arXiv:1705.09653v1 [astro-ph.HE] 26 May 2017 Dbs21) o hs ihahgl ceti ri of modulated orbit eccentric emitting highly a with companion and e those For Be hole) 2013). compact (Dubus black or a stellar-mass O γ harbours or massive star that a (neutron (HMXBs) object binaries X-ray rsn)cntigretariayflrsse from seen (if flares disc circumstellar through TeV extraordinary Be therein) to trigger the radio references can and/or the 2013; wind wind present) Dubus and pulsar stellar e.g., 2015 as see, the modelling; al. cases et in these Tam required in usually star is neutron (probably cir ta.20;Sitne l 09 ogon tal. et Bongiorno 2009; 2009; al. al. et al. et Tam 2015, et Skilton Hinton al. 2009; 2011; et al. Chernyakova 148; et al. J0632+057/MWC Acciari et 2015; Abdo HESS al. al. and Mold´on et 2011; et Caliandro 2005; al. al. 2015; et et Tam Aharonian 2004; 2011; al. et Wang ryeiso tGVMVadee e energies TeV even and GeV/MeV at emission -ray ainlUiest,Deen eulco Korea of Republic Daejeon, University, National China Technology, and Science of versity 10001 Beijing District, China Chaoyang R. Road, P. Datun 20A Sciences, China Yangfangwan R. P. 396 650216, Sciences, Kunming of District, Academy Guandu Chinese Objects, China tial R. P. 650216, Kunming District, Guandu Yangfangwang, China 519082, Zhuhai Taiwan 30013, Hsinchu University, Hua Tsing UK 3RH, OX1 Oxford Road, Keble [email protected] USA; 48824, MI Lansing, East versity, am-a iaisaeasbls fhigh-mass of subclass a are binaries Gamma-ray rpittpstuigL using 2017 typeset 29, Preprint May version Draft & 9 8 7 6 5 4 3 2 1 0 eateto srnm n pc cec,Chungnam Science, Space and Astronomy of Department Uni- Huazhong Astronomy, and Physics Particle of Institute of Academy Chinese Mega-Science, Celes- Astronomical for of Center Evolution and Structure the for 396 Laboratory Sciences, Key of Academy Chinese University, Yat-sen Observatories, Sun Yunnan Astronomy, and Physics of School National Physics, of Department and Astronomy of Oxford Institute of University Physics, of Department Astrophysics, Uni- State Michigan Astronomy, and Physics of Department . ,teprato asg ftecmatobject compact the of passage periastron the 8, in ae n21/6soe htisXryeiso a enbrigh been has emission X-ray its that showed 2015/16 in taken tions hnta eoe21,poal eeln oeo-on activities new on-going Our some revealing wind. probably stellar 2013, the before that than rgtnn rn,btrte eea togXryflrso we on flares the X-ray namely strong baseline, several brightening rather slowly but trend, brightening tla id ftecmaint rdc h ae.Teeprecur wind These pulsar flares. the de extreme the this that a produce from show outburst implying to giant companion respectively, predicted state, the the states, flaring of low winds the the stellar during and index flaring power-law the during taken nPRJ0242/T123 the 213, J2032+4127/MT91 PSR on ujc headings: Subject ae rmkVt e r xetd eetsuiswith studies Recent 20 expected. November are in TeV be to to predicted keV was system from the flares of periastron coming erpr u recent our report We SWIFT γ .L Li L. K. ry eg,PRB1259 PSR (e.g., -rays XMM- , 1. A INTRODUCTION T E tl mltajv 12/16/11 v. emulateapj style X 1 .K .Kong H. K. A. , NEWTON -as iais—plas niiul(S 23+17 tr:in stars: — J2032+4127) (PSR individual pulsars: — binaries X-rays: M9 1)—sas id,outflows winds, stars: — 213) (MT91 Swift AND , , Swift NuSTAR 2,3 − XTlgtuv hw osrn vdneo igevigorous single a of evidence strong no shows lightcurve /XRT .H .Tam T. H. P. , 3L 2883; 63/LS NUSTAR rf eso a 9 2017 29, May version Draft γ n XMM- and , rybnr addt ihapro f4–0yas The years. 