Abstracts by session

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Phenomenology of Outbursts

Be stars, Be disks & Models

High Energy Sources

BeXRBs in external Galaxies

Multiwaveband observations

Accretion Physics

Space Missions



Session: Phenomenology of Outbursts

The BeXRB Zoo - An Update

Peter Kretschmar

We present a systematic study of Be X-ray outbursts, where the outburst lightcurves were fitted with simple, phenomenological model shapes. We discuss similarities and differences across sources and typical timescales.

States and Transitions in Be/X-ray pulsars during giant outbursts

Pablo Reig

The definition of source states in low-mass X-ray binaries (LMXB) and black-hole binaries (BHB) has been a very useful way to describe the rich phenomenology exhibited by these systems in the X-ray band. In such studies, color-color diagrams (CDs) and hardness-intensity diagrams (HIDs) in correlation with the analysis of the source aperiodic variability emerged as powerful tools to characterize those systems because it was observed that the source spectral and timing parameters evolve smoothly along the diagrams, describing a pattern. While this methodology has been applied to numerous LMXBs and BHBs, no analogue work exists on high-mass X-ray binaries (HMXB). We present the first detailed correlated spectral and timing study of hard transient HMXBs containing Be stars during giant outbursts. We find that these systems do exhibit different source states and spectral branches.

Transient Be X-ray binary pulsars with MAXI

Tatehiro Mihara

MAXI onboard the International Space Station has been monitoring the whole X-ray sky in every 92 minutes. It can detect the start of an outburst of Be X-ray binaries pulsars (BeXBP) and observe it to the end with good sensitivity. It is useful not only to grasp the whole activity of BeXBP, but also to trigger detailed follow-up observations. About 25 BeXBP are listed in the watch-list in MAXI home page (http://maxi.riken.jp/). MAXI detected about 40 outbursts from about 15 BeXBP. We present MAXI light curves and some advanced studies including normal and giant outburst of A0535+26, cyclotron observations of GX 304-1 and GRO J1008-57 with Suzaku and RXTE and spin acceleration by accretion of GX 304-1 and MAXI J1409-619.

GBM Observations of Be X-ray Binary Outbursts.

Colleen A. Wilson-Hodge/M.H. Finger/P.A. Jenke

Since 2008 we have been monitoring accreting pulsars using the Gamma ray Burst Monitor (GBM) on Fermi. This monitoring program includes daily blind full sky searches for previously unknown or previously quiescent pulsars and source specific analysis to track the frequency evolution of all detected pulsars. To date we have detected outbursts from 23 transient accreting pulsars, including 21 confirmed or likely Be/X-ray binaries. I will describe our techniques and highlight results for selected pulsars.

Superorbital Periodic Modulation in Wind-Accretion High-Mass X-ray Binaries

Robin Corbet

We report the discovery using data from the Swift Burst Alert Telescope (BAT) of superorbital modulation in the wind-accretion supergiant high-mass X-ray binaries 4U 1909+07 (= X 1908+075), IGR J16418-4532, and IGR J16479-4514. Together with already known superorbital periodicities in 2S 0114+650 and IGR J16493-4348, the systems exhibit a monotonic relationship between superorbital and orbital periods. These systems include both supergiant fast X-ray transients (SFXTs) and classical supergiant systems, and have a range of inclination angles. This suggests an underlying physical mechanism which is connected to the orbital period. In addition to these sources with clear detections of superorbital periods, IGR J16393-4643 (= AX J16390.4-4642) is identified as a system that may have superorbital modulation due to the coincidence of low-amplitude peaks in power spectra derived from BAT, RXTE PCA, and INTEGRAL light curves. 1E 1145.1-6141 may also be worthy of further attention due to the amount of low-frequency modulation of its light curve. However, we find that the presence of superorbital modulation is not a universal feature of wind-accretion supergiant X-ray binaries. Two suggested mechanisms to drive superorbital modulation are pulsations in the primary star and a 3 body system. However, both of these models appear to have problems and detailed multiwavelength data over a superorbital cycle are required to investigate the cause(s) of the modulation.

