Unusual quiescent X-ray activity from XTE J0421+560 (original) (raw)
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Unusual quiescent X-ray activity from XTE J0421+560 (CI Cam)
Astron Astrophys, 2000
We report on BeppoSAX observations of the X-ray transient XTE J0421+560 in quiescence 156, 541, and ∼690 days after the maximum of the 1998 April outburst. In the first observation the source was soft with a power-law photon index, α, of 4.0± 1.9 0.9 and absorption, N H , of (1.1± 4.9 1.1 ) × 10 21 atom cm −2 . In the second observation, the source brightened by a factor ∼15 in the 1-10 keV energy range, became significantly harder with α = 1.86± 0.27 0.32 and was strongly absorbed with N H = (4.0 ± 0.8) × 10 23 atom cm −2 . There is evidence for a narrow emission line in both spectra at ∼7 keV. In the third observation, the source had faded by a factor ∼ > 8 from the previous observation to below the BeppoSAX detection level. It is possible that these variations result from orbital motion of a compact object around the B[e] star companion with the intense, absorbed, spectrum arising during passage through dense circumstellar material. If this is the case, the system may be continuing to exhibit periodic activity.
Astronomy and Astrophysics, 2002
We have observed the X-ray transient XTE J0421+56 in quiescence with XMM-Newton. The observed spectrum is highly unusual being dominated by an emission feature at ∼6.5 keV. The spectrum can be fit using a partially covered power-law and Gaussian line model, in which the emission is almost completely covered (covering fraction of 0.98 +0.02 −0.06) by neutral material and is strongly absorbed with an NH of (5 +3 −2) × 10 23 atom cm −2. This absorption is local and not interstellar. The Gaussian has a centroid energy of 6.4 ± 0.1 keV, a width σ < 0.28 keV and an equivalent width of 940 +650 −460 eV. It can be interpreted as fluorescent emission line from iron. Using this model and assuming XTE J0421+56 is at a distance of 5 kpc, its 0.5-10 keV luminosity is 3.5 × 10 33 erg s −1. The Optical Monitor onboard XMM-Newton indicates a V magnitude of 11.86 ± 0.03. The spectra of X-ray transients in quiescence are normally modeled using advection dominated accretion flows, power-laws, or by the thermal emission from a neutron star surface. The strongly locally absorbed X-ray emission from XTE J0421+56 is therefore highly unusual and could result from the compact object being embedded within a dense circumstellar wind emitted from the supergiant B[e] companion star. The uncovered and unabsorbed component observed below 5 keV could be due either to X-ray emission from the supergiant B[e] star itself, or to the scattering of high-energy X-ray photons in a wind or ionized corona, such as observed in some low-mass X-ray binary systems.
The 1998 outburst of the X?ray transient XTE J2012+381 as observed with BeppoSAX
Astronomy and Astrophysics, 2002
We report on the results of a series of X-ray observations of the transient black hole candidate XTE J2012+381 during the 1998 outburst performed with the BeppoSAX satellite. The observed broad-band energy spectrum can be described by the superposition of an absorbed disk black body, with an iron line plus a high energy component, modelled with either a power law or a Comptonisation tail. The source showed pronounced spectral variability between our five observations. While the soft component in the spectrum remained almost unchanged throughout our campaign, we detected a hard spectral tail which extended to 200 keV in the first two observations, but became barely detectable up to 50 keV in the following two. A further re-hardening is observed in the final observation. The transition from a hard to a soft and then back to a hard state occurred around an unabsorbed 0.1-200 keV luminosity of 10 38 erg s −1 (at 10 kpc). This indicates that state transitions in XTE 2012+281 are probably not driven only by mass accretion rate, but additional physical parameters must play a role in the evolution of the outburst.
