arXiv subjects

Albert K. H. Kong

Publications and source records attributed to Albert K. H. Kong.

At least 55 records · Page 3Linked to original sources

arXiv2020

Optical follow-up of the neutron star-black hole mergers S200105ae and S200115j

LIGO and Virgo's third observing run (O3) revealed the first neutron star-black hole (NSBH) merger candidates in gravitational waves. These events are predicted to synthesize r-process elements creating optical/near-IR "kilonova" (KN) emission. The joint gravitational-wave (GW) and electromagnetic detection of an NSBH merger could be used to constrain the equation of state of dense nuclear matter, and independently measure the local expansion rate of the universe. Here, we present the optical follow-up and analysis of two of the only three high-significance NSBH merger candidates detected to date, S200105ae and S200115j, with the Zwicky Transient Facility (ZTF). ZTF observed $\sim$\,48\% of S200105ae and $\sim$\,22\% of S200115j's localization probabilities, with observations sensitive to KNe brighter than $-$17.5\,mag fading at 0.5\,mag/day in g- and r-bands; extensive searches and systematic follow-up of candidates did not yield a viable counterpart. We present state-of-the-art KN models tailored to NSBH systems that place constraints on the ejecta properties of these NSBH mergers. We show that with depths of $\rm m_{\rm AB}\approx 22$ mag, attainable in meter-class, wide field-of-view survey instruments, strong constraints on ejecta mass are possible, with the potential to rule out low mass ratios, high BH spins, and large neutron star radii.

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arXiv2020

The Palomar Transient Factory Core-Collapse Supernova Host-Galaxy Sample. I. Host-Galaxy Distribution Functions and Environment-Dependence of CCSNe

Several thousand core-collapse supernovae (CCSNe) of different flavors have been discovered so far. However, identifying their progenitors has remained an outstanding open question in astrophysics. Studies of SN host galaxies have proven to be powerful in providing constraints on the progenitor populations. In this paper, we present all CCSNe detected between 2009 and 2017 by the Palomar Transient Factory. This sample includes 888 SNe of 12 distinct classes out to redshift $z\approx1$. We present the photometric properties of their host galaxies from the far-ultraviolet to the mid-infrared and model the host-galaxy spectral energy distributions to derive physical properties. The galaxy mass functions of Type Ic, Ib, IIb, II, and IIn SNe ranges from $10^{5}$ to $10^{11.5}~M_\odot$, probing the entire mass range of star-forming galaxies down to the least-massive star-forming galaxies known. Moreover, the galaxy mass distributions are consistent with models of star-formation-weighted mass functions. Regular CCSNe are hence direct tracers of star formation. Small but notable differences exist between some of the SN classes. Type Ib/c SNe prefer galaxies with slightly higher masses (i.e., higher metallicities) and star-formation rates than Type IIb and II SNe. These differences are less pronounced than previously thought. H-poor SLSNe and SNe~Ic-BL are scarce in galaxies above $10^{10}~M_\odot$. Their progenitors require environments with metallicities of $<0.4$ and $<1$ solar, respectively. In addition, the hosts of H-poor SLSNe are dominated by a younger stellar population than all other classes of CCSNe. Our findings corroborate the notion that low-metallicity \textit{and} young age play an important role in the formation of SLSN progenitors.

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arXiv2020

Kilonova Luminosity Function Constraints based on Zwicky Transient Facility Searches for 13 Neutron Star Mergers

We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo's third observing run. We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook follow-up with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization of 4480 deg^2, median distance of 267 Mpc and false alarm rates ranging from 1.5 to 1e-25 per yr. The ZTF coverage had a median enclosed probability of 39%, median depth of 20.8mag, and median response time of 1.5 hr. The O3 follow-up by the GROWTH team comprised 340 UVOIR photometric points, 64 OIR spectra, and 3 radio. We find no promising kilonova (radioactivity-powered counterpart) and we convert the upper limits to constrain the underlying kilonova luminosity function. Assuming that all kilonovae are at least as luminous as GW170817 at discovery (-16.1mag), we calculate our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than -16.6mag (extrapolated peak magnitude of GW170817) and fade at 1 mag/day (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations, the joint probability of zero detections, assuming all kilonovae are brighter than -16.6mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, <57% (<89%) of putative kilonovae could be brighter than -16.6mag assuming flat (fading) evolution, at 90% confidence. If we further account for the online terrestrial probability for each GW trigger, we find that <68% of putative kilonovae could be brighter than -16.6mag. Comparing to model grids, we find that some kilonovae must have Mej < 0.03 Msun or Xlan>1e-4 or phi>30deg to be consistent with our limits. (Abridged)

