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# Decoding the X-ray flare from MAXI J0709-159 using optical spectroscopy and multi-epoch photometry

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# Decoding the X-ray flare from MAXI J0709-159 using optical spectroscopy and multi-epoch photometry

## Abstract and Figures

We present a follow-up study on the recent detection of two X-ray flaring events by MAXI/GSC observations in soft and hard X-rays from MAXI J0709-159 in the direction of HD 54786 (LY CMa), on 2022 January 25. The X-ray luminosity during the flare was around 10^(37) erg/s (MAXI), which got reduced to 10^(32) erg/s (NuSTAR) after the flare. We took low-resolution spectra of HD 54786 from HCT and VBT facilities in India, on 2022 February 1 and 2. In addition to H-alpha emission, we found emission lines of He I in the optical spectrum of this star. By comparing our spectrum of the object with those from literature we found that He I lines show variability. Using photometric study we estimate that the star is having effective temperature of 20000 K. Although HD 54786 is reported as a supergiant in previous studies, our analysis favours it to be evolving off the main sequence in the Color-Magnitude Diagram. We could not detect any infrared excess, ruling out the possibility of IR emission from a dusty circumstellar disc. Our present study suggests that HD 54786 is a Be/X-ray binary system with a compact object companion, possibly a neutron star.
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Decoding the X-ray ﬂare from MAXI J0709-159 using optical spectroscopy and multi-epoch
photometry
Suman Bhattacharyya ,1Blesson Mathew ,1Savithri H Ezhikode ,1S Muneer,2Selvakumar G,2
Maheswer G,2R. Arun ,1, 2 Hema Anilkumar,1Gourav Banerjee ,1Pramod Kumar S,2Sreeja S Kartha ,1
KT Paul,1and C. Velu2
1Department of Physics and Electronics, CHRIST (Deemed to be University), Hosur Main Road, Bangalore, India
2Indian Institute of Astrophysics, Koramangala, Bangalore, India
ABSTRACT
We present a follow-up study on the recent detection of two X-ray ﬂaring events by MAXI/GSC
observations in soft and hard X-rays from MAXI J0709-159 in the direction of HD 54786 (LY CMa),
on 2022 January 25. The X-ray luminosity during the ﬂare was around 1037 erg/s (MAXI ), which
got reduced to 1032 erg/s (NuSTAR) after the ﬂare. We took low-resolution spectra of HD 54786
from HCT and VBT facilities in India, on 2022 February 1 and 2. In addition to Hαemission, we
found emission lines of Heiin the optical spectrum of this star. By comparing our spectrum of the
object with those from literature we found that Heilines show variability. Using photometric study
we estimate that the star is having eﬀective temperature of 20000 K. Although HD 54786 is reported
as a supergiant in previous studies, our analysis favours it to be evolving oﬀ the main sequence in the
Color-Magnitude Diagram. We could not detect any infrared excess, ruling out the possibility of IR
emission from a dusty circumstellar disc. Our present study suggests that HD 54786 is a Be/X-ray
binary system with a compact object companion, possibly a neutron star.
Keywords: Be stars (142); Spectroscopy (1558); X-ray sources (1822), X-ray binary stars (1811),
1. INTRODUCTION
Be/X-ray binaries (BeXRBs) form a major subclass
of high-mass X-ray binaries which consist of a Be star
and a compact object (e.g. Reig 2011;Rappaport & van
den Heuvel 1982). The compact object accretes mat-
ter from the decretion disc of the Be star. The pos-
sible compact objects are neutron star (NS) (Okazaki
& Negueruela 2001), white-dwarf (WD) (Kennea et al.
2021) and black hole (BH) (Casares et al. 2014), yet
NSs are most frequently observed companion than other
kinds (Belczynski & Ziolkowski 2009). BeXRBs can be
persistent or transient in nature. Whereas in the latter
case it exhibits outburst events, which are classiﬁed as
Type I, where X-ray luminosity Lx1037erg s-1 and oc-
cur regularly, separated by the orbital period, and Type
II, where Lx1037erg s-1, this outbursts are less frequent
and are not modulated on the orbital period (Stella et al.
1986). The transient X-ray behavior is caused by the
interaction of the compact object with the material in
the circumstellar disc (Monageng et al. 2017;Okazaki &
Negueruela 2001). A Be star is a special class of massive
B-type main sequence star, surrounded by a geometri-
cally thin, equatorial, gaseous, decretion disc (Meilland
et al. 2007). Be stars are primarily identiﬁed by means
of Hαemission feature formed in the ionized decretion
disc of Be stars, by means of the recombination process
(e.g. Banerjee et al. 2021;Mathew et al. 2012).