The Astrophysical Journal, 2001
We report on results of a target of opportunity observation of the X-ray transient XTE J1118+480 performed on 2000 April 14-15 with the Narrow Field Instruments (0.1-200 keV) of the BeppoSAX satellite. The measured spectrum is a power law with a photon index of ∼ 1.7 modified by an ultrasoft X-ray excess and a high-energy cutoff above ∼ 100 keV. The soft excess is consistent with a blackbody with temperature of ∼ 40 eV and a low flux, while the cut-off power law is well fitted by thermal Comptonization in a plasma with an electron temperature of ∼ 10 2 keV and an optical depth of order of unity. Consistent with the weakness of the blackbody, Compton reflection is weak. Though the data are consistent with various geometries of the hot and cold phases of the accreting gas, we conclude that a hot accretion disk is the most plausible model. The Eddington ratio implied by recent estimates of the mass and the distance is ∼ 10 −3 , which may indicate that advection is probably not the dominant cooling mechanism. We finally suggest that the reflecting medium has a low metallicity, consistent with location of the system in the halo.
ASTROPHYSICS X-ray/optical observations of XTE J0421+560/CI Cam in quiescence
2000
We report on a BeppoSAX observation of the transient X-ray source XTE J0421+560 during quiescence performed ∼150 days after the 1998 April outburst. The source had an unabsorbed 0.5-10 keV flux of 6.7 × 10 −12 erg cm −2 s −1 and was still remarkably soft with most of the emission below 2 keV. The X-ray spectrum can be fit with the same twotemperature model as the outburst observations. There is evidence for the presence of an Iron emission feature at ∼7 keV. We report also on a series of optical observations performed using the 72 cm Teramo-Normale Telescope (TNT) of the Teramo Observatory, and the 1.5 m Loiano Telescope of the Bologna Observatory. The optical spectrum includes very strong Balmer emission lines, He I, [Fe II], and [O I] features. From the observed L x /L opt ∼ 10 −3 the quiescent optical emission cannot be due to reprocessing of the X-rays, but has to be generated in the optical companion or the circumstellar material. Moreover, the quiescent X-ray luminosity cannot be due to the optical star if it is of spectral type OB. Although the nature of the compact object present in the XTE J0421+560/CI Cam system cannot be firmly established, we speculate that it is most probably a white dwarf.
Detection of a large Be circumstellar disk during X-ray quiescence of XTE J1946+274
We present a multiwavelength study of the Be/X-ray binary system XTE J1946+274 with the main goal of better characterizing its behavior during X-ray quiescence. We aim to shed light on the mechanism which triggers the X-ray activity for this source. XTE J1946+274 was observed by Chandra-ACIS during quiescence in 2013 March 12. In addition, this source has been monitored from the ground-based astronomical observatories of El Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain) and Sierra Nevada (Granada, Spain) since 2011 September, and from the TUBITAK National Observatory (Antalya, Turkey) since 2005 April. We have performed spectral and photometric temporal analyses in order to investigate the quiescent state and transient behavior of this binary system. In 2006, a long mass ejection event took place from the Be star, lasting for about seven years. We also found that a large Be circumstellar disk was present during quiescence, although major X-ray activity was not obs...
Optical Observations of XTE J1118 + 480 during the 2000 Outburst
Publications of the Astronomical Society of Japan, 2002
We report on photometric and spectroscopic observations of a possible halo black-hole X-ray nova, XTE J1118 + 480 (= KV UMa) during outburst. Our photometric monitoring during the main outburst revealed that the optical maximum as well as the onset of the outburst precede those in the X-ray region. This indicates that the event was an "outside-in" type outburst and that its optical flux was dominated by viscous heating, itself, and not the effect of X-ray irradiation. Based on these results, we suggest an outburst scenario analogous to superoutbursts in SU UMa-type dwarf novae. This scenario predicts a superhump phenomenon, which we indeed detected throughout the outburst. We determined its period to be 0.170529 ± 0.000006 d, which is slightly longer than the orbital periods suggested from spectroscopic observations. We have furthermore revealed the first evidence of a continuous period decreasing in X-ray novae. The most prominent feature in our optical spectrum is a double-peak He II 4686Å emission line having an asymmetric profile with an outstanding blue side peak. Using a Doppler mapping method, we found that the He II emission originates from the accretion disk, which particularly concentrates on the hot spot. The time that the blue peak becomes strongest corresponds to a superhump peak. This implies that we see an elongated side of an eccentric disk at that time and, hence, it may cause an asymmetric emission profile. Substituting the observed fractional superhump excess for a theoretically expected relation between it and the mass ratio, we estimate that the black-hole mass is larger than 9.5 M. XTE J1118 + 480 thus has a large mass of a compact object compared with the typical black-hole X-ray novae.