Recent hardness and spectral analysis results of X-Persei

Zeynep Acuner

We present the recent hardness and spectral analysis results of the Be type high-mass X-ray binary X-Persei.Using the observations made over the period 1998 to 2002 by Proportional Counter Array of the Rossi X-ray Timing Explorer,we have traced the changes in soft (3-7 to 7-10 keV) and hard colors (10-15 to 15-20 keV) and compared these trends with the previously published timing and spectral properties. We have discussed the possibility of different accretion modes and the temporary formation of an accretion disk around the pulsar 4U 0352+309 via the quasi-spherical accretion from Be companion's stellar wind.

How circumstellar discs affect mass accretion in Be X-ray binaries

Itumeleng Monageng

Long-term spectroscopic monitoring of Galactic Be X-ray binaries (BeXBs) is performed using the Liverpool Telescope and the Southern African Large Telescope on northern and southern objects, respectively. Analysis of different properties of the H-alpha emission line is presented to study circumstellar disc size variations and its influence on mass accretion resulting in X-ray activity. The work is done in the context of the viscous decretion disc model presented by Okazaki & Negueruela (2001), which predicts that the circumstellar discs around Be stars in binary systems are truncated by resonant torques from the neutron star in its orbit. Type I outbursts are seen to occur when the disc is truncated close to/beyond the mean critical Roche lobe radius at periastron passage of the neutron star, in agreement with model predictions. Type II outbursts, however, do not show any correlation (or anti-correlation) with the disc size, as they are seen to occur at relatively small and large sizes of the disc. Additional information on the H-alpha emission line profile variations, such as the line-shape variations of high-resolution spectra, is required to investigate the origin of type II outbursts in order to make robust predictions of them.

The contrasting behaviour of BeXRBs: how do the orbital parameters affect what we observe?

Lee Townsend

The vast archive of X-ray observations of the Magellanic Clouds provide a unique opportunity to study the properties of a large number of BeXRBs simultaneously. In studying these data, it is becoming more apparent that the orbital parameters and state of the circumstellar disk combine to produce observable properties unique to that system at that time. I will present two very different outbursts from two recently active systems in the SMC and discuss what we can conclude about their orbits and disk state, as well as what we can learn about any given system from it's observable properties.

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Session: Be stars, Be disks & models

Interferometric Observations of Be X-Ray emitters.

Philippe Stee

I will present a multi-technique observing campaign on the Be X-Ray emitter Gamma Cas including CHARA/VEGA and VLTI/AMBER measurements as well as more recent results on HD 110432 which is classified as a “γ Cas X-ray analog.

Simulations of inhomogeneous stellar winds interacting with pulsar winds in high-mass binaries

Valenti Bosch-Ramon

High-mass binaries hosting a non-accreting pulsar present strong interaction between the stellar and the pulsar wind. This interaction can lead to powerful non-thermal emission whose properties can be affected by the structure of the stellar wind. We have studied numerically the influence of inhomogeneities in the stellar wind, as those related to the presence of a circumstellar disc such as the ones found in Be stars, on thestructure of the two-wind interaction region. With this purpose, we have carried out axisymmetric relativistic hydrodynamical simulations, with Lorentz factors of up to 6, to study the impact in the two-wind interaction structure of an overdense region of the stellar wind. We have also followed the evolution of this inhomogeneity as it faces the impact of the pulsar wind. In this talk, we present the results of these simulations, and discuss the impact that stellar wind inhomogeneities may have in the non-thermal emission.

3D simulations of pulsar wind-stellar wind interacion in gamma-ray binaries

Manel Perucho

I will present our current work on simulations of pulsar wind-stellar wind collisions in massive binary systems hosting a young pulsar. This interaction between the relativistic wind of the pulsar and the strong wind from a massive star has been proposed as a possible mechanism to explain high-energy emission from galactic sources. Our simulations are performed with the RHD code Ratpenat within supercomputing resources at the University of València. I will put some time to discuss the computational challenge that this kind of simulations imply, and report on our first results and relevant conclusions regarding emission of very-high energy radiation from those systems.

