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Chandra imaging spectroscopy of 1E 1740.7 - 2942  [PDF]
E. Gallo,R. Fender
Physics , 2002, DOI: 10.1046/j.1365-8711.2002.05957.x
Abstract: We have observed the black hole candidate 1E 1740.7 - 2942, the brightest persistent hard X-ray source within a few degrees of the Galactic centre, for 10 ksec with Chandra (ACIS-I) on August 2000. Attempting to compensate for pile-up effects we found the spectra were well-fit by an absorbed power law, with photon indices Gamma = 1.54 ^{+0.42}_{-0.37} (readout streak) and Gamma = 1.42^{+0.14}_{-0.14} (annulus), consistent with a black hole low/hard state. We have analysed a public observation performed by Chandra which utilised short frames in order to avoid severe pile-up effects: subtracting the core point spread function from the whole image, we did not find evidence for any elongated feature perpendicular to the radio jet axis, as reported in a recent analysis of the same data. Moreover, comparing the radial profiles with those of an unscattered X-ray point source, we found indication of an extended, previously undetected, X-ray scattering halo. The measured halo fractional intensity at 3 keV is between 30 and 40 percent within 40 arcsec but drops below detectable levels at 5 keV. Finally, by placing a limit on the X-ray flux from the radio emitting lobe which has been identified as the hot spot at the end of the northern jet of 1E 1740.7 - 2942, we are able to constrain the magnetic energy density in that region.
Flux and spectral variations of 1E${\thinspace}$1740.7$-$2942 over the years 2003$-$2012  [PDF]
Manuel Castro,Flavio D'Amico,Francisco Jablonski,Jo?o Braga
Physics , 2015,
Abstract: The black hole system 1E${\thinspace}$1740.7$-$2942 is usually the brightest hard X-ray source (above 20 keV) near the Galactic Center, but presents some epochs of low emission (below the INTEGRAL detection limit, for example). In this work, we present the results of studies on 1E${\thinspace}$1740.7$-$2942 over 10 years, using the instruments ISGRI/IBIS and JEM-X, both on board the INTEGRAL observatory. We fit the spectra with both the compTT and cutoffpl models. According to the fits and taking the mean value over the 10 years, we have obtained a plasma temperature in the range $\sim$20$\,-\,$90$\thinspace$ keV, and an average powerlaw index of 1.41 ($\sigma$=0.25). We have also made a Lomb$\,-\,$Scargle periodogram of the flux in the 50$\,-\,$20${\thinspace}$keV band and found two tentative periods at 2.90 and 3.99${\thinspace}$days. We present here the preliminary results of this ongoing work.
The precessing jets of 1E 1740.7-2942  [PDF]
Pedro L. Luque-Escamilla,Josep Marti,Josee Martinez-Aroza
Physics , 2015, DOI: 10.1051/0004-6361/201527238
Abstract: Context. 1E 1740.7-2942 is believed to be one of the two prototypical microquasars towards the Galactic center region whose X-ray states strongly resemble those of Cygnus X-1. Yet, the bipolar radio jets of 1E 1740.7-2942 are very reminiscent of a radio galaxy. The true nature of the object has thus remained an open question for nearly a quarter of a century. Aims. Our main goal here is to confirm the Galactic membership of 1E 1740.7-2942 by searching for morphological changes of its extended radio jets in human timescales. This work was triggered as a result of recent positive detection of fast structural changes in the large-scale jets of its 'twin' GRS 1758-258. Methods. We carried out an in-depth exploration of the VLA public archives and fully recalibrated all 1E 1740.7-2942 extended data sets in the C configuration of the array. We obtained and analyzed matching beam radio maps for five epochs, covering years 1992, 1993, 1994, 1997 and 2000, with an angular resolution of a few arcseconds. Results. We clearly detected structural changes in the arcminute jets of 1E 1740.7-2942 on timescales of roughly a year, which set a firm distance upper limit of 12 kpc. Moreover, a simple precessing model was simultaneously fitted to the five observing epochs available. The observed changes in the jet flow are strongly suggestive of a precession period of ~1.3 years. Conclusions. The fitting of the precession model to the data yields a distance of ~5 kpc. This value, and the observed changes, rule out any remaining doubts about the 1E 1740.7-2942 Galactic nature. To our knowledge, this microquasar is the second whose jet precession ephemeris become available after SS433. This kind of information is relevant to the physics of compact objects, since the genesis of the precession phenomenon occurs very close to the interplay region between the accretion disk and the compact object in the system.