Recently, on 2022 January 25 at 10:42:28 UT, the
Monitor of All-sky X-ray Image (MAXI) Gas slit camera
(GSC) (Negoro et al. 2016) nova alert system triggered
a bright uncatalogued X-ray transient source with an
X-ray ﬂux of 266 ±32 mCrab over the scan. The source
of ﬂare is situated at an RA of 107.280 deg and Dec of
-15.923 deg. The intensity of X-ray ﬂux was compara-
tively low before (09:09 UT) and after (12:15 UT) the
event (Serino et al. 2022). The source was named as
MAXI J0709-159. Following that, at 13:44 UT (on the
same day), Neutron Star Interior Composition Explorer
Mission (NICER; Gendreau et al. 2016) reported X-ray
emission from the position of RA = 107.392 deg and Dec
= -16.108 deg, with an X-ray ﬂux of 10 mCrab in the
energy band of 0.212 keV (Iwakiri et al. 2022).
Kobayashi et al. (2022) found that the position of the
Be star HD 54786 matches with that of the region from
where X-ray emission is detected by NICER. Based on
the Gas Slit Camera (GSC) energy spectrum study, they
arXiv:2206.04473v2 [astro-ph.SR] 14 Jun 2022
2Bhattacharyya et al.
suggested HD 54786 to be a companion of MAXI J0709-
159, and that the system might be a supergiant Fast
X-ray Transient (SFXT) or a Cir X-1 type object. This
was further corroborated from the study of Negoro et al.
(2022), who analyzed the X-ray emission using the Nu-
clear Spectroscopic Telescope Array (NuSTAR; Harrison
et al. 2013) observations from 00:21 to 11:21 UT on 2022
January 29.
The ﬁrst spectroscopic study of HD 54786 is by Mer-
rill & Burwell (1949) who identiﬁed hydrogen emission
lines in the spectrum. Morgan et al. (1955) classiﬁed this
star as a blue-supergiant with luminosity class I. Subse-
quently, it was listed in the Be star catalog of Jaschek
& Egret (1982). Later, Chojnowski et al. (2015) also
identiﬁed this star as a Be star. Houk & Smith-Moore
(1988) estimated its spectral type to be B1/B2.
This letter follows up on the recent X-ray ﬂaring event
of HD 54786, combining optical spectroscopy and pho-
tometry over various epochs, to better understand the
nature of the prospective BeXRB system.
2. OBSERVATIONS
2.1. Optical spectroscopy
We collected two low resolution spectra of HD 54786,
from HFOSC instrument mounted on the 2.01-m Hi-
malayan Chandra Telescope (HCT)1located at Hanle,
Ladakh, India. We observed the star on two consecutive
dates, 2022 February 01 & 02. The spectral coverage is
from 55008800 ˚
A, where both the red region spectra
are obtained with Grism 8 (58008300 ˚
A) and 167 µm
slit, providing an eﬀective resolution of 7 ˚
A at Hα.
To validate the spectra obtained from HCT, we ac-
quired two more simultaneous low resolution spectra at
the same epochs from the OMR instrument, mounted
on the 2.34-m reﬂecting telescope situated at the Vainu
Bappu Observatory (VBO), Kavalur, Tamil Nadu, In-
dia. In VBT the photographic sensor employed is an
Andor CMOS high speed read out sensor, comprising of
1024 ×256 pixels having pixel size of 26 µm. The spec-
tra are obtained in the wavelength region of 55008500
˚
A, at settings particularly centered at Hαline with a
resolving power of 1000.
For both observations, dome ﬂats taken with halogen
lamps were utilized for ﬂat-ﬁelding the images. Bias
subtraction, ﬂat-ﬁelding and spectrum extraction were
performed using IRAF routines. The wavelength cali-
brated spectra are continuum ﬁtted using IRAF tasks.
Additionally, the resulting spectra from VBT are further
normalized using python routine.
1http://www.iiap.res.in/iao/hfosc.html
2.2. Gaia astrometric data
The astrometric data of HD 54786 is taken from the
latest data release of Gaia (Gaia EDR3; Gaia Collabo-
ration et al. 2021). The geometrical distance estimates
by Bailer-Jones et al. (2021) is used in this study. The
CMD analysis using the Gaia EDR3 data has given our
work a greater context (section 3.2).
