Large Area X-ray Proportional Counter Research Articles

Spectral and type I X-ray burst studies of 4U 1702−429 using AstroSat observations

ABSTRACT 4U 1702−429, an atoll-type neutron star low-mass X-ray binary, was observed twice by the AstroSat/Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counters (LAXPC-20) on 2018 April 27 and 2019 August 8. Persistent emission spectra of the source were well fitted with the model combination - constant × tbabs (thcomp × diskbb+powerlaw). The parameters obtained from the spectral analysis revealed the source to be in a hard spectral state during the observations. Time-resolved spectral analyses were performed on the three type I X-ray bursts detected from the source. Burst analysis showed that the source underwent a photospheric radius expansion. Consequently, the radius of the neutron star and distance to the source (with isotropic and anisotropic burst emission) were obtained as 12.65$\substack{+0.90\\-0.86}$ km and 6.92$\substack{+0.16\\-0.09}$ and 8.43$\substack{+0.20\\-0.10}$ kpc, respectively.

AstroSat and NICER timing view of the Z−type neutron star X-ray binary GX 340 + 0

ABSTRACT The timing properties of the Z-type low-mass X-ray binaries provide insights into the emission components involved in producing the unique Z-shaped track in the hardness–intensity diagrams of these sources. In this work, we investigate the AstroSat and NICER observations of the GX 340 + 0 covering the complete ‘Z’-track from the horizontal branch (HB) to the extended flaring branch (EFB). For the first time, we present the Z-track as seen in soft X-rays using the AstroSat/Soft X-ray Telescope and NICER (the soft colour is defined as a ratio of 3–6 to 0.5–3 keV). The shape of the track is distinctly different in soft X-rays, strongly suggesting the presence of additional components active in soft X-rays. The detailed timing analysis revealed significant quasi-periodic oscillation throughout the HB and the normal branch (NB) using large area X-ray proportional counter and the first NICER detection of 33.1 ± 1.1 Hz HB oscillation (HBO) in 3–6 keV. The oscillations at the HB/NB vertex are observed to have higher frequencies (41–52 Hz) than the HBOs (16–31 Hz) and NB oscillations (6.2–8 Hz) but significantly lower rms (∼1.6 per cent). The HBO is also limited to the energy range 3–20 keV, indicating an association of HBO origin with the non-thermal component. It is also supported by earlier studies that found the strongest X-ray polarization during HB.

Spectral and temporal features of GX 13+1 as revealed by AstroSat

ABSTRACT GX 13+1, a neutron star low-mass X-ray binary that exhibits the properties of both atoll and Z sources, is studied using data from Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat. The source traces a ν shaped track in its hardness-intensity diagram (HID). Spectral modelling of the data in the 0.7–30.0 keV energy range, with the model – $\tt {constant}$ × $\tt {tbabs}$ × $\tt {thcomp}$ × $\tt {bbodyrad}$ + $\tt {relxillNS}$, yields orbital inclination angle (θ) of 77$\substack{+10\\-8}$°. Flux resolved spectral analysis reveals the ν shaped pattern in the plots of spectral parameters kTe, kTbb, and Γ versus Fbol, closely resembling the pattern traced in LAXPC HID. This indicates changes in the spectral properties of the corona and the boundary layer/accretion disc. Assuming that the accretion disc truncates at the Alfvén radius, the upper limit of the magnetic field strength (B) at the poles of neutron star in GX 13+1 is calculated to be 5.10 × 108 G (for kA = 1 and η = 0.1), which is close to that of atoll sources. Furthermore, thickness of the boundary layer is estimated to be 5.70 km, which results in the neutron star radius value of ≲14.50 km. Quasi-periodic oscillations (QPOs) at 56 ± 4 and 54 ± 4 Hz are detected in Regions D and E of HID, respectively. The frequencies of these QPOs are similar to the characteristic frequency of horizontal branch oscillation and these do not exhibit a positive correlation with mass accretion rate.

