Article

# Decoding the X-Ray Flare from MAXI J0709-159 Using Optical Spectroscopy and Multiepoch Photometry

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## Abstract

We present a follow-up study on the recent detection of two X-ray flaring events by MAXI/Gas Slit Camera observations in soft and hard X-rays from MAXI J0709–159 in the direction of HD 54786 (LY CMa), on 2022 January 25. The X-ray luminosity during the flare was around 1037 erg s−1 (MAXI), which got reduced to 1032 erg s−1 (NuSTAR) after the flare. We took low-resolution spectra of HD 54786 from the 2.01 m Himalayan Chandra Telescope and the 2.34 m Vainu Bappu Telescope (VBT) facilities in India, on 2022 February 1 and 2. In addition to Hα emission, we found emission lines of He i in the optical spectrum of this star. By comparing our spectrum of the object with those from the literature we found that He i lines show variability. Using photometric studies we estimate that the star has an effective temperature of 20,000 K. Although HD 54786 is reported as a supergiant in previous studies, our analysis favors it to be evolving off the main sequence in the color–magnitude diagram. We could not detect any infrared excess, ruling out the possibility of IR emission from a dusty circumstellar disk. Our present study suggests that HD 54786 is a Be/X-ray binary system with a compact object companion, possibly a neutron star.

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The All-Sky Automated Survey for Supernovae (ASAS-SN) provides long baseline (∼4 yrs) light curves for sources brighter than V≲ 17 mag across the whole sky. The Transiting Exoplanet Survey Satellite (TESS) has started to produce high-quality light curves with a baseline of at least 27 days, eventually for most of the sky. The combination of ASAS-SN and TESS light curves probes both long and short term variability in great detail, especially towards the TESS continuous viewing zones (CVZ) at the ecliptic poles. We have produced ∼1.3 million V-band light curves covering a total of |${\sim }1000 \, \rm deg^2$| towards the southern TESS CVZ and have systematically searched these sources for variability. We have identified ∼11, 700 variables, including ∼7, 000 new discoveries. The light curves and characteristics of the variables are all available through the ASAS-SN variable stars database (https://asas-sn.osu.edu/variables). We also introduce an online resource to obtain pre-computed ASAS-SN V-band light curves (https://asas-sn.osu.edu/photometry) starting with the light curves of the ∼1.3 million sources studied in this work. This effort will be extended to provide ASAS-SN light curves for ∼50 million sources over the entire sky.
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Measurement of the Galactic neutral atomic hydrogen (HI) column density, NHI, and brightness temperatures, Tb, is of high scientific value for a broad range of astrophysical disciplines. In the past two decades, one of the most-used legacy HI datasets has been the Leiden/Argentine/Bonn Survey (LAB). We release the HI 4$\pi$ survey (HI4PI), an all-sky database of Galactic HI, which supersedes the LAB survey. The HI4PI survey is based on data from the recently completed first coverage of the Effelsberg-Bonn HI Survey (EBHIS) and from the third revision of the Galactic All-Sky Survey (GASS). EBHIS and GASS share similar angular resolution and match well in sensitivity. Combined, they are ideally suited to be a successor to LAB. The new HI4PI survey outperforms the LAB in angular resolution (16.2', FWHM) and sensitivity (RMS: 43 mK). Moreover, it has full spatial sampling and thus overcomes a major drawback of LAB, which severely undersamples the sky. We publish all-sky column density maps of the neutral atomic hydrogen in the Milky Way, along with full spectroscopic data, in several map projections including HEALPix.
Article
We present long term H$\alpha$ monitoring results of five Be/X-ray binaries to study the Be disc size variations and their influence on Type II (giant) X-ray outbursts. The work is done in the context of the viscous decretion disc model which predicts that Be discs in binary systems are truncated by resonant torques induced by the neutron star in its orbit. Our observations show that type II outbursts are not correlated(nor anti-correlated) with the disc size, as they are seen to occur both at relatively small and large Be disc radii. We discuss these observations in context of alternate interpretation of Be disc behaviour, such as precession, elongation and density effects, and with cognisance of the limitations of our disc size estimates.
