Browsing by Author "Mukai, K."
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Item The 2019 outburst of the 2005 classical nova V1047 Cen: a record breaking dwarf nova outburst or a new phenomenon?(2021-08-17) Aydi, E.; Sokolovsky, K. V.; Bright, J. S.; Tremou, E.; Nyamai, M. M.; Evans, A.; Strader, J.; Chomiuk, L.; Myers, G.; Hambsch, F-J.; Page, K. L.; Buckley, D. A. H.; Woodward, C. E.; Walter, F. M.; Mróz, P.; Vallely, P. J.; Geballe, T. R.; Banerjee, D. P. K.; Gehrz, R. D.; Fender, R. P.; Gromadzki, M.; Kawash, A.; Knigge, C.; Mukai, K.; Munari, U.; Orio, M.; Ribeiro, V. A. R. M.; Sokoloski, J. L.; Starrfield, S.; Udalski, A.; Woudt, P. A.We present a detailed study of the 2019 outburst of the cataclysmic variable V1047 Cen, which hosted a classical nova eruption in 2005. The peculiar outburst occurred 14 years after the classical nova event, lasted for more than 400 days, and reached an amplitude of around 6 magnitudes in the optical. Early spectral follow-up revealed what could be a dwarf nova (accretion disk instability) outburst in a classical nova system. However, the outburst duration, high velocity (>2000 km s⁻¹) features in the optical line profiles, luminous optical emission, and the presence of prominent long-lasting radio emission, together suggest a phenomenon more exotic and energetic than a dwarf nova outburst. There are striking similarities between this V1047 Cen outburst and those of "combination novae" in classical symbiotic stars. We suggest that the outburst may have started as a dwarf nova that led to the accretion of a massive disk, which in turn triggered enhanced nuclear shell burning on the white dwarf and eventually led to generation of a wind/outflow. From optical photometry we find a \bf{possible} orbital period of 8.36 days, which supports the combination nova scenario and makes the system an intermediate case between typical cataclysmic variables and classical symbiotic binaries. If true, such a phenomenon would be the first of its kind to occur in a system that has undergone a classical nova eruption and is intermediate between cataclysmic variables and symbiotic binaries.Item Chandra Observation of Luminous and Ultraluminous X-ray Binaries in MIOIMukai, K.; Pence, W. D.; Snowden, S. L.; Kuntz, K. D.X-ray binaries in the Milky Way axe among the brightest objects on the Xray sky. With the increasing sensitivity of recent missions, it is now possible to study X-ray binaries in nearby galaxies. We present data on six ultraluminous binaries in the nearby spiral galaxy, M101, obtained with Chandra ACIS-S. Of these, five appear to be similar to ultraluminous sources in other galaxies, while the brightest source, P098, shows some unique characteristics. We present our interpretation of the data in terms of an optically thick outflow, and discuss implications.Item CTCV J2056-3014: An X-Ray-faint Intermediate Polar Harboring an Extremely Fast-spinning White Dwarf(IOP, 2020-07-30) Oliveira, R. Lopes de; Bruch, A.; Rodrigues, C. V.; Oliveira, A. S.; Mukai, K.We report on XMM-Newton X-ray observations that reveal CTCV J2056-3014 to be an unusual accretion-powered, intermediate polar (IP) system. It is a member of the class of X-ray-faint IPs whose space density remains unconstrained but potentially very high, with L x,0.3–12 keV of 1.8 × 10³¹ erg s⁻¹. We discovered a coherent 29.6 s pulsation in X-rays that was also revealed in our reanalysis of published optical data, showing that the system harbors the fastest-spinning, securely known white dwarf (WD) so far. There is no substantial X-ray absorption in the system. Accretion occurs at a modest rate (~6 × 10⁻¹² M ⊙ yr⁻¹) in a tall shock above the WD, while the star seems to be spinning in equilibrium and to have low magnetic fields. Further studies of CTCV J2056-3014 potentially have broad implications on the origin of magnetic fields in WDs, on the population and evolution of magnetic cataclysmic variables, and also on the physics of matter around rapidly rotating magnetic WDs.Item Developing the Physical Understanding of Intermediate Polars: An X-Ray Study of TV Col and V2731 Oph(IOP Publishing, 2019-08-02) Oliveira, R. Lopes de; Mukai, K.The X-rays in intermediate polars (IPs) originate in a compact region near the surface of a magnetic white dwarf (WD) and interact with the complex environment surrounding the emission region. Here we report a case study of two IPs, TV Col and V2731 Oph, with selected archival X-ray observations (NuSTAR, Swift, Suzaku, and XMMNewton). For TV Col, we were successful in simultaneously accounting for the primary X-rays, the secondary X-rays due to Compton scattering and fluorescence, and the effects of local absorbers. In this case, we were able to demonstrate that the shock height is small, based on the high reflection amplitude; hence, the maximum temperature of the post-shock region can be used to derive the WD mass of 0.735 ± 0.015 Me. Despite the high specific accretion rate required to explain the small shock height, we do not detect any spin modulation in our NuSTAR data, consistent with the modest amount of complex absorption seen spectroscopically. We argue that our results are robust because they are based on the joint temporal–spectral analysis of broadband X-ray