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تسجيل مستخدم جديدWhat ignites on the neutron star of 4U 0614+091?
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E. Kuulkers
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[abridged] The LMXB 4U 0614+091 is a source of sporadic thermonuclear (type I) X-ray bursts. We find bursts with a wide variety of characteristics in serendipitous wide-field X-ray observations by EURECA/WATCH, RXTE/ASM, BeppoSAX/WFC, HETE-2/FREGATE, INTEGRAL/IBIS/ISGRI, and Swift/BAT, as well as pointed observations by RXTE/PCA and HEXTE. Most of them reach a peak flux of ~15 Crab, but a few only reach a peak flux below a Crab. One of the bursts shows a very strong photospheric radius-expansion phase. This allows us to evaluate the distance to the source: 3.2 kpc. The burst durations are between 10 sec to 5 min. However, after one of the intermediate-duration bursts, a faint tail is seen to at least ~2.4 hours after the start of the burst. One very long burst lasted for several hours. This superburst candidate was followed by a normal type-I burst only 19 days later. This is, to our knowledge, the shortest burst-quench time among the superbursters. A superburst in this system is difficult to reconcile if it accretes at ~1% L_Edd. The intermediate-duration bursts occurred when 4U 0614+091s persistent emission was lowest and calm, and when bursts were infrequent (on average one every month to 3 months). The average burst rate increased significantly after this period. The maximum average burst recurrence rate is once every week to 2 weeks. The burst behaviour may be partly understood if there is at least an appreciable amount of helium present in the accreted material from the donor star. If the system is an ultra-compact X-ray binary with a CO white-dwarf donor, as has been suggested, this is unexpected. If the bursts are powered by helium, we find that the energy production per accumulated mass is about 2.5 times less than expected for pure helium matter.
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Multi-wavelength spectral energy distributions of Low Mass X-ray Binaries in the hard state are determined by the emission from a jet, for frequencies up to mid-infrared, and emission from the accretion flow in the optical to X-ray range. In the last
years, the flat radio-to-mid-IR spectra of Black Hole (BH) X-ray binaries was described using the internal shocks model, which assumes that the fluctuations in the velocity of the ejecta along the jet are driven by the fluctuations in the accretion flow, described by the X-ray Power Density Spectrum (PDS). In this work we attempt to apply this model for the first time to a Neutron Star (NS) LMXB, i.e. 4U 0614+091. We used the multi-wavelength data set obtained in 2006, comprising data from radio to X-ray, and applied a model which includes an irradiated disc model for the accretion flow and an updated version of the internal shocks code for the ejection. The new version of the code allows to change the geometry of the jet for the case of non-conical jets. Only two alternative scenarios provide a satisfactory description of the data: using the X-ray PDS but in a non-conical geometry for the jet, or either using a conical geometry but with a flicker-noise PDS. Both scenarios would imply some differences with the results obtained with similar models on BH X-ray binaries, shedding light on the possibility that jets in NS and BH binaries might somehow have a different geometry or a different coupling with the accretion flow.
Atoll sources are accreting neutron star (NS) low-mass X-ray binaries. We present a spectral analysis of four persistent atoll sources (GX 3+1, 4U 1702$-$429, 4U 0614+091, and 4U 1746$-$371) observed for $sim20$ ks each with NuSTAR to determine the e
xtent of the inner accretion disk. These sources range from an apparent luminosity of $0.006-0.11$ of the Eddington limit (assuming the empirical limit of $3.8times10^{38}$ ergs s$^{-1}$). Broad Fe emission features shaped by Doppler and relativistic effects close to the NS were firmly detected in three of these sources. The position of the disk appears to be close to the innermost stable circular orbit (ISCO) in each case. For GX 3+1, we determine $R_{in}=1.8^{+0.2}_{-0.6} R_{mathrm{ISCO}}$ (90% confidence level) and an inclination of $27^{circ}-31^{circ}$. For 4U 1702$-$429, we find a $R_{in}=1.5_{-0.4}^{+1.6} R_{mathrm{ISCO}}$ and inclination of $53^{circ}-64^{circ}$. For 4U 0614+091, the disk has a position of $R_{in}=1.3_{-0.2}^{+5.4} R_{mathrm{ISCO}}$ and inclination of $50^{circ}-62^{circ}$. If the disk does not extend to the innermost stable circular orbit, we can place conservative limits on the magnetic field strength in these systems in the event that the disk is truncated at the Alfv{e}n radius. This provides the limit at the poles of $Bleq6.7times10^{8}$ G, $3.3times10^{8}$ G, and $14.5times10^{8}$ G for GX 3+1, 4U 1702$-$429, and 4U 0614+091, respectively. For 4U 1746$-$371, we argue that the most plausible explanation for the lack of reflection features is a combination of source geometry and strong Comptonization. We place these sources among the larger sample of NSs that have been observed with NuSTAR.