45–50 of period a with candidate binary -ray o state low ABSTRACT 2, g, γ BEVTOSO S 23+17M9 213 J2032+4127/MT91 PSR OF OBSERVATIONS , rybnr uigteprato asg nlt 2017. late in passage periastron the during binary -ray 4 .Hou X. , The . Newton onluminosity: down ihasrn pndw aeof rate spin-down strong a with ogrpro f4–0yas codn otertiming their to even According radio/ an strong years. suggested a 45–50 solutions, and of of model period period binary long longer to the a thought refined initially with (2017) was binary J2032 a While be 2015). al. et (Lyne hwn hti sayugpla facaatrsi age characteristic a of pulsar young a of is it that showing strong 2012). al. et Casares 2011; with 2017). al. et Ho a rtdsoee sa as discovered first was nraigtedseigyapasi the rapidly a in a for appears brightening, 2017 seemingly the al. et brightening trend Since Takata increasing (see X-ray the modelling). winds between extraordinary detailed stellar interaction and This intimate pulsar an indicates strongly 2015. after ihthe with ilb nlt 07(.. J 86 nthe in 58069 MJD binary (i.e., the 2017 of late best-fit position periastron in The next pulsar’s be pulsar. the will inferred the that the 1.5 of shows at companion about ephemeris high-mass found association; the is stellar as us) OB2 from Cyg kpc the of member 07 aaae l 07,wihi eiicn of reminiscent is which 2017), al. et (Ho al. et mid-2016 Takata to September 2017; 2015 from lightcurve ∼ F aioe l 09,rsetvl,adltrietfida a as further identified with later and system respectively, binary 2009), NRAO al. et the Camilo and 2009) X S 23+17M9 1 J02hratr sa is hereafter) (J2032 213 J2032+4127/MT91 PSR oe l 21)fuda -a onepr fJ2032 of counterpart X-ray an found (2017) al. et Ho 0tmsbihe (i.e., brighter times 10 ∼ (1–5) = NuSTAR 0 y.A kyr. 200 Chandra 5,6,7 γ rybnr addt ihahg cetiiy It eccentricity. high a with candidate binary -ray -a and X-ray em ag raTelescope Area Large Fermi .Takata J. , Chandra × kyt otl ieclswt a with timescales monthly to ekly oswudb rca nstudying in crucial be would sors 10 n XMM- and and srevrnetadasofter a and environment nser − ewe h usrwn and wind pulsar the between V 7 rudwihhigh-energy which around 17, 14 Lijiang 11 = E Swift r cm erg 8 ˙ and n .Y Hui Y. C. and , e yafcosof factors a by ter neatdwt stronger with interacted ∼ γ re akTelescope Bank Green γ . 5mgB tr T123(a 213 MT91 star, Be mag 95 XT hc a an (i.e., faint was which /XRT, ryadrdoeitn pulsar emitting radio and -ray F Swift ryplainat pulsation -ray Newton − X γ pia observations optical 10 2 ryadrdoobservations radio and -ray s ≈ 35 − 1 3 r s erg -a observa- X-ray ∼ eoe21,btwas but 2013, before ) × observations 6 ∼ 10 − × 0yas oe al. et Ho years, 20 9 LT boe al. et Abdo (LAT; 1 − a detected, was ) 10 dividual 13 − r cm erg P 13 ∼ s Swift oe 2 Model 10 s 6 = − 2 − (GBT; . /XRT (spin- 2 8Hz 98 s − 1 of ) 2 Li et al.