Dynamical Modeling of the Be/Gamma-ray Sources HESS~J0632$+$057 and AGL~J2241$+$4454

Atsuo Okazaki

Several Be binaries have been established as gamma-ray sources. In this talk, we focus on two of such sources, HESS J0632$+$057 and AGL J2241$+$44549. HESS~J0632$+$057 is a gamma-ray binary (a binary with peak energy above 1\,MeV) composed of a Be star and a compact object of unknown nature. The orbit is wide ($P_\mathrm{orb} \sim 315\,\mathrm{d}$) and highly eccentric ($e=0.83$). It exhibits two X/gamma-ray outbursts per orbit, both of which occur when the compact object is far from periastron. Carrying out 3D SPH simulations and high-energy calculations, we find that the basic observed features of this system are caused by the interaction between a huge Be disk and a relativistic pulsar wind. AGL J2241$+$4454 is a Be binary recently found to host a black hole in a low-eccentricity ($e \sim 0.1$) orbit (Casares et al. 2014). Currently, no information is available about the B-disk size and inclination from the orbital plane. Thus, as a first step towards the understanding of the interaction between the black hole and the Be disk, we perform SPH simulations for various inclination angles and discuss the resulting mass-accretion rate and accretion flow characteristics.

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Session: High Energy

Be disk variability by the interaction in TeV-gamma ray binaries

Yuki Moritani

TeV-gamma ray binaries are puzzling systems with unknown compact object and massive star. There are two models advocated for the nature of the systems and hence the mechanism of very high energetic emissions: pulsar wind model and accretion model. At present, five systems is classed as TeV-gamma ray binaries, but the model has been established in only one system, in spite of a variety of observational and theoretical researches. Recently, we have simulated the interaction between the Be disk and the compact object with SPH code, in terms of each model. The predicted Be disk structure has different variation between the two models. Therefore, observing such a variability should give a clue to the nature of the compact object. Motivated by this, we have monitored two TeV-gamma ray binaries, LS I +61 303 and HESS J0632+057, using optical high-dispersion spectrography. In this talk, we discuss the interaction between the Be disk and the compact object, and the nature of the systems.

Changes in the circumstellar envelope of the gamma-ray binary LS I +61 303

Xavier Paredes-Fortuny

Three gamma-ray binaries, namely PSR B1259-63, HESS J0632+057 and LS I +61 303, contain compact objects orbiting massive Be stars. Around periastron passage the compact objects should produce significant changes in the structure of the Be disks due to gravitational forces and eventually by ram pressure from the putative pulsar wind. Indeed, variability in the H alpha emission line has been detected in all these systems, and photometric optical periodic variability has been detected in the case of LS I +61 303. We are performing a systematic monitoring of accurate optical photometry for the northern sources with the Telescope Fabra ROA Montsec. In the case of LS I +61 303 the ongoing observations span from 2012 July 30 to 2014 February 5 and cover twelve 26.5 days orbital cycles. The obtained results reveal the known orbital periodic variability and cycle to cycle variability. The orbital variability displays a maximum to minimum difference of 0.08 mag, which is compatible with a change of the disk emission (and area) of a 20%, increasing towards apastron. The cycle to cycle variability and the observed night to night variability amount to 0.02 mag, implying changes of the disk emission (and area) of a 5%. A preliminary interpretation of these results is provided.

Variable very low frequency radio emission from the gamma-ray binary LS I +61 303

Benito Marcote

Gamma-ray binaries are systems consisting on a compact object and a massive star, which display a spectral energy distribution dominated by the gamma rays. The gamma-ray binary LS I +61 303 hosts a Be star with a compact object, whose nature still remains unknown. Here we present the results obtained with LOFAR and GMRT observations of the very low frequency radio emission of LS I +61 303, which can provide constraints to understand the ongoing absorption processes. We have detected for the first time the source at 150 MHz with LOFAR, showing variability on week time scales. With GMRT at 600 MHz we have detected an orbital modulation similar to that reported at GHz frequencies, which is not present at 230 MHz. We provide a preliminary interpretation of the obtained results in the context of the LS I +61 303 radio outbursts and discuss possible absorption effects at low radio frequencies.

Multi-frequency phenomenology of the superorbital variability of LS I +61 303

Diego F. Torres

LS I +61 303 is a high mass X-ray binary hosting a B0 Ve star with an equatorial outflowing disk. Besides the orbital period of 26.496 days, LS I +61 303 is known to have a long-term (1667 days) superorbital modulation. The superorbital variability of LS I +61 303 was first discovered in radio and Halpha emission line, and our recent studies have searched for it in X-rays, GeV, and TeV bands. In this talk we will mention these recent findings, and present new analysis of the years-long radio and hard X-ray data on the source. Even when non-simultaneous, enough hard X-ray data at the same orbital and super orbital phases of the radio measurements already exist to consider possible relationships between the spectral indices in the two bands. Using these data sets, we will uncover hints for the superorbital variability in the hard X-ray flux and show how an orbital phase drift of radio peak flux and index along the superorbital period is observed in the radio data. We explore its influence on a previously reported double peak structure of radio orbital lightcurve, posing it as a plausible explanation, and making further considerations of the long-term power spectrum of the system's emission.

