We report the discovery of X-ray pulsations at 105.2 Hz (9.5 ms) from the transient X-ray binary IGR J16597–3704 using NuSTAR and Swift. The source was discovered by INTEGRAL in the globular cluster ...NGC 6256 at a distance of 9.1 kpc. The X-ray pulsations show a clear Doppler modulation that implies an orbital period of ~46 min and a projected semi-major axis of ~5 lt-ms, which makes IGR J16597–3704 an ultracompact X-ray binary system. We estimated a minimum companion mass of 6.5 × 10−10 M⊙, assuming a neutron star mass of 1.4 M⊙, and an inclination angle of <75° (suggested by the absence of eclipses or dips in its light curve). The broad-band energy spectrum of the source is well described by a disk blackbody component (kT ~ 1.4 keV) plus a comptonised power-law with photon index ~2.3 and an electron temperature of ~30 keV. Radio pulsations from the source were unsuccessfully searched for with the Parkes Observatory.
We report on the timing analysis of the 2015 outburst of the intermittent accreting millisecond X-ray pulsar SAX J1748.9−2021 observed on March 4 by the X-ray satellite XMM–Newton. By phase ...connecting the time of arrivals of the observed pulses, we derived the best-fitting orbital solution for the 2015 outburst. We investigated the energy pulse profile dependence finding that the pulse fractional amplitude increases with energy while no significant time lags are detected. Moreover, we investigated the previous outbursts from this source, finding previously undetected pulsations in some intervals during the 2010 outburst of the source. Comparing the updated set of orbital parameters, in particular the value of the time of passage from the ascending node, with the orbital solutions reported from the previous outbursts, we estimated for the first time the orbital period derivative corresponding with
$\dot{P}_{{\rm orb}}=(1.1\pm 0.3)\times 10^{-10}$
s s−1. We note that this value is significant at 3.5σ confidence level, because of significant fluctuations with respect to the parabolic trend and more observations are needed in order to confirm the finding. Assuming the reliability of the result, we suggest that the large value of the orbital-period derivative can be explained as a result of a highly non-conservative mass transfer driven by emission of gravitational waves, which implies the ejection of matter from a region close to the inner Lagrangian point. We also discuss possible alternative explanations.
Context.
GX 9+9 (4U 1728−16) is a low mass X-ray binary source harboring a neutron star. Although it belongs to the subclass of the bright Atoll sources together with GX 9+1, GX 3+1, and GX 13+1, its ...broadband spectrum is poorly studied and apparently does not show reflection features in the spectrum.
Aims.
To constrain the continuum well and verify whether a relativistic smeared reflection component is present, we analyze the broadband spectrum of GX 9+9 using
Beppo
SAX and
XMM-Newton
spectra covering the 0.3−40 keV energy band.
Methods.
We fit the spectrum adopting a model composed of a disk-blackbody plus a Comptonized component whose seed photons have a blackbody spectrum (Eastern Model). A statistically equivalent model is composed of a Comptonized component whose seed photons have a disk-blackbody distribution plus a blackbody that mimics a saturated Comptonization likely associated with a boundary layer (Western model). Other trials did not return a good fit.
Results.
The spectrum of GX 9+9 was observed in a soft state and its luminosity is 2.3 × 10
37
erg s
−1
assuming a distance to the source of 5 kpc. In the Eastern Model scenario, we find the seed-photon temperature and electron temperature of the Comptonized component to be 1.14
−0.07
+0.10
keV and 2.80
−0.04
+0.09
keV, respectively, while the optical depth of the Comptonizing corona is 8.9 ± 0.4. The color temperature of the inner accretion disk is 0.86
−0.02
+0.08
keV and 0.82 ± 0.02 keV for the
Beppo
SAX and
XMM-Newton
spectrum, respectively. In the Western Model scenario, instead, we find that the seed-photon temperature is 0.87 ± 0.07 keV and 1.01 ± 0.08 keV for the
Beppo
SAX and
XMM-Newton
spectrum, respectively. The electron temperature of the Comptonized component is 2.9 ± 0.2 keV, while the optical depth is 9.4
−1.1
+1.5
. The blackbody temperature is 1.79
−0.18
+0.09
keV and 1.85
−0.15
+0.07
keV for the
Beppo
SAX and
XMM-Newton
spectrum, respectively. The addition of a relativistic smeared reflection component improved the fit in both the scenarios, giving compatible values of the parameters, even though a significant broad emission line in the Fe-K region is not observed.
