Sizing up the population of gamma-ray binaries Dubus, Guillaume; Guillard, Nicolas; Petrucci, Pierre-Olivier ...
Astronomy and astrophysics (Berlin),
12/2017, Letnik:
608
Journal Article
Recenzirano
Odprti dostop
Context. Gamma-ray binaries are thought to be composed of a young pulsar in orbit around a massive O or Be star with their gamma-ray emission powered by pulsar spin-down. The number of such systems ...in our Galaxy is not known. Aims. We aim to estimate the total number of gamma-ray binaries in our Galaxy and to evaluate the prospects for new detections in the GeV and TeV energy range, taking into account that their gamma-ray emission is modulated on the orbital period. Methods. We modelled the population of gamma-ray binaries and evaluated the fraction of detected systems in surveys with the Fermi-LAT (GeV), H.E.S.S., HAWC and CTA (TeV) using observation-based and synthetic template light curves. Results. The detected fraction depends more on the orbit-average flux than on the light-curve shape. Our best estimate for the number of gamma-ray binaries is 101\hbox{$_{-52}^{+89}$}+89-52 systems. A handful of discoveries are expected by pursuing the Fermi-LAT survey. Discoveries in TeV surveys are less likely. However, this depends on the relative amounts of power emitted in GeV and TeV domains. There could be as many as ≈ 200 HESS J0632+057-like systems with a high ratio of TeV to GeV emission compared to other gamma-ray binaries. Statistics allow for as many as three discoveries in five years of HAWC observations and five discoveries in the first two years of the CTA Galactic Plane survey. Conclusions. We favour continued Fermi-LAT observations over ground-based TeV surveys to find new gamma-ray binaries. Gamma-ray observations are most sensitive to short orbital period systems with a high spin-down pulsar power. Radio pulsar surveys (SKA) are likely to be more efficient in detecting long orbital period systems, providing a complementary probe into the gamma-ray binary population.
We analyze 11 Nuclear Spectroscopic Telescope Array and Swift observations of the black hole X-ray binary GX 339-4 in the hard state, 6 of which were taken during the end of the 2015 outburst and 5 ...during a failed outburst in 2013. These observations cover luminosities from 0.5% to 5% of the Eddington luminosity. Implementing the most recent version of the reflection model relxillCp, we perform simultaneous spectral fits on both data sets to track the evolution of the properties in the accretion disk, including the inner edge radius, the ionization, and the temperature of the thermal emission. We also constrain the photon index and electron temperature of the primary source (the "corona"). We observe a maximum truncation radius of 37 Rg in the preferred fit for the 2013 data set, and a marginal correlation between the level of truncation and luminosity. We also explore a self-consistent model under the framework of coronal Comptonization, and find consistent results regarding the disk truncation in the 2015 data, providing a more physical preferred fit for the 2013 observations.
Context.
Warm coronas offer a plausible explanation behind the soft X-ray excess in active galactic nuclei (AGNs). This paper presents the self-consistent modeling of an accretion disk with an ...optically thick corona, where the gas is heated by magneto-rotational instability dynamo (MRI) and is simultaneously cooled by radiation which undergoes free-free absorption and Compton scattering.
Aims.
We determined the parameters of a warm corona in an AGN using disk-corona structure model that takes into account magnetic and radiation pressure. We aim to show the role of thermal instability (TI) as a constraint for warm, optically thick X-ray coronas in AGNs.
Methods.
With the use of relaxation code, we calculated the vertical solution of the disk driven by MRI, together with radiative transfer in hydrostatic and radiative equilibrium. This has allowed us to point out how TI affects the corona for a wide range of global parameters.
Results.
We show that magnetic heating is strong enough to heat the upper layers of the accretion disk atmosphere, which form the warm corona covering the disk. Magnetic pressure does not remove TI caused by radiative processes operating in X-ray emitting plasma. TI disappears only in case of accretion rates higher than 0.2 of Eddington, and high magnetic field parameter
α
B
> 0.1.
Conclusions.
TI plays the major role in the formation of the warm corona above magnetically driven accretion disk in an AGN. The warm, Compton cooled corona, responsible for soft X-ray excess, resulting from our model typically exhibits temperatures in the range of 0.01–2 keV and optical depths of even up to 50, in agreement with recent observations.