AstroSat observation of the magnetar SGR J1830−0645 during its first detected X-ray outburst

ABSTRACT We present here timing and spectral analyses of SGR J1830−0645 based on an AstroSat observation carried out on 2020 October 16, about a week after the onset of its first detected X-ray outburst. Using data taken with the Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC), we have detected 0.9–10 keV coherent pulsations at a period of ∼10.4 s. The pulse profiles were single-peaked, asymmetric, and consisted of minor peaks attributable to hotspots on the neutron star surface. The pulsed fraction evolved significantly with energy, increasing to energies around 5 keV with a steep drop thereafter. The 0.9–25 keV SXT–LAXPC energy spectrum is best described with two thermal components having temperatures ∼0.46 and ∼1.1 keV (emission radii of ∼2.4 and ∼0.65 km, respectively, assuming a distance of 4 kpc) along with a power-law component having a photon index of ∼0.39. We report the detection of 67 X-ray bursts having an average duration of ∼33 ms. The brightest burst lasted for about 90 ms and had a 3–25 keV fluence of ∼5 × 10−9 erg cm−2.

NuSTAR and AstroSat observations of GX 9+1: spectral and temporal studies

ABSTRACT We have studied the spectro-temporal properties of the neutron star low-mass X-ray binary GX 9+1 using data from NuSTAR/Focal Plane Module and AstroSat/Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC). The hardness–intensity diagram of the source showed it to be in the soft spectral state (banana branch) during both observations. NuSTAR spectral analysis yielded an inclination angle (θ) = 29${^{+3}_{-4}}^{\circ }$ and inner disc radius (Rin) ≤ 19 km. Assuming that the accretion disc was truncated at the Alfvén radius during the observation, the upper limits of the magnetic dipole moment (μ) and the magnetic field strength (B) at the poles of the neutron star in GX 9+1 were calculated to be 1.5 × 10 26 G cm3 and 2.1 × 10 8 G, respectively (for kA = 1). Furthermore, the thickness of the boundary layer was found to be ≃7.5 km, which yielded the radius of the neutron star to be ≤11.5 km. Flux-resolved spectral analysis with AstroSat data showed the source to be disc dominated (Fdisc/Ftotal ∼ 0.9) with a monotonic increase in mass accretion rate ($\dot{m}$) along the banana branch. The analysis also showed the presence of absorption edges at ∼1.9 and ∼2.4 keV, likely due to Si xiii and S xv, respectively. Temporal analysis with LAXPC-20 data in the 0.02–100 Hz range revealed the presence of noise components, which could be characterized by broad Lorentzian components.

Correlations between QPO frequencies and spectral parameters of GRS 1915+105 using AstroSat observations

ABSTRACT In this work, we study the correlation between quasi-periodic oscillation (QPO) frequency and the spectral parameters during various X-ray states in the black hole binary GRS 1915+105 which matches well with the predicted relativistic dynamic frequency (i.e. the inverse of the sound crossing time) at the truncated radii. We have used broad-band data of Large Area X-ray Proportional Counter and Soft X-ray Telescope instruments onboard AstroSat. Spectral fitting shows that the accretion rate varies from ∼0.1 to ∼5.0 × 1018 gm s−1 and the truncated radius changing from the last stable orbit of an almost maximally spinning black hole, ∼1.2 to ∼19 gravitational radii. For this wide range, the frequencies of the C-type QPO (2–6 Hz) follow the trend predicted by the relativistic dynamical frequency model and interestingly, the high-frequency QPO at ∼70 Hz also follows the same trend, suggesting they originate from the innermost stable circular orbit with the same mechanism as the more commonly observed C-type QPO. While the qualitative trend is as predicted, there are quantitative deviations between the data and the theory, and the possible reasons for these deviations are discussed.

Spectro-temporal and type I X-ray burst analysis of GX 3+1 using AstroSat observations

