Aims.The Galactic bulge region is a rich host of variable high-energy point sources. Since 2005, February 17 we are monitoring the source activity in the Galactic bulge region regularly and ...frequently, i.e., about every three days, with the instruments onboard INTEGRAL. Thanks to the large field of view, the imaging capabilities and the sensitivity at hard X-rays, we are able to present for the first time a detailed homogeneous (hard) X-ray view of a sample of 76 sources in the Galactic bulge region. Methods.We describe the successful monitoring program and show the first results from the start of the monitoring up to 2006, April 21, i.e., for a period of about one and a half year, during three visibility seasons. We focus on the short (hour), medium (month) and long-term (year) variability in the hard X-ray bands, i.e., 20-60 keV and 60-150 keV. When available, we discuss the simultaneous observations in the soft X-ray, 3-10 keV and 10-25 keV, bands. Results.Almost all the sources in the Galactic bulge region we detect in the 20-60 keV and 60-150 keV bands are variable. During the last two and a half weeks of the third visibility season most of the known persistent (hard) X-ray sources in the Galactic Center region were not detected. Of our sample of sources, per visibility season we detect 32/33 sources in the 20-60 keV band and 8/9 sources in the 60-150 keV band above a signal to noise of 7. On average, we find per visibility season one active bright ($\ga$100 mCrab, 20-60 keV) black-hole candidate X-ray transient and three active weaker ($\la$25 mCrab, 20-60 keV) neutron star X-ray transients. Most of the time a clear anti-correlation can be seen between the soft and hard X-ray emission in some of the X-ray bursters. Hard X-ray flares or outbursts in X-ray bursters, which have a duration of the order of weeks are accompanied by soft X-ray drops. On the other hand, hard X-ray drops can be accompanied by soft X-ray flares/outbursts. During the course of our program we found a number of new sources, IGR J17354-3255, IGR 17453-2853, IGR J17454-2703, IGR J17456-2901b, IGR J17536-2339, and IGR J17541-2252. We report here on some of the high-energy properties of these sources. Conclusions.The high-energy light curves of all the sources in the field of view, and the high-energy images of the region, are made available through the WWW, as soon as possible after the observations have been performed, at http://isdc.unige.ch/Science/BULGE/.
We report on INTEGRAL, Swift, and XMM-Newton observations of IGR J17511–3057 performed during the outburst that occurred between March 23 and April 25, 2015. The source reached a peak flux of 0.7(2) ...× 10-9 erg cm-2 s-1 and decayed to quiescence in approximately a month. The X-ray spectrum was dominated by a power law with photon index between 1.6 and 1.8, which we interpreted as thermal Comptonization in an electron cloud with temperature >20 keV. A broad (σ ≃ 1 keV) emission line was detected at an energy (E = 6.9+0.2-0.3 keV) compatible with the K-α transition of ionized Fe, suggesting an origin in the inner regions of the accretion disk. The outburst flux and spectral properties shown during this outburst were remarkably similar to those observed during the previous accretion event detected from the source in 2009. Coherent pulsations at the pulsar spin period were detected in the XMM-Newton and INTEGRAL data at a frequency compatible with the value observed in 2009. Assuming that the source spun up during the 2015 outburst at the same rate observed during the previous outburst, we derive a conservative upper limit on the spin-down rate during quiescence of 3.5 × 10-15 Hz s-1. Interpreting this value in terms of electromagnetic spin-down yields an upper limit of 3.6 × 1026 G cm3 to the pulsar magnetic dipole (assuming a magnetic inclination angle of 30°). We also report on the detection of five type-I X-ray bursts (three in the XMM-Newton data, two in the INTEGRAL data), none of which indicated photospheric radius expansion.
