A broadband look of the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 using AstroSat and XMM-Newton
MMNRAS , 1–9 (2020) Preprint 14 January 2020 Compiled using MNRAS L A TEX style file v3.0
A broadband look of the Accreting Millisecond X-rayPulsar SAX J1748.9-2021 using
AstroSat and
XMM-Newton
Rahul Sharma (cid:63) , Aru Beri , , Andrea Sanna and Anjan Dutta Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India DST-INSPIRE Faculty, IISER Mohali, Punjab, India 140306 School of Physics and Astronomy, University of Southampton, Southampton, Hampshire, SO17 1BJ United Kingdom Universit´a degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
SAX J1748.9-2021 is a transient accretion powered millisecond X-ray pulsar lo-cated in the Globular cluster NGC 6440. We report on the spectral and timing analy-sis of SAX J1748.9-2021 performed on
AstroSat data taken during its faint and shortoutburst of 2017. We derived the best-fitting orbital solution for the 2017 outburstand obtained an average local spin frequency of 442.361098(3) Hz. The pulse profileobtained from 3–7 keV and 7–20 keV energy bands suggest constant fractional am-plitude ∼ .
5% for fundamental component, contrary to previously observed energypulse profile dependence. Our
AstroSat observations revealed the source to be in a hardspectral state. The 1–50 keV spectrum from SXT and LAXPC on-board
AstroSat canbe well described with a single temperature blackbody and thermal Comptonization.Moreover, we found that the combined spectra from
XMM-Newton (EPIC-PN) and
AstroSat (SXT+LAXPC) indicated the presence of reflection features in the form ofiron (Fe K α ) line that we modeled with the reflection model xillvercp . One of thetwo X-ray burst observed during the AstroSat /LAXPC observation showed hard X-ray emission ( >
30 keV) due to Compton up-scattering of thermal photons by the hotcorona. Time resolved analysis performed on the bursts revealed complex evolution inemission radius of blackbody for second burst suggestive of mild photospheric radiusexpansion.
Key words: accretion, accretion discs – stars: neutron – X-ray: binaries – X-rays:bursts – X-rays: individual (SAX J1748.9-2021)
Low Mass X-ray Binaries (LMXBs) are composed of a com-pact object (a black hole or a neutron star) that accretesmatter from a low mass companion star, (cid:46) M (cid:12) . In someneutron star (NS) LMXBs, X-ray pulsations of the orderof millisecond have been detected (see e.g., Chakrabarty &Morgan 1998; Wijnands & van der Klis 1998; Galloway et al.2002; Markwardt et al. 2002; Papitto et al. 2013b; Sannaet al. 2018c,d). These systems are called accretion poweredmillisecond X-ray pulsars (AMXPs) (see e.g., Patruno &Watts 2012; Campana & Di Salvo 2018, for reviews). Themagnetic field estimated in these systems is of the order of10 − Gauss (see e.g., Cackett et al. 2009; Mukherjee (cid:63)
E-mail: [email protected] et al. 2015; Ludlam et al. 2017; Sharma et al. 2019). Cur-rently, 22 AMXPs are known and all of them are transientin nature, observed during outbursts in the past 21 years(Marino et al. 2019).SAX J1748.9-2021 is an AMXP discovered with
Beppo-SAX during its 1998 outburst (in ’t Zand et al. 1999). It islocated in the globular cluster NGC 6440 at a distance of ∼ . . − . M (cid:12) and 0 . − . R (cid:12) (see, Cadelano et al. 2017). Since 1998only six outbursts have been observed in SAX J1748.9-2021(in ’t Zand et al. 1999; in’t Zand et al. 2001; Markwardt& Swank 2005; Patruno et al. 2010; Pintore et al. 2016,2018; Negoro et al. 2017; Sharma et al. 2019). SAX J1748.9-2021 showed intermittent pulsations at ∼ . c (cid:13) a r X i v : . [ a s t r o - ph . H E ] J a n R. Sharma et al. ∼ .
76 h and projected semi-major axis of ∼ . AstroSat (Agrawal 2006; Singh et al. 2014), waslaunched in 2015. It has five principal payloads on-board:(i) the Soft X-ray Telescope (SXT), (ii) the Large Area X-ray Proportional Counters (LAXPCs), (iii) the Cadmium-Zinc-Telluride Imager (CZTI), (iv) the Ultra-Violet Imag-ing Telescope (UVIT), and (v) the Scanning Sky Monitor(SSM). Here, we have performed a broadband spectroscopyusing simultaneous
XMM-Newton and
AstroSat (SXT andLAXPC) data of SAX J1748.9-2021 observed during its lat-est outburst of 2017. We also report results from the timingand burst analysis carried out with
AstroSat /LAXPC data.