Fig. 1.— Upper Panel shows (i) the Swift/XRT lightcurve (0.3–10 keV) of J2032 (black bars); and (ii) the equivalent widths (EWs) of the Hα emission lines measured by the MDM 1.3/2.4-m and Liverpool 2-m telescopes in Ho et al. (2017) (green squares) and the Lijiang 2.4-m telescope in this work (purple asterisks). In addition, the X-ray intensities measured by the Chandra (Ho et al. 2017), NuSTAR, and XMM-Newton observations were converted to XRT’s count rates based on the best-fits power-law models and shown as circles in the plot. The shadowed regions indicate the possible periods when the X-ray source was in the low state. The dashed line represents the −1.2±0.1 increasing trend of the low state (i.e., FX ∝ tp , where tp is the number of days from the periastron passage on MJD 58069; see a more detailed description in §7.1). Lower Panel plots the Chandra (Ho et al. 2017) and Swift/XRT data taken before 2013 March, with the same dashed trend line as shown in the upper panel.

PSR B1259−63/LS 2883 just before the disc passage TABLE 1 (see, e.g., Tam et al. 2015; Chernyakova et al. 2015). X-ray Spectral Properties of the three field sources from the Chandra data taken on 2016 February 24 Swift NuSTAR 22 −2 a In this letter, we report our recent , , Source NH (10 cm ) Γor kT Norm. Newton Lijiang − and XMM- X-ray and optical obser- Cyg OB2 4b 1.0 0.5 keV 1.0 × 10 4 vations of J2032 and clarify the current status of the MT91 221 0.5 2.4 1.1 × 10−5 system based on the results. CXOU J203213.5 0.8 2.4 8.6 × 10−6 aSee https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual/Models.html for the definitions. 2. THE 2016 CHANDRA OBSERVATION bThe XSPEC model mekal was used. We re-analysed the 4.9 ksec Chandra observation work. taken on 2016 February 24 (ID: 18788). While it has been well studied by Ho et al. (2017) for J2032, we focus 3. on the three bright nearby X-ray sources (Cyg OB2 4, SWIFT /XRT OBSERVATIONS MT91 221, and CXOU J203213.5+412711), which are In March 2016, we launched a bi-weekly monitoring just marginality resolved by XMM-Newton and Swift, campaign to follow up the X-ray brightening seen by and unresolved by NuSTAR. Latest spectral information Swift and Chandra (Ho et al. 2017). We once switched of these sources from the Chandra data is extremely use- the observing cadence to one week from December ful to eliminate their undesired contributions in our data. 2016 to February 2017. But we changed it back to the two-week cadence in March, which is the best for the The CIAO (v4.7.2) task specextract was used to study. We also note that there is another Swift program extract the spectra with circular source regions of on J2032, probably with a longer cadence (PI: Coe). r = 1′′. 5 and source-free background regions of r = 10′′. Some of the Swift observations (i.e., data taken before The sources can be described by an absorbed power-law 2016 September) have been reported in Ho et al. (2017) or an absorbed thermal mekal model (the best-fit and Takata et al. (2017) and we extend the analysis parameters are listed in Table 1). These models will be with all the XRT observations taken before 2017 April included in the NuSTAR and XMM-Newton spectral fits 14 in this work. (with frozen parameters) to subtract the field sources’ contributions. For J2032, we used and discussed the The exposures are ranging from < 1 ksec to 5 ksec. results presented in Ho et al. (2017) throughout this Most of them are useful in building a long-term X-ray 3

