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Thermonuclear X-ray Bursts with late secondary peaks observed from 4U 1608−52
Tolga G
¨
uver,
1, 2
Tu
˘
gba Boztepe,
3
Ersin G
¨
o
˘
g
¨
us¸,
4
Manoneeta Chakraborty,
5
Tod E. Strohmayer,
6
Peter Bult,
7, 8
Diego Altamirano,
9
Gaurava K. Jaisawal,
10
Tu
˘
gc¸e Kocabıyık,
3
C. Malacaria,
11, 12, ∗
Unnati Kashyap,
5
Keith C. Gendreau,
8
Zaven Arzoumanian,
8
and Deepto Chakrabarty
13
1
Istanbul University, Science Faculty, Department of Astronomy and Space Sciences, Beyazıt, 34119,
˙
Istanbul, Turkey
2
Istanbul University Observatory Research and Application Center, Istanbul University 34119,
˙
Istanbul Turkey
3
Istanbul University, Graduate School of Sciences, Department of Astronomy and Space Sciences, Beyazıt, 34119,
˙
Istanbul, Turkey
4
Faculty of Engineering and Natural Sciences, Sabancı University, Orhanlı-Tuzla 34956,
˙
Istanbul, Turkey
5
Discipline of Astronomy, Astrophysics and Space Engineering (DAASE), Indian Institute of Technology Indore, Khandwa Road, Simrol,
Indore 453552, India
6
Astrophysics Science Division and Joint Space-Science Institute, NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA
7
Department of Astronomy, University of Maryland, College Park, MD 20742, USA
8
Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
9
School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
10
National Space Institute, Technical University of Denmark, Elektrovej 327-328, DK-2800 Lyngby, Denmark
11
NASA Marshall Space Flight Center, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805, USA
12
Universities Space Research Association, Science and Technology Institute, 320 Sparkman Drive, Huntsville, AL 35805, USA
13
MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
(Received XX, 2020; Revised XX, 2020; Accepted XX, 2020)
Submitted to ApJ
ABSTRACT
We report the temporal and spectral analysis of three thermonuclear X-ray bursts from 4U 1608−52,
observed by the Neutron Star Interior Composition Explorer (NICER) during and just after the out-
burst observed from the source in 2020. In two of the X-ray bursts, we detect secondary peaks, 30 and
18 seconds after the initial peaks. The secondary peaks show a fast rise exponential decay-like shape
resembling a thermonuclear X-ray burst. Time-resolved X-ray spectral analysis reveals that the peak
flux, blackbody temperature, and apparent emitting radius values of the initial peaks are in agreement
with X-ray bursts previously observed from 4U 1608−52, while the same values for the secondary peaks
tend toward the lower end of the distribution of bursts observed from this source. The third X-ray
burst, which happened during much lower accretion rates did not show any evidence for a deviation
from an exponential decay and was significantly brighter than the previous bursts. We present the
properties of the secondary peaks and discuss the events within the framework of short recurrence time
bursts or bursts with secondary peaks. We find that the current observations do not fit in standard
scenarios and challenge our understanding of flame spreading.
1. INTRODUCTION
Neutron stars in low mass X-ray binaries (LMXBs)
often exhibit sudden flashes of X-rays, called type-I X-
ray bursts. These events, typically last only about tens
of seconds and show a characteristic morphology, that
is, fast rise and exponential decay. Time-resolved X-ray
spectral studies of these events show that the burst spec-
Corresponding author: Tolga G¨uver
tolga.guver@istanbul.edu.tr
∗
NASA Postdoctoral Fellow
tra can be described with an evolving Planckian func-
tion (kT∼1–3 keV) and the total energy released in such
bursts can be anywhere between 10
38−39
erg (see Lewin
et al. (1993), Strohmayer & Bildsten (2006) and Gal-
loway & Keek (2017) for detailed reviews on type-I X-
ray bursts).
X-ray bursts are attributed to the unstable fusion of
hydrogen and/or helium present in the material accreted
onto the neutron star, therefore, they are also termed as
thermonuclear X-ray bursts. Galloway et al. (2008) and
Galloway et al. (2020) provide up-to-date catalogs and
analyses of an extensive sample of thermonuclear bursts
arXiv:2101.11637v1 [astro-ph.HE] 27 Jan 2021
2 G
¨
uver et al.
spanning a wide range of characteristics. The burning
primarily depends on the mass accretion rate to the sur-
face (Fujimoto et al. 1981), although various other phys-
ical factors like metallicity and the compactness of the
neutron star also play a role (Bildsten 1998; Narayan &
Heyl 2003).