data. The spectrum of V2731 Oph is more highly absorbed. Through our analysis of the Suzaku data, we present a spectral model with nitrogen overabundance without the previously claimed soft blackbody that should be further explored. We have been unable to constrain the reflection amplitude for V2731 Oph; this and the detection of spin modulation above 10 keV suggest that it may have a tall shock. Hence, we only derive a lower limit to the mass of its WD (>0.9 Me).Item Dissecting a disk-instability outburst in a symbiotic star: NuSTAR and Swift observations of T Coronae Borealis during the rise to the “super-active” state(The Institute of Physics, 2019-08-11) Luna, G. J. M.; Nelson, T.; Mukai, K.; Sokoloski, J. L.The current super-active state of the recurrent nova T CrB has been observed with unprecedented detail. Previously published observations provide strong evidence that this state is due to an enhancement of the flow of material through the accretion disk, which increased the optical depth of its most internal region, the boundary layer. NuSTAR and Swift observed T CrB in 2015 September, roughly halfway through the rise to optical maximum. In our analysis of these data, we have found that: i) the UV emission, as observed with Swift/UVOT in 2015, was already as bright as it became in 2017, after the optical peak; ii) the soft X-ray emission (E . 0.6 keV) observed in 2017 after the optical peak, on the other hand, had not yet developed during the rising phase in 2015; iii) the hard X-ray emitting plasma (E & 2 keV) had the same temperature and about half the flux of that observed during quiescence in 2006. This phenomenology is akin to that observed during dwarf novae in outburst, but with the changes in the spectral energy distribution happening on a far longer time scale.Item Flaring, Dust Formation, And Shocks In The Very Slow Nova ASASSN-17pf (LMCN 2017-11a)(2019-03-21) Aydi, E.; Chomiuk, L.; Strader, J.; Swihart, S. J.; Bahramian, A.; Harvey, E. J.; Britt, C. T.; Buckley, D. A. H.; Chen, P.; Dage, K.; Darnley, M. J.; Dong, S.; Hambsch, F-J.; Holoien, T. W.-S.; Jha, S. W.; Kochanek, C. S.; Kuin, N. P. M.; Li, K. L.; Monard, L. A. G.; Mukai, K.; Page, K. L.; Prieto, J. L.; Richardson, N. D.; Shappee, B. J.; Shishkovsky, L.; Sokolovsky, K. V.; Stanek, K. Z.; Thompson, T.We present a detailed study of the 2017 eruption of the classical nova ASASSN-17pf (LMCN 2017-11a), which is located in the Large Magellanic Cloud, including data from AAVSO, ASAS-SN, SALT, SMARTS, SOAR, and the Neil Gehrels Swift Observatory. The optical light-curve is characterized by multiple maxima (flares) on top of a slowly evolving light-curve (with a decline time, t2> 100 d). The maxima correlate with the appearance of new absorption line systems in the optical spectra characterized by increasing radial velocities. We suggest that this is evidence of multiple episodes of mass-ejection with increasing expansion velocities. The line profiles in the optical spectra indicate very low expansion velocities (FWHM ∼ 190 km s−1), making this nova one of the slowest expanding ever observed, consistent with the slowly evolving light-curve. The evolution of the colors and spectral energy distribution show evidence of decreasing temperatures and increasing effective radii for the pseudo-photosphere during each maximum. The optical and infrared light-curves are consistent with dust formation 125 days post-discovery. We speculate that novae showing several optical maxima have multiple mass-ejection episodes leading to shocks that may drive γ-ray emission and dust formationItem Galactic Extinction: How Many Novae Does It Hide and How Does It Affect the Galactic Nova Rate?(AAS, 2021-11-16) Kawash, A.; Chomiuk, L.; Rodriguez, J. A.; Strader, J.; Sokolovsky, K. V.; Aydi, E.; Kochanek, C. S.; Stanek, K. Z.; Mukai, K.; De, K.; Shappee, B.; Holoien, T. W.-S.; Prieto, J. L.; Thompson, T. A.There is a long-standing discrepancy between the observed Galactic classical nova rate of ∼10 yr−1 and the predicted rate from Galactic models of ∼30–50 yr−1. One explanation for this discrepancy is that many novae are hidden by interstellar extinction, but the degree to which dust can obscure novae is poorly constrained. We use newly available all-sky three-dimensional dust maps to compare the brightness and spatial distribution of known novae to that predicted from relatively simple models in which novae trace Galactic stellar mass. We find that only half (53%) of the novae are expected to be easily detectable (g ≲ 15) with current all-sky optical surveys such as the All-Sky Automated Survey for Supernovae (ASAS-SN). This fraction is much lower than previously estimated, showing that dust does substantially affect nova detection in the optical. By comparing complementary survey results from the ASAS-SN, OGLE-IV, and Palomar Gattini IR surveys using our modeling, we find a tentative Galactic nova rate of ∼30 yr−1, though this could be as high as ∼40 yr−1, depending on the assumed distribution of novae within the Galaxy. These preliminary estimates will be improved in future work through more sophisticated modeling of nova detection in ASAS-SN and other surveys.Item IGR J14257−6117, a magnetic accreting white dwarf with a very strong strong X-ray orbital