Unveiling the environment surrounding LMXB SAX J1808.4-3658

Low-mass X-ray binaries (LMXBs) are a natural workbench to study accretion disk phenomena and optimal background sources to measure elemental abundances in the Interstellar medium (ISM). In high-resolution XMM-Newton spectra, the LMXB SAX J1808.4-3658 showed in the past a neon column density significantly higher than expected given its small distance, presumably due to additional absorption from a neon-rich circumstellar medium (CSM). It is possible to detect intrinsic absorption from the CSM by evidence of Keplerian motions or outflows. For this purpose, we use a recent, deep (100 ks long), high-resolution Chandra/LETGS spectrum of SAX J1808.4-3658 in combination with archival data. We estimated the column densities of the different absorbers through the study of their absorption lines. We used both empirical and physical models involving photo- and collisional-ionization in order to determine the nature of the absorbers. The abundances of the cold interstellar gas match the solar values as expected given the proximity of the X-ray source. For the first time in this source, we detected neon and oxygen blueshifted absorption lines that can be well modeled with outflowing photoionized gas. The wind is neon rich (Ne/O>3) and may originate from processed, ionized gas near the accretion disk or its corona. The kinematics (v=500-1000 km/s) are indeed similar to those seen in other accretion disks. We also discovered a system of emission lines with very high Doppler velocities (v~24000 km/s) originating presumably closer to the compact object. Additional observations and UV coverage are needed to accurately determine the wind abundances and its ionization structure.Comment: 12 pages, 10 figures, accepted for publication on A&

X-ray Flux Related Timing and Spectral Features of 2S 1417-62

RXTE observations of the X-ray transient pulsar 2S 1417-62 between 1999 November and 2000 August with a total exposure of $\sim 394$ ksec were analyzed. Observations include a main outburst followed by a series of mini outbursts. Changes in pulse morphology and pulse fraction were found to be related to the changes in X-ray flux. Particularly low X-ray flux regions were found to have significantly lower pulse fractions with different pulse morphologies. The 3-60 keV PCA-HEXTE main outburst spectrum was modeled with an absorbed power law model with high energy cut-off and a Gaussian Iron line complex feature. Using the same spectral model, individual 3-20 keV PCA spectra were found to be softer and less absorbed in low X-ray flux regions between outbursts. Spectral studies showed that hydrogen column density was correlated, and the power law index was anti-correlated with the 3-20 keV X-ray flux. X-ray flux related spectral and timing features in 2S 1417-62 except for low X-ray flux regions were interpreted as a sign of disc accretion with a similar accretion geometry with a varying mass accretion rate ($\dot{M}$), whereas spectral and timing features of the low X-ray flux regions were interpreted as a sign of possible temporary accretion geometry change prior to the next periastron where $\dot{M}$ increases again to restore the original accretion geometry.Comment: 14 pages, 6 figures, accepted for publication in MNRA

The Hot and Energetic Universe: End points of stellar evolution

White dwarfs, neutron stars and stellar mass black holes are key laboratories to study matter in most extreme conditions of gravity and magnetic field. The unprecedented effective area of Athena+ will allow us to advance our understanding of emission mechanisms and accretion physics over a wide range of mass accretion rates, starting from lower and sub-luminous quiescent X-ray binaries up to super-Eddington ultra-luminous sources. Athena+ will measure stellar black hole spins in a much higher number of binaries than achievable now, opening the possibility to study how spin varies with black hole history. The high throughput and energy resolution of the X-IFU will be instrumental in establishing how disc wind properties depend on accretion state, in determining wind launching mechanism and in quantifying the impact of the wind induced mass loss on binary evolution and environment. Triggers and high quality optical and radio data originating from large wide field contemporaneous instruments will provide essential complementary information on jet launching mechanisms and on the physics of rotation powered pulsars, for instance. In addition, Athena+ will furnish multiple, independent measurements of the neutron star mass/radius relation in a wide range of environments and conditions so as to constrain the debated equation of state.Comment: Supporting paper for the science theme "The Hot and Energetic Universe" to be implemented by the Athena+ X-ray observatory (http://www.the-athena-x-ray-observatory.eu). 9 pages, 4 figure

The Large Observatory For X-ray Timing: LOFT

LOFT, the Large Observatory for X-ray Timing, is a new space mission concept devoted to observations of Galactic and extra-Galactic sources in the X-ray domain with the main goals of probing gravity theory in the very strong field environment of black holes and other compact objects, and investigating the state of matter at supra-nuclear densities in neutron stars. The instruments on-board LOFT, the Large area detector and the Wide Field Monitor combine for the first time an unprecedented large effective area (~10 m2 at 8 keV) sensitive to X-ray photons mainly in the 2-30 keV energy range and a spectral resolution approaching that of CCD-based telescopes (down to 200 eV at 6 keV). LOFT is currently competing for a launch of opportunity in 2022 together with the other M3 mission candidates of the ESA Cosmic Vision Progra

Observatory science with eXTP

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s

Chandra observation of SAX J1818.6-1703: confirmation of optical counterpart & suggestion of non-transient nature

SAX J1818.6-1703, discovered in 1998 with the BeppoSAX Wide Field Cameras (in 't Zand et al. 1998, IAUC #6840), was found with INTEGRAL and RXTE to exhibit few-hours long outbursts similar to the Supergiant Fast X- ray Transients (Grebenev & Sunyaev 2005, AstL 31, 672; Sguera et al. 2005, A&A 444, 221). Negueruela & Smith (2006; ATel #831), on the basis of this analogy, identified within the 1.5 arcmin radius error region determined by Grebenev & Sunyaev a likely optical/NIR counterpart whose I-band spectrum is typical for an OB supergiant

Discovery of millihertz quasi-periodic oscillations in the X-ray binary EXO 0748−676

We report the discovery of millihertz quasi-periodic oscillations (mHz QPOs) from the bursting, high-inclination atoll neutron star low-mass X-ray binary (NS LMXB) EXO 0748-676 with the Rossi X-ray Time Explorer (RXTE). This class of QPO, originally discovered in three NS LMXBs, has been interpreted as a consequence of a special mode of nuclear burning on the NS surface. Using all the RXTE archival observations of the source, we detected significant (>3σ) mHz QPOs in 11 observations. The frequency of the oscillations was between ∼5 and ∼13 mHz. We also found a decrease of the QPO frequency with time in two occasions; in one of these the oscillations disappeared with the onset of an X-ray burst, similar to what was reported in other sources. Our analysis of the X-ray colours revealed that EXO 0748-676 was in a soft spectral state when it exhibited the QPOs. This makes EXO 0748-676 the sixth source with mHz oscillations associated with marginally stable burning, and the second one that shows a systematic frequency drift. Our results suggest that the mechanism that produces the drift might always be present if the mHz QPOs are observed in the so-called intermediate state.Facultad de Ciencias Astronómicas y GeofísicasInstituto Argentino de Radioastronomí

Data for: A catalogue of unusually long thermonuclear bursts on neutron stars

This data repository for the catalog of unusually long bursts contains the light curves and time-resolved spectral analysis results discussed in the paper. Downloadable plots and the data itself are available