VLT search for the infrared counterpart of 1E 1740.7-2942  [PDF]
Josep Marti,I. Felix Mirabel,Sylvain Chaty,Luis F. Rodriguez
Physics , 2000,
Abstract: We report the results of our search for the near infrared counterpart of the microquasar 1E 1740.7-2942 using the VLT (Based on observations collected at the European Southern Observatory, Chile (ESO No 63.H-0261)). For the first time, several counterpart candidates have been found in our Ks-band images that may be consistent with the best radio and X-ray positions available for 1E 1740.7-2942. However, the non-detection of variability between two observing epochs and the positional uncertainty still remaining at the nearly less than 1 arcsec level prevent us from identifying an unambiguous counterpart. Alternatively, the VLT images set new upper limits significantly deeper than previously reported that constrain the binary companion to be later than B8 V or earlier than G5 III.
Further studies of 1E 1740.7-2942 with ASCA  [PDF]
Masaaki Sakano,Kensuke Imanishi,Masahiro Tsujimoto,Katsuji Koyama,Yoshitomo Maeda
Physics , 1999, DOI: 10.1086/307441
Abstract: We report the ASCA results of the Great Annihilator 1E 1740.7-2942 obtained with five pointing observations in a time span of 3.5 years. The X-ray spectrum for each period is well fitted with a single power-law absorbed by a high column of gas. The X-ray flux changes by a factor of 2 from period to period, but the other spectral parameters show no significant change. The photon index is flat with \Gamma = 0.9--1.3. The column densities of hydrogen N_H is $\sim$ 1.0 x 10^{23} H cm^{-2} and that of iron N_{Fe} is $\sim$ 10^{19} Fe cm^{-2}. These large column densities indicate that 1E 1740.7-2942 is near at the Galactic Center. The column density ratio leads the iron abundance to be 2 times larger than the other elements in a unit of the solar ratio. The equivalent width of the K\alpha-line from a neutral iron is less than 15 eV in 90% confidence. This indicates that the iron column density within several parsecs from 1E 1740.7-2942 is less than 5 x 10^{17} Fe cm^{-2}. In addition, the derived hydrogen column density is about 1/6 of that of giant molecular clouds in the line of sight. All these facts support that 1E 1740.7-2942 is not in a molecular cloud, but possibly in front of it; the X-rays are not powered by accretion from a molecular cloud, but from a companion star like ordinary X-ray binaries.
Comptonization in 1E 1740.7-2942 spectra from 2 to 200 keV  [PDF]
Manuel Castro,Taís Maiolino,Flavio D'Amico,J?rn Wilms,Katja Pottschmidt,Jo?o Braga
Physics , 2013,
Abstract: Studies of the long-term spectral variations have been used to constrain the emission processes of black hole candidates. However, a common scenario which is able to explain the emission from soft to hard X-rays has been proposed only recently. Here, we use XMM and INTEGRAL data on 1E 1740.7-2942 in order to demonstrate that Comptonization plays an important role in producing high energy photons, as predicted by the current modeling scenario.
Orbital and Super-Orbital Periods of 1E 1740.7-2942 and GRS 1758-258  [PDF]
D. M. Smith,W. A. Heindl,J. H. Swank
Physics , 2002, DOI: 10.1086/344701
Abstract: Five years of Rossi X-ray Timing Explorer (RXTE) observations of the Galactic black-hole candidates 1E 1740.7-2942 and GRS 1758-258 show a periodic modulation with amplitude 3-4% in each source at 12.73 +/- 0.05 dy and 18.45 +/- 0.10 dy, respectively. We interpret the modulations as orbital, suggesting that the objects have red-giant companions. Combining the RXTE data with earlier data (Zhang, Harmon & Liang 1997) from the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory, we find a long period or quasi-period of about 600 dy in 1E 1740.7-2942, and a suggestion of a similar 600-dy period in GRS 1758-258. These timescales are longer than any yet found for either precessing systems like Her X-1 and SS 433 or binaries like LMC X-3 and Cyg X-1 with more irregular long periods.