Abstract
AX J1745.6-2901 is a low-mass X-ray binary with an accreting neutron star, showing clear evidence for highly ionized absorption. Strong ionized Fe Kα and Kβ absorption lines are always ...observed during the soft state, whereas they disappear during the hard state. We computed photoionization stability curves for the hard and the soft states, under different assumptions on the adopted spectral energy distributions and the physical parameters of the plasma. We observe that the ionized absorber always lies on a stable branch of the photoionization stability curve during the soft state, while it becomes unstable during the hard state. This suggests that photoionization instability plays a key role in defining the observable properties of the ionized absorber. The same process might explain the disappearance of the high ionization absorber/wind during the hard state in other accreting neutron stars and black holes.
Context
. The microquasar SS 433 is the only known steady supercritical accretor in the Galaxy. It is well-known for its relativistic baryonic jets, but the system also drives equatorial outflows. ...These have been routinely detected in radio images, and components associated with a circumbinary disk have also been suggested in optical emission lines.
Aims
. We aim to spatially resolve the regions producing the stationary emission lines of SS 433 to shed light on its circumbinary structure and outflows. With an estimated binary orbit size of ≲0.1 mas, this requires optical interferometry.
Methods
. We use the optical interferometer VLTI+GRAVITY to spatially resolve SS 433 in the near-infrared
K
band at high spectral resolution (
R
≈ 4000) on three nights in July 2017. This is the second such observation, after the first one in July 2016.
Results
. The stationary Br
γ
line in the 2017 observation is clearly dominated by an extended ∼1 mas ∼ 5 AU circumbinary structure perpendicular to the jets with a strong rotation component. The rotation direction is retrograde relative to the jet precession, in accordance with the slaved disk precession model. The structure has a very high specific angular momentum and is too extended to be a stable circumbinary disk in Keplerian rotation; interpreting it as such leads to a very high enclosed mass
M
≳ 400
M
⊙
. We instead interpret it as the centrifugal ejection of the circumbinary disk, with the implication that there must be an efficient transfer of specific angular momentum from the binary to the disk. We suggest that the equatorial outflows sometimes seen in radio images result from similar episodes of circumbinary disk centrifugal ejection. In addition to the equatorial structure, we find a very extended ∼6 mas ∼ 30 AU spherical wind component to the Br
γ
line: the entire binary is engulfed in an optically thin spherical line emission envelope.
Using the simultaneous Infra-Red (IR) and X-ray light curves obtained by Kalamkar et al., we perform a Fourier analysis of the IR/X-ray timing correlations of the black hole X-ray binary (BHB) GX ...339-4. The resulting IR vs X-ray Fourier coherence and lag spectra are similar to those obtained in previous studies of GX 339-4 using optical light curves. In particular, above 1 Hz, the lag spectrum features an approximately constant IR lag of about 100 ms. We model simultaneously the radio to IR Spectral Energy Distribution (SED), the IR Power Spectral Density (PSD), and the coherence and lag spectra using the jet internal shock model ISHEM assuming that the fluctuations of the jet Lorentz factor are driven by the accretion flow. It turns out that most of the spectral and timing features, including the 100-ms lag, are remarkably well-reproduced by this model. The 100-ms time-scale is then associated with the travel time from the accretion flow to the IR emitting zone. Our exploration of the parameter space favours a jet which is at most mildly relativistic (|$\bar {\Gamma}\lt 3$|), and a linear and positive relation between the jet Lorentz factor and X-ray light curve i.e. Γ(t) − 1∝(t). The presence of a strong Low-Frequency Quasi-Periodic Oscillation (LFQPO) in the IR light curve could be caused by jet precession driven by Lense–Thirring precession of the jet-emitting accretion flow. Our simulations confirm that this mechanism can produce an IR LFQPO similar to that observed in GX 339-4.
ABSTRACT
In our self-similar, analytical, magnetohydrodynamic (MHD) accretion–ejection solution, the density at the base of the outflow is explicitly dependent on the disc accretion rate – a unique ...property of this class of solutions. We had earlier found that the ejection index $p \gt \sim 0.1 (\dot{M}_{\rm{acc}} \propto r^p)$ is a key MHD parameter that decides if the flow can cause absorption lines in the high resolution X-ray spectra of black hole binaries. Here, we choose three dense warm solutions with p = 0.1, 0.3, 0.45 and carefully develop a methodology to generate spectra which are convolved with the Athena and XRISM response functions to predict what they will observe seeing through such MHD outflows. In this paper two other external parameters were varied – extent of the disc, $\rm {r_o|_{\rm{max}}} = 10^5, \, 10^6 \, \, \rm {r_G}$, and the angle of the line of sight, i ∼ 10–25°. Resultant absorption lines (H and He-like Fe, Ca, Ar) change in strength and their profiles manifest varying degrees of asymmetry. We checked if (a) the lines and (ii) the line asymmetries are detected, in our suit of synthetic Athena and XRISM spectra. Our analysis shows that Athena should detect the lines and their asymmetries for a standard 100 ks observation of a 100 mCrab source – lines with equivalent width as low as a few eV should be detected if the 6–8 keV counts are larger than 104–105 even for the least favourable simulated cases.
