Observations of Her X-1 in low states during SRG/eROSITA all-sky survey

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) instrument onboard the Russian-German ‘Spectrum-Roentgen-Gamma’ (SRG) mission observed the Her X-1/HZ Her binary system in multiple scans over the source during the first and second SRG all-sky surveys. Both observations occurred during a low state of the X-ray source when the outer parts of the accretion disk blocked the neutron star from view. The orbital modulation of the X-ray flux was detected during the low states. We argue that the detected X-ray radiation results from scattering of the emission of the central source by three distinct regions: (a) an optically thin hot corona with temperature ~(2−4) × 106 K above the irradiated hemisphere of the optical star; (b) an optically thin hot halo above the accretion disk; and (c) the optically thick cold atmosphere of the optical star. The latter region effectively scatters photons with energies above 5–6 keV.

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X-Ray Transients in the SRG/eROSITA All-Sky Survey

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318002430 ◽

2017 ◽

Vol 14 (S339) ◽

pp. 145-145

Author(s):

A. Rau

Keyword(s):

Survey Data ◽

Time Domain ◽

Scanning Strategy ◽

Telescope Array ◽

X Ray ◽

Sky Survey ◽

Wide Range ◽

Imaging Telescope

AbstracteROSITA (the extended ROentgen Survey with an Imaging Telescope Array) onboard the Spectrum Roentgen Gamma mission will perform a deep all-sky X-ray survey. During the first four years of operation the satellite will scan the entire sky every once every half year, visiting any position between 8 and 500 times. The eROSITA scanning strategy will test a wide range of times-scales, from seconds to years, and thus provide a powerful window into the X-ray transient and variable sky. This contribution summarised the key science opportunities for time-domain studies with eROSITA, and presented strategies for finding transients in the all-sky survey data.

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On the nature of the 35-day cycle in the X-ray binary Her X-1/HZ Her

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319002151 ◽

2018 ◽

Vol 14 (S346) ◽

pp. 281-287

Author(s):

N. Shakura ◽

D. Kolesnikov ◽

K. Postnov ◽

I. Volkov ◽

I. Bikmaev ◽

...

Keyword(s):

Neutron Star ◽

Accretion Disk ◽

Orbital Plane ◽

Outer Edge ◽

Model Parameters ◽

Free Precession ◽

X Ray ◽

Synchronization Mechanism ◽

Optical Star ◽

Optical Flux

AbstractThe X-ray binary Her X-1 consists of an accreting neutron star and the optical companion HZ Her. The 35-day X-ray variability of this system is known since its discovery in 1972 by the UHURU satellite and is believed to be caused by forced precession of the warped accretion disk tilted to the orbital plane. We argue that the observed features of the optical variability of HZ Her can be explained by free precession of the neutron star with a period close to that of the forced disk precession. The model parameters include a) the intensity (power) of the stream of matter flowing out of the optical star; b) the X-ray luminosity of the neutron star; c) the optical flux of the accretion disk; d) the X-ray irradiation pattern on the donor star; e) the tilt of the inner and outer edge of the accretion disk. A possible synchronization mechanism based on the coupling between the neutron star free precession and the dynamical action of non-stationary gas streams is discussed shortly.

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Hardness Ratio in Evolving Low-Mass X-ray Binary Systems

Symposium - International Astronomical Union ◽

10.1017/s0074180900160723 ◽

1987 ◽

Vol 125 ◽

pp. 199-199

Author(s):

J. Shaham ◽

M. Tavani

Keyword(s):

Neutron Star ◽

Accretion Disk ◽

Binary Systems ◽

Spectral Component ◽

Soft Component ◽

Hard Component ◽

X Ray ◽

Low Mass ◽

Additional Support ◽

X Ray Binaries

Spectral observations of low-mass X-ray binaries (LMXBs) show that the soft component usually dominates over the hard one. These results provide additional support to an interpretation based on models of LMXBs in which the neutron star while, on the average, spinning up, is also experiencing a spinning down torque. Under these conditions, a fraction of the luminosity associated with the gravitational release of energy on the surface of the accreting neutron star may manifest itself as luminosity originating in the inner part of the accretion disk. It is probably possible to separate the two contributions; the stellar luminosity can be associated with the hard component of the spectrum and the disk luminosity, related to the exchange of energy due to the torque between the rapidly spinning neutron star and the accretion disk, can be associated with the soft spectral component.