Almost all of the existing scenarios suggest that the
material needs to accumulate for a certain period of time
for the next X-ray burst to happen and most of the fuel
is burned during a burst, with only a thin layer of ashes
remaining (Lewin et al. 1987; van Paradijs et al. 1988;
Fisker et al. 2008; Jos´e et al. 2010). The recurrence time
of X-ray bursts depend strongly on the mass accretion
rate and typically can be hours or longer. However, X-
ray bursts with even shorter recurrence times have been
observed. These events are generally referred to as short
waiting time (SWT) bursts, where the waiting time is
defined to be less than 45 minutes (Keek & Heger 2017)
however, bursts with recurrence times as short as only
a few minutes have been observed from several sources
including EXO 0748−676 (Gottwald et al. 1987; Boirin
et al. 2007; in’t Zand et al. 2009), GS 0836−429 (Aoki
et al. 1992), 4U 1608−52 (Murakami et al. 1980), 4U
1705−440 (Langmeier et al. 1987), the Rapid Burster
(Bagnoli et al. 2014), GRS 1747−312 (in’t Zand et al.
2003) and several others (Keek et al. 2010; Linares et al.
2012). In some cases triple or even quadruple SWT
bursts have been observed (Keek et al. 2010) includ-
ing an event observed from 4U 1608−52 (Zhang et al.
2009).
The SWTs between X-ray bursts are not very well
understood within the framework of classical thermonu-
clear flash scenarios, because of the fact that the waiting
times in between the X-ray bursts are often too short to
accrete enough material and start another ignition. The
secondary bursts within these events are thought to be
caused by the burning of residual fuel that remained
from the primary burst. This was first demonstrated
by (Fujimoto et al. 1981) using several mechanisms like
an incomplete nuclear flame, fuel storage, and a mix-
ing mechanism with new accreted material. Keek &
Heger (2017) suggested that second bursts must be pow-
ered by fuel remaining from the previous explosion given
the fact that these events are typically less bright and
have shorter durations. SWT burst events have been
observed more frequently in sources where the accreted
material is hydrogen-rich and the neutron star spin fre-
quency is larger than >500 Hz (Keek et al. 2010). Boirin
et al. (2007) suggested that stochastic processes associ-
ated with fast rotation may play an important role in
the occurrence of SWT burst events.
In addition to bursts with short recurrence times,
there is also another group of X-ray bursts where sec-
ondary peaks are observed, these events are called
double-peaked bursts (see, e.g. Sztajno et al. 1985;
Bhattacharyya & Strohmayer 2006a,b). Some exam-
ples of observations of these rare events include X-
ray bursts from 4U 1636−536 (Zhang et al. 2009),
4U 1608−52 (Penninx et al. 1989; Galloway & Keek
2017; Jaisawal et al. 2019), GX 17+2 (Kuulkers et al.
2002) and 4U 1709−267 (Jonker et al. 2004). Unlike
in standard type-I bursts, during these events, X-ray
intensity reaches to a peak followed by a decline and
another subsequent rise. The secondary peak can reach
to similar intensity levels as the initial peak and the
peaks are separated by a few seconds. Most recently
Li et al. (2020) studied 16 multi-peaked X-ray bursts
observed from 4U 1636−536 with Rossi X-ray Timing
Explorer (RXTE). They find an anti-correlation be-
tween the peak flux of the secondary peaks and the
separation time between the peaks. They also find that
the ratio of the peak fluxes of the peaks in bursts are
correlated with the temperature of the thermal compo-
nent in the pre-burst spectra and conclude that double
peaks maybe related to the accretion rate in the disc or
the temperature of the neutron star.
Bhattacharyya & Strohmayer (2006a,b) suggested
that in these cases, the X-ray burst is ignited at or
around one of the poles and propagates towards the
equator. As the burning front propagates towards the
equator, either due to acting Coriolis forces (Bhat-
tacharyya & Strohmayer 2007) or due to the effect of
the magnetic field at the surface (Payne & Melatos
2006) it stalls for a few seconds, causing a decline in the
X-ray intensity. As the burning front continues to prop-
agate towards the opposite pole the intensity increases
again. Note that, Cavecchi et al. (2013, 2015) investi-
gate possible mechanisms for stalling near the equator
and conclude that, the Coriolis effects alone may not be
enough to stall the front, however, their simulations do
not take into account the effects of ongoing accretion
and magnetic fields. As an alternative, Fisker et al.