2.3. X-ray
HD 54786 has been continuously monitored with
MAXI for years. As the recent MAXI archival data
were not available, we used the publicly available pro-
cessed GSC (230 keV) light curves (version 7Lrkn 4h),
which is frequently updated every 4 hours. NuSTAR
also observed the source during the quiescent period for
18 ks on 2022 January 29. The two co-aligned focal
plane modules, FPMA & FPMB, detected hard X-ray
photons (379 keV) from the source. We generated
the spectra and light curves from FPMA and FPMB
observations using standard pipelines in the NuSTAR
Data Analysis Software (NUSTARDAS V2.0.0; CALDB
Version 20220131). The source spectra were extracted
for circular regions of radii 32.3200while the background
spectra were generated for annular regions (rin=32.3200 ,
rout=60.0000 ) centered on the source.
3. RESULTS
3.1. Gaia EDR3 analysis
The distance of HD 54786 was reported to be diﬀerent
in previous studies. So, we used the Gaia EDR3 distance
estimate of 2955+244
175 pc (Bailer-Jones et al. 2021) for
further analysis.
From the Gaia database and the ﬁeld image of 2MASS
(Fig. 1), we note that there is another star (source B)
at a separation of 6.5” in the south-west direction of HD
54786. This is not prominent in the optical image (e.g.
SDSS9) but detectable in 2MASS J,H and KScompos-
ite image. The astrometric parameters such as parallax
and proper motions of two stars must be similar and
within uncertainty limits for being considered to be in a
binary system (Arun et al. 2021a). We found that the
distances of the two sources are matching within uncer-
tainties. However, their proper motion values are con-
siderably diﬀerent. Hence, for the present study we will
not consider them as associated and further discussion
will purely be on HD 54786.
3.2. Gaia CMD analysis
HD 54786 is reported as a supergiant by Houk &
Smith-Moore (1988), belonging to the luminosity class
Ib. We constructed the Gaia color-magnitude diagram
X-ray flaring of HD 54786 3
Figure 1. Figure shows images of the region around HD 54786 (FoV 1.3’) from SDSS9 survey (left) in grey scale and 2MASS
survey (right) in J, H and KScolor composite, respectively. The source B is distinguishable in the 2MASS image but not in the
SDSS9 image.
(CMD) for this star to identify its location. Consider-
ing its Gaia EDR3 distance and AVvalue of 0.93 (as
estimated from the Green’s map; Green et al. 2019), we
plotted the corresponding MGversus G(BP RP )0 CMD
for the star. Additionally, we over-plotted the proba-
bility distribution (Gaussian ﬁtted at three contour lev-
els) of previously studied B-type (Huang et al. 2010),
Be stars (Bhattacharyya et al. 2021, and references
therein), Giant stars (Hohle et al. 2010) and supergiants
(Georgy et al. 2021) to better understand the location of
HD 54786. From Pecaut & Mamajek (2013), the ZAMS
(zero-age main sequence) line does not extend beyond
B9 spectral type in MGand G(BP RP )0 values. Hence,
we utilised the closely matching 60 Myr isochrone track
from MESA (Modules for Experiments in Stellar As-
trophysics) isochrones and evolutionary tracks (MIST)
(Choi et al. 2016;Dotter 2016), which is over-plotted
in the CMD with V/Vcrit = 0.4, since that is the only
model available in the MIST database for a rotating
system. Moreover, we adopted the metallicity value of
[Fe/H] = 0, corresponding to solar metallicity Z=
0.0142 (Asplund et al. 2009) for the tracks. The CMD
showing the location of HD 54786, with the representa-
tive locations of main sequence B-type stars, Be stars,
giants and supergiants is presented in Fig. 2a.
Our analysis shows that the location of HD 54786 is
near to the top of the distribution of B and Be stars.
It is also noticed that the star resides inside the dis-
tribution of giant stars but below that of supergiants.
This indicates that HD 54786 might be an evolved star
with respect to B/Be types. However, its location also
conveys that it is not a supergiant star.
3.3. Spectral energy distribution
As a next step, we ﬁtted the spectral energy distribu-
tion (SED) for HD 54786 to re-estimate its spectral type.
