The examination of two 2010 Chandra ACIS (Advanced CCD Imaging Spectrometer) exposures of the Circinus galaxy resulted in the discovery of two pulsators: CXO J141430.1−651621 and ...CXOU J141332.9−651756. We also detected 26 ks pulsations in CG X-1, consistently with previous measures. For ∼40 other sources, we obtained limits on periodic modulations. In CXO J141430.1−651621, which is ∼2 arcmin outside the Circinus galaxy, we detected signals at 6120 ± 1 s and 64.2 ± 0.5 ks. In the longest observation, the source showed a flux of ≈1.1 × 10−13 erg cm−2 s−1 (absorbed, 0.5–10 keV) and the spectrum could be described by a power law with photon index Γ ≃ 1.4. From archival observations, we found that the luminosity is variable by ≈50 per cent on time-scales of weeks to years. The two periodicities pin down CXO J141430.1−651621 as a cataclysmic variable of the intermediate polar subtype. The period of CXOU J141332.9−651756 is 6378 ± 3 s. It is located inside the Circinus galaxy, but the low absorption indicates a Galactic foreground object. The flux was ≈5 × 10−14 erg cm−2 s−1 in the Chandra observations and showed ≈50 per cent variations on weekly/yearly scales; the spectrum is well fitted by a power law with Γ ≃ 0.9. These characteristics and the large modulation suggest that CXOU J141332.9−651756 is a magnetic cataclysmic variable, probably a polar. For CG X-1, we show that if the source is in the Circinus galaxy, its properties are consistent with a Wolf–Rayet (WR) plus black hole (BH) binary. We consider the implications of this for ultraluminous X-ray sources and the prospects of Advanced LIGO and Virgo. In particular, from the current sample of WR–BH systems, we estimate an upper limit to the detection rate of stellar BH–BH mergers of ∼16 yr−1.
We report on the results of XMM–Newton and Swift observations of SMC X-2 during its last outburst in 2015 October, the first one since 2000. The source reached a very high luminosity (L ∼ 1038 erg ...s−1), which allowed us to perform a detailed analysis of its timing and spectral properties. We obtained a pulse period P
spin = 2.372267(5) s and a characterization of the pulse profile also at low energies. The main spectral component is a hard (Γ ≃ 0) power-law model with an exponential cut-off, but at low energies we detected also a soft (with kT ≃ 0.15 keV) thermal component. Several emission lines are present in the spectrum. Their identification with the transition lines of highly ionized N, O, Ne, Si, and Fe suggests the presence of photoionized matter around the accreting source.
We have developed a stellar wind model for OB supergiants to investigate the effects of accretion from a clumpy wind on the luminosity and variability properties of high-mass X-ray binaries. Assuming ...that the clumps are confined by ram pressure of the ambient gas and exploring different distributions for their mass and radii, we computed the expected X-ray light curves in the framework of the Bondi–Hoyle accretion theory, modified to take into account the presence of clumps. The resulting variability properties are found to depend not only on the assumed orbital parameters but also on the wind characteristics. We have then applied this model to reproduce the X-ray light curves of three representative high-mass X-ray binaries: two persistent supergiant systems (Vela X−1 and 4U 1700−377) and the supergiant fast X-ray transient IGR J11215−5952. The model can reproduce the observed light curves well, but requiring in all cases an overall mass loss from the supergiant about a factor of 3–10 smaller than the values inferred from ultraviolet lines studies that assume a homogeneous wind.
ABSTRACT
We report results from an investigation at hard X-rays (above 18 keV) and soft X-rays (below 10 keV) of a sample of X-ray transients located on the Galactic plane and detected with the ...bursticity method, as reported in the latest 1000 orbits INTEGRAL/IBIS catalogue. Our main aim has been to individuate those with X-rays characteristics strongly resembling Supergiant Fast X-ray Transients (SFXTs). As a result, we found four unidentified fast X-ray transients which now can be considered good SFXT candidates. In particular, three transients (IGR J16374–5043, IGR J17375–3022, and IGR J12341–6143) were very poorly studied in the literature before the current work, and our findings largely improved the knowledge of their X-ray characteristics. The other transient (XTE J1829–098) was previously studied in detail only below 10 keV, conversely the current work provides the first detailed study in outburst above 18 keV. In addition we used archival infrared observations of the transients to pinpoint, among the field objects, their best candidate counterpart. We found that their photometric properties are compatible with an early-type spectral classification, further supporting our proposed nature of SFXTs. Infrared spectroscopy is advised to confirm or disprove our interpretation. The reported findings allowed a significant increase of the sample of candidate SFXTs known to date, effectively doubling their number.