(2004) suggest that the stall may be caused by the wait-
ing points in the nuclear reaction chain, however, such
a scenario falls short of a complete explanation of the
burst profile characteristics.
The transient low mass X-ray binary 4U 1608−52
is a well-known X-ray burster, since its first detec-
tion with the two Vela-5 satellites (Belian et al. 1976).
4U 1608−52 is classified as an atoll source based on spec-
tral and timing properties by Hasinger & van der Klis
(1989). The detection of the burst oscillations from the
source suggests that the spin period of the neutron star
Thermonuclear X-ray Bursts with Late Secondary Peaks from 4U 1608−52 3
is ∼620 Hz (Muno et al. 2002). Wachter et al. (2002)
used the observed periodic modulation in the I band
data to claim an orbital period for the system as 12.9 h.
Wachter et al. (2002) argued that if the companion is
assumed to be a main-sequence star, then existing ob-
servations indicate an F to G type donor. Using the
observed X-ray bursts with RXTE, first G¨uver et al.
(2010) and later
¨
Ozel et al. (2016) determined a num-
ber of physical parameters of the system. Using red
clump giants and soft X-ray observations, the distance
of 4U 1608−52 is determined to be most likely at 4 kpc
or even larger (
¨
Ozel et al. 2016). The Eddington limit of
the source was also measured at 3.54±0.38×10
38
erg s
−1
(or 18.5±2.0×10
−8
erg s
−1
cm
−2
) using the touchdown
fluxes measured during thermonuclear X-ray bursts.
Throughout the paper, we use these values as our refer-
ence.
The Multi-INstrument Burst ARchive (MINBAR) re-
ports 147 bursts in the RXTE, BeppoSAX, and INTE-
GRAL archives from this source (Galloway et al. 2020).
Using Hakucho, Murakami et al. (1980) detected X-ray
bursts separated by about 10 minutes. Furthermore,
Keek et al. (2010) reported the detection of SWT burst
events, using an earlier version of the MINBAR cata-
log. In the final version of the MINBAR catalog (Gal-
loway et al. 2020) there are several bursts separated by
as short as 6 minutes (with MINBAR burst IDs: 2218,
1619, 2885, 7497, 7498) observed either by RXTE/PCA
or INTEGRAL JEM-X. Jaisawal et al. (2019) reported
the detection with NICER of a secondary peak in the
cooling tail of an X-ray burst showing photospheric ra-
dius expansion, in a soft state. The soft X-ray sensitivity
of NICER allows for the comparison and testing of mod-
els based on the effects of absorption to those that rely
on stalling of flame propagation or additional burning
(Jaisawal et al. 2019).
Starting from May 27th to roughly 19th of August
2020, 4U 1608−52 has been in an outburst state. This
outburst has been monitored by Swift/BAT and the
Monitor of All-sky X-ray Image (MAXI, Matsuoka et al.
2009). Starting from June 4th, NICER performed a
high cadence campaign to monitor the evolution of
4U 1608−52 during the whole outburst. A detailed anal-
ysis of the NICER dataset regarding the spectroscopic
evolution of the system during the outburst is ongoing
and will be reported in a forthcoming paper. Within
these observations we detected two X-ray bursts each
showing clear secondary peaks, 30 and 18 seconds after
the initial peaks and another third burst just after the
outburst. In this paper we concentrate on the temporal
and spectral analyses of these X-ray bursts, especially
the secondary peaks and their nature.
2. OBSERVATION AND DATA ANALYSIS
On-board the International Space Station (ISS),
NICER contains 56 (52 of which were active during the
observations presented here) X-ray concentrator optics
and silicon drift detector pairs to provide a large effec-
tive area in the 0.2−12 keV band (Gendreau et al. 2012,
2017). According to MAXI, 4U 1608−52 started an
outburst on May 27th, 2020. NICER started observing
the source on June 4th, 2020 and continued to monitor
the evolution of the outburst comprehensively. MAXI
lightcurve and the NICER coverage is shown in Figure
1 together with the detected thermonuclear bursts. A
total unfiltered exposure time of 283 ks, with 78 dif-
ferent observations is obtained with NICER, up to the
14 September 2020 (see Figure 1). These observations
cover the ObsIDs, 3050070101, and all observations with
ids 365702xxxx, where xxxx changes from 0101 to 9906.
