[vsnet-outburst 26389] Re: [vsnet-alert 25192] V822 Cen = Cen X-4 probable new outburst after 41 years
Josch Hambsch
hambsch at telenet.be
Thu Jan 7 19:41:52 JST 2021
I have taken it up in my observing schedule. Weather-wise though January and
February are the worst in San Pedro de Atacama, where my remote telescope is
located.
Josch
-----Original Message-----
From: vsnet-alert [mailto:vsnet-alert-bounces at ooruri.kusastro.kyoto-u.ac.jp]
On Behalf Of Taichi Kato
Sent: Donnerstag, 7. Januar 2021 01:15
To: vsnet-alert at ooruri.kusastro.kyoto-u.ac.jp;
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Subject: [vsnet-alert 25192] V822 Cen = Cen X-4 probable new outburst after
41 years
V822 Cen = Cen X-4 probable new outburst after 41 years
If this lead to a full outburst, it would be the most important
phenomenon. In 1979, the object reached B=12.8 and we have long been
waiting for another outburst to occur.
The optical rise may take some time and time-resolved photometry is
requested to detect possible superhumps, just as in ASASSN-18ey = MAXI
J1820+070.
===
ATEL #14302 ATEL
#14302
Title: XB-NEWS detects a probable new outburst from Cen X-4 after
41 years
Author: Payaswini Saikia, Maria Cristina Baglio, David M. Russell
(NYU Abu Dhabi), Jeroen Homan (Eureka Scientific &
SRON), St efan
Waterval, D. M. Bramich (NYU Abu Dhabi), Fraser Lewis
(Faulkes T elescope
Project & Astrophysics Research Institute, LJMU), Jakob
van den
Eijnden (Univ. of Oxford)
Queries: ps164 at nyu.edu
Posted: 6 Jan 2021; 10:09 UT
Subjects:Optical, Ultra-Violet, X-ray, Binary, Neutron Star, Transient
Cen X-4 is a neutron star X-ray transient discovered in 1979 during an
outburst. It has since been in quiescence for almost 41 years. Recently,
ATel #14254 indicated that Cen X-4 might enter an outburst in the near
future, based on a gradual, significant, optical brightening detected in the
long term (12 years) optical monitoring of the source with the Las Cumbres
Observatory (LCO) 2-m and 1-m robotic telescopes. The last data reported in
ATel #14254 were obtained in August 2020, when the optical magnitudes of the
source were V = 18.31 +/- 0.17, i' = 17.40 +/- 0.11, as compared to a
fainter V = 18.43 +/- 0.10, i' = 17.46 +/- 0.09 during 2017-2018. Since
the start of September, the source has been Sun-constrained.
We have been closely monitoring the source with LCO, since it re-emerged
from the Sun constraint at the end of 2020 December. For the analysis of the
LCO data, we are making use of the real-time data analysis pipeline, the
"X-ray Binary New Early Warning System" (XB-NEWS; see <a
href="https://ui.adsabs.harvard.edu/abs/2019AN....340..278R/abstract">Russel
l
et al. 2019</a>, <a
href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.498.3429G/abstract">Goodwi
n
et al. 2020</a> and ATel #13451 for details).
We resumed our LCO observations on MJD 59213.3 (2020 December 30) after the
Sun constraint ended, and found the source to be significantly brighter than
before with optical magnitudes V=17.86 +/- 0.06 and i'=17.04 +/- 0.02. The
most recent observations made on MJD 59219 (2021 January 5) show a steep
rise in the optical emission, with V = 17.58 +/- 0.03, i'=16.69
+/- 0.02, g' = 17.66 +/- 0.03, r' = 17.02 +/- 0.02 and Y = 21.70 +/-
0.08. From our long-term monitoring of Cen X-4 with LCO, we find that the
modulation of the source is approximately 0.1 mag amplitude, which is much
smaller than what is required to explain the amplitude of the variability
(see ATel #14254). This suggests that Cen X-4 is entering a new outburst
after ~41 years in quiescence.
After the start of the optical flux rise, the Neil Gehrels Swift Observatory
(Swift) / UVOT observed with the UVW1 filter, resulting in an AB magnitude
of 19.74 +/- 0.09 on MJD 59211 (2020 December 28). This is consistent with
the range of quiescent magnitudes shown in Bernardini et al. 2013, that
reported on large variations in the UVW1 band, spanning the range of (AB)
magnitudes ~21.3-19.4. Later, observations with the UVM2 filter have been
performed, obtaining an AB magnitude of 19.64 +/- 0.11 on MJD 59214 (2020
December 31) and 19.33 +/- 0.08 on MJD 59218 (2021 January 4), thus
suggesting a rise also at UV wavelengths. These results also show that the
system is brighter now than it was in quiescence, since Cackett et al. 2013
reported a UVM2 magnitude of 19.85 +/- 0.14 on MJD 56048 (2012 May 1).
The optical monitoring of the source with LCO is still ongoing. Further
quasi-simultaneous multiwavelength observations are encouraged in order to
confirm the outburst and trace the rise of the source. As Cen X-4 is very
close-by (distance ~ 1.2 kpc), it is a promising candidate to study how
multi-wavelength emission processes are coupled at the fainter, early stages
of an low-mass X-ray binary outburst. The LCO observations are part of an
on-going monitoring campaign of ~ 50 low-mass X-ray binaries (<a
href="https://arxiv.org/abs/0712.2751">Lewis et al. 2008</a>) with LCO and
the Faulkes Telescopes. We acknowledge the support of the NYU Abu Dhabi
Research Enhancement Fund under grant RE124.