We performed a systematic analysis of all INTEGRAL observations from 2003 to 2009 of 14 supergiant fast X-ray transients (SFXTs), implying a net exposure time of about 30 Ms. For each source we ...obtained light curves and spectra (3–100 keV), discovering several new outbursts. We discuss the X-ray behaviour of SFXTs emerging from our analysis in the framework of the clumpy wind accretion mechanism we proposed. We discuss the effect of X-ray photoionization on accretion in close binary systems such as IGR J16479−4514 and IGR J17544−2619. We show that, because of X-ray photoionization, there is a high probability of an accretion disc forming from the capture of angular momentum in IGR J16479−4514, and we suggest that the formation of transient accretion discs could be partly responsible for the flaring activity in SFXTs with narrow orbits. We also propose an alternative way to explain the origin of flares with peculiar shapes observed in our analysis applying the model of Lamb et al., which is based on accretion via the Rayleigh–Taylor instability and was originally proposed to explain Type II bursts.
We report on the results of an
XMM–Newton
observation of the supergiant fast X-ray transient (SFXT) IGR J08408-4503 performed in June 2020. The source is composed of a compact object (likely a ...neutron star) orbiting around an O8.5Ib-II(f)p star, LM Vel. The X-ray light curve shows a very low level of emission, punctuated by a single, faint flare. We analysed spectra measured during the flare and during quiescence. The quiescent state shows a continuum spectrum that is well deconvolved to three spectral models: two components are from a collisionally ionized plasma (with temperatures of
k
T
1
= 0.24 keV and
k
T
2
= 0.76 keV), together with a power-law model (photon index, Γ, of ∼2.55), dominating above ∼2 keV. The X-ray flux emitted at this lowest level is 3.2 × 10
−13
erg cm
−2
s
−1
(0.5–10 keV, corrected for the interstellar absorption), implying an X-ray luminosity of 1.85 × 10
32
erg s
−1
(at 2.2 kpc). The two-temperature collisionally ionized plasma is intrinsic to the stellar wind of the donor star, while the power-law can be interpreted as emission due to residual, low-level accretion onto the compact object. The X-ray luminosity contributed by the power-law component only, in the lowest state, is (4.8 ± 1.4)×10
31
erg s
−1
, which is the lowest quiescent luminosity detected from the compact object in an SFXT. Thanks to this very faint X-ray state caught by
XMM–Newton
, X-ray emission from the wind of the donor star LM Vel could be well-established and studied in detail for the first time, along with a very low level of accretion onto the compact object. The residual accretion rate onto the compact object in IGR J08408-4503 can be interpreted as the Bohm diffusion of (possibly magnetized) plasma entering the neutron star magnetosphere at low Bondi capture rates from the supergiant donor wind at the quasi-spherical, radiation-driven settling accretion stage.
AX J1910.7+0917: the slowest X-ray pulsar Sidoli, L; Israel, G. L; Esposito, P ...
Monthly notices of the Royal Astronomical Society,
08/2017, Letnik:
469, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Abstract
Pulsations from the high-mass X–ray binary AX J1910.7+0917 were discovered during Chandra observations performed in 2011. We report here more details on this discovery and discuss the source ...nature. The period of the X-ray signal is P = 36200 ± 110 s, with a pulsed fraction, PF, of 63 ± 4 per cent. Given the association with a massive B-type companion star, we ascribe this long periodicity to the rotation of the neutron star (NS), making AX J1910.7+0917 the slowest known X-ray pulsar. We also report on the spectroscopy of XMM−Newton observations that serendipitously covered the source field, resulting in a highly absorbed (column density almost reaching 1023 cm−2), power-law X-ray spectrum. The X-ray flux is variable on a time-scale of years, spanning a dynamic range ≳ 60. The very long NS spin period can be explained within a quasi-spherical settling accretion model
that applies to low luminosity, wind-fed, X-ray pulsars.