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Session: BeXRBs in external galaxies

BeXRBs in the SMC: how many are there?

Haberl Frank

The Small Magellanic Cloud (SMC) is unique with respect to its high number of known Be/X-ray binaries (BeXRBs). More than 140 BeXRBs and candidates can be found in the literature. While many of them show properties which clearly demonstrate their BeXRB nature (X-ray pulsations, typical X-ray spectrum and variability in X-rays and optical, proper optical counterpart) it is not so obvious for others. I present a new compilation of BeXRBs in the SMC together with statistical investigations which allows to divide them into different classes of different probability for being genuine BeXRBs.

Investigating the Be X-ray Binary population of the Small Magellanic Cloud

Grigoris Maravelias

Although the number of identified Be X-ray Binaries (BeXRBs) has increased dramatically in the last decade, only recently we started to obtain a better picture of their spectral-type distributions, which in turn can provide us with information and constrains regarding their evolution and nature. In order to further investigate this picture we performed a systematic spectroscopic campaign for High-Mass X-ray Binaries (HMXBs) identified in Chandra and XMM-Newton surveys of the Small Magellanic Cloud (SMC), using the AAOmega/2dF spectrograph at the Anglo-Australian Observatory. We classified 21 sources, 5 of which are new BeXRBs and one is the well known supergiant B[e] star S18 (or AzV 154). We suggest that it is the optical counterpart of the X-ray source CXOU J005409.57-724143.5, which is either a highly obscured wind-fed supergiant X-ray binary or a colliding-wind system. The classified sources extend our census of BeXRBs to almost quiescent luminosities and later spectral types. A comparison of the populations of BeXRBs in the SMC and the Milky Way with respect to their spectral types

shows that there is evidence (at 99% confidence level) for difference in their populations, while their orbital parameters (periods and eccentricities) show no such evidence. In order to expand the sample of BeXRBs we performed a systematic survey on selected active regions of the SMC. Since the optical counterparts of the BeXRBs display Halpha excess they can be identified through Halpha imaging. Thus, we observed 7 regions with the Wide Field Imager at MPG/ESO telescope. The analysis of these data yields approximately 24000 identified Halpha emitting stars. We discuss the photometric parameters of these sources (Halpha EW, broad-band optical and IR colors), and their correlation with the X-ray sources identified in the Chandra and XMM-Newton surveys.

Spin period change, pulse profiles and the magnetic fields of neutron stars in Be X-ray binaries in the SMC.

Helen Klus

We report on the long term average spin period, rate of change of spin period and X-ray luminosity during outbursts for 42 Be X-ray binary systems in the Small Magellanic Cloud. We also collect and calculate parameters of each system and use this data to determine that all systems contain a neutron star which is accreting via a disc, rather than a wind, and that if these neutron stars are near spin equilibrium, then over half of them, including all with spin periods over about 100 seconds, have magnetic fields over the quantum critical level of 4.4×10 13 G. If these neutron stars are not close to spin equilibrium, then their magnetic fields are inferred to be much lower, on the order of 10 6 -10 10 G, comparable to the fields of neutron stars in low mass X-ray binaries. Both results are unexpected and have implications for the rate of magnetic field decay and the isolated neutron star population. In order to investigate this further we look at the pulse profiles of individual systems which can indicate the position and structural geometry of the emitting regions.

