ASTROPHYSICS X-ray/optical observations of XTE J0421+560/CI Cam in quiescence (original) (raw)
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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.
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.
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.
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.
Unusual quiescent X-ray activity from XTE J0421+560
Astronomy and Astrophysics
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, alpha, of 4.0 ± 1.90.9 and absorption, NH, of (1.1 ± 4.91.1) × 1021 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 alpha = 1.86 ± 0.270.32 and was strongly absorbed with NH = (4.0 ± 0.8) × 1023 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 continuin...
The Astrophysical Journal, 2003
We present optical photometry of XTE J2123-058 during its quiescent state taken in 1999 and 2000. The dominant feature of our R-band light curve is the ellipsoidal modulation of the secondary star, however, in order to fit this satisfactorily we require additional components which comprise an X-ray heated Roche-lobe filling secondary star, and an accretion disk bulge, i.e. where the gas stream impacts the accretion disk. The observed dip near phase 0.8 is interpreted as the eclipse of inner parts of the accretion disk by the bulge. This scenario is highly plausible given the high binary inclination. Our fits allow us to constrain the size of the quiescent accretion disk to lie in the range 0.26-0.56 R L1 (68 percent confidence). Using the distance of 9.6 kpc and the X-ray flux inferred from the heated hemisphere of the companion, we obtain an unabsorbed Xray luminosity of 1.2×10 33 erg s −1 for XTE J2123-058 in quiescence. From the observed quiescent optical/IR colors we find that the power-law index (-1.4) for the spectral distribution of the accretion disk compares well with other quiescent X-ray transients. We also re-analyse the optical light curves of the soft X-ray transient XTE J2123-058 taken during its outburst and decay in 1998. We use a robust method to fit the data using a refined X-ray binary model. The model computes the light arising from a Roche-lobe filling star and flared accretion disk irradiated by X-rays, and calculates the effects of shadowing and mutual star/disk eclipses. We obtain relatively accurate values for the binary inclination and mass ratio, which when combined with spectroscopic results obtained in paper II gives a neutron star mass in the range 1.04-1.56 M ⊙ (68% confidence).
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.
The Variable Quiescent X-Ray Emission of the Transient Neutron Star XTE J1701-462
Advances in Colloid and Interface Science, 2011
We present the results of continued monitoring of the quiescent neutron star low-mass X-ray binary XTE J1701-462 with Chandra and Swift. A new Chandra observation from 2010 October extends our tracking of the neutron star surface temperature from ~800 days to ~1160 days since the end of an exceptionally luminous 19 month outburst. This observation indicates that the neutron star crust may still be slowly cooling toward thermal equilibrium with the core; another observation further into quiescence is needed to verify this. The shape of the overall cooling curve is consistent with that of a broken power law, although an exponential decay to a constant level cannot be excluded with the present data. To investigate possible low-level activity, we conducted a monitoring campaign of XTE J1701-462 with Swift during 2010 April-October. Short-term flares - presumably arising from episodic low-level accretion - were observed up to a luminosity of ~1e35 erg/s, ~20 times higher than the normal quiescent level. We conclude that flares of this magnitude are not likely to have significantly affected the equilibrium temperature of the neutron star and are probably not able to have a measurable impact on the cooling curve. However, it is possible that brighter and longer periods of low-level activity have had an appreciable effect on the equilibrium temperature.
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...
Multifrequency observations of XTE J0421+560/CI Cam in outburst
Astronomy and Astrophysics, 1998
We report on two X-ray observations of the transient source XTE J0421+560 performed by BeppoSAX, and on a series of observations performed by the 0.7 m Teramo-Normale Telescope. Outburst peak occurrence time and duration depend on photon energy: the outburst peak is achieved first in the X-ray band, then in the optical and finally in the radio. An exponential decay law fits well the X-ray data except in the TOO2 0.5-1.0 keV band, where erratic time variability is detected. During TOO1 the e-folding time scale decreases with energy up to ∼20 keV, when it achieves a saturation; during TOO2 it decreases up to ∼2 keV and then increases. This change is correlated with a spectral change, characterized by the onset of a soft ( < ∼ 2 keV) component in TOO2 (Orr et al. 1998). This component might originate from the relativistic jets, while the hard component is more likely associated to processes occurring in the circumstellar matter and/or near the compact object. Optical observations show that the object appears intrinsically red even during the outburst. The nature of the compact object is discussed.