ABSTRACT GX 3+1, an atoll type neutron star low-mass X-ray binary, was observed four times by Soft X-ray Telescope and The Large Area X-ray Proportional Counter on-board AstroSat between 2017 October 5 and 2018 August 9. The hardness-intensity-diagram of the source showed it to be in the soft spectral state during all the four observations. The spectra of the source could be adequately fit with a model consisting of blackbody ($\tt {bbody}$) and power--law ($\tt {powerlaw}$) components. This yielded the blackbody radius and mass accretion rate to be ∼8 km and ∼2 × 10−9 M⊙ y−1, respectively. In one of the observations, a type I X-ray burst having a rise and e-folding time of 0.6 and 5.6 s, respectively, was detected. Time-resolved spectral analysis of the burst showed that the source underwent a photospheric radius expansion. The radius of the emitting blackbody in GX 3+1 and its distance were estimated to be 9.19 $\substack{+0.97\\ -0.82}$ km and 10.17 $\substack{+0.07\\ -0.18}$ kpc, respectively. Temporal analysis of the burst yielded upper limits of the fractional root mean square amplitude of 7${{\ \rm per\ cent}}$, 5${{\ \rm per\ cent}}$, and 6${{\ \rm per\ cent}}$ during burst start, burst maximum, and right after the radius expansion phase, respectively.

Broad-band mHz QPOs and spectral study of LMC X-4 with AstroSat

ABSTRACT We report the results of broad-band timing and spectral analysis of data from an AstroSat observation of the high-mass X-ray binary LMC X-4. The Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) instruments onboard the AstroSatobserved the source in 2016 August. A complete X-ray eclipse was detected with the LAXPC. The 3–40 keV power density spectrum showed the presence of coherent pulsations along with a ∼26 mHz quasi-periodic oscillation feature. The spectral properties of LMC X-4 were derived from a joint analysis of the SXT and LAXPC spectral data. The 0.5–25 keV persistent spectrum comprised of an absorbed high-energy cut-off power law with photon index of Γ ∼ 0.8 and cut-off at ∼16 keV, a soft thermal component with kTBB ∼ 0.14 keV, and Gaussian components corresponding to Fe Kα, Ne ix, and Ne x emission lines. Assuming a source distance of 50 kpc, we determined 0.5–25 keV luminosity to be ∼2 × 1038 erg s−1.

AstroSat observation of rapid type-I thermonuclear burst from low-mass X-ray binary GX 3$$+$$1

We report the results of an observation of low-mass X-ray binary GX 3 $$+$$ 1 with AstroSat’s large area X-ray proportional counter (LAXPC) and soft X-ray telescope (SXT) instruments on-board for the first time. We have detected type-I thermonuclear burst ( $$\sim $$ 15 s) present in the LAXPC 20 light curve with a double peak feature at higher energies and our study of the hardness–intensity diagram reveals that the source was in a soft banana state. The pre-burst emission could be described well by a thermally Comptonized model component. The burst spectra is modeled adopting a time-resolved spectroscopic method using a single color blackbody model added to the pre-burst model, to monitor the parametric changes as the burst decays. Based on our time-resolved spectroscopy, we claim that the detected burst is a photospheric radius expansion (PRE) burst. During the PRE phase, the blackbody flux is found to be approximately constant at an averaged value of $$\sim $$ 2.56 in $$10^{-8}$$ ergs s $$^{-1}$$ cm $$^{-2}$$ units. On the basis of literature survey, we infer that AstroSat/LAXPC 20 has detected a burst from GX 3 $$+$$ 1 after more than a decade, which is also a PRE one. Utilizing the obtained burst parameters, we provide a new estimation to the source distance, which is $$\sim $$ $$9.3 \pm 0.4$$ kpc, calculated for an isotropic burst emission. Finally, we discuss and compare our findings with the published literature reports.

Timing and spectral studies of Cen X-3 in multiple luminosity states using AstroSat

ABSTRACT We present the results of timing and spectral analysis of the high-mass X-ray binary pulsar, Cen X-3, with the help of observations carried out using the Large Area X-ray Proportional Counter onboard AstroSat. As part of our analysis, we sampled the source properties during four different observation epochs covering two widely different intensity states. We obtain a timing solution and report precise measurements of the spin and orbital parameters corresponding to these observational epochs. The pulse profiles during the two intensity states reveal dramatically varying shapes within a time span of 1 month. We report the detection of one of the lowest measured frequencies of quasi-periodic oscillations at 0.026 ± 0.001 Hz for Cen X-3 during its low-intensity state. We also find correlated periodic and aperiodic noise components in the power density spectra. We further carried out a phase-averaged and a pulse phase-resolved spectral study, where we find that the best-fitting continuum spectrum is well described by an absorbed Comptonization model along with a blackbody. Cen X-3 exhibited the presence of the ∼28 keV cyclotron resonant scattering feature absorption line and a ∼6.6 keV Fe emission line in both the intensity states. Significant variations in the line-forming regions and mode of accretion for Cen X-3 within time spans of a month make Cen X-3 a highly dynamic persistent binary.