On the Origin of Radio Emission in the X-Ray States of XTE J1650500 during the 2001-2002 Outburst
The Astrophysical …, 2004
We report on simultaneous radio and X-ray observations of the black hole candidate XTE J1650−500 during the course of its 2001-2002 outburst. The scheduling of the observations allowed us to sample the properties of XTE J1650−500 in different X-ray spectral states, namely the hard state, the steep power-law state and the thermal dominant state, according to the recent spectral classification of McClintock & Remillard. The hard state is consistent with a compact jet dominating the spectral energy distribution at radio frequencies; however, the current data suggest that its contribution as direct synchrotron emission at higher energies may not be significant. In that case, XTE J1650−500 may be dominated by Compton processes (either inverse Comptonization of thermal disk photons and/or SSC from the base of the compact jet) in the X-ray regime. We, surprisingly, detect a faint level of radio emission in the thermal dominant state that may be consistent with the emission of previously ejected material interacting with the interstellar medium, similar (but on a smaller angular scale) to what was observed in XTE J1550−564 by Corbel and co-workers. Based on the properties of radio emission in the steep power-law state of XTE J1650−500, and taking into account the behavior of other black hole candidates (namely GX 339−4, XTE J1550−564, and XTE J1859+226) while in the intermediate and steep power-law states, we are able to present a general pattern of behavior for the origin of radio emission in these two states that could be important for
X-ray observation of XTE J2012+381 during the 1998 outburst
Journal of Astrophysics and Astronomy, 2000
The outburst of X ray transient source XTE J2012+381 was detected by the RXTE All Sky Monitor on 1998 May 24th. Following the outburst, X ray observations of the source were made in the 2 18keV energy band with the Pointed Proportional Counters of the Indian X ray Astronomy Experiment (IXAE) on board the Indian satellite IRS P3 during 1998 June 2nd 10th. The X ray flux of the source in the main outburst decreased exponentially during the period of observation. No large amplitude short term variability in the intensity is detected from the source. The power density spectrum obtained from the timing analysis of the data shows no indication of any quasi periodic oscillations in 0.002 0.5 Hz band. The hardness ratio i.e. the ratio of counts in 6 18 keV to 2 6 keV band, indicates that the X ray spectrum is soft with spectral index >2. From the similarities of the X ray properties with those of other black hole transients, we conclude that the X ray transient XTE J2012+381 is likely to be a black hole.
X-ray/optical observations of XTE J0421+ 560/CI Cam in quiescence
Arxiv preprint astro-ph/ …, 2000
We report on a BeppoSAX observation of the transient X-ray source XTE J0421+560 during quiescence performed ∼150 days after the 1998 April outburst. The source had an unabsorbed 0.5-10 keV flux of 6.7 × 10 −12 erg cm −2 s −1 and was still remarkably soft with most of the emission below 2 keV. The X-ray spectrum can be fit with the same twotemperature model as the outburst observations. There is evidence for the presence of an Iron emission feature at ∼7 keV. We report also on a series of optical observations performed using the 72 cm Teramo-Normale Telescope (TNT) of the Teramo Observatory, and the 1.5 m Loiano Telescope of the Bologna Observatory. The optical spectrum includes very strong Balmer emission lines, He I, [Fe II], and [O I] features. From the observed L x /L opt ∼ 10 −3 the quiescent optical emission cannot be due to re-processing of the X-rays, but has to be generated in the optical companion or the circumstellar material. Moreover, the quiescent X-ray luminosity cannot be due to the optical star if it is of spectral type OB. Although the nature of the compact object present in the XTE J0421+560/CI Cam system cannot be firmly established, we speculate that it is most probably a white dwarf.