A deep X-ray view of our second nearest-star forming galaxy

Vallia Antoniou

We present the first results from the Chandra Deep observations of the Small Magellanic Cloud (SMC). This project aims in the identification of the nature of the X-ray sources detected in the 1.1 Ms Chandra survey of the SMC, which will detect all X-ray sources brighter than about few times 10^32 erg/s in 11 fields representing young (10-100 Myr) stellar populations of different ages, thus providing the deepest X-ray luminosity functions (XLF) for X-ray binaries (XRBs) ever recorded. The main scientific driver behind these observations is to study how accreting XRBs form and evolve and to provide observational constraints on the mass-transfer mechanisms. In particular, these observations will allow us to investigate the properties of the overall accreting XRB population, and the relative contribution of the various components in the low-metallicity (Z~0.2Z⊙) environment of the SMC. All X-ray data have been processed through a pipeline developed by our team at CfA and we now have the source parameters (including photometry, spectral fits, and light-curves). The first X-ray source lists have been cross-correlated with comprehensive optical and IR photometric and spectroscopic catalogs (such as the OGLE and SAGE). We have determined the most likely optical counterpart for those sources, while based on the combination of their X-ray and optical/IR properties, we identified candidate Be-XRBs and interlopers such as foreground stars and AGN. We study the population of high-mass XRBs (HMXBs), their clustering with star-forming regions and OB associations and the long-term variability of the identified sources based on their comparison with previous shallow surveys of the SMC with XMM-Newton and Chandra.

The faint end of the massive X-ray binary population of the SMC

Vanessa McBride

A recent XMM Newton survey of the Small Magellanic Cloud has probed the X-ray population down to a luminosity of ~1E33 erg/s, revealing a large number of new, faint X-ray sources. Based on their X-ray and optical colours, a sample of these are identified as massive X-ray binaries, which comprise the entire known SMC X-ray binary population. We present optical spectroscopy of this sample and compare the results to the currently known populations of massive X-ray binaries in the SMC, LMC and the Galaxy.

Long-term properties of Be/X-ray binaries in the SMC.

Andry Rajoelimanana

The Magellanic Clouds, particularly the SMC, host a large number of high-mass X-ray binaries (HMXBs) when compared to our Galaxy. The majority of these HMXBs are Be/X-ray binaries (BeX) in which a neutron star orbits a rapidly rotating Be star in a very wide (P ~ 20-200 d) and eccentric orbit. I will present my work on the long-term temporal properties of these systems, using optical light curves from the MACHO and OGLE monitoring of the Small Magellanic Cloud. All the BeX in our sample display long-term quasi-periodic variations on timescales of ~ 200-3000 d. These superorbital modulations are believed to be related to the formation and depletion of the circumstellar disc around the Be star. I will also present some recent SALT/RSS optical spectroscopic observations of these systems during their optical high and low states.

New BeXRBs in the LMC

Vasilopoulos Georgios

Nearby galaxies are well suited for investigating X-ray source populations in different environments besides our own Galaxy. Moreover, sources in these galaxies have well determined distances and are less absorbed than sources in the galactic plane. The Large (LMC) and the Small (SMC) Magellanic Clouds (MC) are the nearest gas-rich star-forming galaxies and their gravitational interactions are believed to have tidally triggered recent bursts of star formation. The XMM-Newton large program for the LMC has just been completed and covers an area of about 10 square degrees, while reaching a limiting luminosity of 10^32 erg/s and provide a unique data set for X-ray source population studies. Among the most interesting results of the survey is the discovery of new Be/XRBs. Here, we present an overview of some newly confirmed Be/XRB pulsars in the LMC (e.g. RX J0520.5-6932). To investigate the spectral and temporal characteristics of the sources, we used XMM-Newton and Swift X-ray data, along with optical observations (Spectroscopy and lighcurves). Apart from the obvious interest in the properties of individual sources, the detection of a more complete sample of Be/XRBs in the LMC is necessary for the study of their population and the comparison with other galaxies, like the SMC that already counts close to 100 confirmed or candidate Be/XRBs.

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Session: Multiwaveband observations

Multi-wavelength Studies of Be/X-ray Binaries

Jingzhi YAN

In this talk, I will present our optical photometric and spectroscopic observations on several Be/X-ray binaries, A 0535+26, MXB 0656-072, LSV+44 71, and X Per. An anti-correlation between the optical brightness and the intensity of the Halpha line was found these systems. The anti-correlation might be connected with the mass ejection event from the Be star, which could trigger the following X-ray outbursts.

Optical and X-ray observations of XTE J1946+274

Mehtap Özbey Arabacı

In this study we have investigated the relation between the optical brightness, Halpha equivalent width and X-ray flux of XTE J1946+274 , by analysing the optical photometric and spectroscopic observations together with the archival X-ray observations. The source is one of the poorly studied Be/X-ray systems in the optical band therefore with the long-term photometric (2007-2013) and spectroscopic (2007-2014) observations a comprehensive study of the companion will be presented for the first time.