Fig. 2.— Left: Joint NuSTAR (black: FPMA and red: FPMB) and Swift/XRT (green) X-ray spectrum taken on 2016 September 9–10, +1.3 22 −2 +0.14 −12 −2 −1 with an absorbed power-law of Γ = 2.7 ± 0.2, NH = 2.5−0.9 × 10 cm , and F3−78keV = 1.25−0.13 × 10 erg cm s (unabsorbed). Right: XMM-Newton X-ray spectrum (black: PN, red: MOS1, and green: MOS2) taken on 2016 November 6–7, with an absorbed +0.08 22 −2 +0.07 −12 −2 −1 power-law of Γ = 1.9 ± 0.1, NH = 0.70−0.07 × 10 cm , and F0.3−10keV = 0.87−0.06 × 10 erg cm s (unabsorbed). lightcurve, but the data qualities are still insufficient (FPMA+B). A simultaneous 4 ksec Swift observation for meaningful spectral analyses and such analyses are was also obtained to extend the analysis down to 0.3 keV. therefore skipped in this work. For the XRT lightcurve extraction, we used the Swift’s on-line analysis tool10 The HEAsoft (v6.19) task nuproducts with the (Evans et al. 2007, 2009) to take a good care of the CALDB (v20160731) was used to extract spectra and bad pixels, vignetting, and point spread function (PSF) lightcurves from the FPMA/FPMB observations in the corrections of the data. All parameters were left at default energy range of 3–78 keV (channels: 35–1909). program default values with the option binning by We adopted a circular source region of radius r = 30′′, observation chosen. Figure 1 shows the XRT lightcurve which is recommended for faint sources by the NuSTAR after (i) removing the bad data (i.e., upper limits due to team. To minimize the effect of the stray light, we extremely short exposures, some data bins with S/N< 3, selected two source-free regions of r = 30′′ at the and a fake detection in 2006 due to a noisy background), respective positions of the source in the ghost patterns (ii) re-binning the data points taken within 24 hours, for the background extractions. and (iii) subtracting the expected contributions from the three bright X-ray sources (i.e., 1.7 × 10−3 cts/s, The spectra (together with the simultaneous estimated by PIMMS with the parameters in Table 1). Swift/XRT spectrum extracted by the Swift’s on-line analysis tool) can be well described 2 As previously mentioned, the spectral information of (χν = 53.2/64) by an absorbed simple power- the XRT data is bad. Given that J2032 showed strong +1.3 22 −2 law of Γ = 2.7 ± 0.2, NH = 2.5−0.9 × 10 cm , spectral variability (cf. Table 2), we discuss the XRT +0.14 −12 −2 −1 and F3−78keV = 1.25−0.13 × 10 ergcm s (or lightcurve using the XRT count rate throughout the +3.2 −12 −2 −1 paper to avoid providing misleading information. We F0.3−10keV = 6.1−1.9 × 10 ergcm s ; absorption here give a counts-to-flux conversion factor of 9.5×10−11 corrected), with no obvious high-energy exponential erg cm−2 cts−1 (absorption corrected), computed based cutoff feature (Figure 2). The fitting result does not on the best-fit model for the XMM-Newton data (see §5 significantly change, if the Swift data is not included and Table 2), for a rough reference. (Table 2). An additional mekal thermal component with a plasma temperature of T ≈ 0.5 keV can slightly improve the joint NuSTAR-Swift fit by ∆χ2 = 2.4. We 4. NUSTAR OBSERVATION simulated 10000 spectra based on the best-fit simple power-law and then fitted the simulated spectra with We obtained a 45 ksec (live time) NuSTAR ToO both power+mekal and power models. 46% of the observation to study the J2032 on 2016 September 9–10 simulations improve better than ∆χ2 = 2.4, indicating (Figure 1). In the NuSTAR FPMA/FPMB images, that improvement is not significant. For the NuSTAR the stray light from the HMXB Cygnus X-3, 30′away lightcurve, we binned it with 5 ksec to achieve about from J2032, created ghost ray patterns through single 100 counts per bin and no strong variability can be seen. reflections (Kristin Madsen, private communication). Fortunately, the contamination, especially for ener- gies > 5 keV, is not too severe, and the source was 5. XMM-NEWTON OBSERVATION clearly detected with a net count rate of ∼ 0.02 cts/s A 43 ksec XMM-Newton ToO observation operated under the prime full window mode with the medium 10 http://www.swift.ac.uk/user_objects/ optical blocking filter was obtained on 2016 November 4 Li et al.