After applying standard filtering criteria, using HEA-
SOFT v.6.27.2, NICERDAS v7a, and the calibration
files as of 2020/07/27, we had a total of 174 ks clean
exposure time. We searched for X-ray bursts including
the unfiltered data and found that within this rich data
set, NICER detected three type-I X-ray bursts that oc-
curred on June 23rd and 26th at around the peak of
the outburst and on September 13th just after the end
of the outburst. These bursts were detected in the ob-
servations with ObsIDs 3657021501, 3657021801, and
3657029905, which had 2.6 ks, 5.3 ks, and 232 s of ef-
fective exposure times, respectively. Note that the stan-
dard screening of the data eliminates the time of the last
X-ray burst. We, therefore, used the unfiltered data for
the analysis of this burst. Throughout the paper, we
refer to these X-ray burst events as bursts I, II and III
and call the two peaks in the first two events as the ini-
tial and secondary. We present the lightcurves of these
X-ray bursts in Figure 3 and further discuss below.
An in-depth analysis of the outburst will be presented
elsewhere, however to put the observed X-ray bursts
in perspective we show in Figure 1 the 2 − 20 keV
lightcurve of the outburst as observed by MAXI start-
ing from 1st of May to November 11th 2020. Compared
to the outbursts observed recently, the 2020 outburst
has been one of the longest and brightest events on
record. Using X-ray lightcurves extracted from the ana-
lyzed NICER data binned to have a time resolution of
128 s, we also calculated X-ray hardness-intensity and
color-color diagrams following the energy selection of
Jaisawal et al. (2019). We present these diagrams in
Figure 2.
2.1. X-ray lightcurves
4 G
¨
uver et al.
Figure 1. 2−20 keV MAXI lightcurve of 4U 1608−52 from daily average measurements, starting from May (05/01/2020)
to December (12/03/2020). The grey dotted lines indicate the dates of NICERobservations and the red dashed lines shows
NICER burst times. The sudden rise seen in the MAXI lightcurve around 50 days after the start of the outburst is also shown
and is a superburst not reported before (Iwakiri et al. 2021 in preparation).
We applied barycenter corrections to the event lists us-
ing the barycorr tool, JPL DE430 planetary ephemeris
and the coordinates of the source as R.A. 16
h
12
m
43
s
,
decl. 52
◦
25
0
23.
00
2 (J2000). Using the barycentred
events we generated 0.3−3.0, 3.0−12.0, 0.3−12.0, keV
lightcurves, with a time resolution of 0.25 s (see Figure
3). The X-ray bursts are clearly visible 120, 5964, and
5795 seconds after the start of the observations, respec-
tively. In the first two burst, a secondary peak is also
clearly visible in all bands, while the last burst lack any
evidence of such a structure. In burst I, a secondary
peak or possibly a second burst is visible roughly 30 s
after the initial burst started. In burst II, the secondary
peak is even closer to the initial peak and starts ∼ 18 s
after the burst.
We modeled the 0.3 − 12 keV burst lightcurves of
4U 1608−52 to better characterize the observed bursts.
We determined the pre-burst count rates calculating the
average of data 50 s before each burst. For the secondary
peak in burst I, we determined the average using the
data 8 s data prior to it. These values, together with
the peak count rates are presented in Table 1 and show
that the initial peak did not yet fully return to the per-
sistent count rate, but very close to it. In burst II the
secondary peak happens during the tail. In each case
the peak count rates of the secondary peaks reached
two-thirds of the initial peaks.
We calculated the rise times for each X-ray burst
defining them as the time between the first bin within
1σ of the peak count-rate and the last time bin within
1σ of the persistent rate. In order to determine the
exponential decay time-scales, we fit the decay of the X-
ray bursts with an exponential function plus a constant,
with the constant fixed to the pre-burst level. Note that
to better model the decay of burst II, we removed a 10-
second interval covering the secondary peak. The total
decay of burst III lasted for 205 s if we measure from
peak to the pre-burst level. This whole interval can be
modelled with a double exponential but to be able to
better compare with the previous X-ray bursts we only
modeled the decay of the first 50 s after the peak. The
best fit values are given in Table 1 and further under-
line the fast rise and exponential decay nature of the
secondary peaks themselves.
2.2. Time Resolved Spectral Analysis of the Bursts
To analyze the spectral evolution of the X-ray bursts
and better characterize the nature of the secondary
peaks, we performed a time-resolved spectral analysis.