AstroSat /LAXPC
LAXPC is one of the primary instrument aboard
AstroSat .It consists of three co-aligned identical proportional coun-ters (LAXPC10, LAXPC20 and LAXPC30) that work inthe energy range of 3–80 keV. Each LAXPC detector inde-pendently record the arrival time of each photon with a timeresolution of 10 µ s and has five layers, each with 12 detectorcells (for details see Yadav et al. 2016; Antia et al. 2017).Table 1 gives the log of observations that have been usedin this work. Due to the gain instability issue caused by gasleakage, we have not used LAXPC30 data. LAXPC datawere collected in the Event mode (EA) which contains theinformation about the time, channel number and anodeID S w i f t/ X R T . - k e V ( C oun t s - ) M AX I - k e V ( P ho t on s c m - s - ) Time (MJD)
MAXISwift/XRT
Figure 1.
Timeline of SAX J1748.9-2021 during its 2017 outburstas observed with
Swift /XRT and
MAXI /GSC. The light greyand dark grey regions represent the time of
AstroSat and
XMM-Newton observations, respectively. The blue arrow corresponds tothe start of 2017 outburst (MJD 58025) as reported by Negoroet al. (2017). The first XRT observation was taken close to thepeak of the outburst. The red arrows are the upper limits of thedetection with XRT. of each event. We have used
LaxpcSoft software packageto extract light curves and spectra. LAXPC detectors havedead-time of 42 µ s and the extracted products are dead-timecorrected. The background in LAXPC is estimated from theblank sky observations (see Antia et al. 2017, for details). Wefound that the source was detected up to 50 keV, therefore,to minimize the background we have performed spectroscopyusing the data of top layer (L1, L2) of each detector (alsosee Beri et al. 2019, for details). We have used response filesto obtain channel to energy conversion information whileperforming energy-resolved analysis.We corrected the LAXPC photon arrival times to theSolar system barycentre by using the as1bary tool. Weused the best available position of the source, R.A. (J2000)=17 h m . s
163 and Dec. (J2000) = − ◦ (cid:48) . (cid:48)(cid:48)
40 obtainedwith
Chandra (Pooley et al. 2002). Timing analysis is per-formed on LAXPC10 and LAXPC20 data.
AstroSat /SXT
The Soft X-ray Telescope (SXT) is a focusing X-ray tele-scope with CCD in the focal plane that can performX-ray imaging and spectroscopy in the 0.3–7 keV en-ergy range (Singh et al. 2014, 2016; Singh et al. 2017).SAX J1748.9-2021 was observed in the Photon Counting(PC) mode with SXT (Table 1). Level 1 data were pro-cessed with
AS1SXTLevel2-1.4b pipeline software to pro-duce level 2 clean event files and these files were mergedusing SXT Event Merger Tool (Julia Code ). This merged ∼ astrosat laxpc/LaxpcSoft.html http://astrosat-ssc.iucaa.in/?q=data and analysis ∼ astrosat sxt/dataanalysis.htmlMNRAS , 1–9 (2020) stroSat observation of SAX J1748.9-2021 Table 1.
Log of X-ray observations.
Instrument OBS ID Start Time Stop time Mode Exposure(yy-mm-dd hh:mm:ss) (yy-mm-dd hh:mm:ss) (ks)
AstroSat /LAXPC 9000001594 2017-10-08 07:43:55 2017-10-10 16:53:48 Event Mode 206
AstroSat /SXT 9000001594 2017-10-08 07:57:03 2017-10-10 18:40:09 PC 211
XMM-Newton /EPIC-PN 0795712201 2017-10-09 11:23:17 2017-10-10 03:15:38 Timing 57 event file was used to extract image, light curves and spec-tra using the ftool task xselect heasoft version 6.22. A circular region with radiusof 15 arcmin centered on the source was used. For spec-tral analysis, we have used the background spectrum (Sky-Bkg comb EL3p5 Cl Rd16p0 v01.pha), spectral redistribu-tion matrix file (sxt pc mat g0to12.rmf) and ancillary re-sponse file (sxt pc excl00 v04 20190608.arf) provided by theSXT team . XMM-Newton /EPIC-PN
XMM-Newton has European Photon Imaging Camera(EPIC), Reflection Grating Spectrometer (RGS) and Op-tical Monitor (OM) on-board. The EPIC consists of one PNcamera (Str¨uder et al. 2001) and two MOS detectors (Turneret al. 2001), sensitive in the 0.1–15 keV energy range. The
XMM-Newton observation of SAX J1748.9-2021 has an over-lap in time with the
AstroSat observation (refer to Table 1for details). For current analysis, we have used the EPIC-PN data which was operated in the timing mode. EPIC-PNdata was reduced with SAS v16.1.0 with RDPHA correc-tions (Pintore et al. 2014). The spectra and light curveswere extracted selecting single and double pixel events with
P AT T ERN ≤ F LAG = 0, which retains events op-timally calibrated for spectral analysis. Following Pintoreet al. (2018), source and background events were extractedfrom RAWX=[32:44] and RAWX=[3:5], respectively. Thespectra were rebinned with an oversample of 3 and mini-mum of 25 counts per bin using specgroup task. To avoidthe EPIC-pn (timing mode) calibration uncertainties at lowenergies, we analyzed the spectra in 1.3–10 keV energy range(Pintore et al. 2018).