TABLE 2 X-ray spectral fitting results of PSR J2032+4127/MT91 213

a b c 2 Date Instruments C1 C2 NH Γ F0.3−10keV χ /dof (MJD) (1022 cm−2) (10−12 erg cm−2 s−1)

+0.4 +1.3 +3.2 2016 Sept 9–10 NuSTAR & Swift 1.0 ± 0.1 0.9−0.3 2.5−0.9 2.7 ± 0.2 6.1−1.9 53.2/64 +0.3 +5.6 2016 Sept 9–10 NuSTAR 1.0 ± 0.1 ··· < 5.3 2.6−0.2 5.0−1.8 52.8/62 +0.08 +0.07 2016 Nov 6–7 XMM-Newton 0.91 ± 0.06 0.95 ± 0.06 0.70−0.07 1.9 ± 0.1 0.87−0.06 278.9/272 aThe cross-calibration factor of FPMB (or MOS1) w.r.t. FPMA (or PN). bThe cross-calibration factor of XRT (or MOS2) w.r.t. FPMA (or PN). cThe fluxes have been absorption corrected.

6–7 (Figure 1). Following the analysis threads in the September, it had brightened to CXRT ≈ 0.008 cts/s XMM-Newton Science Operation centre11, we used the after a 2.5-year observing gap. The X-ray emission metatask xmmextractor in SAS (v15.0.0) to extract the then increased more rapidly from CXRT ≈ 0.007 cts/s scientific products from the raw data in the Observation to ≈ 0.024 cts/s in 2016 April–July (Ho et al. 2017; Data File (ODF). Takata et al. 2017). While a continuous increase was theoretically expected (see, e.g., Takata et al. 2017), The live times were 35 ksec and 41 ksec for PN the X-ray emission returned back to CXRT ≈ 0.006 and MOS1/2, respectively. After filtering the high cts/s in three months, confirmed by the XMM-Newton background periods, usable live times are reduced to observation (Figure 1). The flux was increasing again 27 ksec (PN), 39 kec (MOS1), and 38 kec (MOS2). afterwards, but a few declines are again shown later J2032 was detected in all EPIC cameras with net (Figure 1). More than one type of variation should be count rates of 0.1 cts/s for PN and 0.03 cts/s for involved to result in the complexity seen in the X-ray each MOS in 0.3–10 keV. Similar to the NuSTAR lightcurve. lightcurve, no hourly variability can be seen in the PN (1 ksec binned) and MOS1+2 (1.5 ksec binned) lightcurves. We fitted an absorbed simple power-law 7.1. The Long-Term Variability model to the spectra and found the best-fit parameters By taking a closer look of the Swift/XRT lightcurve, ± +0.08 × 22 −2 ofΓ = 1.9 0.1, NH = 0.70−0.07 10 cm , and one can easily identify some local flux minima and +0.07 −12 −2 −1 F0.3−10keV =0.87−0.06 × 10 ergcm s (absorption the most obvious ones are indicated by the shadowed 2 corrected; χν = 278.9/272) (Figure 2), which are all regions in Figure 1. We tried to fit these XRT minima very different from that of the NuSTAR+Swift spectral in the shadow and the quiescent fluxes measured before fit (Table 2 and Figure 3). We also tried to add a mekal 2013 (including the three Chandra measurement taken component to improve the fit, but the reduced χ was in 2002–2010; Ho et al. 2017) to a simple power-law, −1.2±0.1 found to be even higher. All the best-fit parameters and the data can be well connected with FX ∝ tp (including the NuSTAR+Swift’s) are shown in Table 2. (Figure 1), where tp is the number of days from the periastron passage (i.e., MJD 58069; Ho et al. 2017). 6. THE LIJIANG 2.4-M OBSERVATIONS Apparently, these low flux intervals could belong to the same emission state, which will be called the low state To study the evolving Hα emission line from the in the following discussion. circumstellar disc of MT91 213 (Ho et al. 2017), two 120 sec and 180 sec spectra were taken with the Yunnan The momentum ratio of the stellar wind to the pulsar Faint Object Spectrograph and Camera (YFOSC) on wind is one of the major factors to determine the X-ray the Lijiang 2.4-m telescope on 2016 November 20 and luminosity of the wind-wind interacting shock in a γ-ray December 11, respectively. The spectral resolutions are binary. The dependence can be even higher under a medium with Grism 15 (183 nm/mm) & Slit 3 (1′′. 