Anticorrelated lags in a neutron star Z source GX 5-1: AstroSat’s View

ABSTRACT We report the cross-correlation function studies of a neutron star low-mass X-ray binary, a Z source GX 5-1, using Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) energy bands onboard AstroSat. For the first time, we report the lag between soft (0.8–2.0 keV, SXT) and hard X-ray energy bands (10–20 and 16–40 keV, LAXPC) in GX 5-1 and detected lags of the order of a few tens to hundreds of seconds in the horizontal branch. We interpreted them as the readjustment time-scale of the inner region of the accretion disc. We used various two components and three-component spectral models to unfold the spectra and observed the changes in soft and hard component fluxes that were exhibiting horizontal branch oscillation variations. It was observed that the bbody component assumed to be originating from the boundary layer over the NS and was also found to vary along with the HBO variation where lags were detected. We constrained the size of the comptonizing region of the order 15–55 km, assuming that lags were due to variation in the size of the corona. We noticed a similar size of the comptonizing region after employing other models and suggest that the overall size of corona must be of the order of a few tens of km to explain the lags, HBO variation, and respective spectral variations. In a case study, it was noted that the BL size increases as GX 5-1 vary from the top of the HB to the upper vertex.

Improved Background Model for the Large Area X-Ray Proportional Counter (LAXPC) Instrument on board AstroSat

We present an improved background model for the Large Area X-ray Proportional Counter (LAXPC) detectors on board AstroSat. Because of the large collecting area and high pressure, the LAXPC instrument has a large background count rate, which varies during the orbit. Apart from the variation with latitude and longitude during the orbit there is a prominent quasi-diurnal variation which has not been previously modeled. Using over 5 yr of background observations, we determined the period of the quasi-diurnal variation to be 84,495 s and using this period it is possible to account for the variation and also identify time intervals where the fit is not good. These lead to a significant improvement in the background model. The quasi-diurnal variation can be ascribed to the changes in charged particle flux in the near-Earth orbit.

Probing spectral and temporal evolution of the neutron star low-mass X-ray binary 4U 1724–30 with AstroSat

ABSTRACT We report the broad-band spectro-temporal study of the poorly studied accreting neutron star (NS) low mass X-ray binary (LMXB) 4U 1724–30 using data from Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counters (LAXPC) instruments on board AstroSat. The dim persistent LMXB source was observed with AstroSat over 4 epochs in 2017, all of which corresponded to a low-luminosity non-thermal emission dominated (hard/island) emission state with modest spectral evolution. All the X-ray broad-band spectra can be modelled by a combination of thermal emission from the NS boundary layer (BL) or NS surface and a non-thermal emission component possibly originating from the inverse Comptonization of the disc seed photons. We investigate the presence of frequency and energy-dependent variabilities to probe the origin of the disc/coronal fluctuations. We also report the detection of a Type-I X-ray burst displaying a photospheric radius expansion (PRE). During the burst, a hard X-ray shortage in the 30–80 keV energy band and the enhancement of the persistent emission reveal the burst feedback on the overall accretion process. Using the touch-down burst flux ∼4.25 × 10−8 erg s−1 cm−2, the distance of the source is estimated as ∼8.4 kpc.

AstroSat detection of a quasi-periodic oscillation at ∼42 Hz in Cygnus X-2

ABSTRACT We report the results of AstroSat observations of Cygnus X-2 during 2016 February. The source’s power density spectrum generated using Large Area X-ray Proportional Counter (LAXPC) data revealed the presence of a prominent quasi-periodic oscillation (QPO) at ∼42 Hz with broad-band continuum noise at lower frequencies at ∼10 Hz. The large effective area of LAXPC at ≳30 keV allowed for an unprecedented study of the energy dependence of the QPO and the broad noise continuum. The fractional rms increases with energy, and its shape is similar for both the QPO and the continuum noise, suggesting a common radiative origin. However, while the QPO exhibits hard time-lags, with the high-energy photons lagging the low ones by a few milliseconds, the continuum noise shows the opposite behaviour. The photon spectrum from Soft X-ray Imaging Telescope and LAXPC in 0.7–30 keV band comprises the soft component from a disc and a hard Comptonized component from a hot corona. While the energy dependence of the rms shows that the QPO and the continuum noise variability are dominated by the Comptonized component, the change in sign of the time-lag suggests that the dynamic origin of the QPO may be in the disc while the noise continuum may originate from the corona.