We present a framework for understanding the dynamical and spectral properties of X-ray binaries, where the presence of an organized large-scale magnetic field plays a major role. Such a field is ...threading the whole accretion disc with an amplitude measured by the disc magnetization μ(r, t) =B2z/(μoPtot), where Ptot is the total, gas and radiation, pressure. Below a transition radius rJ, a jet emitting disc (JED) is settled and drives self-collimated non-relativistic jets. Beyond rJ, no jet is produced despite the presence of the magnetic field and a standard accretion disc (SAD) is established. The radial distribution of the disc magnetization μ adjusts itself to any change in the disc accretion rate , thereby modifying the transition radius rJ. We propose that a SAD-to-JED transition occurs locally, at a given radius, in a SAD when μ=μmax≃ 1 while the reverse transition occurs in a JED only when μ=μmin≃ 0.1. This bimodal behaviour of the accretion disc provides a promising way to explain the hysteresis cycles followed by X-ray binaries during outbursts.
The relationship between warm absorber (WA) outflows of active galactic nuclei and nuclear obscuration activities caused by optically thick clouds (obscurers) crossing the line of sight is still ...unclear. NGC 3227 is a suitable target for studying the properties of both WAs and obscurers because it matches the following selection criteria: WAs in both ultraviolet (UV) and X-rays, suitably variable, bright in UV and X-rays, and adequate archival spectra for making comparisons with the obscured spectra. In the aim of investigating WAs and obscurers of NGC 3227 in detail, we used a broadband spectral-energy-distribution model that is built in findings of the first paper in our series together with the photoionization code of
SPEX
software to fit the archival observational data taken by
XMM-Newton
and
NuSTAR
in 2006 and 2016. Using unobscured observations, we find four WA components with different ionization states (log
ξ
erg cm s
−1
∼ −1.0, 2.0, 2.5, 3.0). The highest-ionization WA component has a much higher hydrogen column density (∼10
22
cm
−2
) than the other three components (∼10
21
cm
−2
). The outflow velocities of these WAs range from 100 to 1300 km s
−1
, and show a positive correlation with the ionization parameter. These WA components are estimated to be distributed from the outer region of the broad line region (BLR) to the narrow line region. It is worth noting that we find an X-ray obscuration event in the beginning of the 2006 observation, which was missed by previous studies. We find that it can be explained by a single obscurer component. We also study the previously published obscuration event captured in one observation in 2016, which needs two obscurer components to fit the spectrum. A high-ionization obscurer component (log
ξ
∼ 2.80; covering factor
C
f
∼ 30%) only appears in the 2016 observation, which has a high column density (∼10
23
cm
−2
). A low-ionization obscurer component (log
ξ
∼ 1.0 − 1.9;
C
f
∼ 20%−50%) exists in both 2006 and 2016 observations, which has a lower column density (∼10
22
cm
−2
). These obscurer components are estimated to reside within the BLR by their crossing time of transverse motions. The obscurers of NGC 3227 are closer to the center and have larger number densities than the WAs, which indicate that the WAs and obscurers might have different origins.
ABSTRACT
MCG-5-23-16 is a Seyfert 1.9 galaxy at redshift z = 0.008 49. We analyse here the X-ray spectra obtained with X-ray Multi-Mirror (XMM)-Newton and Nuclear Spectroscopic Telescope Array ...(NuSTAR) data, which are the first contemporaneous observations with these two X-ray telescopes. Two reflection features, producing a narrow core and a broad component of the Fe Kα, are clearly detected in the data. The analysis of the broad iron line shows evidence of a truncated disc with inner radius $R_{\rm in}=40^{+23}_{-16}$Rg and an inclination of $41^{+9}_{-10}$°. The high quality of the NuSTAR observations allows us to measure a high-energy cut-off at $E_{\rm cut}=131^{+10}_{-9}$ keV. We also analyse the reflection grating spectrometer spectrum, finding that the soft X-ray emission is produced by two photoionized plasma emission regions, with different ionization parameters and similar column densities. Remarkably, the source only shows moderate continuum flux variability, keeping the spectral shape roughly constant in a time-scale of ∼20 yr.
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