For the extraction of the X-ray burst spectra we fol-
lowed a method that is very similar to what has been
done by Galloway et al. (2008) or G¨uver et al. (2012b).
We extracted spectra with 0.5 s time resolution up to
the peak of each X-ray burst. From the peak, we ex-
tracted X-ray spectra with exposure times of 0.5 s, 1.0 s
or 2.0 s depending on the total count rate. We used
the up to date response and ancillary response files as
in NICER CALDB release xti20200722, however, note
Thermonuclear X-ray Bursts with Late Secondary Peaks from 4U 1608−52 5
Table 1. Some characteristic properties of the bursts.
Burst I Burst II Burst III
Initial Peak Secondary Peak Initial Peak Secondary Peak
Time (MJD) 59019.576478 59022.096125 59105.212414
Time Between Peaks (s) 28.00±0.25 18.00±0.25 –
Prior Count Rate 2978±8 3269±21 2893±8 – 63.7±1.0
Persistent Flux
a
2.99±0.01 2.58±0.01 0.22±0.01
Peak Count Rate
b
(s
−1
) 6096±156 4052±127 6660±163 4264±131 2240±94
Peak Flux
a
6.2±0.5 1.87±0.2 8.72
+0.6
−0.8
1.4±0.2 12.64±1.42
Rise Time (s) 1.4 0.76 1.93 1.75 4.0
τ 9.13±0.15 10.3±0.5 9.6±0.3 7.2±0.6 11.42±0.26
a
Unabsorbed bolometric flux in units of ×10
−8
erg s
−1
cm
−2
.
b
Including the persistent count rate of the source.
0 2 4 6 8 10 12
SC (1.1-2 keV / 0.5-1.1 keV)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
HC (3.8-6.8 keV / 2-3.8 keV)
8.0 8.5 9.0 9.5 10.0 10.5 11.0
0.3
0.4
0.5
0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65
Hardness (3.8-6.8keV / 2.0-3.8 keV)
500
1000
1500
2000
2500
3000
Intensity (Count Rate in 0.5-10.0 keV)
Figure 2. Hardness-Intensity diagram extracted from all
of the observations in which we searched for an X-ray burst
during the outburst. Detected X-ray bursts are shown with
blue star, red triangle, and orange circle symbol for burst I,
II, and III, respectively.
that we removed the data from Focal Plane Modules 14
and 34 and therefore adjusted the response and ancillary
response files accordingly.
Time-resolved spectral analysis of X-ray bursts is af-
fected by the persistent emission of the low mass X-ray
binary and needs to be taken into account carefully (Gal-
loway et al. 2008; G¨uver et al. 2012b,a; Worpel et al.
2013, 2015). We extracted X-ray spectra prior to each
X-ray burst with an exposure time of 100 s and mod-
eled this pre-burst emission. For the spectral analy-
sis, we utilized Sherpa (Freeman et al. 2001) distributed
with the CIAO v4.12. We created background files for
each observation using the nibackgen3C50
1
tool. For
burst I and burst II, we fit the energy spectrum ob-
tained from this interval with an absorbed disk black-
body (diskbb) and a blackbody (bbodyrad) model (Gil-
fanov et al. 2003). For burst III, however adding a black-
body to the disk blackbody component resulted in un-
constrained parameters, we therefore used a power-law
model, instead of a blackbody to better characterize the
X-ray spectrum. Note that this last burst is observed
at a low-hard state compared to the earlier bursts (see
Figure 2). To take into account the Hydrogen column
density we assumed the abundance of the interstellar
medium and used the tbabs model (Wilms et al. 2000).
With these models, we are able to obtain very good fits
to the X-ray spectra in the 0.5 − 10 keV range. We
present the results of these fits in Table 2.
We calculated the X-ray flux in the 0.01−200 keV
band based on the best fit parameters and found the
bolometric unabsorbed fluxes of the source prior to burst
I, II, and III as F
bol
=2.99±0.04×10
−8
erg s
−1
cm
−2
,
F
bol
=2.58±0.01×10
−8
erg s
−1
cm
−2
, and F
bol
=0.22±0.01×10
−8
erg s
−1
cm
−2
respectively. Using the Eddington limit
from
¨
Ozel et al. (2016) these fluxes indicate that the
persistent emission was 16.16% and 13.95%, 1.19% Ed-
dington.
1
https://heasarc.gsfc.nasa.gov/docs/nicer/tools/
nicer bkg est tools.html
