ABSTRACT
We present new results from INTEGRAL and Swift observations of the hitherto poorly studied and unidentified X-ray source XTE J1906+090. A bright hard X-ray outburst (luminosity of ...∼1036 erg s−1 above 20 keV) has been discovered with INTEGRAL observations in 2010, this being the fourth outburst ever detected from the source. Such events are sporadic, the source duty cycle is in the range (0.8–1.6) per cent as inferred from extensive INTEGRAL and Swift monitoring in a similar hard X-ray band. Using five archival unpublished Swift/X-Ray Telescope (XRT) observations, we found that XTE J1906+090 has been consistently detected at a persistent low X-ray luminosity value of ∼1034 erg s−1, with limited variability (a factor as high as 4). Based on our findings, we propose that XTE J1906+090 belongs to the small and rare group of persistent low-luminosity Be X-ray binaries.
ABSTRACT
The candidate PeVatron MGRO J1908+06, which shows a hard spectrum beyond 100 TeV, is one of the most peculiar γ-ray sources in the Galactic plane. Its complex morphology and some possible ...counterparts spatially related with the very high energy (VHE) emission region, preclude to distinguish between a hadronic and leptonic nature of the γ-ray emission. In this paper, we illustrate a new multiwavelength analysis of MGRO J1908+06, with the aim to shed light on its nature and the origin of its ultra-high-energy emission. We performed an analysis of the 12CO and 13CO molecular line emission demonstrating the presence of dense molecular clouds spatially correlated with the source region. We also analysed 12 yr of Fermi-Large Area Telescope (LAT) data between 10 GeV and 1 TeV finding a counterpart with a hard spectrum (Γ ∼ 1.6). Our reanalysis of XMM–Newton data allowed us to put a more stringent constraint on the X-ray flux from this source. We demonstrate that a single accelerator cannot explain the whole set of multiwavelength data, regardless of whether it accelerates protons or electrons, but a two-zone model is needed to explain the emission from MGRO J1908+06. The VHE emission seems most likely the superposition of a TeV pulsar wind nebula powered by PSR J1907+0602, in the southern region, and of the interaction between the supernova remnant G40.5−0.5 and the molecular clouds, in the northern region.
Context. Local-Group galaxies provide access to samples of X-ray source populations of whole galaxies. The XMM-Newton survey of the Small Magellanic Cloud (SMC) completely covers the bar and eastern ...wing with a 5.6 deg2 area in the (0.2−12.0) keV band. Aims. To characterise the X-ray sources in the SMC field, we created a catalogue of point sources and sources with moderate extent. Sources with high extent (≥40′′) have been presented in a companion paper. Methods. We searched for point sources in the EPIC images using sliding-box and maximum-likelihood techniques and classified the sources using hardness ratios, X-ray variability, and their multi-wavelength properties. Results. The catalogue comprises 3053 unique X-ray sources with a median position uncertainty of 1.3′′ down to a flux limit for point sources of ~10-14 erg cm-2 s-1 in the (0.2−4.5) keV band, corresponding to 5 × 1033 erg s-1 for sources in the SMC. We discuss statistical properties, like the spatial distribution, X-ray colour diagrams, luminosity functions, and time variability. We identified 49 SMC high-mass X-ray binaries (HMXB), four super-soft X-ray sources (SSS), 34 foreground stars, and 72 active galactic nuclei (AGN) behind the SMC. In addition, we found candidates for SMC HMXBs (45) and faint SSSs (8) as well as AGN (2092) and galaxy clusters (13). Conclusions. We present the most up-to-date catalogue of the X-ray source population in the SMC field. In particular, the known population of X-ray binaries is greatly increased. We find that the bright-end slope of the luminosity function of Be/X-ray binaries significantly deviates from the expected universal high-mass X-ray binary luminosity function.
ABSTRACT
HD 49798/RX J0648.0–4418 is the only confirmed X-ray binary in which the mass donor is a hot subdwarf star of O spectral type and, most likely, it contains a massive white dwarf (1.28 ± 0.05 ...M⊙) with a very fast spin period of 13.2 s. Here, we report the results of new XMM–Newton pointings of this peculiar binary, carried out in 2018 and in 2020, together with a reanalysis of all the previous observations. The new data indicate that the compact object is still spinning-up at a steady rate of (−2.17 ± 0.01) × 10−15 s s−1, consistent with its interpretation in terms of a young contracting white dwarf. Comparison of observations obtained at similar orbital phases, far from the ecplise, shows evidence for long-term variability of the hard (>0.5 keV) spectral component at a level of ∼(70 ± 20) per cent, suggesting the presence of time-dependent inhomogeneities in the weak stellar wind of the HD 49798 subdwarf. To investigate better the soft spectral component that dominates the X-ray flux from this system, we computed a theoretical model for the thermal emission expected from an atmosphere with element abundances and surface gravity appropriate for this massive white dwarf. This model gives a best fit with effective temperature of Teff = 2.25 × 105 K and an emitting area with a radius of ∼1600 km, larger than that found with blackbody fits. This model also predicts a contribution of the pulsed emission from the white dwarf in the optical band significantly larger than previously thought and possibly relevant for optical variability studies of this system.