Monitoring of BeXRBs with the Faulkes Telescopes

Pere Blay

We are developing a monitoring campaign with the Faulkes Telescopes. It involves the participation of High School students in the observation and analysis of the images. We are monitoring changes in R and I filters and in the R-Halpha index, in order to study their long term variations and search for possible correlations of this variations with data from other wavelengths.

Multiwavelength monitoring of SAXJ2103.5+4545

Ascension Camero

We will present the first results of our multiwavelength campaign for SAX J2103.5+4545. This source was observed by Swift-XRT four times in 2007 from April 25 to May 5, and during quiescence in 2012 August 31. In addition, this system was monitored from the ground-based astronomical observatories of El Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain) and Sierra Nevada (Granada, Spain) since 2011 August, and from the TUBITAK National Observatory (Antalya, Turkey) since 2009 June. We have performed spectral and photometric temporal analysis in order to investigate the transient behaviour exhibited by this source since 2007. These new observations were put into the context of historical data and discussed in terms of the neutron star Be-disc interaction.

A Be star with a black hole companion

Ignacio Negueruela

The Be star MWC 656 (= HD 215227) has been proposed as the optical counterpart of the gamma-ray source AGLJ2241+4454. Analysis of its lightcurve suggested a periodicity of 60.4 d. Radial velocity measurements confirmed that the star is in a binary with this periodicity. We report the detection of a double-peaked He II 4686 emission line in the optical spectra that is the signature of an accretion disk. Its radial velocity curve mirrors that of the Be star, proving that it arises from the unseen companion. This, together with an improved radial velocity curve of the Be star through fitting sharp Fe II profiles from the equatorial disk reveals a 3.8–6.9 M black hole orbiting MWC 656. The black hole is X-ray quiescent and fed by a radiatively inefficient accretion flow. This implies that Be binaries with black hole companions are difficult to detect by conventional X-ray surveys, explaining why none had been detected until now. They are likely much more numerous than previously thought.

Revealing the X-ray counterpart of the first Be/BH binary system

Pere Munar-Adrover/Josep Maria ParedesJ.M. Paredes/ M. Ribó/K. Iwasawa/V. Zabalza/J. Casares

MWC 656 is a Be star with a black hole (BH) companion, being the first and unique Be/BH binary system found. We have observed MWC 656 with XMM-Newton and detected X-ray emission for the first time. The discovery of the X-ray counterpart implies that MWC 656 is also the first Be/BH X-ray binary found. The spectral analysis requires a model fit with two components: a thermal one probably arising in the hot wind of the Be star, and a non-thermal one probably arising from the vicinity of the BH. The derived non-thermal luminosity of the system, together with radio luminosity upper limits, makes MWC 656 compatible with the radio/X-ray luminosity correlation found for LMXBs. MWC 656 is located at the level of the faintest detected LMXBs, thus suggesting that this correlation might also be valid for HMXBs with X-ray luminosities down to ~10^-8 L_Edd.

Optical orbital variability in the first Be/BH binary system

Marc Ribó

MWC 656 has recently been discovered as the first binary system hosting a Be star and a black hole (BH), which also displays faint non-thermal X-ray emission. We are conducting a monitoring of optical photometry of MWC 656 with the Telescope Fabra ROA Montsec. We report here observations spanning around 1.5 years and covering 7 different orbital cycles. We detect the known orbital variability of the source, with an amplitude compatible with previous observations. We discuss here on the nature of this variability considering the low eccentricity of the binary, the reported orbital variability of the equivalent width of the He II line originated in the accretion disk, and the presence of a hot spot in the outer part of the accretion disk. We also provide an updated orbital period combining our data set with archival optical photometry.

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Session: Accretion Physics

GRO J1008-57 - a textbook BeXRB

Matthias Kühnel

The neutron star Be X-ray transient GRO J1008-57 is a particularly interesting source that is the perfect example of the textbook behaviour for this type of objects: each time the neutron star passes the periastron, type I X-ray bursts are observed and the neutron star spins up. Astonishing, the evolution of the X-ray spectrum over any outburst depends on only one single parameter, the source's flux. To the authors' knowledge, this behaviour has never been observed for an accreting neutron star before and its explanation will require future theoretical work. In particular, the power-law photon index and the black-body flux are the only parameters showing a dependency on the overall flux. Nevertheless, there are already promising agreements to recent theoretical attempts to describe the spectrum of accreting neutron stars, such as subcritical thermal and bulk Comptonization. GRO J1008-57 is thus the ideal candidate to study differences to other sources and to revise the simple picture of Be X-ray transients.