Conference Paper
During 2014 and 2015, NASA's Neutron star Interior Composition Explorer (NICER) mission proceeded success- fully through Phase C, Design and Development. An X-ray (0.2-12 keV) astrophysics payload destined for the International Space Station, NICER is manifested for launch in early 2017 on the Commercial Resupply Services SpaceX-11 flight. Its scientific objectives are to investigate the internal structure, dynamics, and energetics of neutron stars, the densest objects in the universe. During Phase C, flight components including optics, detectors, the optical bench, pointing actuators, electronics, and others were subjected to environmental testing and integrated to form the flight payload. A custom-built facility was used to co-align and integrate the X-ray "con- centrator" optics and silicon-drift detectors. Ground calibration provided robust performance measures of the optical (at NASA's Goddard Space Flight Center) and detector (at the Massachusetts Institute of Technology) subsystems, while comprehensive functional tests prior to payload-level environmental testing met all instrument performance requirements. We describe here the implementation of NICER's major subsystems, summarize their performance and calibration, and outline the component-level testing that was successfully applied.
Article
This is the first of a series of papers presenting the Modules for Experiments in Stellar Astrophysics (MESA) Isochrones and Stellar Tracks (MIST) project, a new comprehensive set of stellar evolutionary tracks and isochrones computed using MESA, a state-of-the-art open-source 1D stellar evolution package. In this work, we present models with solar-scaled abundance ratios covering a wide range of ages ($5 \leq \rm \log(Age)\;[yr] \leq 10.3$), masses ($0.1 \leq M/M_{\odot} \leq 300$), and metallicities ($-2.0 \leq \rm [Z/H] \leq 0.5$). The models are self-consistently and continuously evolved from the pre-main sequence to the end of hydrogen burning, the white dwarf cooling sequence, or the end of carbon burning, depending on the initial mass. We also provide a grid of models evolved from the pre-main sequence to the end of core helium burning for $-4.0 \leq \rm [Z/H] < -2.0$. We showcase extensive comparisons with observational constraints as well as with some of the most widely used existing models in the literature. The evolutionary tracks and isochrones can be downloaded from the project website at http://waps.cfa.harvard.edu/MIST/.
Article
I describe a method to transform a set of stellar evolution tracks onto a uniform basis and then interpolate within that basis to construct stellar isochrones. The method accommodates a broad range of stellar types, from substellar objects to high-mass stars, and phases of evolution, from the pre-main sequence to the white dwarf cooling sequence. I discuss situations in which stellar physics leads to departures from the otherwise monotonic relation between initial stellar mass and lifetime and how these may be dealt with in isochrone construction. I close with convergence tests and recommendations for the number of points in the uniform basis and the mass between tracks in the original grid required in order to achieve a certain level of accuracy in the resulting isochrones. The programs that implement these methods are free and open-source; they may be obtained from the project webpage.
Article
We present our NextGen Model Atmosphere grid for low-mass stars for effective temperatures larger than 3000 K. These LTE models are calculated with the same basic model assumptions and input physics as the VLMS part of the NextGen grid so that the complete grid can be used, e.g., for consistent stellar evolution calculations and for internally consistent analysis of cool star spectra. This grid is also the starting point for a large grid of detailed NLTE model atmospheres for dwarfs and giants. The models were calculated from 3000 to 10,000 K (in steps of 200 K) for 3.5 ≤ log g ≤ 5.5 (in steps of 0.5) and metallicities of -4.0 ≤ [M/H] ≤ 0.0.
Article
He I lines wavelength 5876 and wavelength 10830 were found in emission from several Be stars. The UV continuum above the ionization potential for He I is found to be enough to produce the observed emission only for stars earlier than B1 and to explain the observed emission for stars of later type, a source of helium ionizing photons is needed. It is shown that the EUV or X-ray photons from a binary compact companion star, accreting matter from the Be star gas envelope, can explain the observed He I line emission.
Article
We present the results of X-ray observations of gamma Cassiopeiae made with the ASCA satellite. Since the discovery of X-ray emission from this system, several possible models for the origin of X-rays have been proposed, including the corona of gamma Cas, itself, a binary neutron-star companion, and a binary white-dwarf companion. However, there has been no clear way to distinguish which, if any, of these models is correct. In this paper we exploit the spectral resolution of the ASCA SIS detectors which, for the first time, enables the iron-line emission from this system to be studied in detail. The line and continuous spectrum of gamma Cas is compared with the X-ray spectra of hot coronae of early type stars, low-mass X-ray binaries, high-mass X-ray binaries, and white-dwarf binaries. The observed properties of rapid fluctuations on timescales down to 10 s and a spectrum with a temperature of 10.6 keV, together with iron line emission, are very similar to those of binary white-dwarf systems. Assuming a white-dwarf binary origin for the X-rays, we estimated the luminosity of this system as a function of the binary separation using recent measurements of the circumstellar environment of gamma Cas. Despite previous arguments that the X-ray luminosity is too large to be accounted for by accretion onto a white dwarf, we find that such a model is, in fact, consistent with the observed properties of the system.