Observational constraints on annihilation sites in 1E 1740.7-2942 and Nova Muscae  [PDF]
I. V. Moskalenko,E. Jourdain
Physics , 1997, DOI: 10.1063/1.54157
Abstract: The region of the Galactic center contains several sources which demonstrate their activity at various wavelengths and particularly above several hundred keV. Escape of positrons from such a source or several sources into the interstellar medium, where they slow down and annihilate, can account for the 511 keV narrow line observed from this direction. 1E 1740.7-2942 object has been proposed as the most likely candidate to be responsible for this variable source of positrons. Besides, Nova Muscae shows a spectrum which is consistent with Comptonization by a thermal plasma kT<100 keV in its hard X-ray part, while a relatively narrow annihilation line observed by SIGMA on January 20-21, 1991 implies that positrons annihilate in a much colder medium. We estimate the electron number density and the size of the emitting regions suggesting that annihilation features observed by SIGMA from Nova Muscae and 1E 1740.7-2942 are due to the positron slowing down and annihilation in thermal plasma. We show that in the case of Nova Muscae the observed radiation is coming from a pair plasma stream, N(e+)~N(e-), rather than from a gas cloud. We argue that two models are probably relevant to the 1E source: annihilation in (hydrogen) plasma N(e+)<~N(e-) at rest, and annihilation in the pair plasma stream, which involves matter from the source environment.
Confirming the thermal Comptonization model for black hole X-ray emission in the low-hard state  [PDF]
M. Castro,F. D'Amico,J. Braga,T. Maiolino,K. Pottschmidt,J. Wilms
Physics , 2014, DOI: 10.1051/0004-6361/201323290
Abstract: Hard X-ray spectra of black hole binaries in the low/hard state are well modeled by thermal Comptonization of soft seed photons by a corona-type region with $kT$\thinspace$\sim 50${\thinspace}keV and optical depth around 1. Previous spectral studies of 1E{\thinspace}1740.7$-$2942, including both the soft and the hard X-ray bands, were always limited by gaps in the spectra or by a combination of observations with imaging and non-imaging instruments. In this study, we have used three rare nearly-simultaneous observations of 1E{\thinspace}1740.7$-$1942 by both XMM-Newton and INTEGRAL satellites to combine spectra from four different imaging instruments with no data gaps, and we successfully applied the Comptonization scenario to explain the broadband X-ray spectra of this source in the low/hard state. For two of the three observations, our analysis also shows that, models including Compton reflection can adequately fit the data, in agreement with previous reports. We show that the observations can also be modeled by a more detailed Comptonization scheme. Furthermore, we find the presence of an iron K-edge absorption feature in one occasion, which confirms what had been previously observed by Suzaku. Our broadband analysis of this limited sample shows a rich spectral variability in 1E{\thinspace}1740.7$-$2942 at the low/hard state, and we address the possible causes of these variations. More simultaneous soft/hard X-ray observations of this system and other black-hole binaries would be very helpful in constraining the Comptonization scenario and shedding more light on the physics of these systems.
Long-Term X-ray Monitoring of 1E 1740.7-2942 and GRS 1758-258  [PDF]
D. S. Main,D. M. Smith,W. A. Heindl,J. Swank,M. Leventhal,I. F. Mirabel,L. F. Rodriguez
Physics , 1999, DOI: 10.1086/307935
Abstract: We report on long-term observations of the Galactic-bulge black hole candidates 1E 1740.7-2942 and GRS 1758-258 with the Rossi X-Ray Timing Explorer. 1E 1740.7-2942 has been observed 77 times and GRS 1758-258 has been observed 82 times over the past 1000 days. The flux of each object has varied by no more than a factor of 2.5 during this period, and the indices of the energy spectra have varied by no more than 0.4. The power spectra are similar to other black-hole candidates: flat-topped noise, breaking to a power law. Each object has exhibited a brightening that lasted for several months, and we have a found a time lag between the photon power-law index and the count rate. In both sources, the spectrum is softest during the decline from the brightening. This behavior can be understood in the context of thin-disk and advection-dominated accretion flows coexisting over a wide range of radii, with the implication that both sources have low-mass companions and accrete via Roche-lobe overflow.
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