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arXiv2020

Spitzer Observations of the Predicted Eddington Flare from Blazar OJ 287

Binary black hole (BH) central engine description for the unique blazar OJ 287 predicted that the next secondary BH impact-induced bremsstrahlung flare should peak on 2019 July 31. This prediction was based on detailed general relativistic modeling of the secondary BH trajectory around the primary BH and its accretion disk. The expected flare was termed the Eddington flare to commemorate the centennial celebrations of now-famous solar eclipse observations to test general relativity by Sir Arthur Eddington. We analyze the multi-epoch Spitzer observations of the expected flare between 2019 July 31 and 2019 September 6, as well as baseline observations during 2019 February-March. Observed Spitzer flux density variations during the predicted outburst time display a strong similarity with the observed optical pericenter flare from OJ 287 during 2007 September. The predicted flare appears comparable to the 2007 flare after subtracting the expected higher base-level Spitzer flux densities at 3.55 and 4.49 $μ$m compared to the optical R-band. Comparing the 2019 and 2007 outburst lightcurves and the previously calculated predictions, we find that the Eddington flare arrived within 4 hours of the predicted time. Our Spitzer observations are well consistent with the presence of a nano-Hertz gravitational wave emitting spinning massive binary BH that inspirals along a general relativistic eccentric orbit in OJ 287. These multi-epoch Spitzer observations provide a parametric constraint on the celebrated BH no-hair theorem.

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arXiv2019

GROWTH on S190814bv: Deep Synoptic Limits on the Optical/Near-Infrared Counterpart to a Neutron Star-Black Hole Merger

On 2019 August 14, the Advanced LIGO and Virgo interferometers detected the high-significance gravitational wave (GW) signal S190814bv. The GW data indicated that the event resulted from a neutron star--black hole (NSBH) merger, or potentially a low-mass binary black hole merger. Due to the low false alarm rate and the precise localization (23 deg$^2$ at 90\%), S190814bv presented the community with the best opportunity yet to directly observe an optical/near-infrared counterpart to a NSBH merger. To search for potential counterparts, the GROWTH collaboration performed real-time image subtraction on 6 nights of public Dark Energy Camera (DECam) images acquired in the three weeks following the merger, covering $>$98\% of the localization probability. Using a worldwide network of follow-up facilities, we systematically undertook spectroscopy and imaging of optical counterpart candidates. Combining these data with a photometric redshift catalog, we ruled out each candidate as the counterpart to S190814bv and we placed deep, uniform limits on the optical emission associated with S190814bv. For the nearest consistent GW distance, radiative transfer simulations of NSBH mergers constrain the ejecta mass of S190814bv to be $M_\mathrm{ej} < 0.04$~$M_{\odot}$ at polar viewing angles, or $M_\mathrm{ej} < 0.03$~$M_{\odot}$ if the opacity is $κ< 2$~cm$^2$g$^{-1}$. Assuming a tidal deformability for the neutron star at the high end of the range compatible with GW170817 results, our limits would constrain the BH spin component aligned with the orbital momentum to be $ χ< 0.7$ for mass ratios $Q < 6$, with weaker constraints for more compact neutron stars. We publicly release the photometry from this campaign at http://www.astro.caltech.edu/~danny/static/s190814bv.