For generating the SED, we used Johnson U magnitude,
B, V, g’, r’, and i’ magnitudes from APASS (Henden
et al. 2015); G, GRP and GBP magnitdes from Gaia
EDR3 and the infrared J, H and Ksmagnitudes from
2MASS. The SED is generated with the python rou-
tine used in Arun et al. (2021b). Fig. 2b presents the
SED plot generated for HD 54786. From a grid of BT-
NextGen (AGSS2009) (Hauschildt et al. 1999) theoreti-
cal stellar atmospheres at solar metallicity and log(g) =
34 (since it seems to be evolved from the main sequence
in the CMD), corresponding to various temperatures, we
found that the best ﬁt (by means of chi-squared mini-
mization) corresponds to an eﬀective temperature (Tef f )
of 20000 ±500 K with log(g) of 3.
The obtained Teff matches with a spectral type of
B2 (Pecaut & Mamajek 2013), if it is a main sequence
(MS) star. However, the stellar radius estimated from
the SED is R*/R= 11.8. This value is higher than
that of a MS star (5.7 R) with a similar Teff (Pecaut &
Mamajek 2013). Cox (2000) reported that a supergiant
with the similar spectral type will have a radius of
19 R. Our estimated log(g) and stellar radius values
suggest that the star is not on the MS phase, but is in
the evolved region (oﬀ the MS), which is in agreement
with our CMD analysis. Hence, we will not resort to
4Bhattacharyya et al.
ab
Figure 2. Panel (a): The Gaia CMD of HD 54786 having absolute Gaia G and color corrected (BP-RP) magnitudes available
from Gaia Collaboration et al. (2021). The probability distribution (Gaussian ﬁtted at three contour levels) of the B, Be stars,
Giants and supergiants are shown in blue, orange, green and red shaded colors, respectively. The black dashed line in the plot
represents the isochrone of 60 Myr with V/Vcrit = 0.4 and [Fe/H] = 0 (The top black dashed line is the blue loop part of the same
isochrone). Panel (b): In the SED, the ﬂux values of the star HD 54786 is ﬁtted with the theoretical BT-NextGEN(ASGG2009)
model at Teﬀ = 20000 K and log(g) = 3, using the chi-squared minimization method.
identifying any speciﬁc spectral type for this star in the
present work. Instead, we agree that the spectral type
is similar to the previous estimates of B1/B2 (Houk &
Smith-Moore 1988), and we will put forth the Tef f and
log(g) value as the derivatives from this study.
Furthermore, we do not notice any signiﬁcant IR ex-
cess in the SED, suggesting the absence of dust emis-
sion from the circumstellar disc. If the disc contains
a dust component, it should appear as ﬂux excess in
the 2MASS colors. The lack of dusty disc eliminates
the possibility of the extended accretion disc. However,
based on the Hαemission, a decretion disc surrounding
the Be star is possible, with the excess caused by free-
free emission or thermal bremsstrahlung, which is more
apparent in the mid-IR range. Although mid-IR data
is available in ALLWISE survey (Cutri et al. 2013), we
did not consider it since there is a nearby star that will
contaminate the WISE magnitudes.
3.4. Prominent spectral features identiﬁed in HD 54786
We then identiﬁed all major spectral features observed
in the spectra of HD 54786 taken using VBT and HCT.
We found that the Hαline is visible in single-peak emis-
sion in all spectra, for each of the epochs. The average
signal-to-noise ratio (SNR) of every spectrum taken is
greater than 120. In case of HCT, we detected that Hα
equivalent width (EW) was -17.6 ˚
A on 2022 February
01, whereas it was -16.9 ˚
A on 2022 February 02. The
HαEW from HCT spectra decreased by a value of 0.7
˚
A within one day. Fig. 5(top panel) represents the Hα
EW variation with time. Apart from Hα, emission lines
of Hei: 5876, 6678, 7065, Oi: 7772, 8446 ˚
A and P13 -
P17 of the Paschen series are also noticed on both the
dates. In addition, we also observed telluric O2absorp-
tion bands of 6867 and 7593 ˚
A (Smette et al. 2015) on
both the cases. Fig. 3shows the representative spectra
of HD 54786 obtained using HCT and VBT.
Interestingly, HαEW of HD 54786 was found to be -23
˚
A when observed on 2022 January 28 by Nesci (2022),
which is taken 3 days after the X-ray ﬂaring. It is in-
triguing to notice that the HαEW was higher on the
nights closer to the ﬂaring event and is decreasing to -
16.9 ˚
A on 2022 February 02. However, any major change
of EW is not noticed for any other emission line feature
other than Hα, during the period of our observations.