Article
Massive stars are of interest as progenitors of supernovae, i.e. neutron stars and black holes, which can be sources of gravitational waves. Recent population synthesis models can predict neutron star and gravitational wave observations but deal with a fixed supernova rate or an assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernova progenitors, i.e. with O- and early B-type stars, and also all supergiants within 3 kpc. We restrict our sample to those massive stars detected both in 2MASS and observed by Hipparcos, i.e. only those stars with parallax and precise photometry. To determine the luminosities we calculated the extinctions from published multi-colour photometry, spectral types, luminosity class, all corrected for multiplicity and recently revised Hipparcos distances. We use luminosities and temperatures to estimate the masses and ages of these stars using different models from different authors. Having estimated the luminosities of all our stars within 3 kpc, in particular for all O- and early B-type stars, we have determined the median and mean luminosities for all spectral types for luminosity classes I, III, and V. Our luminosity values for supergiants deviate from earlier results: Previous work generally overestimates distances and luminosities compared to our data, this is likely due to Hipparcos parallaxes (generally more accurate and larger than previous ground-based data) and the fact that many massive stars have recently been resolved into multiples of lower masses and luminosities. From luminosities and effective temperatures we derived masses and ages using mass tracks and isochrones from different authors. From masses and ages we estimated lifetimes and derived a lower limit for the supernova rate of ≈20 events/Myr averaged over the next 10 Myr within 600 pc from the sun. These data are then used to search for areas in the sky with higher likelihood for a supernova or gravitational wave event (like OB associations) (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Article
The interest in X/γ-ray Astronomy has grown enormously in the last decades thanks to the ability to send X-ray space missions above the Earth’s atmosphere. There are more than half a million X-ray sources detected and over a hundred missions (past and currently operational) devoted to the study of cosmic X/γ rays. With the improved sensibilities of the currently active missions new detections occur almost on a daily basis. Among these, neutron-star X-ray binaries form an important group because they are among the brightest extra-solar objects in the sky and are characterized by dramatic variability in brightness on timescales ranging from milliseconds to months and years. Their main source of power is the gravitational energy released by matter accreted from a companion star and falling onto the neutron star in a relatively close binary system. Neutron-star X-ray binaries divide into high-mass and low-mass systems according to whether the mass of the donor star is above ∼8 or below ∼2 M⊙, respectively. Massive X-ray binaries divide further into supergiant X-ray binaries and Be/X-ray binaries depending on the evolutionary status of the optical companion. Virtually all Be/X-ray binaries show X-ray pulsations. Therefore, these systems can be used as unique natural laboratories to investigate the properties of matter under extreme conditions of gravity and magnetic field. The purpose of this work is to review the observational properties of Be/X-ray binaries. The open questions in Be/X-ray binaries include those related to the Be star companion, that is, the so-called “Be phenomenon”, such as, timescales associated to the formation and dissipation of the equatorial disc, mass-ejection mechanisms, V/R variability, and rotation rates; those related to the neutron star, such as, mass determination, accretion physics, and spin period evolution; but also, those that result from the interaction of the two constituents, such as, disc truncation and mass transfer. Until recently, it was thought that the Be stars’ disc was not significantly affected by the neutron star. In this review, I present the observational evidence accumulated in recent years on the interaction between the circumstellar disc and the compact companion. The most obvious effect is the tidal truncation of the disc. As a result, the equatorial discs in Be/X-ray binaries are smaller and denser than those around isolated Be stars.