We report the analysis of the first superburst from a transiently accreting neutron star system with the All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer. The superburst occurred 55 days ...after the onset of an accretion outburst in 4U 1608-522. During that time interval, the accretion rate was at least 7% of the Eddington limit. The peak flux of the superburst is 22 to 45% of the Eddington limit, and its radiation energy output is between $4\times 10^{41}$ and $9\times 10^{41}$ erg for a distance of 3.2 kpc. Fits of cooling models to the superburst light curve indicate an ignition column depth between $1.5\times 10^{12}$ and $4.1\times 10^{12}$ g cm-2. Extrapolating the accretion history observed by the ASM, we derive that this column was accreted over a period of 26 to 72. The superburst characteristics are consistent with those seen in other superbursting low-mass X-ray binaries. However, the transient nature of the hosting binary presents significant challenges for superburst theory, requiring additional ingredients for the models. The carbon that fuels the superburst is thought to be produced mostly during the accretion outbursts and destroyed in the frequent type-I X-ray bursts. Mixing and sedimentation of the elements in the neutron star envelope may significantly influence the balance between the creation and destruction of carbon. Furthermore, predictions for the temperature of the neutron star crust fail to reach the values required for the ignition of carbon at the inferred column depth.
We present a new method for determining masses and radii of neutron stars residing in thermonuclear X-ray burst sources. To illustrate this method, we apply it to a burst from the source 4U 1820−30 ...recorded by the Rossi X-Ray Timing Explorer. Fits of the observed X-ray spectra to grids of Comptonized model atmospheres yield estimates for the mass and radius of the neutron star, M= 1.3 ± 0.6 M⊙ and R= 11+3
− 2 km, respectively.
Context. The microquasar GX 339−4 experienced a new outburst in 2010: it was observed simultaneously at various wavelengths from radio up to soft γ-rays. We focus on observations that are ...quasi-simultaneous with those made with the INTEGRAL and RXTE satellites: these were collected in 2010 March–April during our INTEGRAL target of opportunity programme, and during some of the other INTEGRAL observing programmes with GX 339−4 in the field-of-view. Aims. X-ray transients are extreme systems that often harbour a black hole, and are known to emit throughout the whole electromagnetic spectrum when in outburst. The goals of our programme are to understand the evolution of the physical processes close to the black hole and to study the connections between the accretion and ejection. Methods. We analysed radio, NIR, optical, UV, X-ray and soft γ-ray observations. We studied the source evolution in detail by producing light curves, hardness-intensity diagrams and spectra. We fitted the broadband data with phenomenological, then physical, models to study the emission coming from the distinct components. Results. Based on the energy spectra, the source evolved from the canonical hard state to the canonical soft state. The source showed X-ray spectral variations that were correlated with changes in radio, NIR and optical emission. The bolometric flux increased from 0.8 to 2.9 × 10-8 erg cm-2 s-1 while the relative flux and contribution of the hot medium decreased on the average. Reprocessing in the disc was likely to be strong at the end of our observations. Conclusions. The source showed a behaviour similar to that of previous outbursts, with some small deviations in the hard X-ray parameter evolution. The radio, NIR and optical emission from jets was detected and observed to fade as the source softened. The results are discussed within the context of disc and jet models.
Aims. We study the spectrum of the cosmic X-ray background (CXB) in energy range ~5-100 keV. Methods. Early in 2006 the INTEGRAL observatory performed a series of four 30 ks observations with the ...Earth disk crossing the field of view of the instruments. The modulation of the aperture flux due to occultation of extragalactic objects by the Earth disk was used to obtain the spectrum of the Cosmic X-ray Background (CXB). Various sources of contamination were evaluated, including compact sources, Galactic Ridge emission, CXB reflection by the Earth atmosphere, cosmic ray induced emission by the Earth atmosphere and the Earth auroral emission. Results.The spectrum of the cosmic X-ray background in the energy band 5-100 keV is obtained. The shape of the spectrum is consistent with that obtained previously by the HEAO-1 observatory, while the normalization is ~10% higher. This difference in normalization can (at least partly) be traced to the different assumptions on the absolute flux from the Crab Nebulae. The increase relative to the earlier adopted value of the absolute flux of the CXB near the energy of maximum luminosity (20-50 keV) has direct implications for the energy release of supermassive black holes in the Universe and their growth at the epoch of the CXB origin.