0) on ′′ consideration of a non-constant magnetization of the December 11, and Grism 14 (92 nm/mm) & Slit 3 (1. 8) shock along the distance from the pulsar (Takata et al. on November 20. After the standard data reduction 2017). In J2032, the slowly brightening low state are processes with IRAF, the Hα emission line was clearly likely the consequence as the pulsar approaches the detected in both datasets, although the double-peaked Be star and interacts with the stronger stellar wind. line profile (Ho et al. 2017) is unresolved. Using the Because of the current large distance between the pulsar eqwidth task in the rvsao package, we computed the and the Be star, the rate of X-ray flux increase would equivalent widths (EW) of the Hα emission lines to be be slow. However, this is still sufficient to develop −5.6A˚ and −5.3A˚ on November 20 and December 11, the two distinct flux levels before/after the 2.5-year respectively (Figure 1). observing gap in 2013–2015, as observed by Swift/XRT and Chandra. 7. DISCUSSION It is worth noting that the XMM-Newton data was Before 2013 March, the X-ray source was marginally taken during the low state. The best-fit hydrogen col- 21 −2 detected by XRT at CXRT ∼ 0.001 cts/s. In 2015 umn density (i.e., NH =7 × 10 cm ) is well consistent with the foreground value estimated by the optical 11 https://www.cosmos.esa.int/web/xmm-newton/sas-threads 21 −2 color excess of MT91 213 (i.e., NH = 7.7 × 10 cm ; 5

in 3–4 months during the first few X-ray flares (see also Figure 1). Our Lijiang spectra indicate that the circumstellar disc shrank back to Rdisc ≈ 0.3 AU (con- verted from EW using the equation in Hanuschik 1989) a few months later while the X-ray source was likely in the low state. We suspect that the disc expansion is an indication of the hypothetical strong clumpy wind to trigger the observed flares. On the other hand, the activity of the circumstellar disc could be induced by the approaching pulsar as shown in PSR B1259−63/LS 2883 (Chernyakova et al. 2014, 2015), although the separation between the pulsar and the Be star in J2032 was much larger.

Finally, we note that PSR B1259−63/LS 2883 did Fig. 3.— In the hydrogen column density (NH) and photon index plane, significant differences in both axes are seen between the not show such pre-periastron-passage flares previously contours (68%, 90%, and 99%) for the NuSTAR and XMM-Newton (see, e.g., Chernyakova et al. 2006). However, when fits. PSR B1259−63 entered the circumstellar disc in 2004, Camilo et al. 2009; Ho et al. 2017). In addition, the the X-ray emission, NH, and the photon index were all best-fit photon index (i.e., Γ = 1.9) is very close to increasing (Chernyakova et al. 2006). In J2032, a very that of those Chandra observations taken before 2016 similar spectral change is seen when transiting from the (i.e., Γ = 2 with the foreground N ; Ho et al. 2017), low state to the flaring state (Table 2 and Figure 3), H − supporting our suggestion that the source was in the although the photon indexes of PSR B1259 63/LS 2883 same low state during the Chandra observations. are generally harder (i.e., increased from Γ = 1.2 to 1.8) than that of J2032 (i.e., from Γ = 1.9 to 2.7). It would be intriguing to ask whether this is a common feature in 7.2. The Short Variability γ-ray binaries when the pulsars entering from a lighter medium to a denser medium. Besides the possible long-term brightening trend, multiple flares on weekly to monthly timescales are obviously present in the XRT lightcurve (Figure 1). 