3.4.1. Spectral features observed in data from the BeSS
database
We queried in the BeSS database (Neiner et al. 2011)
to check whether any observation of this star was done
by amateur astronomers. We found a total of three spec-
tra for HD 54786 taken by amateur astronomers Dejean
Pastor (spectrum taken on 2020 January 15), Cazzato
(2018 February 09) and Buil (2012 March 02), in the
BeSS database. The spectra are represented in Fig. 3.
X-ray flaring of HD 54786 5
Figure 3. Representative spectra of HD 54786 showing diﬀerent spectral features in the wavelength range of 5500 - 8500 ˚
A.
The spectra from HCT, VBT and BeSS spectrum are listed in top, middle and lower panel.
It was found that HαEW was the maximum (around
-28.3 ˚
A) on 2012 March 02. This value decreased to -2.7
˚
A on 2018 February 09. Interestingly, HαEW again in-
creased to -12.1 ˚
A on 2020 January 15. Hβis also seen
in emission (EW = -3.1 ˚
A) on 2012 March 02, whereas
it shows weak absorption on the other two dates. More-
over, Hei5876, 6678 and 7065 ˚
A lines are noticed in
emission only on 2012 March 02, whereas in the other
two occasions (2018 February, 09 and 2020 January, 15,
respectively) we are not seeing these three lines in emis-
sion.
3.4.2. Variability in Heiemission lines
Comparing the spectra obtained by us using the HCT
we found a considerable variability exhibited by Hei
5876, 6678 and 7065 ˚
A emission lines. All these lines
were visible in emission on 2012 March 02 when am-
ateur astronomer Buil observed HD 54786 (C11 LISA
ATIK314L+). On the contrary, all were present in ab-
sorption in the BeSS spectra on 2018 February 09 and
2020 January 15. However, we found all of them in emis-
sion when observed with HCT on 2022 February 01 and
02.
It is known that Heiemission lines are found rarely
in Be stars (Banerjee et al. 2021) and are produced in
high temperature regions with T 15,000 K (Kwan &
Fischer 2011). Bahng & Hendry (1975) proposed that
these lines may either be a temporary phenomenon or
non-LTE eﬀects are responsible for their selective exci-
tation. Since we ﬁnd that Heilines are showing intensity
changes over a short period of time, it may be a tem-
porary phenomenon, as proposed by Bahng & Hendry
(1975). Apparao & Tarafdar (1994) proposed that in Be
stars, Heiemission lines can be produced by the inﬂu-
ence of a compact binary companion which is accreting
matter from the disc of the primary (here the Be star)
star.
The occasional appearance and disappearance of Hei
emission lines in the series of spectra used in our study
indicate that the disc temperature of HD 54786 becomes
higher during the ﬂaring event and has a recurring na-
ture. However, appearance of these lines in emission on
2022 February 01 and 02 hints towards some sudden in-
crease in disc temperature. This can happen if some
ﬂare activity takes place from a nearby companion star,
which is what has been observed recently in case of HD
54786. Hence, our study suggests the possibility that
X-ray ﬂaring probably resulted in the formation of Hei
emission lines, which may disappear as time progresses.
We did not see such a correlation earlier since X-ray data
was not available then. We are doing follow-up observa-
6Bhattacharyya et al.
tions for HD 54786 to check whether any X-ray ﬂaring
event leads to the formation of Heiemission lines or not.
3.5. X-ray spectral analysis
We performed the X-ray spectral analysis of the NuS-
TAR observation of the source for the quiescent period
using the X-ray spectral ﬁtting package Xspec (version
12.12.0; Arnaud 1996). The spectra were grouped to
have a minimum of 15 counts per bin. Above 25 keV,
the spectra were dominated by the background. As a
result, we looked at the spectra in the 325 keV en-
ergy range. We used an empirical power-law model
modiﬁed by galactic absorption to ﬁt the spectra. The
galactic absorption was modelled with the neutral hy-
drogen column density NHﬁxed at 5.6×1021 cm2,
obtained using the HI4PI survey (HI4PI Collaboration
et al. 2016). We did not consider the intrinsic absorption
as it was not considerably signiﬁcant. To account for the
cross-normalisation disparities between the FPMA and
FPMB data, a constant factor (0.94+0.37
0.28) was applied.