We report on the first detection of very high-energy gamma-ray emission from the Crab Nebula by a Cherenkov telescope in dual-mirror Schwarzschild-Couder (SC) configuration. This result has been ...achieved by means of the 4 m ASTRI-Horn telescope, operated on Mt. Etna, Italy, and developed in the context of the Cherenkov Telescope Array Observatory preparatory phase. The dual-mirror SC design is aplanatic and characterized by a small plate scale, which allows us to implement large cameras with a large field of view, with small-size pixel sensors and a high level of compactness. The curved focal plane of the ASTRI camera is covered by silicon photo-multipliers, managed by an unconventional front-end electronic system that is based on a customized peak-sensing detector mode. The system includes internal and external calibration systems, hardware and software for control and acquisition, and the complete data archiving and processing chain. These observations of the Crab Nebula were carried out in December 2018 during the telescope verification phase for a total observation time (after data selection) of 24.4 h, equally divided between on- and off-axis source exposure. The camera system was still under commission and its functionality was not yet completely exploited. Furthermore, due to recent eruptions of the Etna Volcano, the mirror reflection efficiency was reduced. Nevertheless, the observations led to the detection of the source with a statistical significance of 5.4
σ
above an energy threshold of ∼3 TeV. This result provides an important step toward the use of dual-mirror systems in Cherenkov gamma-ray astronomy. A pathfinder mini-array based on nine ASTRI-like telescopes with a large field-of-view is in the course of implementation.
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.
Aims. Despite recent progress in the field, there are still many open questions regarding γ-ray binaries. In this paper we provide an overview of non-transient γ-ray binaries and discuss how ...observations with the Cherenkov Telescope Array (CTA) will contribute to their study. Methods. We simulated the spectral behaviour of the non-transient γ-ray binaries using archival observations as a reference. With this we tested the CTA capability to measure the spectral parameters of the sources and detect variability on various timescales. Results. We review the known properties of γ-ray binaries and the theoretical models that have been used to describe their spectral and timing characteristics. We show that the CTA is capable of studying these sources on timescales comparable to their characteristic variability timescales. For most of the binaries, the unprecedented sensitivity of the CTA will allow studying the spectral evolution on a timescale as short as 30 min. This will enable a direct comparison of the TeV and lower energy (radio to GeV) properties of these sources from simultaneous observations. We also review the source-specific questions that can be addressed with these high-accuracy CTA measurements.
We report on an X-ray observation of the Be X-ray binary pulsar RX J0059.2–7138, performed by XMM–Newton in 2014 March. The 19 ks long observation was carried out about three months after the ...discovery of the latest outburst from this Small Magellanic Cloud transient, when the source luminosity was LX
∼1038 erg s−1. A spin period of P
spin=2.762 383(5) s was derived, corresponding to an average spin-up of
$\dot{P}_{\mathrm{spin}} = -(1.27\pm 0.01)\times 10^{-12}$
s s−1 from the only previous period measurement, obtained more than 20 years earlier. The time-averaged continuum spectrum (0.2–12 keV) consisted of a hard power-law (photon index ∼0.44) with an exponential cut-off at a phase-dependent energy (∼20–50 keV) plus a significant soft excess below ∼0.5 keV. In addition, several features were observed in the spectrum: an emission line at 6.6 keV from highly ionized iron, a broad feature at 0.9–1 keV likely due to a blend of Fe L-shell lines, and narrow emission and absorption lines consistent with transitions in highly ionized oxygen, nitrogen and iron visible in the high-resolution RGS data (0.4–2.1 keV). Given the different ionization stages of the narrow-line components, indicative of photoionization from the luminous X-ray pulsar, we argue that the soft excess in RX J0059.2–7138 is produced by reprocessing of the pulsar emission in the inner regions of the accretion disc.
We report the results provided by the XMM-Newton observation of the X-ray binary pulsar SXP59.0 during its most recent outburst in April 2017. The source was detected at fX(0.2–12 keV) = 8 × 10−11erg ...cm−2 s−1, one of its highest flux levels reported to date. The measured pulse period was Pspin = 58.949(1) s, very similar to the periods measured in most of the previous observations. The pulsed emission was clearly detected over the whole energy range between 0.2 and 12 keV, but the pulse profile is energy dependent and the pulsed fraction increases as the energy increases. Although the time-averaged EPIC spectrum is dominated by a power-law component (with photon index Γ = 0.76 ± 0.01), the data show an evident soft excess, which can be described with the sum of a black-body and a hot thermal plasma component (with temperatures kTBB = 171+11−14 k T BB = 171 − 14 + 11 $ kT_{\rm BB} = 171^{+11}_{-14} $ eV and kTAPEC = 1.09+0.16−0.09 k T APEC = 1.09 − 0.09 + 0.16 $ kT_{\rm APEC} = 1.09^{+0.16}_{-0.09} $ keV, respectively). Moreover, the EPIC and RGS spectra show narrow emission lines due to N, O, Ne, Mg, and Fe. The phase-resolved spectral analysis of the EPIC data shows that the flux of the black-body component varies with the pulse phase, while the plasma component is almost constant. We show that the black-body component can be attributed to the reprocessing of the primary emission by the optically thick material at the inner edge of the accretion disc, while the hot plasma component is due to a diffuse gas far from the accretion region and the narrow emission lines of the RGS spectrum are most probably due to photoionized matter around the accreting source.
Abstract
ZTF J213056.71+442046.5 is the prototype of a small class of recently discovered compact binaries composed of a white dwarf and a hot subdwarf that fills its Roche lobe. Its orbital period ...of only 39 minutes is the shortest known for the objects in this class. Evidence for a high orbital inclination (
i
= 86°) and for the presence of an accretion disk has been inferred from a detailed modeling of its optical photometric and spectroscopic data. We report the results of an XMM-Newton observation carried out on 2021 January 7. ZTF J213056.71+442046.5 was clearly detected by the Optical Monitor, which showed a periodic variability in the UV band (200–400 nm), with a light curve similar to that seen at longer wavelengths. Despite accretion on the white dwarf at an estimated rate of the order of 10
−9
M
⊙
yr
−1
, no X-rays were detected with the EPIC instrument, with a limit of ∼10
30
erg s
−1
on the 0.2–12 keV luminosity. We discuss possible explanations for the lack of a strong X-ray emission from this system.
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