Pulse phase resolved spectroscopy of cyclotron lines in accretion powered pulsars

Chandreyee Maitra

We present results of pulse phase resolved spectroscopy of the Cyclotron Resonance Scattering Features (CRSF) of some bright accretion powered X-ray pulsars like 1A 1118-61, Vela X-1, A0535+26, XTE J1946+274, 4U 1907+09, 4U 1626-67 and GX 301-2 using Suzaku observations. The study is performed using different spectral models for the continuum and have obtained similar patterns of variations of the CRSF in all the cases, thus demonstrating the robustness of our results. Pulse phase dependence of the CRSF in XTE J1946+274 has been obtained for the first time, and phase resolved variations of the CRSF in 4U 1907+09 has been compared at factor of ~ 2 difference in luminosity. We have also studied the pulse profiles of these objects near the CRSF energy, and have noticed an increased pulse fraction and/or a change in the pulse shape near the CRSF energy for some sources. The implications of the results and prospects of future observations with ASTROSAT and ASTRO-H are discussed.

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Session: Space Missions

Neutron Star Interior Composition Explorer (NICER)

Colleen A. Wilson-Hodge/K. Gendreau/Z. Arzoumanian

The Neutron Star Interior Composition Explorer (NICER) is an approved NASA Explorer Mission of Opportunity dedicated to the study of the extraordinary gravitational, electromagnetic, and nuclear-physics environments embodied by neutron stars. Scheduled to be launched in 2016 as an International Space Station payload, NICER will explore the exotic states of matter, using rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft (0.2-12 keV) X-ray band. Grazing-incidence “concentrator” optics coupled with silicon drift detectors, actively pointed for a full hemisphere of sky coverage, will provide photon-counting spectroscopy and timing registered to GPS time and position, with high throughput and relatively low background. The NICER project plans to implement a Guest Observer Program, which includes competitively selected user targets after the first year of flight operations. I will describe NICER and discuss ideas for potential Be/X-ray binary science.

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RXTE and Swift observations of SWIFT J1729.9-343

Altan Baykal

We analyse Rossi X-ray Timing Explorer (RXTE) and Swift observations of SWIFT J1729.9-3437 after its outburst from 2010 July 20 to 2010 August 12. We calculate a spin frequency and spin frequency derivative of 1.8734(8) × 10-3 Hz and 6.42(6) × 10-12 Hz s-1, respectively from the quadratic fit of the pulse arrival times. The quadratic fit residuals fit well to a circular orbital model with a period of 15.3(2) d and a mass function of about 1.3 M⊙, but they can also be explained by a torque noise strength of 6.8 × 10-18 Hz s-2. Pulse profiles switch from double peaked to single peaked as the source flux continues to decrease. We find that the pulse shape generally shows no strong energy dependence.

Formation of Be stars in binaries and the spin period evolution in Be/X-ray binaries

Xiangdong Li

We investigate the formation of Be stars originating from binaries in the Galaxy. Through binary population synthesis calculations, we present the predicted numbers and characteristics of Be stars in binary systems with different types of companions, including helium stars, white dwarfs, neutron stars, and black holes. We find that in Be/neutron star binaries the Be stars can have a lower limit of mass $ \sim 8 M_{\odot}$ if they are formed due to stable mass transfer (i.e., without the occurrence of common envelope evolution). We also demonstrate that isolated Be stars may originate from both mergers of two main-sequence stars and disrupted Be binaries during supernova explosions, but mergers seem to play a more important role. It was recently suggested that the distribution of the spin periods in For Be/X-ray binaries contains two distinct subpopulations peaked at $\sim 10$ s and $\sim 200$ s, which are related to electron-capture and core-collapse supernovae respectively. By comparing the outburst behavior of the two subpopulations, we propose that the bimodal spin period distribution is likely to be ascribed to different accretion modes of the neutron stars, with an advection-dominated or radiatively-cooling dominated disk, respectively.