Detection of low-frequency breaks in power density spectrum of GX 339−4 in faint low/hard state observations using AstroSat data

ABSTRACT We present the spectro-timing analysis of the black hole binary system GX 339−4 using AstroSat data extracted at the beginning of outbursts in 2017 and 2019. The joint spectral fitting of Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) data revealed that the source was in a faint low/hard state for both 2017 and 2019 observations, with nearly equal photon indexes of ∼1.57 and ∼1.58 and Eddington ratios, L/LEdd, of 0.0011 and 0.0046, respectively. The addition of a reflection component into the spectral modelling improved the fit (Δχ2 ≈ 6 for 2017 and Δχ2 ≈ 7 for 2019), pointing towards the presence of weak reflection features arising due to irradiation of the accretion disc. The power density spectrum consisted of strong band-limited noise with a break at low frequencies, described by a combination of a few zero-centred Lorentzians. The fitting revealed a low-frequency break at ∼6 mHz for 2017 and ∼11 mHz for 2019 observations, whose detection is validated by results from independent detectors (LAXPCs and SXT). The break frequency is roughly consistent with results obtained from earlier observations that showed an evolution of the frequency with flux, which is in accordance with the truncated disc model. Associating the break frequency with the viscous time-scale of the accretion disc, we estimated a truncation radius of ∼93 and ∼61 gravitational radius for 2017 and 2019 observations, respectively.

Broad-band time-resolved spectroscopy of thermonuclear X-ray bursts from 4U 1636−536 using AstroSat

ABSTRACT Broad-band spectral studies of type I X-ray bursts can put strong constraints on the physics of burst spectra as well as their interaction with the environment. We present the results obtained from the broad-band time-resolved spectroscopy of 15 thermonuclear bursts detected simultaneously from the neutron star atoll source 4U 1636−536 using the Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) onboard AstroSat. During the observations with AstroSat, the low-mass X-ray binary 4U 1636−536 is observed to show a modest spectral evolution within the island state. The broad-band burst spectra are observed to show an excess in addition to the thermal emission from the neutron star surface near the peak of the bursts. We investigate the interpretation of the excess observed near the peak of the burst as re-emission/reprocessing of the photons by the accretion disc/corona or scattering of the photons in the neutron star atmosphere or the enhanced persistent emission due to the Poynting–Robertson drag. This is the first reported broad-band simultaneous study of type I bursts using LAXPC and SXT onboard AstroSat. This kind of study may provide a better understanding of the burst–accretion interaction and how the bursts influence the overall accretion process contributed by the accretion disc as well as the corona.

AstroSat observation of non-resonant type-C QPOs in MAXI J1535-571

Galactic transient black hole candidate (BHC) MAXI J1535-571 was discovered on 2017 September 02 simultaneously by {\it MAXI}/GSC and {\it Swift}/BAT instruments. It has also been observed by India's first multi-wavelength astronomy-mission satellite {\it AstroSat}, during the rising phase of its 2017-18 outburst. We make both the spectral and the temporal analysis of the source during 2017 September 12-17 using data of {\it AstroSat}'s Large Area X-ray Proportional Counter (LAXPC) in the energy range of $3-40$~keV to infer the accretion flow properties of the source. Spectral analysis is done with the physical two-component advective flow (TCAF) solution-based {\it fits} file. From the nature of the variation of the TCAF model fitted physical flow parameters, we conclude and confirm that the source was in the intermediate spectral state during our analysis period. We observe sharp type-C quasi-periodic oscillations (QPOs) in the frequency range of $\sim 1.75-2.81$~Hz. For a better understanding of the nature and evolution of these type-C QPOs, a dynamic study of the power density spectra is done. We also investigate the origin of these QPOs from the shock oscillation model. We find that non-satisfaction of Rankine-Hugoniot conditions for non-dissipative shocks and not their resonance oscillations is the cause of the observed type-C QPOs.