8. CONCLUSION NuSTAR Our observation provides a good X-ray With the NuSTAR and XMM-Newton X-ray obser- spectroscopic study on one of these flares. Comparing vations, we identify two very different spectral states, Newton with the low state spectrum taken by XMM- , namely the low state (i.e., low X-ray flux and N with NuSTAR H the spectrum is significantly softer in photon a hard spectrum) and the flaring state (i.e., high X-ray index with a heavier NH absorption (Figure 3). This flux and NH with a soft spectrum). The Swift/XRT high NH strongly implies a denser medium around the lightcurve suggests that the low state has been slowly pulsar during the flare, probably caused by an occasional evolving, possibly following F ∝ t−1.2, while the strong wind from the Be star. Using the binary orbit X p presented in Ho et al. (2017) and the mass-loss rate of flares are likely on weekly to monthly timescales. In 2 addition, these flares could be correlated to the size of m˙ = 4π r vwρw (where r is the distance from the star, the circumstellar disc of MT91 213, indicated by the ∼ vw 1000 km/s is the wind speed, and ρw is the density Hα emission line studies (see also Ho et al. 2017). The of the wind at r) for a steady and spherically symmetric physical origin of these flares and the implication of the wind, we integrated the density along the line-of-sight slowly brightening low state are still not entirely clear. ∼ −5 −4 −1 and found thatm ˙ 10 –10 M⊙ yr is required Hopefully, continuous multi-wavelength monitoring ∼ 22 −2 to accumulate the intrinsic NH to 10 cm . This observations (e.g., from Swift and Fermi) will be useful inferred rate is several orders of magnitude higher than in studying these flares as well as any pre-periastron the typical value for B type stars (Krtiˇcka 2014), sug- activities before the periastron passage in late 2017. gesting that the wind is likely compact and clumpy (i.e., the pulsar was hitting compact wind clumps, instead of a homogeneous wind). When impacting the pulsar, this strong clumpy wind probably pushed the shock toward We thank (i) Neil Gehrels and Brad Cenko for approv- the pulsar side to cause a stronger magnetic field at ing our Swift monitoring campaign, (ii) Fiona Harrison the emission region (Takata et al. 2017). In this case, for approving the NuSTAR DDT request and Kristin NuSTAR might observe the emissions from the particles Madsen for the technical support, and (iii) Norbert in both the slow and fast cooling regimes in the flaring Schartel for approving the XMM-Newton DDT request state, while only the emission from the slow cooling and Rosario Gonzalez-Riestra for scheduling the obser- regime was observed by XMM-Newton in the low state, vation. We acknowledge the support of the staff of the possibly explaining the observed divergence in photon Lijiang 2.4m telescope. Funding for the telescope has index. been provided by Chinese Academy of Sciences and the Peoples Government of Yunnan Province. The scientific In the Hα line study of Ho et al. (2017), the cir- results reported in this article are based in part on data cumstellar disc of the Be star was expanding from obtained from the Chandra Data Archive. AKHK is Rdisc ≈ 0.2 to 0.4 AU (i.e. EW: from −3.3 to −10.2A)˚ supported by the Ministry of Science and Technology 6 Li et al. of Taiwan through grant 105-2112-M-007-033-MY2, supported by NSFC grants of Chinese Government 105-2119-M-007-028-MY3, and 106-2918-I-007-005. XH under 11573010 and U1631103. CYH is supported by is supported by National Natural Science Foundation the National Research Foundation of Korea through of China through grant 11503078. PHT is supported grants 2014R1A1A2058590 and 2016R1A5A1013277. by the National Science Foundation of China (NSFC) through grants 11633007 and 11661161010. JT is Facilities: Swift, NuSTAR, XMM, YAO:2.4m, and CXO

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