Conclusions.
From the reflection component we estimated an inclination angle of about 43
−4
+6
deg and 51
−2
+9
deg for the Eastern and Western Model, respectively. The value of the reflection fraction Ω/2
π
is 0.18 ± 0.04 and 0.21 ± 0.03 for the Eastern and Western Model, respectively, suggesting that the Comptonized corona should be compact and close to the innermost region of the system.
Iron emission lines at 6.4–6.97 keV, identified with Kα radiative transitions, are among the strongest discrete features in the X-ray band. These are 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 Suzaku
observation, 100-ks effective exposure, of the atoll source and X-ray burster 4U 1705−44, where we clearly detect signatures of a reflection component which is distorted by the high-velocity motion in the accretion disc. The reflection component consists of a broad iron line at about 6.4 keV and a Compton bump at high X-ray energies, around 20 keV. All these features are consistently fitted with a reflection model, and we find that in the hard state the smearing parameters are remarkably similar to those found in a previous XMM–Newton observation performed in the soft state. In particular, we find that the inner disc radius is R
in = 17 ± 5R
g (where R
g is the gravitational radius, GM/c
2), the emissivity dependence from the disc radius is r
−2.5 ± 0.5, the inclination angle with respect to the line of sight is i = 43° ± 5°, and the outer radius of the emitting region in the disc is R
out > 200R
g. We note that the accretion disc does not appear to be truncated at large radii, although the source is in a hard state at ∼3 per cent of the Eddington luminosity for a neutron star. We also find evidence of a broad emission line at low energies, at 3.03 ± 0.03 keV, compatible with emission from mildly ionized argon (Ar XVI–XVII). Argon transitions are not included in the self-consistent reflection models that we used and we therefore added an extra component to our model to fit this feature. The low-energy line appears compatible with being smeared by the same inner disc parameters found for the reflection component.
Abstract
We report on the spectral and timing properties of the accreting millisecond X-ray pulsar IGR J00291+5934 observed by XMM–Newton and NuSTAR during its 2015 outburst. The source is in a hard ...state dominated at high energies by a Comptonization of soft photons (∼0.9 keV) by an electron population with kT
e
∼ 30 keV, and at lower energies by a blackbody component with kT ∼ 0.5 keV. A moderately broad, neutral Fe emission line and four narrow absorption lines are also found. By investigating the pulse phase evolution, we derived the best-fitting orbital solution for the 2015 outburst. Comparing the updated ephemeris with those of the previous outbursts, we set a 3σ confidence level interval −6.6 × 10−13 s s−1
$< \dot{P}_{{\rm orb}} < 6.5 \times 10^{-13}$
s s−1 on the orbital period derivative. Moreover, we investigated the pulse profile dependence on energy finding a peculiar behaviour of the pulse fractional amplitude and lags as a function of energy. We performed a phase-resolved spectroscopy showing that the blackbody component tracks remarkably well the pulse profile, indicating that this component resides at the neutron star surface (hotspot).
We report the detection of X-ray pulsations at 2.1 ms from the known X-ray burster IGR J17379–3747 using XMM-Newton. The coherent signal shows a clear Doppler modulation from which we estimate an ...orbital period of ~1.9 h and a projected semi-major axis of ~8 lt-ms. Taking into account the lack of eclipses (inclination angle of <75°) and assuming a neutron star mass of 1.4 M⊙, we have estimated a minimum companion star of ~0.06 M⊙. Considerations on the probability distribution of the binary inclination angle make the hypothesis of a main-sequence companion star less likely. On the other hand, the close correspondence with the orbital parameters of the accreting millisecond pulsar SAX J1808.4–3658 suggests the presence of a bloated brown dwarf. The energy spectrum of the source is well described by a soft disk black-body component (kT ~ 0.45 keV) plus a Comptonisation spectrum with photon index ~1.9. No sign of emission lines or reflection components are significantly detected. Finally, combining the source ephemerides estimated from the observed outbursts, we obtained a first constraint on the long-term orbital evolution of the order of Ṗorb = (−2.5 ± 2.3) × 10−12 s s−1.