Accretion Flow Properties of GRS 1915+105 During Its θ Class Using AstroSat Data

Abstract The Galactic microquasar GRS 1915+105 shows rich variability that is categorized into different classes. In this paper, we report the temporal and spectral analysis of GRS 1915+105 to study the properties of the accretion flow when the light curve shows θ class variability. For this purpose, we use the Large Area X-ray Proportional Counter data from the Target of Opportunity observations of India’s first multiwavelength astronomy satellite AstroSat. The θ class is marked by the recurrent appearance of U-shaped regions in the light curve, where the photon count rate first decreases rapidly and then increases slowly. For our analysis, we use U-shaped regions of the first two orbits (02345 and 02346) on 2016 March 4. In both cases, the dynamic power-density spectra (PDS) showed significant power at around 4–5 Hz, suggesting the presence of a low-frequency quasi-periodic oscillation (QPO) around that frequency interval. The QPO frequency is found to increase with time when the energy flux is also enhanced. From the evolution of the spectra, we determine the evolution of the accretion flow parameters in these two observations. Fitting the spectra with the transonic flow solution-based two-component advective flow (TCAF) model in the 4–25 keV energy band shows that the Keplerian disk accretion rate increases with the increase in radiation intensity, while the location of the centrifugal pressure-driven shock front decreases. In both these data, a gradual increment of power-law photon index with intensity is observed, suggesting the progressive softening of the source.

Exploring the inner-disc region of the atoll source 4U 1705-44 using AstroSat’s SXT and LAXPC observations

ABSTRACT For the first time, simultaneous broad-band spectral and timing study of the atoll source 4U 1705-44 was performed using AstroSat Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) data (0.8–70 keV). Based on the HID, the source was in the soft banana state during these observations. Spectral modelling was performed using the full reflection framework and an inner disc radii of 14 Rg was obtained. A hard power-law tail was noticed in the soft state and hot component fluxes and varying power-law indices point towards a varying corona/sub-Keplerian flow. Based on the spectral fits, the boundary layer radius and magnetospheric radius were constrained to be ∼14–18 and ∼9–19 km, respectively. Cross-correlation function studies were performed between the 0.8–3 keV soft SXT light curve and 10–20 keV hard LAXPC light curve and correlated and anticorrelated lags were found, which was used to constrain the coronal height to 0.6–20 km (β = 0.1). Since the inner disc radius is not varying during the observations, we conclude that the detected lags are possibly caused by a varying structure of corona/boundary layer in the inner region of the accretion disc. Based on the observations, a geometrical model is proposed for explaining the detected lags in the atoll source 4U 1705-44.

Revealing Thermal Comptonization of Accretion Disk Photons in IC 4329A with AstroSat

Abstract We present five simultaneous UV/X-ray observations of IC 4329A by AstroSat performed over a 5 month period. We utilize the excellent spatial resolution of the Ultra-Violet Imaging Telescope on board AstroSat to reliably separate the intrinsic active galactic nucleus (AGN) flux from the host galaxy emission and correct for the Galactic and internal reddening, as well as the contribution from the narrow- and broad-line regions. We detect large-amplitude UV variability, which is unusual for a large black hole mass AGN like IC 4329A, over such a small period. In fact, the fractional variability amplitude is larger in the UV band than in the X-ray band. This demonstrates that the observed UV variability is intrinsic to the disk and not due to X-ray illumination. The joint X-ray spectral analyses of five sets of Soft X-ray Telescope and Large Area X-ray Proportional Counter spectral data reveal a soft X-ray excess component, a narrow iron line (with no indication of a significant Compton hump), and a steepening power law (ΔΓ ∼ 0.21) with increasing X-ray flux. The soft excess component could arise due to thermal Comptonization of the inner disk photons in a warm corona with kT e ∼ 0.26 keV. The UV emission we detect acts as the primary seed photons for the hot corona, which produces the broadband X-ray continuum. The X-ray spectral variability is well described by the cooling of this corona from kT e ∼ 42 to ∼32 keV with increasing UV flux, while the optical depth remains constant at τ ∼ 2.3.