modulation(Oxford University Press, 2018-04-28) Bernardini, F.; de Martino, D.; Mukai, K.; Falanga, M.IGR J14257−6117 is an unclassified source in the hard X-ray catalogues. Optical follow-ups suggest it could be a Cataclysmic Variable (CV) of the magnetic type. We present the first high signal-to-noise (S/N) X-ray observation performed by XMM–Newton at 0.3–10 keV, complemented with 10–80 keV coverage by Swift/BAT, aimed at revealing the source nature. We detected for the first time a fast periodic variability at 509.5 s and a longer periodic variability at 4.05 h, ascribed to the white dwarf (WD) spin and binary orbital periods, respectively. These unambiguously identify IGR J14257−6117 as a magnetic CV of the intermediate polar (IP) type. The energy-resolved light curves at both periods reveal amplitudes decreasing with increasing energy, with the orbital modulation reaching ∼100 per cent in the softest band. The energy spectrum shows optically thin thermal emission with an excess at the iron complex, absorbed by two dense media (NH ∼ 10²²⁻²³ cm⁻²), partially covering the X-ray source. These are likely localized in the magnetically confined accretion flow above the WD surface and at the disc rim, producing the energy-dependent spin and orbital variabilities, respectively. IGR J14257−6117 joins the group of strongest orbitally modulated IPs now counting four systems. Drawing similarities with low-mass X-ray binaries displaying orbital dips, these IPs should be seen at large orbital inclinations allowing azimuthally extended absorbing material fixed in the binary frame to intercept the line of sight. For IGR J14257−6117, we estimate 50⁰ < i < 70⁰. Whether also the mass accretion rate plays a role in the large orbital modulations in IPs cannot be established with the present data.Item Magnetic Cataclysmic Variables discovered in hard X-rays(2019-06-27) Falanga, M.; de Martino, D.; Bernardini, F.; Mukai, K.Among hard X-ray galactic sources detected by INTEGRAL and Swift surveys, those discovered as accreting white dwarfs have surprisingly boosted in number, representing 20% of the galactic sample. The majority are identified as magnetic cataclysmic variabiles of the intermediate polar type suggesting this subclass as an important constituent of galactic population of X-ray sources. In this conference-proceeding, we review the X-ray emission properties as observed with our ongoing XMM-Newton programme of newly discovered INTEGRAL and/or Swift sources that enlarged almost by a factor of two, identifying cataclysmic variabiles commonalities and outliers.Item Optical spectroscopy and X-ray observations of the D-type symbiotic star EF Aql(2020-05-07) Stoyanov, K. A.; Iłkiewicz, K.; Luna, G. J. M.; Mikołajewska, J.; Mukai, K.; Martí, J.; Latev, G.; Boeva, S.; Zamanov, R. K.We performed high-resolution optical spectroscopy and X-ray observations of the recently identified Mira-type symbiotic star EF Aql. Based on high-resolution optical spectroscopy obtained with SALT, we determine the temperature (∼55 000 K) and the luminosity (∼ 5.3 L⊙) of the hot component in the system. The heliocentric radial velocities of the emission lines in the spectra reveal possible stratification of the chemical elements. We also estimate the mass-loss rate of the Mira donor star. Our Swift observation did not detect EF Aql in X-rays. The upper limit of the X-ray observations is 10⁻¹² erg cm⁻² s⁻¹, which means that EF Aql is consistent with the faintest X-ray systems detected so far. Otherwise we detected it with the UVOT instrument with an average UVM2 magnitude of 14.05. During the exposure, EF Aql became approximately 0.2 UVM2 magnitudes fainter. The periodogram analysis of the V-band data reveals an improved period of 320.4±0.3 d caused by the pulsations of the Mira-type donor star. The spectra are available upon request from the authors.Item The true nature of Swift J0746.3-1608: a possible Intermediate Polar showing accretion state changes(2018-12-21) Bernardini, F.; de Martino, D.; Mukai, K.; Falanga, M.Optical and X-ray observations suggested that the 9.38 h binary, SWIFT J0746.3- 1608 could be a Cataclysmic Variable of the magnetic or nova-like type, or a low mass X-ray binary. Its optical, UV, and X-ray light curves are strongly variable over years. We report on a recent XMM-Newton observation (28 April 2018), when the source had recovered from a deep low state that likely begun mid-late 2011. We detect for the first time a signal at about 38 min that we interpret as the rotation of the accreting white dwarf primary. Its amplitude decreases with increasing energy, indicating localised photoelectric absorption from cold material. The X-ray spectrum shows optically thin thermal emission with excess at the iron complex, absorbed by a dense medium partially covering the X-ray source. Based on these features, we propose that SWIFT J0746.3-1608 is a magnetic CV of the Intermediate Polar (IP) type. The long-term light curves at different wavelengths show high and low states, a rare phenomenon in the IP subclass and observed so far in only three other systems. The long orbital period, the peculiar long term variability, and its proposed magnetic nature, makes SWIFT J0746.3-1608 an interesting evolutionary test case.