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arXiv2019

Investigation of X-ray timing and spectral properties of ESO 243-49 HLX-1 with long-term Swift Monitoring

The long-term Swift monitoring of ESO 243-49 HLX-1 provides an opportunity to investigate the detailed timing and spectral behaviour of this hyper-luminous X-ray source. Swift has detected 7 outbursts since 2009 mid-August. Using different dynamical timing algorithms, we confirm an increasing trend for the time intervals between outbursts, which is manifest in the delays between the latest outbursts. The X-ray spectra of HLX-1 in quiescence can be described with a single power-law model while the thermal component dominates the X-ray emission during outburst. There is only marginal evidence for photon index (or spectral hardness) changes between quiescent states with about 1σ deviation. With the updated temporal and spectral features, we re-examine different scenarios to explain the origin of the quasi-periodic modulation of HLX-1. A significantly increasing trend without obvious stochastic fluctuations on the timescale of the detected quasi-period may not fully support an orbital period origin as might be due to mass transfer episodes from a donor star at periastron of an extremely eccentric orbit. The outburst profile seems to be consistent with the effect of tidal-induced-precession of an accretion disc or an oscillating wind scenario in the inner disc. Based on these models, we speculate that the true orbital period is much shorter than the detected quasi-periodicity.

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arXiv2019

GROWTH on S190425z: Searching thousands of square degrees to identify an optical or infrared counterpart to a binary neutron star merger with the Zwicky Transient Facility and Palomar Gattini IR

The third observing run by LVC has brought the discovery of many compact binary coalescences. Following the detection of the first binary neutron star merger in this run (LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. The initial skymap of this single-detector gravitational wave (GW) trigger spanned most of the sky observable from Palomar Observatory. Covering 8000 deg$^2$ of the initial skymap over the next two nights, corresponding to 46\% integrated probability, ZTF system achieved a depth of $\approx$\,21 $m_\textrm{AB}$ in $g$- and $r$-bands. Palomar Gattini-IR covered 2200 square degrees in $J$-band to a depth of 15.5\,mag, including 32\% integrated probability based on the initial sky map. The revised skymap issued the following day reduced these numbers to 21\% for the Zwicky Transient Facility and 19\% for Palomar Gattini-IR. We narrowed 338,646 ZTF transient "alerts" over the first two nights of observations to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod, were particularly compelling given that their location, distance, and age were consistent with the GW event, and their early optical lightcurves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young core-collapse supernovae. The remaining candidates were ruled-out as supernovae. Palomar Gattini-IR did not identify any viable candidates with multiple detections only after merger time. We demonstrate that even with single-detector GW events localized to thousands of square degrees, systematic kilonova discovery is feasible.

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arXiv2019

X-Ray spectral evolution of PSR~J2032+4127 during the 2017 periastron passage

We report X-ray data analysis results obtained from Chandra, XMM-Newton, NuSTAR and Swift observations of PSR J2032+4127 taken before, during, and after the periastron on 2017 November 13. We found the first clear evidence of a change in the X-ray spectral index over the passage period, thanks to a broad and sensitive spectral coverage by XMM-Newton and NuSTAR. We analysed the joint XMM-Newton and NuSTAR observation epochs with power-law and broken power-law model. We have obtained change in spectral parameters before and after the periastron passage for both models. The spectra get softened after the passage. The evolution of the spectral index and break energy before and after the periastron may indicate a change in the physical state of shock-accelerated electrons.

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arXiv2019

Star-formation rates of two GRB host galaxies at z~2 and a [CII] deficit observed with ALMA

Event rate of long Gamma-Ray Bursts (GRBs) is expected to be an useful tracer of the cosmic star-formation history. For this purpose, it is necessary to understand what kind of star formations/galaxies are traced by GRBs. Here we report rest-frame far-infrared (FIR) continuum detections of GRB 070521 and 080207 host galaxies at $z\sim2$ with ALMA band 8 and 9. The FIR photometries provide the reliable star-formation rates (SFRs), because FIR emission is free from dust extinction and possible radio contamination from long-lived afterglows of GRBs. The spectral energy distribution fittings indicate 49.85$^{+72.33}_{-2.86}$ and 123.4$^{+25.19}_{-21.78}$ $M_{\odot}$ yr$^{-1}$ for the 070521 and 080207 hosts, respectively. The derived SFRs place them on the \lq \lq main sequence\rq \rq\ of normal star-forming galaxies at $z\sim2$. The derived SFRs are significantly lower than that of radio observations. It is inferred that the observed radio fluxes in a previous study are contaminated by the afterglows. ALMA marginally detected [C~{\sc ii}]\,158\,$μ$m emission line from the GRB 080207 host galaxy with S/N $\sim$ 4. This is the first detection of [C~{\sc ii}]\,158\,$μ$m of a GRB host at $z>2$, and the second detection among known GRBs. The luminosity ratio of [C~{\sc~ii}]\,158$μ$m to FIR is 7.5$\times 10^{-4}$, which is one of the smallest values among galaxies at $z\sim 1-2$ with the same FIR luminosity. The \lq \lq [C~{\sc ii}] deficit\rq \rq\ could be a new physical property to characterise GRB hosts at $z\sim1-2$. Possible parameters controlling the deficit include the metallicity, initial mass function, and gas density.

