Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram

We analyze a large collection of RXTE archive data from April 1997 to August 2003 of the bright X-ray source Scorpius X-1 in order to study the broadband spectral evolution of the source for different values of the inferred mass accretion rate by studying energy spectra from selected regions in the Z-track of its Color-Color Diagram. A two-component model, consisting of a soft thermal component interpreted as thermal emission from an accretion disk and a thermal Comptonization component, is unable to fit the whole 3--200 keV energy spectrum at low accretion rates. Strong residuals in the highest energy band of the spectrum require the addition of a third component that can be fitted with a power-law component, that could represent a second thermal Comptonization from a much hotter plasma, or a hybrid thermal/non-thermal Comptonization. We discuss the physical implications derived from the results of our analysis, with a particular emphasis on the hardest part of the X-ray emission and its possible origins.Comment: 18 pages. Accepted for publication in Ap

Spin down during quiescence of the fastest known accretion-powered pulsar

We present a timing solution for the 598.89 Hz accreting millisecond pulsar, IGR J00291+5934, using Rossi X-ray Timing Explorer data taken during the two outbursts exhibited by the source on 2008 August and September. We estimate the neutron star spin frequency and we refine the system orbital solution. To achieve the highest possible accuracy in the measurement of the spin frequency variation experienced by the source in-between the 2008 August outburst and the last outburst exhibited in 2004, we re-analysed the latter considering the whole data set available. We find that the source spins down during quiescence at an average rate of dot{νsd=(-4.1±1.2)×10^-15} Hz s-1. We discuss possible scenarios that can account for the long-term neutron star spin-down in terms of either magneto-dipole emission, emission of gravitational waves, and a propeller effect. If interpreted in terms of magneto-dipole emission, the measured spin down translates into an upper limit to the neutron star magnetic field, B ≲ 3 × 10^8 G, while an upper limit to the average neutron star mass quadrupole moment of Q ≲ 2 × 10^36 g cm2 is set if the spin down is interpreted in terms of the emission of gravitational waves

A relativistically smeared spectrum in the neutron star X-ray Binary 4U 1705-44: Looking at the inner accretion disc with X-ray spectroscopy

Iron emission lines at 6.4-6.97 keV, identified with fluorescent Kalpha transitions, are among the strongest discrete features in the X-ray band. These are therefore one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disc around a compact object. In this paper we present a recent XMM observation of the X-ray burster 4U 1705-44, where we clearly detect a relativistically smeared iron line at about 6.7 keV, testifying with high statistical significance that the line profile is distorted by high velocity motion in the accretion disc. As expected from disc reflection models, we also find a significant absorption edge at about 8.3 keV; this feature appears to be smeared, and is compatible with being produced in the same region where the iron line is produced. From the line profile we derive the physical parameters of the inner accretion disc with large precision. The line is identified with the Kalpha transition of highly ionised iron, Fe XXV, the inner disc radius is Rin = (14 \pm 2) R_g (where R_g is the Gravitational radius, GM/c^2), the emissivity dependence from the disc radius is r^{-2.27 \pm 0.08}, the inclination angle with respect to the line of sight is i = (39 \pm 1) degrees. Finally, the XMM spectrum shows evidences of other low-energy emission lines, which again appear broad and their profiles are compatible with being produced in the same region where the iron line is produced.Comment: 6 pages, including 3 figures. Final version, including proof corrections, as it will appear in MNRA

A ionized reflecting skin above the accretion disk of GX 349+2

The broad emission features in the Fe-Kalpha region of X-ray binary spectra represent an invaluable probe to constrain the geometry and the physics of these systems. Several Low Mass X-ray binary systems (LMXBs) containing a neutron star (NS) show broad emission features between 6 and 7 keV and most of them are nowi nterpreted as reflection features from the inner part of an accretion disk in analogy to those observed in the spectra of X-ray binary systems containing a Black Hole candidate. The NS LMXB GX 349+2 was observed by the XMM-Newton satellite which allows, thanks to its high effective area and good spectral resolution between 6 and 7 keV, a detailed spectroscopic study of the Fe-Kalpha region. We study the XMM data in the 0.7-10 keV energy band. The continuum emission is modelled by a blackbody component plus a multicolored disk blackbody. A very intense emission line at 1 keV, three broad emission features at 2.63, 3.32, 3.9 keV and a broader emission feature in the Fe-Kalpha region are present in the residuals. The broad emission features above 2 keV can be equivalently well fitted with Gaussian profiles or relativistic smeared lines (diskline in XSPEC). The Fe-Kalpha feature is better fitted using a diskline component at 6.76 keV or two diskline components at 6.7 and 6.97 keV, respectively. The emission features are interpreted as resonant transitions of S xvi, Ar xviii, Ca xix, and highly ionized iron. Modelling the line profiles with relativistic smeared lines, we find that the reflecting plasma is located at less than 40 km from the NS, a value compatible with the inner radius of the accretion disk inferred from the multicolored disk blackbody component ($24 \pm 7$ km). The inclination angle of GX 349+2 is between 40 and 47 deg.Comment: 9 pages, 8 figures. Accepted by A&