A photon index (slope of the hard X-ray power-law) of
Γ=2.00+0.47
0.43 was obtained from the model ﬁt with a ﬁt-
statistic of χ2(degrees of freedom)=18.26(16). To mea-
sure the unabsorbed ﬂux we added another component
cﬂux to the power law model. We measured the unab-
sorbed X-ray ﬂux in the energy range of 325 keV to be
7.03+1.81
1.68 ×1013erg cm2s1. The best-ﬁt model and
unfolded spectra are shown in Fig 4.
100101
Energy (keV)
10 4
10 3
Flux (kev cm 2s1)
Absorbed power-law NuSTAR data
Figure 4. NuSTAR (FPMA & FPMB) spectra modelled
with constant ×T Babs ×powerlaw in the 3–25 keV band.
The solid line shows the power-law continuum corrected for
the Galactic absorption.
The X-ray (210 keV) ﬂux values of the source for
the ﬂaring period are obtained from Kobayashi et al.
(2022). We then calculated the luminosity of the two
detected peaks during the ﬂaring, which was reported
by MAXI/GSC at 10:42 and 13:48 UT on 2022 January
25. The luminosity for the ﬁrst and second ﬂares are es-
timated to be 6.42+2.82
1.67 ×1036 erg s-1 and 1.67+4.29
1.05 ×1037
erg s-1, respectively. Moreover, we calculated the unab-
sorbed 2–10 keV ﬂux from the NuSTAR data from 00:21
to 11:21 UT on 2022 January 29, about four days after
the initial MAXI detection. The spectral ﬁt is found to
be 5.33+1.47
1.32 ×1013erg cm2s1and the X-ray lumi-
nosity, for a distance of 2955 pc, to be 5.57+1.54
1.38 ×1032
erg s-1.
3.6. Is HD 54786 a γCas variable or BeXRB?
Based on irregular photometric variability, HD 54786
is reported as a γCas variable (Alfonso-Garz´on et al.
2012). However, it is diﬃcult to diﬀerentiate γCas vari-
ables and BeXRBs based on optical variability alone.
They can diﬀer signiﬁcantly depending on their X-ray
characteristics (Reig 2011). In this section we evaluate
whether HD 54786 is a γCas variable or a BeXRB bi-
nary system.
Typically, γCas variables (mostly a Be + white dwarf
system) show harder X-ray spectra, best ﬁt with a thin
plasma emission model (Smith et al. 2016). On the con-
trary, a power law model is usually observed in high
mass X-ray binaries (HMXBs), where BeXRB is a sub-
class. Moreover, γCas variables do not exhibit large
X-ray outbursts (Reig 2011). The hard X-ray emission
from BeXRBs are usually non-thermal in nature, which
is not the case in γCas variables (Kubo et al. 1998).
For HD 54786, the X-ray luminosity of the source was
found to be of the order 1037 erg s-1, when observed by
MAXI/GSC on two diﬀerent times on the same date,
i.e., on 2022 January 25. The luminosity then dropped
to 1032 erg s-1 on 2022 January 29, when observed by
NuSTAR. The X-ray ﬂare of such a scale suggests that
HD 54786 belongs to the class of BeXRB, where the
companion is a compact object, possibly a neutron star
(e.g. Reig 2011;Okazaki & Negueruela 2001). The X-ray
luminosity as estimated by us implies that the outburst
is of Type I (Stella et al. 1986). However, further study
using time period analysis and periodic pulsations will
be necessary to conﬁrm this.
4. DISCUSSION
Our study suggests that HD 54786 might be an
evolved Be star rather than a main sequence star or a
supergiant. Using SED analysis we estimated its Tef f ,
log(g) and radius (R*/R) values to be 20000 K, 3 and
11.8, respectively. We found a considerable decrease in
HαEW from -23 ˚
A on 2022 January 28 (noticed by Nesci
(2022) 3 days after the X-ray ﬂare) to -16.9 ˚
A on 2022
February 02. It is interesting to notice that the HαEW
X-ray flaring of HD 54786 7
Figure 5. Variation of EW and V-mag shown by HD 54786. In the top panel, the red cross marks represent the Hα
EW. In the bottom panel, the grey, black and blue cross marks show V-mag values obtained from Alfonso-Garz´on et al. (2012),
ASAS and AAVSO, respectively. The vertical red lines and green line represent the time of spectral observations and the X-ray
ﬂaring event, respectively. It is noticed that its V-mag steadily increases over the span of observations. The epoch of BeSS
spectra from 2018 February (HJD 2458150) and the photometry measurements from ASAS are seen to overlap in the ﬁgure.