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The Nature of the Compact Object in the Hard X-Ray Source 4U2206+54

International Astronomical Union Colloquium ◽

10.1017/s0252921100152546 ◽

2004 ◽

Vol 194 ◽

pp. 208-208

Author(s):

J. M. Torrejón ◽

I. Kreykenbohni ◽

A. Orr ◽

L. Titarchuk ◽

I. Negueruela

Keyword(s):

Neutron Star ◽

Accretion Disk ◽

Compact Object ◽

High Energy ◽

Energy Data ◽

X Ray ◽

Hot Plasma ◽

Cyclotron Line ◽

First Time

We present an analysis of archival RXTE and BeppoSAX data of the X-ray source 4U2206+54. For the first time, high energy data (≥ 30 keV) is analyzed. The data is well described by comptonization models in which seed photons with temperatures between 1.1 keV arid 1.5 keV are comptonized by a hot plasma at 50 keV thereby producing a hard tail which extends up to 100 keV. From luminosity arguments it is shown that the area of the soft photons source must be small (r ≈ 1 km) and that the presence of an accretion disk in this system is unlikely. Here we report on the possible existence of a cyclotron line around 30 keV . The presence of a neutron star in the system is strongly favored by the available data.

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Distribution and Space Density of Soft X-ray Emitting Polars in the Solar Neighbourhood

International Astronomical Union Colloquium ◽

10.1017/s0252921100071062 ◽

1997 ◽

Vol 166 ◽

pp. 247-250

Author(s):

H.-C. Thomas ◽

K. Beuermann

Keyword(s):

Cataclysmic Variables ◽

Interstellar Space ◽

X Rays ◽

Absorption Column ◽

Local Bubble ◽

X Ray ◽

Sky Survey ◽

Imaging Telescope ◽

Lower Limits ◽

Typical Temperature

The ROSAT All Sky Survey (RASS) was the first one performed with an imaging telescope in the soft X-ray regime and has led to the discovery of numerous new objects whose emission is dominated by soft X-rays. Among these are white dwarfs and a subclass of the cataclysmic variables (CVs), the Polars or AM Herculis binaries. From a pre-ROSAT census of only 17, the number of known sources of this class has increased to some 55 (Beuermann and Thomas 1993, Beuermann 1997). Distances or lower limits to the distance are available for some 35 of these, based on the detection or non-detection of the TiO-Features in their optical red spectra. The derived distances range from below 100 pc up to ~ 600 pc, implying that many of these objects are located within the “Local Bubble” of low gas density in interstellar space. As the soft X-ray emission can be reasonably well represented by blackbody emission with a typical temperature of kTbb ≃ 25 eV, spectral fits to the ROSAT PSPC spectra from either the All-Sky-Survey (RASS) or from subsequent pointed ROSAT observations allow to determine the foreground absorption column density in the direction of the polars.

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X-ray selected Wolf-Rayet stars: The discovery of Th35-42 and prospects for future X-ray surveys

Symposium - International Astronomical Union ◽

10.1017/s0074180900202507 ◽

1995 ◽

Vol 163 ◽

pp. 481-485

Author(s):

S. Mereghetti ◽

T. Belloni ◽

F. Haberl ◽

W. Voges

Keyword(s):

Neutron Star ◽

Emission Line ◽

Optical Spectroscopy ◽

Early Type ◽

X Ray ◽

Sky Survey ◽

Colliding Winds ◽

Einstein Observatory

The X-ray source 1E 1024.0-5732, serendipitously discovered with the Einstein Observatory, was previously interpreted as a rapidly spinning neutron star, accreting matter from its massive early-type companion, the emission-line star Th35-42. However, new ROSAT data do not support the presence of a neutron star in this source. A more likely scenario, also indicated by recent optical spectroscopy, involves X-ray emission from the colliding winds of a WR+O binary. Surprisingly, this star remained unnoticed during an extensive optical search for new Wolf-Rayet stars, carried out in this region of sky. Stimulated by the discovery of this first X-ray selected WR star, we have undertaken a search for similar objects in the data from the ROSAT All Sky Survey.

