MAXI/GSC observations of new superbursts from Ser X-1
and 4U 1705−44
Wataru B. Iwakiri,1 Motoko Serino,1 and Jean in ’t Zand2

12RIKEN MAXI team, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
E-mail(WI): wataru.iwakiri@riken.jp
Abstract

Two hours-long flares were detected with MAXI/GSC, each from a type-I X-ray bursting low-mass
X-ray binary. The first one was observed on December 12th, 2011, from Ser X-1. The second one wasobserved on 2016 Oct 22 from 4U 1705−44. On the basis of the results from the timing and spectral
analysis, we concluded that the flares are the fourth superburst from Ser X-1 and first superburst from 4U
1705−44, respectively.
Key words: thermonuclear burning — stars: neutron

2nd
3rd

5×10 42 − 4 keV
Photons/sec/cm2

0.6
0.4
0.2

4− 10 keV
Photons/sec/cm2

00.6
0.4
0.2
0
5.59×1045.5901×104
MJD

5.5902×104

5.5903×104

NEWand simultaneously fitted it with the burst spectrum extracted from each scan. The spectrum of each of the first
three scan transits is well fitted by an absorbed black+0.3
body model with temperatures kT = 2.1+0.3−0.2 , 1.5−0.2 and0.2

1st0.8Fig. 2. The lightcurves from Ser X-1 observed by MAXI/GSC in the2 – 4 keV (upper panel) and 4 – 10 keV (lower panel).

0.1

Flux (Crab)

0.3

1. Superburst candidate from Ser X-1
Ser X-1 is a persistent source whose persistent brightness
is about 0.15 – 0.25 Crab (Figure 1). Three superbursts
were reported previously from this source (Cornelisse etal. 2002; Kuulkers 2009). The new event was found in
MAXI/GSC observations on 2011 Dec 12 (MJD 55901).
The event was detected at 08:10 UT and lasted for atleast 4 hours with a fast-rise and exponential-decay lightcurve (Figure 2).

5.2×10 4

5.4×10 4
MJD

5.6×104

Fig. 1. Long-trend light curve of Ser X-1 obtained by RXTE/ASM(Black : 1.5 – 12.0 keV) and MAXI/GSC (Red : 2.0 – 10.0 keV) in
Crab units. The arrows denote the detection times of superbursts.

To obtain the information on the spectral variation
during a burst, we perform the spectral analysis. Assuming that the persistent emission did not change during
the burst, we extracted the spectrum from 10 consecutive scans just before the burst as the persistent emission

1.1+0.6−0.4 keV at 08:10, 09:42 and 11:15 UT, respectively.
The hydrogen column density was fixed at 0.5×1022
cm−2 (Bhattacharyya & Strohmayer 2007). The results prove cooling along the decay. The bolometricflux declined from (6.7 ± 1.1) ×10−9 to (1.9 ± 0.7)×10−9
erg/s/cm2 over the course of 3 hours. From the obtained
bolometric fluxes, the e-folding decay time is 2.3±0.7
hours. Since the interval of each MAXI scan is 92 min
and the observation time of each scan transit is about 40s, the peak flux could be larger than the observed values.
Taking into account the interval of each MAXI scan, the0.4
0.3
0.2

Flux (Crab)

0.1
0
5×104

5.2×104

5.4×104
MJD (day)

5.6×104

5.8×104

Fig. 3. Long-trend light curve of 4U 1705−44 obtained by RXTE/ASM (Black : 1.5 – 12.0 keV) and MAXI/GSC (Red : 2.0 – 10.0 keV) inCrab units. The arrow denotes the detection time of the superburst.

1.2
2 − 4 keV
Photons/sec/cm2

2. Superburst candidate from 4U 1705−44
4U 1705−44 is persistent source and the long-trend
lightcurve (Figure 3) clearly shows the low and high intensity state. Because of the high-α value observed fromthis source (α is the ratio of the average persistent flux
times the average wait times between two type-I burst
and the burst fluence), it was predicted that 4U 1705−44
is a prospective superburster by in ′ t Zand et al. (2003).
However, no superburst was observed from this sourceyet.
The new candidate superburst from 4U 1705−44 wasfound in observations on 2016 Oct 22 (MJD 57683). Figure 4 shows the burst lightcurves. The observed newevent detected at 20:21 UT lasted for at least 2 hours
with a fast-rise and exponential-decay light curve. Theenergy-resolved lightcurves show that the X-ray emission
softens throughout the decay. We performed the spectral analysis following the same method described in theprevious section. The spectrum obtained during the firstand second scan transits are well fitted by an absorbed
blackbody model with temperatures of kT = 1.8 ± 0.1
and 1.8 ± 0.2 keV. The hydrogen column density was
fixed at 1.9×1022 cm−2 (Piraino et al. 2007). This resultshows that the blackbody temperature did not decreasesignificantly but is typical of superbursts. The bolomet-ric flux declined from (1.4 ± 0.1) ×10−8 to (0.7 ± 0.1)
×10−8 erg/s/cm2 over the course of 1.5 hours. The efolding decay time is 2.2 ± 0.6 hours. From these results
we conclude that this is the first superburst observed
from 4U 1705−44.
Interestingly, their type-I burst activity depends on
the intensity state; type-I bursts are more frequently occurred in the low intensity state (Langmeier et al. 1987).
On the other hand, we detected the superburst candidatein the high intensity state. It might be related to the prediction that the carbon fuel of the superburst could bedestroyed in type-I X-ray burst (Woosley et al. 2004).

4 − 10 keV
Photons/sec/cm 2extrapolating peak flux is (7 – 13) ×10−9 erg/s/cm2 . On
the basis of these results, we conclude that this is the 4thsuperburst event from Ser X-1.
We also calculated the ignition column depth y andenergy release per unit mass E using the cooling model
given by (Cumming & Macbeth 2004). Assuming thedistance is 6 kpc, the derived y and E are (1.7 – 2.5)
×1012 g/cm2 and (1.4 – 6.5) ×1017 ergs/g, respectively.
Comparing with the 1st superburst results (Cumminget al. 2006), the ignition depth is significantly deeper.
These results are summarized in Table 1 in in ′ t Zand(2017). The recurrence time between the 3rd and the 4thsuperburst is 1178 days although there is a possibility of
missing events during data gaps.

1
0.8
0.6
0.4
0.2
1.5

1

0.5
5.7682×10 4

5.7683×104

5.7684×104
MJD

5.7685×104

5.7686×104

Fig. 4. The lightcurves from 4U 1705−44 observed by MAXI/GSC
in the 2 – 4 keV (upper panel) and 4 – 10 keV (lower panel).

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