Optical LCO light curves of Cen X-4:
https://xbnews.abudhabi.nyu.edu/static/data/website.plots/quick.results.Cen_
X-4_2021Jan.png
ATEL #14303 ATEL
#14303
Title: Swift and NICER X-ray monitoring of the probable new outburst
of Cen X-4
Author: J. van den Eijnden (Univ. of Oxford), J. Homan (Eureka Scientific
& SRON), Payaswini Saikia, Maria Cristina Baglio, David
M. Russell,
Stefan Waterval, D. M. Bramich (NYU Abu Dhabi), Fraser Lewis
(Faulkes
Telescope Project & Astrophysics Research Institute,
LJMU), P.
Roche (Cardiff University)
Queries: a.j.vandeneijnden at uva.nl
Posted: 6 Jan 2021; 10:13 UT
Subjects:X-ray, Neutron Star, Transient
Cen X-4 is a close-by (~1.2 kpc) low-mass X-ray binary, hosting a neutron
star accretor. After its discovery outburst in 1979, it remained in
quiescence for over four decades. Recently, based on 12 years of optical
monitoring with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic
telescopes, ATel #14254 suggested the possible onset of a new outburst in
the near future. A sharp rise in the optical and UV has been observed in the
past week, suggesting that a new outburst has now started (ATel #14302).
Since Cen X-4 emerged from Sun constraint on 28 December 2020, we have
monitored the X-ray binary at X-ray wavelengths with the Neil Gehrels Swift
observatory
(Swift) and the Neutron Star Interior Composition ExploreR (NICER). Swift
and NICER light curves can be found at the link below.
Swift observed Cen X-4 on 28 and 31 December 2020, and 4 January 2021, with
exposure times between ~0.5 and 1 ks. The X-ray Telescope (XRT) operated in
Photon Counting mode for all observations. We used the online Swift pipeline
(Evans et al. 2007, A&A, 469, 379) to extract the XRT light curve.
Cen X-4 is significantly detected in all observations, and is brightening
over time: we measure count rates of 0.155 +/- 0.017 ct/s, 0.266 +/- 0.037
ct/s, and 0.569 +/- 0.034 ct/s in the 1st, 2nd, and 3rd observation,
respectively.
The latter two count rates exceed the historical maximum count rate of Cen
X-4 in quiescence observed by Swift (~0.22 ct/s; Tudor et al. 2017, MNRAS,
470, 324).
NICER has been performing daily observations of Cen X-4 since January
1 2021, with exposure times between ~0.6 and ~4.2 ks. The 0.5-10 keV light
curves show strong variability (by factors of up to four) on time scales of
a few tens to a few hundred seconds. To reduce the intrinsic scatter we
produced a light curve from daily averaged count rates. A rising trend is
seen, with a large jump in count rate between January 4 and January 5.
In order to measure the X-ray flux, we fitted the NICER spectrum taken on 2
January 2021 and the Swift/XRT spectrum from 4 January 2021. Following
Chakrabarty et al. (2014, ApJ, 797, 92), who modeled XMM-Newton/NuSTAR
spectrum of Cen X-4, we fit both spectra in XSPEC with a model combining
interstellar absorption, a neutron star atmosphere (nsatmos, with
neutron-star mass of 1.9 solar mass and a radius of 9 km) and a cutoff power
law (cutoffpl).
This model provided a good description of the quiescent spectra. For the
NICER and Swift spectra, respectively, we fix the absorption column density
N_H to 9E+20 cm<sup>-2</sup> and the cutoff energy to 10 keV. We measured
power law indices of Gamma ~ 2.0 ( NICER) and Gamma ~ 1.5 (Swift). The
effective atmosphere temperature in the Swift observation was higher than in
the NICER observation, ~89 eV and ~80 eV respectively, while the 0.5-10 keV
flux in the cutoff power-law component had doubled. All measured parameters
are broadly consistent with those reported by Chakrabarty et al. (2014).
The unabsorbed 0.5 - 10 keV X-ray fluxes measured by NICER and Swift are,
respectively, F_X ~ 7.4E-12 erg s<sup>-1</sup> cm<sup>-2</sup> and F_X ~
1.5E-11 erg s<sup>-1</sup> cm<sup>-2</sup>. Assuming a distance of 1.2 kpc,
these fluxes correspond to X-ray luminosities of L_X ~ 1.2E33 erg
s<sup>-1</sup> and L_X ~ 2.6E33 erg s<sup>-1</sup>.
The sudden X-ray brightening (to levels not seen before with Swift) suggests
that Cen X-4 is entering a new outburst, supporting the report in ATel
#14302 of an outburst start based on optical and UV observations. Given the
proximity of Cen X-4 and the rarity of its outbursts, we strongly encourage
multi-wavelength follow up. X-ray monitoring with Swift and NICER will
continue.
We are grateful to the Swift and NICER teams for rapidly approving,
scheduling, and performing these observations.
Swift and NICER light curves of Cen X-4:
https://www.dropbox.com/s/kiuj9ch0etrt3
2i/cen_x-4_swift_nicer.pdf?dl=0
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