We report the results of an XMM-Newton and NuSTAR coordinated observation of the Supergiant Fast X-ray Transient (SFXT) IGR J11215-5952, performed on 2016 February 14, during the expected peak of its ...brief outburst, which repeats every ∼165 days. Timing and spectral analysis were performed simultaneously in the energy band 0.4-78 keV. A spin period of 187.0 ( 0.4) s was measured, consistent with previous observations performed in 2007. The X-ray intensity shows a large variability (more than one order of magnitude) on timescales longer than the spin period, with several luminous X-ray flares that repeat every 2-2.5 ks, some of which simultaneously observed by both satellites. The broadband (0.4-78 keV) time-averaged spectrum was well deconvolved with a double-component model (a blackbody plus a power law with a high energy cutoff) together with a weak iron line in emission at 6.4 keV (equivalent width, EW, of 40 10 eV). Alternatively, a partial covering model also resulted in an adequate description of the data. The source time-averaged X-ray luminosity was 1036 erg s−1 (0.1-100 keV; assuming 7 kpc). We discuss the results of these observations in the framework of the different models proposed to explain SFXTs, supporting a quasi-spherical settling accretion regime, although alternative possibilities (e.g., centrifugal barrier) cannot be ruled out.
We report on the follow-up
XMM–Newton
observation of the persistent X-ray pulsar CXOU J225355.1+624336, which was discovered with the CATS@BAR project on archival
Chandra
data. The source was ...detected at
f
X
(0.5−10 keV) = 3.4 × 10
−12
erg cm
−2
s
−1
, a flux level that is fully consistent with previous observations performed with ROSAT,
Swift
, and
Chandra
. When compared with previous measurements, the measured pulse period
P
= 46.753(3) s implies a constant spin down at an average rate of
Ṗ
= 5.3 × 10
−10
s s
−1
. The pulse profile is energy dependent, showing three peaks at low energy and a less structured profile above about 3.5 keV. The pulsed fraction slightly increases with energy. We described the time-averaged EPIC spectrum with four different emission models: a partially covered power law, a cutoff power law, and a power law with an additional thermal component (either a black body or a collisionally ionised gas). In all cases we obtained equally good fits, so it was not possible to prefer or reject any emission model on a statistical basis. However, we disfavour the presence of thermal components since their modeled X-ray flux, resulting from a region larger than the neutron star surface, would largely dominate the X-ray emission from the pulsar. The phase-resolved spectral analysis showed that a simple flux variation cannot explain the source variability and proved that there is a spectral variability along the pulse phase. The results of the
XMM–Newton
observation confirmed that CXOU J225355.1+624336 is a Be X-ray binary (BeXB) with a low luminosity (
L
X
∼ 10
34−35
erg s
−1
), limited variability, and a constant spin down. Therefore, these results reinforce its source classification as a persistent BeXB.
We report on the discovery of 41 new pulsating sources in the data of the Chandra Advanced CCD Imaging Spectrometer, which is sensitive to X-ray photons in the 0.3–10 keV band. The archival data of ...the first 15 yr of Chandra observations were retrieved and analysed by means of fast Fourier transforms, employing a peak-detection algorithm able to screen candidate signals in an automatic fashion. We carried out the search for new X-ray pulsators in light curves with more than 50 photons, for a total of about 190 000 light curves out of about 430 000 extracted. With these numbers, the ChAndra Timing Survey at Brera And Roma astronomical observatories (CATS @ BAR) – as we called the project – represents the largest ever systematic search for coherent signals in the classic X-ray band. More than 50 per cent of the signals were confirmed by further Chandra (for those sources with two or more pointings), XMM–Newton or ROSAT data. The period distribution of the new X-ray pulsators above ∼2000 s resembles that of cataclysmic variables, while there is a paucity of sources with shorter period and low fluxes. Since there is not an obvious bias against these detections, a possible interpretation is in terms of a magnetic gating mechanism in accreting neutron stars. Finally, we note that CATS @ BAR is a living project and the detection algorithm will continue to be routinely applied to the new Chandra data as they become public. Based on the results obtained so far, we expect to discover about three new pulsators every year.
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