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arXiv2019

GROWTH on S190510g: DECam Observation Planning and Follow-Up of a Distant Binary Neutron Star Merger Candidate

The first two months of the third Advanced LIGO and Virgo observing run (2019 April-May) showed that distant gravitational wave (GW) events can now be readily detected. Three candidate mergers containing neutron stars (NS) were reported in a span of 15 days, all likely located more than 100 Mpc away. However, distant events such as the three new NS mergers are likely to be coarsely localized, which highlights the importance of facilities and scheduling systems that enable deep observations over hundreds to thousands of square degrees to detect the electromagnetic counterparts. On 2019-05-10 02:59:39.292 UT the GW candidate S190510g was discovered and initially classified as a BNS merger with 98% probability. The GW event was localized within an area of 3462 deg2, later refined to 1166 deg2 (90%) at a distance of 227 +- 92 Mpc. We triggered Target of Opportunity observations with the Dark Energy Camera (DECam), a wide-field optical imager mounted at the prime focus of the 4m Blanco Telescope at CTIO in Chile. This Letter describes our DECam observations and our real-time analysis results, focusing in particular on the design and implementation of the observing strategy. Within 24 hours of the merger time, we observed 65% of the total enclosed probability of the final skymap with an observing efficiency of 94%. We identified and publicly announced 13 candidate counterparts. S190510g was re-classified 1.7 days after the merger, after our observations were completed, with a "binary neutron star merger" probability reduced from 98% to 42% in favor of a "terrestrial" classification.

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arXiv2019

A Multi-epoch X-ray Study of the Spiral Galaxy NGC 7331

X-ray point sources in galaxies are dominated by X-ray binaries (XRBs) that are variables or transients and whether their variability would alter the X-ray luminosity functions (XLF) is still in debate. Here we report on NGC 7331 as an example to test this with 7 Chandra observations. Their detection limit is 7$\times10^{37}$ erg s$^{-1}$ in the 0.3 -- 8.0 keV energy range by assuming a power-law spectral model with a photon index of 1.7. We detected 55 X-ray sources. Thirteen of them are variables for which 3 of them are transients and some of the sources possess a bimodal luminosity-hardness ratio feature, which is often observed among X-ray binaries. Nine more ultra-luminous X-ray sources are found comparing to previous studies and 8 are likely to be low-mass or high-mass XRBs. Twenty-one optical counterpart candidates are found based on the Hubble Space Telescope images, but we cannot rule out the possibility of positional coincidence. The spectral analysis of SN 2014C shows a trend of increasing soft photons and decreasing hydrogen column densities as its outer shell expands. We fit the 7 incompleteness-corrected XLFs to both a power-law (PL) and a power-law with an exponential cut-off (PLC) model using Bayesian method, which is the first time used in XLF fitting. The hierarchical PLC model can describe the XLF of NGC 7331 best with a slope of $\sim$ 0.5 and a luminosity cut-off around 8$\times$10$^{38}$ erg s$^{-1}$. This study proves that multi-epoch observations decrease the deviation due to the variable luminous sources in XLFs.