Insights into the High-Mass X-ray Binary Population of the Large Magellanic Cloud

Vallia Antoniou

In contrast to the Small Magellanic Cloud (SMC), the Large Magellanic Cloud (LMC), our nearest star-forming galaxy with metallicity between the Galaxy and the SMC, has received little attention in X-rays so far. With the aim to compare the accreting X-ray binary (XRB) populations in two of our nearest star-forming galaxies, we recently compiled the most complete census of high-mass X-ray binaries (HMXBs) in the LMC. We found 43 members of which 14 are XRB pulsars, while we also identified their most likely optical counterpart (previously, half of these sources lacked an identification). Using this census, we investigated the link between the young accreting XRBs and their parent stellar populations. It was known that HMXBs can be used as star-formation (SF) rate indicators, but these first studies have been focused only on bright systems (Galaxy: >10^38 erg s−1; Magellanic Clouds: >10^36 erg s−1) and SF values for the whole galaxy. By including Magellanic Cloud sources with X-ray luminosities at least two order of magnitudes fainter than the above limits and by utilizing the detailed, spatially resolved, SF history maps of these galaxies, we were able to provide observational constraints on ill-understood parameters related to their formation and evolution (such as the kick velocities imparted into the neutron star during the supernova explosion) and to derive their formation efficiency. This work was mainly supported by the National Aeronautics and Space Administration under Grant No. NNX10AH47G issued through the Astrophysics Data Analysis Program.

A Photometric Classification of the SAGE LMC Point Source Catalog

Vallia Antoniou

The Spitzer SAGE Large Magellanic Cloud (LMC) photometric catalog lists over 7.2 million sources detected with the Spitzer space telescope. This catalog has been cross-correlated with the near-IR 2MASS and IRSF surveys, as well as with optical photometry from the MCPS database. This combined catalog is an invaluable tool for the study of LMC stellar populations, and to understand the life cycle of gas and dust in the LMC. To fully leverage the broad wavelength coverage of this database, we have developed a novel technique for the photometric classification of the SAGE LMC point source catalog, based on a weighted k-Nearest Neighbor method. This technique allows the classification of most LMC sources based on templates that have been identified spectroscopically, without a-priori assumptions about their spectral characteristics. We present here the details of our classification method, and the main results of our work, while we also discuss the IR properties of the high-mass X-ray binary population of our nearest star-forming galaxy.

Density in Be disks of Be/X-ray binaries estimated from X-ray outburst light curves observed with MAXI all-sky survey

Toshihiro Takagi

We estimated gas densities of Be disks in Be/X-ray binaries using X-ray outburst light curves obtained with MAXI GSC all-sky survey. These outbursts are considered to emerge when the neutron star passes through the Be disk, where the X-ray emission varies according to the mass-accretion rate. Assuming the Hoyle-Lyttleton accretion, we can calculate the density of the Be disk through which the neutron star orbited. Using data of the 2009 December outburst of A 0535+262, we obtained the estimate of the density at 1.3e-15 g cm-3. The value is comparable to that suggested from the Be-disk model bases on optical H-alpha observations. We also estimated the Be-disk densities of GX 304-1, GRO J1008-57, and 4U 0115+63 at 6.8e-16 g cm-3, 2.7e-15 g cm-3, and 1.9e-14 g cm-3, respectively, from their outburst light-curve data. These difference may suggest the spacial density variation in the Be disks.

Modelling Be Star Circumstellar Disks with Bedisk/Beray

Aaron Sigut

The Bedisk/Beray suite of codes can model the thermal structure and observational diagnostics of the circumstellar disks surrounding Be stars. Given the density structure of the disk, predicted observables include monochromatic images, spectral energy distributions, and emission line profiles for hydrogen, helium, and metallic lines. Full gravitational darkening (temperature variation and distortion) of the rapidly rotating, central star is supported. This contribution will illustrate many of the capabilities of Bedisk/Beray code suite.

IPHAS Be stars on Perseus arm and beyond in the BCD classification system

Gkouvelis Leonardos

IPHAS Be stars on Perseus arm and beyond in the BCD classification system

Cyclotron Lines in Be XRBs

Joern Wilms

I will give a summary of recent observations and modeling of cyclotron lines in Be X-ray binaries.

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