Figure 1 shows the light curve of SAX J1748.9-2021 dur-ing its 2017 outburst as observed with the X-ray tele-scope (XRT) on-board the Neil Gehrels Swift
Observatory(Gehrels et al. 2004) and with Gas Slit Camera (GSC) on-board the Monitor of All-sky X-ray Image ( MAXI ; Mat-suoka et al. 2009; Mihara et al. 2011). Figure 2 shows thebackground corrected light curve extracted from LAXPC10(upper panel) binned at 100 sec. The LAXPC light curves ∼ astrosat sxt/dataanalysis.html Created from the online tool: Build XRT products (Evans et al.2007) of UK Swift Science Data Centre. http://maxi.riken.jp/top/lc.html C oun t R a t e ( c t s − ) H a r dn e ss R a ti o Time (s) since MJD 58034.32216
Figure 2.
Top: bottom:
Hardnessratio (=hard/soft) between 3–10 keV and 10–30 keV binned at100 sec. Red arrows indicate the times during which there is anoverlap between
AstroSat and
XMM-Newton . Black arrows rep-resents the time when X-ray bursts were observed. show the persistent emission separated by data gaps dueto Earth occultation and South Atlantic Anomaly (SAA)passage. Two Type-I X-ray bursts are also observed in theLAXPC light curves (position marked with black arrows inFigure 2). However, these X-ray bursts were not seen in theSXT light curves (not plotted) as these times when X-raybursts were observed have been filtered during the Goodtime filtering. The X-ray burst seen in the PN light curvehas been reported earlier (Pintore et al. 2018). Therefore,we have excluded this X-ray burst from our analysis.During the
AstroSat observation, the 3–80 keV back-ground subtracted count rate of persistent emission fromLAXPC10 decreased to ∼ half from 63 count s − at the startof observation to 33 count s − at the end of observation(Figure 2). Similar trend was observed with LAXPC20 also.However, we did not observe any change in the hardness ra-tio calculated using light curves in the two energy bands 3–10keV and 10–30 keV during the observation (bottom panel ofFigure 2), suggesting that source did not seem to change thespectral state with decay in the count rate. The 0.5–10 keVcount rate of XMM-Newton decreased to 34 count s − from38 count s − , during the XMM-Newton observation.
We started by correcting the
AstroSat /LAXPC time se-ries for the binary orbital motion (see e.g. Burderi et al.2007, for details on the method) through the available source
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MNRAS000 , 1–9 (2020)