We present U, B and V light curves (taken from the literature) of the low-mass X-ray binary Cygnus X-2. We show that the most significant photometric periods seen in the B and V light curves are ...consistent with half of the orbital period found from spectroscopy (P=9.8444 d). The ‘lower envelopes’ of the light curves folded on the orbital period are ellipsoidal (i.e. they have two maxima and two minima per orbital cycle). We fit an ellipsoidal model to the lower envelopes of the B and V light curves to derive inclination constraints. This model includes light from an accretion disc, and accounts for eclipses and X-ray heating. Using the extreme assumption that there is no disc light, we derive a lower limit on the inclination of i≥ 49°. If we assume the accretion disc is steady-state where its radial-temperature profile goes as T(r)∝ r−3/4, we find an inclination of i=62°.5 ± 4°. However, the predicted ratio of the disc flux to the total flux in B (the ‘disc fraction’) is larger than what is observed (≈0.55 compared with ≤0.3). If we use a flatter radial-temperature profile of the disc expected for strongly irradiated discs T(r) ∝ r−3/7, then we find an inclination of i = 54° 6 and a disc fraction in B of ≈ 0.30. However, in this case the value of χ2 is much larger (48.4 with 36 degrees of freedom compared with 40.9 for the steady-state case). Adopting i=62°.5 ±4° and using a previous determination of the mass ratio (q=Mc/Mx=0.34 ± 0.04) and the optical mass function f(M)=0.69 ± 0.03 M⊙, we find that the mass of the neutron star is Mx=1.78 ± 0.23 M⊙ and the mass of the secondary star is Mc=0.60 ± 0.13 M⊙. We derive a distance of d=7.2 ± 1.1 kpc, which is significantly smaller than a recent distance measurement of d=11.6 ± 0.3 kpc derived from an observation of a type I radius-expansion X-ray burst, but consistent with earlier distance estimates.
MAXI J1659−152 was discovered on 2010 September 25 as a new X-ray transient, initially identified as a gamma-ray burst, but was later shown to be a new X-ray binary with a black hole as the most ...likely compact object. Dips in the X-ray light curves have revealed that MAXI J1659−152 is the shortest period black hole candidate identified to date. Here we present the results of a large observing campaign at radio, submillimetre, near-infrared (nIR), optical and ultraviolet (UV) wavelengths. We have combined this very rich data set with the available X-ray observations to compile a broad-band picture of the evolution of this outburst. We have performed broad-band spectral modelling, demonstrating the presence of a spectral break at radio frequencies and a relationship between the radio spectrum and X-ray states. Also, we have determined physical parameters of the accretion disc and put them into context with respect to the other parameters of the binary system. Finally, we have investigated the radio-X-ray and nIR/optical/UV-X-ray correlations up to ∼3 yr after the outburst onset to examine the link between the jet and the accretion disc, and found that there is no significant jet contribution to the nIR emission when the source is in the soft or intermediate X-ray spectral state, consistent with our detection of the jet break at radio frequencies during these states.
A superburst from GX 3+1 Kuulkers, E.
Astronomy & astrophysics,
02/2002, Letnik:
383, Številka:
1
Journal Article
Recenzirano
Odprti dostop
I found one long X-ray flare from the X-ray burster GX 3+1 in almost 6 years of observations with the RXTE All Sky Monitor (ASM). The event had a peak flux of about 1.1 Crab (1.5–12 keV), lasted ...between 4.4 and 16.2 hours and exhibited a fluence of more than about 5$\times$1041 erg for a source distance of 5 kpc. During the exponential-like decay, with an exponential decay time of 1.6 hours, spectral softening is seen. The total ASM effective exposure time on GX 3+1 is estimated to be around a year. The flare bears all the characteristics of the recently discovered so-called superbursts in other X-ray burst sources.
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