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What will eROSITA reveal among X-ray faint isolated neutron stars?

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317009590 ◽

2017 ◽

Vol 13 (S337) ◽

pp. 112-115

Author(s):

Adriana M. Pires

Keyword(s):

Neutron Star ◽

Neutron Stars ◽

Radio Pulsar ◽

X Rays ◽

Evolutionary Models ◽

X Ray ◽

Sky Survey ◽

Γ Ray ◽

Unique Potential

AbstractSince the discovery of the first radio pulsar fifty years ago, the population of neutron stars in our Galaxy has grown to over 2,600. A handful of these sources, exclusively seen in X-rays, show properties that are not observed in normal pulsars. Despite their scarcity, they are key to understanding aspects of the neutron star phenomenology and evolution. The forthcoming all-sky survey of eROSITA will unveil the X-ray faint end of the neutron star population at unprecedented sensitivity; therefore, it has the unique potential to constrain evolutionary models and advance our understanding of the sources that are especially silent in the radio and γ-ray regimes. In this contribution I discuss the expected role of eROSITA, and the challenges it will face, at probing the galactic neutron star population.

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UBV-Photometry and 35–day Cycle Model for X-ray Binary HZ Her/Her X–l

International Astronomical Union Colloquium ◽

10.1017/s0252921100042895 ◽

1997 ◽

Vol 163 ◽

pp. 379-383

Author(s):

N. I. Shakura ◽

A. V. Smirnov ◽

N. A. Ketsaris

Keyword(s):

Neutron Star ◽

Detailed Analysis ◽

Accretion Disk ◽

Free Precession ◽

Cycle Model ◽

X Ray ◽

Ubv Photometry

AbstractBased on the detailed analysis of UBV–photometry of HZ Her obtained in 1972–1994, a model of 35–day cycle is proposed in which both forced precession of a tilted accretion disk and free precession of the neutron star are present.

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The Optical Counterpart of SAX J1808.4-3658 in Quiescence: Evidence of an Active Radio Pulsar?

International Astronomical Union Colloquium ◽

10.1017/s0252921100151772 ◽

2004 ◽

Vol 194 ◽

pp. 25-27

Author(s):

L. Burderi ◽

T. Di Salvo ◽

F. D'Antona ◽

N. R. Robba ◽

V. Testa

Keyword(s):

Neutron Star ◽

Accretion Disk ◽

Optical Emission ◽

Rotational Energy ◽

Radio Pulsar ◽

Optical Counterpart ◽

Companion Star ◽

X Ray ◽

Optical Part ◽

Intrinsic Emission

AbstractThe optical counterpart of the binary millisecond X-ray pulsar SAX J 1808.4-3658 during quiescence was detected at V = 21.5 mag, inconsistent with intrinsic emission from the fain companion star. We propose that the optical emission from this system during quiescence is due to the irradiation of the companion star and a remnant accretion disk by the rotational energy released by the fast spinning neutron star, switched on, as magneto-dipole rotator (radio pulsar). In this scenario the companion behaves as a bolometer, reprocessing in optical part of the power emitted by the pulsar. The reprocessed fraction depends only on known binary parameters. Thus the blackbody temperature of the companion can be predicted and compared with the observations. Our computations indicate that the observed optical magnitudes are fully consistent with this hypothesis. In this case the observed optical luminosity may be the first evidence that a radio pulsar is active in this system in quiescence.

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