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arXiv2019

Bayesian Analysis on the X-ray Spectra of the Binary Neutron Star Merger GW170817

For the first time, we present a Bayesian time-resolved spectral study of the X-ray afterglow datasets of GW170817/GRB17017A observed by the Chandra X-ray Observatory. These include all 12 public datasets, from the earliest observation taken at $\rm t \sim 9~d$ to the newest observation at $\rm \sim 359~d$ post-merger. While our results are consistent with the other works using Cash statistic within uncertainty, the Bayesian analysis we performed in this work have yielded Gaussian-like parameter distributions. We also obtained the parameter uncertainties directly from their posterior probability distributions. We are able to confirm that the power-law photon index has remained constant of $Γ\sim 1.6$ throughout the entire year-long observing period, except for the first dataset observed at $\rm t = 8.9~d$ when $Γ=1.04\pm0.44$ is marginally harder. We also found that the unabsorbed X-ray flux peaked at $\rm t \sim 155~d$, temporally consistent with the X-ray flare model suggested recently by Piro et al (2018). The X-ray flux has been fading since $\sim160$ days after the merger and has returned to the level as first discovered after one year. Our result shows that the X-ray spectrum of GW170817/GRB170817A is well-described by a simple power-law originated from non-thermal slow-cooling synchrotron radiation.

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arXiv2018

Why Are Some Gamma-Ray Bursts Hosted by Oxygen-rich Galaxies?

Theoretically long gamma-ray bursts (GRBs) are expected to happen in low-metallicity environments, because in a single massive star scenario, low iron abundance prevents loss of angular momentum through stellar wind, resulting in ultra-relativistic jets and the burst. In this sense, not just a simple metallicity measurement but also low iron abundance ([Fe/H]<-1.0) is essentially important. Observationally, however, oxygen abundance has been measured more often due to stronger emission. In terms of oxygen abundance, some GRBs have been reported to be hosted by high-metallicity star-forming galaxies, in tension with theoretical predictions. Here we compare iron and oxygen abundances for the first time for GRB host galaxies (GRB 980425 and 080517) based on the emission-line diagnostics. The estimated total iron abundances, including iron in both gas and dust, are well below the solar value. The total iron abundances can be explained by the typical value of theoretical predictions ([Fe/H]<-1.0), despite high oxygen abundance in one of them. According to our iron abundance measurements, the single massive star scenario still survives even if the oxygen abundance of the host is very high, such as the solar value. Relying only on oxygen abundance could mislead us on the origin of the GRBs. The measured oxygen-to-iron ratios, [O/Fe], can be comparable to the highest values among the iron-measured galaxies in the Sloan Digital Sky Survey. Possible theoretical explanations of such high [O/Fe] include the young age of the hosts, top-heavy initial mass function, and fallback mechanism of the iron element in supernova explosions.

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arXiv2018

Multi-wavelength observations of a new redback millisecond pulsar candidate: 3FGL J0954.8-3948

We present a multi-wavelength study of the unassociated Fermi-LAT source, 3FGL J0954.8-3948, which is likely the gamma-ray counterpart of a 9.3-hour binary in the field. With more than 9 years of Pass 8 LAT data, we updated the gamma-ray spectral properties and the LAT localization of the gamma-ray source. While the binary lies outside the cataloged 95% error ellipse, the optimized LAT ellipse is 0.1 degrees closer and encloses the binary. The system is likely spectrally hard in X-rays (photon index ~ 1.4) with orbital modulations detected in optical, UV, and possibly X-rays. A steep spectrum radio counterpart (spectral index ~ -1.6) is also found in the TIFR GMRT Sky Survey (TGSS), implying that it is a pulsar system. We obtained a series of SOAR and Gemini spectroscopic observations in 2017/2018, which show a low-mass secondary orbiting in a close circular orbit with K2 = 272 km/s under strong irradiation by the primary compact object. All the observations as well as the modelling of the X/gamma-ray high-energy emission suggest that 3FGL J0954.8-3948 is a redback millisecond pulsar in a rotation-powered state.

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arXiv2018

A multi-wavelength approach to classifying transient events in the direction of M31

