tokens: view of Event 1 observed simultaneously with AstroSat SXT, Chandra LETG/HRC-S, and HST binned to 100s each. The vertical lines in brown and yellow represents the UV and X-ray flare peaks, respectively. The amplitudes of the HST light curves are scaled to be consistent with X-ray light curves. A detailed analysis of the HST data will be presented in Schneider et al. (in preparation). However, a visual inspection of Fig. 4 indicates that the HST FUV light curve precedes the X-ray light curves by about a few minutes. This may indicate that the presence of Neupert effect (Neupert 1968 ) during the rising phase of Event 2. Using the Z-transformed discrete correlation function (ZDCF) (Alexander 1997 ), the cross-correlation of the HST and the X-ray light curves produces a time-lag between the peaks to be 300–400 s, indicating that due to electron impingement the UV light curve precedes the thermal radiation. Figure 4. Event 2 simultaneously observed with the AstroSat SXT in 0.5–4.0 keV (top panel), Chandra LETG/HRC-S zeroth order in 0.1–12.0 keV (middle panel), and HST FUV in 120–170 nm (bottom panel). 3.4 Comparison The quiescent emission measured with Einstein , EXOSAT, and ROSAT in 0.3–4.0 keV energy ranges are 7.2 × 10 −12 , 4–16 × 10 −12 , and 2.6–7.2 × 10 −12 erg s −1 cm −2 , respectively. The quiescent X-ray flux observed by Einstein and EXOSAT was reported by Haisch Linsky ( 1980 ) and Haisch et al. ( 1990 ), respectively. We used the ROSAT observations listed in the ROSAT Positive Sensitive Proportional Counter source catalogue (Voges et al. 1996 , 2000 ). Assuming a coronal temperature of 1 keV, sub-solar abundance of 0.5 and using WebPIMMS, we computed energy conversion factor ECF PSPC = erg cm −2 counts −1 to convert the observed ROSAT counts to flux in canonical 0.1–2.4 keV ROSAT energy band. Furthermore, we use the X-ray flux reported by Fuhrmeister et al. ( 2011 ) based on XMM–Newton observations. In Fig. 5 , we plot quiescent flux measured between 1979 and 2017 as a function of time. Furthermore, we assume similar temperature and abundance as before and used WebPIMMS to estimate the flux in the 0.3–4.0 keV energy range. The flux obtained from AstroSat is consistent with the previous estimates of quiescent flux for Proxima Centauri. Figure 5. Quiescent emission flux as a function of time in 0.3–4.0 keV energy range observed by several X-ray missions between 1979 and 2017. An arbitrary sine curve with a period of 7 yr is represented as the blue dashed curve. While a clear cyclic behaviour with a period of ∼7.0 yr is observed for Proxima Centauri in ASAS observations, a similar variation in the X-ray data is not apparent immediately. However, Wargelin et al. ( 2017 ) reported the evidence for a 7 yr stellar cycle using observations from several X-ray missions, principally Swift . The temporal coverage of X-ray observation, the energy resolution and uncertainties due cross-calibration restrict a detailed analysis of the cyclic
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