During that time, the Hα EW was signiﬁcantly lower (-2.7 ˚
A), with an average V-mag of 9.29.3. Again from 2018 May2018
September (HJD 2458239 to 2458362), V-mag values were between 9.0 and 9.1. Then, a considerable rise in Hα EW (to -12.1
˚
A) in the BeSS spectra of 2020 January (HJD 2458849) was detected, whereas AAVSO V-mag value was found to range between
8.9 and 9.2. Subsequently, the V-mag changed to 8.85 during AAVSO observations in 2021 February (HJD 2459246). However,
a considerable variability of H α EW is seen, i.e., from -23.2 to -16.9 ˚
A, if we compare our observations with that of Nesci (2022).
was higher on the nights closer to the ﬂaring event and
decreased on later dates. The variability we observed
in Hei5876, 6678 and 7065 ˚
A emission lines indicate
that the disc temperature of HD 54786 becomes higher
during the ﬂaring event and has a recurring nature.
Our results further suggest the possibility that X-
ray ﬂaring probably resulted in the formation of Hei
emission lines, which may disappear as time progresses.
Moreover, the X-ray analysis points out that the out-
burst is of Type I and HD 54786 belongs to the class of
BeXRB consisting a compact object companion.
Furthermore, we studied the epoch-wise variation of
V-mag and Hα EW seen in HD 54786 in the context of
the X-ray ﬂare event. This is done to check whether the
variation of V-mag and EW is co-related with the X-
ray ﬂaring event or not. For that, we searched the MAXI
on-demand webpage for the epochs where a variation in
Hαwas observed (2012, 2018 and 2020), and did not
ﬁnd any signiﬁcant X-ray detection during these periods.
Also, we compiled the V-magnitude values from Alfonso-
Garz´on et al. (2012) (between April 2003 and May 2008),
ASAS-SN Catalog of Variable Stars III (Shappee et al.
2014;Jayasinghe et al. 2019) (2016 February to 2018
September), and from the AAVSO database, for the
period from 2019 January to 2021 February. Fig. 5
presents the variation of H α EW and V-mag shown by
HD 54786. Interestingly, our study shows that there
was a gradual increase in V-mag which took place till
2021 February, the last date of observation available be-
fore the X-ray ﬂaring event. Further observational data,
particularly simultaneous observations are required to
understand whether the V-mag and Hα emission are
associated with the X-ray outburst or not. Also, it will
help to assess the periodicity of the HD 53786 BeXRB
system.
5. ACKNOWLEDGEMENTS
We would like to thank the Center for research,
CHRIST (Deemed to be University), Bangalore, India
for providing the necessary support. Moreover, we con-
vey our heartiest gratitude to the staﬀ of the Indian
Astronomical Observatory (IAO), Ladakh and Vainu
Bappu Observatory, Kavalur for taking the immediate
observations using the HCT and VBT, respectively. R.A
is grateful to the Centre for Research, CHRIST (Deemed
to be University), Bangalore for the research grant ex-
tended to carry out the present project (MRP DSC-
1932). B.M acknowledges the support of the Science &
Engineering Research Board (SERB), a statutory body
of the Department of Science & Technology (DST), Gov-
8Bhattacharyya et al.
ernment of India, for funding our research under grant
number CRG/2019/005380. This study has used the
Gaia EDR3 data to adopt the corresponding distances
for program stars. Hence, we express our gratitude
to the Gaia collaboration. We also thank the SIM-
BAD database and the online VizieR library service for
helping us in the relevant literature survey. Further-
more, this work has made use of the BeSS database,
operated at LEISA, Observatory de Meuden, France
(http://basebe.obspm.fr). Hence, we thank the BeSS
database too.
APPENDIX
Table 1. Observation log of HD 54786 from HCT and VBT
Star Name Date and time(UT) of observation HJD RA DEC Exposure Time(s)
(yyyy-mm-dd, hh:mm)
HCT Observation
HD 54786 2022-02-01, 16:22 2459611 07 09 36.97 -16 05 46.1 240
2022-02-02, 13:39 2459612 - - 180
VBT Observation
HD 54786 2022-02-01, 16:00 2459611 07 10 46.97 -16 07 46.1 1800
2022-02-02, 15:08 2459612 - - 2400
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