Article
Photometric and spectroscopic results are presented for the Be star X Per/HD 24534 from near-infrared (near-IR) monitoring in 2010–2011. The star is one of a sample of selected Be/X-ray binaries being monitored by us in the near-IR to study correlations between their X-ray and near-IR behaviour. Comparison of the star’s present near-IR magnitudes with earlier records shows the star to be currently in a prominently bright state with mean J, H, K magnitudes of 5.49, 5.33 and 5.06, respectively. The JHK spectra are dominated by emission lines of He i and Paschen and Brackett lines of H i. Lines of O i 1.1287 and 1.3165 μm are also present and their relative strength indicates, since O i 1.1287 is the stronger among the two lines, that Lyman β fluorescence plays an important role in their excitation. Recombination analysis of the H i lines is done, which shows that the Paschen and Brackett line strengths deviate considerably from case B predictions. These deviations are attributed to the lines being optically thick, and this supposition is verified by calculating the line centre optical depths predicted by recombination theory. Similar calculations indicate that the Pfund and Humphrey series lines should also be expected to be optically thick, which is found to be consistent with observations reported in other studies. The spectral energy distribution of the star is constructed and shown to have an IR excess. Based on the magnitude of the IR excess, which is modelled using a free–free contribution from the disc, the electron density in the disc is estimated and shown to be within the range of values expected in Be star discs.
Article
We perform local excitation calculations to obtain line opacities and emissivity ratios and compare them with observed properties of H, He I, O I, Ca II, and Na I lines to determine the density, temperature, and photon ionization rate. We find that UV photoionization is the most probable excitation mechanism for generating the He I 10830 opacities that produce all the associated absorption features. We also calculate the specific line flux at an observed velocity of v_obs = +/- 150 km/s for both radial wind and infall models. All the model results, together with observed correlations between absorption and emission features and between narrow and broad emission components, are used to deduce the origins of the strong H, He I, and Ca II broad line emission. We conclude that the first two arise primarily in a radial outflow that is highly clumpy. The bulk of the wind volume is filled by gas at a density ~10^9 cm^-3 and optically thick to He I 10830 and H alpha, but optically thin to He I 5876, Pa gamma, and the Ca II infrared triplet. The optically thick He I 5876 emission occurs mostly in regions of density greater than or equal to 10^11 cm^-3 and temperature greater than or equal to 1.5x10^4 K, while the optically thick H alpha and Pa gamma emission occur mostly in regions of density around 10^11 cm^-3 and temperature between 8750 and 1.25x10^4 K. In producing the observed line fluxes at a given v_obs, the covering factor of these emission clumps is sufficiently small not to incur significant absorption of the stellar and veiling continua in either He I or H lines. The strong Ca II broad line emission likely arises in both the magnetospheric accretion flow and the disk boundary layer, where the gases dissipate part of their rotational energies before infalling along magnetic field lines. The needed density and temperature are ~10^12 cm^-3 and less than or equal to 7500 K. Comment: Accepted to MNRAS, 88 pages, 24 figures
Article
In the Galaxy there are 64 Be X-ray binaries known to-date. Out of those, 42 host a neutron star, and for the reminder the nature of a companion is not known. None, so far, is known to host a black hole. There seems to be no apparent mechanism that would prevent formation or detection of Be stars with black holes. This disparity is referred to as a missing Be -- black hole X-ray binary problem. We point out that current evolutionary scenarios that lead to the formation of Be X-ray binaries predict that the ratio of these binaries with neutron stars to the ones with black holes is rather high F_NStoBH=10-50, with the more likely formation models providing the values at the high end. The ratio is a natural outcome of (i) the stellar initial mass function that produces more neutron stars than black holes and (ii) common envelope evolution (i.e. a major mechanism involved in the formation of interacting binaries) that naturally selects progenitors of Be X-ray binaries with neutron stars (binaries with comparable mass components have more likely survival probabilities) over ones with black holes (which are much more likely to be common envelope mergers). A comparison of this ratio (i.e. F_NStoBH=30) with the number of confirmed Be -- neutron star X-ray binaries (42) indicates that the expected number of Be -- black hole X-ray binaries is of the order of only 0-2. This is entirely consistent with the observed Galactic sample. Comment: 13 pages, ApJ 2009 (accepted)
Article
Coude spectra of K CMa in the red region show Hα and He i λλ5876, 6678 in emission. Each of the lines has two emission components but the helium lines have no detectable absorption feature in between. While the Hα emission peaks are separated by 160 km sec⁻¹, the helium lines are separated by 400 km sec⁻¹. A simple model is proposed to account for the behavior of these emission lines.
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