(abridged) In the hunt for rare time-domain events, confusing exotic extragalactic phenomena with more common Galactic foreground events is an important consideration. We show how observations from multiple wavebands, in this case optical and X-ray observations, can be used to facilitate the distinction between the two. We discovered an extremely bright and rapid transient event in optical observations of M31 with the intermediate Palomar Transient Factory (iPTF). The persistent optical counterpart of this transient was previously thought to be a variable star in M31 without any dramatic flux excursions. The iPTF event initially appeared to be an extraordinarily rapid and energetic extragalactic transient, a very fast nova in M31 being one of the exciting possibilities, with a ~3mag positive flux excursion in less than a kilosecond. The nature of the source was resolved with the help of Chandra archival data, where we found an X-ray counterpart and obtained its X-ray spectrum. We find the X-ray spectrum of the quiescent emission can be described by a model of optically thin plasma emission with a temperature of ~7MK, typical for coronal emission from an active star. The combination of the X-ray luminosity, calculated assuming the source is located in M31 (~3.10^36 erg/s), and the color temperature exclude any type of known accreting compact object or active star in M31. We argue instead that the optical transient source is an M type main-sequence, active star located in the disk of the Milky Way at a distance of ~0.5-1 kpc. Its persistent X-ray luminosity is in the ~(1.3-5).10^30 erg/s range and it has the absolute optical magnitude of 9.5-11.0 mag in R-band. The observed optical flare has the equivalent duration of ~95min and total energy of ~(0.3-1).10^35 erg in R-band, which places it among the brightest flares ever observed from an M-type star.

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arXiv2018

Bayesian Inference on the Radio-quietness of Gamma-ray Pulsars

We demonstrate for the first time using a robust Bayesian approach to analyse the populations of radio-quiet (RQ) and radio-loud (RL) gamma-ray pulsars. We quantify their differences and obtain their distributions of the radio-cone opening half-angle $δ$ and the magnetic inclination angle $α$ by Bayesian inference. In contrast to the conventional frequentist point estimations that might be non-representative when the distribution is highly skewed or multi-modal, which is often the case when data points are scarce, Bayesian statistics displays the complete posterior distribution that the uncertainties can be readily obtained regardless of the skewness and modality. We found that the spin period, the magnetic field strength at the light cylinder, the spin-down power, the gamma-ray-to-X-ray flux ratio, and the spectral curvature significance of the two groups of pulsars exhibit significant differences at the 99\% level. Using Bayesian inference, we are able to infer the values and uncertainties of $δ$ and $α$ from the distribution of RQ and RL pulsars. We found that $δ$ is between $10^\circ$ and $35^\circ$ and the distribution of $α$ is skewed towards large values.

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arXiv2018

The long-term optical evolution of the black hole candidate MAXI J1659-152

We present 5 years of optical and infrared data of the black hole candidate MAXI J1659-152 covering its 2010 outburst, decay and quiescence. Combining optical data taken during the outburst decay, we obtain an orbital period of 2.414 $\pm$ 0.005 h, in perfect agreement with the value previously measured from X-ray dips. In addition, we detect a clear H$α$ excess in MAXI J1659-152 with data taken during the outburst decay. We also detect a single hump modulation most likely produced by irradiation. Assuming that the maximum occurs at orbital phase 0.5, we constrain the phase of the X-ray dips to be ~ 0.65. We also detect the quiescent optical counterpart at r' = 24.20 $\pm$ 0.08, I = 23.32 $\pm$ 0.02 and H = 20.7 $\pm$ 0.1. These magnitudes provide colour indices implying an M2-M5 donor star assuming 60% contribution from a disc component in the r'-band.

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arXiv2017

Enhanced gamma radiation toward the rotation axis from the immediate vicinity of extremely rotating black holes

We investigate the acceleration of electrons and positrons by magnetic-field-aligned electric fields in the polar funnel of an accreting black hole (BH). Applying the pulsar outer-gap theory to BH magnetospheres, we find that such a lepton accelerator arises in the immediate vicinity of the event horizon due to frame-dragging, and that their gamma-ray luminosity increases with decreasing accretion rate. Furthermore, we demonstrate that the gamma-ray flux is enhanced along the rotation axis by more than an order of magnitude if the BH spin increases from $a=0.90M$ to $a=0.9999M$. As a result, if a ten-solar-mass, almost-maximally rotating BH is located within 3 kpc, when its accretion rate is between 0.005% and 0.01% of the Eddington rate, its high-energy flare becomes detectable with the Fermi/Large Area Telescope, provided that the flare lasts longer than 1.2 months and that we view the source nearly along the rotation axis. In addition, its very-high-energy flux is marginally detectable with the Cherenkov Telescope Array, provided that the flare lasts longer than a night and that our viewing angle is about 45 degrees with respect to the rotation axis.

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