Spitzer observations of the neutron star (ultra-compact) X-ray binary (XRB) 4U 0614+091 with the Infrared Array Camera reveal emission of non-thermal origin in the range 3.5-8 um. The mid-infrared spectrum is well fit by a power law with spectral ind
ex of alpha=-0.57+/-0.04 (where the flux density is F_nu propto nu^(alpha)). Given the ultra-compact nature of the binary system, we exclude the possibility that either the companion star or the accretion disk can be the origin of the observed emission. These observations represent the first spectral evidence for a compact jet in a low-luminosity neutron star XRB and furthermore of the presence, already observed in two black hole (BH) XRBs, of a `break in the synchrotron spectrum of such compact jets. We can derive a firm upper limit on the break frequency of the spectrum of nu_thin=3.7x10^(13) Hz, which is lower than that observed in BH XRBs by at least a factor of 10. Assuming a high-energy cooling cutoff at ~1 keV, we estimate a total (integrated up to X-rays) jet power to X-ray bolometric luminosity ratio of ~5%, much lower than that inferred in BHs.
We observed the ultra-compact binary candidate 4U 0614+091 for a total of 200 ksec with the high-energy transmission gratings onboard the chandra X-ray Observatory. The source is found at various intensity levels with spectral variations present. X-r
ay luminosities vary between 2.0$times10^{36}$ ergsec and 3.5$times10^{36}$ ergsec. Continuum variations are present at all times and spectra can be well fit with a powerlaw component, a high kT blackbody component, and a broad line component near oxygen. The spectra require adjustments to the Ne K edge and in some occasions also to the Mg K edge. The Ne K edge appears variable in terms of optical depths and morphology. The edge reveals average blue- and red-shifted values implying Doppler velocities of the order of 3500 kms. The data show that Ne K exhibits excess column densities of up to several 10$^{18}$ cm$^{-2}$. The variability proves that the excess is intrinsic to the source. The correponding disk velocities also imply an outer disk radius of the order of $< 10^9$ cm consistent with an ultra-compact binary nature. We also detect a prominent soft emission line complex near the oviii L$alpha$ position which appears extremely broad and relativistic effects from near the innermost disk have to be included. Gravitationally broadened line fits also provide nearly edge-on angles of inclination between 86 and 89$^{circ}$. The emissions appear consistent with an ionized disk with ionization parameters of the order of 10$^4$ at radii of a few 10$^7$ cm. The line wavelengths with respect to oviiia are found variably blue-shifted indicating more complex inner disk dynamics.
We have detected transient X-ray activity from the X-ray burster 4U~0614+091 simultaneously with BATSE/CGRO (20-100 keV) and ASM/RXTE (1-12 keV). The peak fluxes reach approximately 40 mCrab in both instruments over a period of about 20 days. The var
iable emission shows a clear anticorrelation of the hard X-ray flux with the soft X-ray count rate. The observed anticorrelation is another clear counterexample to the notion that only black hole binaries exhibit such correlations. The individual spectra during this period can be fit by power laws with photon indices 2.2+-0.3 (ASM) and 2.7+-0.4 (BATSE), while the combined spectra can be described by a single power law with index 2.09+-0.08. BATSE and the ASM/RXTE are a good combination for monitoring X-ray sources over a wide energy band.
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