tentative X-ray pulsations are observed with a periodicity of 4.4 s and the OGLE I-band light curve of the optical counterpart from more than 17.5 years reveals a period of 2.2324\pm0.0003 d, which we interpret as the orbital period of the binary system. The X-ray spectrum of the SNR is consistent with non-equilibrium shock models as expected for young/less evolved SNRs. From the derived ionisation time scale we estimate the age of the SNR to be <6 kyr. The association of the HMXB with the SNR makes it the youngest HMXB, in the earliest evolutionary stage known to date. A HMXB as young as this can switch on as an accreting pulsar only when the spin period has reached a critical value. Under this assumption, we obtain an upper limit to the magnetic field of < 5x10^{11} G. This implies several interesting possibilities including magnetic field burial, possibly by an episode of post-supernova hyper-critical accretion. Since these fields are expected to diffuse out on a timescale of 10^{3}-10^{4} years, the discovery of a very young HMXB can provide us the unique opportunity to observe the evolution of the observable magnetic field for the first time in X-ray binaries.

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arXiv2019

Microlensing optical depth and event rate toward the Galactic bulge from 8 yr of OGLE-IV observations

The number and properties of observed gravitational microlensing events depend on the distribution and kinematics of stars and other compact objects along the line of sight. In particular, precise measurements of the microlensing optical depth and event rate toward the Galactic bulge enable strict tests of competing models of the Milky Way. Previous estimates, based on samples of up to a few hundred events, gave larger values than expected from the Galactic models and were difficult to reconcile with other constraints on the Galactic structure. Here we used long-term photometric observations of the Galactic bulge by the Optical Gravitational Lensing Experiment (OGLE) to select a homogeneous sample of 8000 gravitational microlensing events. We created the largest and most accurate microlensing optical depth and event rate maps of the Galactic bulge. The new maps ease the tension between the previous measurements and Galactic models. They are consistent with some earlier calculations based on bright stars and are systematically ~30% smaller than the other estimates based on "all-source" samples of microlensing events. The difference is caused by the careful estimation of the source star population. The new maps agree well with predictions based on the Besancon model of the Galaxy. Apart from testing the Milky Way models, our maps may have numerous other applications, such as the measurement of the initial mass function or constraining the dark matter content in the Milky Way center. The new maps will also inform the planning of future space-based microlensing experiments by revising the expected number of events.

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arXiv2019

Neutron Stars and Black Holes in the Small Magellanic Cloud: The SMC NuSTAR Legacy Survey

We present a source catalog from the first deep hard X-ray ($E>10$ keV) survey of the Small Magellanic Cloud (SMC), the NuSTAR Legacy Survey of the SMC. We observed three fields, for a total exposure time of 1 Ms, along the bar of this nearby star-forming galaxy. Fields were chosen for their young stellar and accreting binary populations. We detected 10 sources above a 3$σ$ significance level (4$-$25 keV) and obtained upper limits on an additional 40 sources. We reached a 3$σ$ limiting luminosity in the 4$-$25 keV band of $\sim$ $10^{35}$ erg s$^{-1}$, allowing us to probe fainter X-ray binary (XRB) populations than has been possible with other extragalactic NuSTAR surveys. We used hard X-ray colors and luminosities to constrain the compact-object type, exploiting the spectral differences between accreting black holes and neutron stars at $E>10$ keV. Several of our sources demonstrate variability consistent with previously observed behavior. We confirmed pulsations for seven pulsars in our 3$σ$ sample. We present the first detection of pulsations from a Be-XRB, SXP305 (CXO J005215.4$-$73191), with an X-ray pulse period of $305.69\pm0.16$ seconds and a likely orbital period of $\sim$1160-1180 days. Bright sources ($\gtrsim 5\times 10^{36}$ erg s$^{-1}$) in our sample have compact-object classifications consistent with their previously reported types in the literature. Lower luminosity sources ($\lesssim 5\times 10^{36}$ erg s$^{-1}$) have X-ray colors and luminosities consistent with multiple classifications. We raise questions about possible spectral differences at low luminosity between SMC pulsars and the Galactic pulsars used to create the diagnostic diagrams.

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arXiv2019

Discovery of an Outbursting 12.8 Minute Ultracompact X-Ray Binary

We report the discovery of OGLE-UCXB-01, a 12.8 minute variable object located in the central field of Galactic bulge globular cluster Djorg 2. The presence of frequent, short-duration brightenings at such an ultrashort period in long-term OGLE photometry together with the blue color of the object in Hubble Space Telescope images and the detection of moderately hard X-rays by Chandra observatory point to an ultracompact X-ray binary system. The observed fast period decrease makes the system a particularly interesting target for gravitational-wave detectors such as the planned Laser Interferometer Space Antenna.

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