Spin down during quiescence of the fastest known accretion-powered pulsar

We present a timing solution for the 598.89 Hz accreting millisecond pulsar, IGR J00291+5934, using Rossi X-ray Timing Explorer data taken during the two outbursts exhibited by the source on 2008 August and September. We estimate the neutron star spin frequency and we refine the system orbital solution. To achieve the highest possible accuracy in the measurement of the spin frequency variation experienced by the source in-between the 2008 August outburst and the last outburst exhibited in 2004, we re-analysed the latter considering the whole data set available. We find that the source spins down during quiescence at an average rate of dot{νsd=(-4.1±1.2)×10-15} Hz s-1. We discuss possible scenarios that can account for the long-term neutron star spin-down in terms of either magneto-dipole emission, emission of gravitational waves, and a propeller effect. If interpreted in terms of magneto-dipole emission, the measured spin down translates into an upper limit to the neutron star magnetic field, B ≲ 3 × 108 G, while an upper limit to the average neutron star mass quadrupole moment of Q ≲ 2 × 1036 g cm2 is set if the spin down is interpreted in terms of the emission of gravitational waves

The iron K-shell features of MXB 1728-34 from a simultaneous Chandra-RXTE observation

We report on a simultaneous Chandra and RossiXTE observation of the low-mass X-ray binary atoll bursting source MXB 1728-34 performed on 2002 March 3-5. We fit the 1.2-35 keV continuum spectrum with a blackbody plus a Comptonized component. Large residuals at 6-10 keV can be fitted by a broad (  keV) Gaussian emission line or, alternatively, by two absorption edges associated with lowly ionized iron and Fe XXV/XXVI at ~7.1 keV and ~9 keV, respectively. In this interpretation, we find no evidence of broad emission lines between 6 and 7 keV. We test our alternative model of the iron K shell region by reanalysing a previous BeppoSAX observation of MXB 1728-34, finding a general agreement with our new spectral model

The accretion/ejection link in the neutron star X-ray binary 4U 1820-30 I: a boundary layer-jet coupling?

The accretion flow/jet correlation in neutron star (NS) low-mass X-ray binaries (LMXBs) is far less understood when compared to black hole (BH) LMXBs. In this paper we will present the results of a dense multiwavelength observational campaign on the NS LMXB 4U 1820-30, including X-ray (NICER, NuSTAR, and AstroSat) and quasi-simultaneous radio (ATCA) observations in 2022. 4U 1820-30 shows a peculiar 170 d super-orbital accretion modulation, during which the system evolves between ‘modes’ of high and low X-ray flux. During our monitoring, the source did not show any transition to a full hard state. X-ray spectra were well described using a disc blackbody, a Comptonization spectrum along with a Fe K emission line at ∼6.6 keV. Our results show that the observed X-ray flux modulation is almost entirely produced by changes in the size of the region providing seed photons for the Comptonization spectrum. This region is large (∼15 km) in the high mode and likely coincides with the whole boundary layer, while it shrinks significantly (≲10 km) in low mode. The electron temperature of the corona and the observed rms variability in the hard X-rays also exhibit a slight increase in low mode. As the source moves from high to low mode, the radio emission due to the jet becomes ∼5 fainter. These radio changes appear not to be strongly connected to the hard-to-soft transitions as in BH systems, while they seem to be connected mostly to variations observed in the boundary layer.AM, FCZ, and NR are supported by the H2020 ERC Consolidator Grant ‘MAGNESIA’ under grant agreement number 817661 (PI: Rea) and National Spanish grant PGC2018-095512-BI00. This work was also partially supported by the programme Unidad de Excelencia Maria de Maeztu CEX2020-001058-M, and by the PHAROS COST Action (No. CA16214). MDS and TDR acknowledge support from the INAF grant ‘ACE-BANANA’. AB is grateful to the Royal Society, United Kingdom. She is supported by an INSPIRE Faculty grant (DST/INSPIRE/04/2018/001265) by the Department of Science and Technology, Govt. of India. AB also acknowledges the financial support of ISRO under the AstroSat Archival Data Utilization Programme (No. DS-2B-13013(2)/4/2019-Sec. 2). TDS and AS acknowledge financial support from PRIN-INAF 2019 with the project ‘Probing the geometry of accretion: from theory to observations’ (PI: Belloni). FCZ is supported by a Ramón y Cajal fellowship (grant agreement RYC2021-030888-I). EA acknowledges funding from the Italian Space Agency, contract ASI/INAF n. I/004/11/4. FC acknowledges support from the Royal Society through the Newton International Fellowship programme (NIF/R1/211296). JvdE acknowledges a Warwick Astrophysics prize post-doctoral fellowship made possible thanks to a generous philanthropic donation, and was supported by a Lee Hysan Junior Research Fellowship awarded by St. Hilda’s College, Oxford, during part of this work. We thank Jamie Stevens and ATCA staff for making the radio observations possible. ATCA is part of the Australia Telescope National Facility (https://ror.org/05qajvd42) which is funded by the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge the Wiradjuri people as the Traditional Owners of the ATCA observatory site. NICER is a 0.2–12 keV X-ray telescope operating on the International Space Station, funded by NASA. NuSTAR is a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory and funded by NASA. This publication uses the data from the AstroSat mission of the Indian Space Research Organization (ISRO), archived at the Indian Space Science Data Centre (ISSDC).With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2020-001058-M).Peer reviewe