We report on the discovery of coherent pulsations at a period of 2.9 ms from the X-ray transient MAXI J0911−655 in the globular cluster NGC 2808. We observed X-ray pulsations at a frequency of ...~339.97 Hz in three different observations of the source performed with XMM-Newton and NuSTAR during the source outburst. This newly discovered accreting millisecond pulsar is part of an ultra-compact binary system characterised by an orbital period of 44.3 min and a projected semi-major axis of ~17.6 lt-ms. Based on the mass function, we estimate a minimum companion mass of 0.024 M⊙, which assumes a neutron star mass of 1.4 M⊙ and a maximum inclination angle of 75° (derived from the lack of eclipses and dips in the light-curve of the source). We find that the Roche-lobe of the companion star could either be filled by a hot (5 × 106 K) pure helium white dwarf with a 0.028 M⊙ mass (implying i ≃ 58°) or an old (>5 Gyr) brown dwarf with metallicity abundances between solar/sub-solar and mass ranging in the interval 0.065 to 0.085 (16 < i < 21). During the outburst, the broad-band energy spectra are well described by a superposition of a weak black-body component (kT ~ 0.5 keV) and a hard cut-off power-law with photon index Γ ~ 1.7 and cut-off at a temperature kTe ~ 130 keV. Up until the latest Swift-XRT observation performed on 19th July, 2016, the source had been observed in outburst for almost 150 days, which makes MAXI J0911−655 the second accreting millisecond X-ray pulsar with outburst duration longer than 100 days.
Context. The source 4U 1702-429 (Ara X-1) is a low-mass X-ray binary system hosting a neutron star. Albeit the source is quite bright (~1037 erg s-1) its broadband spectrum has never been studied. ...Neither dips nor eclipses have been observed in the light curve suggesting that its inclination angle is smaller than 60°. Aims. We analysed the broadband spectrum of 4U 1702-429 in the 0.3–60 keV energy range, using XMM-Newton and INTEGRAL data, to constrain its Compton reflection component if it is present. Methods. After excluding the three time intervals in which three type-I X-ray bursts occurred, we fitted the joint XMM-Newton and INTEGRAL spectra obtained from simultaneous observations. Results. A broad emission line at 6.7 keV and two absorption edges at 0.87 and 8.82 keV were detected. We found that a self-consistent reflection model fits the 0.3–60 keV spectrum well. The broadband continuum is composed of an emission component originating from the inner region of the accretion disc, a Comptonised direct emission coming from a corona with an electron temperature of 2.63 ± 0.06 keV and an optical depth τ = 13.6 ± 0.2, and, finally, a reflection component. The best-fit indicates that the broad emission line and the absorption edge at 8.82 keV, both associated with the presence of Fe xxv ions, are produced by reflection in the region above the disc with a ionisation parameter of Log(ξ) ≃ 2.7. We have inferred that the inner radius, where the broad emission line originates, is 64+52-15 km, and the inner radius of the accretion disc is 39+6-8 km. The emissivity of the reflection component and the inclination angle of the system are r-3.2+0.5-5.1and 44+33-6degrees, respectively. The absorption edge at 0.87 keV is associated to the presence of O viii ions and it is produced in a region above the disc with Log(ξ) ≃ 1.9.
ABSTRACT
We present a multiband search for X-ray, optical, and γ-ray emission of the radio binary millisecond pulsar J1836-2354A, hosted in the globular cluster M22. X-ray emission is significantly ...detected in two Chandra observations, performed in 2005 and 2014, at a luminosity of ∼2–3 × 1030 erg s−1, in the 0.5–8 keV energy range. The radio and the X-ray source positions are found consistent within 1σ error box. No detection is found in archival XMM–Newton and Swift/XRT observations, compatible with the Chandra flux level. The low statistics prevents us to assess if the X-ray source varied between the two observations. The X-ray spectrum is consistent with a power-law of photon index ∼1.5. We favour as the most probable origin of the X-ray emission an intrabinary shock scenario. We searched for optical and γ-ray counterparts to the radio source using data from Hubble Space Telescope and Fermi–LAT catalogues, respectively. No optical counterpart down to V = 25.9 and I = 24.7 (3σ) is detected, which suggests a companion mass of 0.1–0.2 M⊙. Combined with the low X-ray luminosity, this is consistent with a black widow nature of PSR J1636-2354A. Inspecting the 8-year Fermi–LAT catalogue, we found a γ-ray source, 4FGL J1836.8–2354, with a positional uncertainty consistent with the globular cluster, but not with the radio position of the millisecond pulsar.
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