R. Sharma et al. N o r m a li s e d I n t e n s it y N o r m a li s e d I n t e n s it y Figure 3.
SAX J1748.9-2021 pulse profiles (black points) ob-tained epoch-folding the
AstroSat /LAXPC data in the energyrange 3–7 keV (top panel) and 7–20 keV (bottom panel) after cor-recting for the updated orbital solution. The best-fitting model(red line) is the superposition of one and two sinusoidal functionswith harmonically related periods, respectively. For clarity, weshow two cycles of the pulse profile. ephemeris obtained during its 2015 outburst (Sanna et al.2016). We then performed epoch-folding search of the wholedataset around the spin frequency value ν = 442.3610957Hz (mean spin value reported during the 2015 outburst), ex-ploring the frequency space with steps of 10 − Hz for a totalof 1001 steps. Moreover, we investigated possible updates onthe orbital solution by exploring parameters with the largestpropagated uncertainty with respect to the orbital solutionof reference. More specifically, we focused on the time of pas-sage from the ascending node characterised by a propagateduncertainty of σ T (cid:63) ∼
780 s, obtained assuming a constantorbital period.We corrected photon time of arrivals adopting the 2015orbital ephemeris, except for T (cid:63) , that we varied in steps of1 seconds in the range T (cid:63) ± σ T (cid:63) . Epoch-folding tech-niques are then applied to search for X-ray pulsation aroundthe spin frequency ν using 8 phase bin to sample the sig-nal. The most significant pulse profile has been obtainedfor ∆ T (cid:63) = 480 . ν = 442 . T (cid:63) value obtained is consistent within errors withthe expected value propagated from the previous solution.Following Riggio et al. (2011); Sanna et al. (2016), we esti-mated the uncertainty on T (cid:63) and ¯ ν using Monte Carlo sim-ulations (100 datasets to allow the 1 σ error estimations),obtaining the values T (cid:63) = T (cid:63) + ∆ T (cid:63) = 58034 . ν = 442 . ν and sampling the signalin 16 phase bins for the energy bands 3–7 keV (top panel)and 7–20 keV (bottom panel). The pulse shape of the 3-7keV energy band is well fitted with a sinusoidal function withbackground corrected fractional amplitude of 0.5%. The 7–20 keV pulse profile requires two harmonically related com-ponents with background corrected fractional amplitude of0.45% and 0.15% for the fundamental and first overtone,respectively.We also checked for burst oscillations during the two observed type-I bursts with LAXPC data, but no significantX-ray pulsation compatible with the spin frequency of thesource seems to be present. Even after combining the twobursts, no significant pulsations were detected. To understand the energy dependence of X-ray bursts, weextracted light curves in different energy bands namely, 3–6keV, 6–12 keV, 12–18 keV, 18–24 keV, 24–30 keV and 30–40keV. Figure 4 shows burst profiles created using the com-bined data of LAXPC10 and LAXPC20. Light curves arebinned with a binsize of 1 sec. SAX J1748.9-2021 has exhib-ited a wide variety of burst profiles (Galloway et al. 2008;Beri et al. 2016; Pintore et al. 2016, 2018; Li et al. 2018). Toquantify the behavior of observed bursts, decay times weremeasured by modeling the bursts profiles using linear risefollowed by an exponential decay. We measured the expo-nential decay time of both bursts in different energy band.We found that the burst duration decreases with increasingenergy. Figure 5 shows the gradual decrease in decay timewith increasing energy due to cooling of the burst to lowertemperature with the decay of burst (Degenaar et al. 2016;Beri et al. 2019). The first X-ray burst was detected up to30 keV while the second was observed up to 40 keV (seeinset in Fig. 4). Following Beri et al. (2019), we also checkedfor the presence of dips due to effect of X-ray burst on thehard X-ray emission in hard X-ray light curves during theobserved bursts (30-80 keV for burst 1 and 40-80 keV forburst 2). No dip in the hard X-ray light curve was observedduring any of the bursts.
To understand the spectral evolution during these X-raybursts, we have performed time-resolved spectroscopy usingspectra extracted with a duration of 1 sec. Spectra obtainedfrom LAXPC10 and LAXPC20 were fitted simultaneously inthe energy band of 3–30 keV. We added a cross-calibrationconstant between the two LAXPC instruments. For all burstintervals, a spectrum extracted from 90 s of data precedingthe burst was extracted as the underlying accretion emis-sion.We fitted each spectrum with a blackbody function( bbodyrad ) in xspec v 12.9.1m (Arnaud 1996).
Tbabs wasused to model interstellar absorption with abundances setto
WILM (Wilms et al. 2000) and the cross-sections to
VERN (Verner et al. 1996). We fixed the interstellar column densityto N H = 0 . × cm − (Pintore et al. 2016).The evolution of count rate in 3–30 keV, blackbody tem-perature ( kT BB ) in keV, blackbody normalisation ( N BB ),emission radius in km, absorbed flux in 3–30 keV in units oferg cm − s − and reduced χ during each burst are plottedin Figure 6 from top to bottom, respectively. Burst 2 wasbrighter than burst 1 and the temperature measured dur-ing the peak of this burst is 2 . ± .
05 keV. Moreover, theevolution of the blackbody radius indicates the presence ofPRE phase. The peak temperature of burst 1 was observedto be 2 . ± .
05 keV. We calculated the bolometric flux( F bol ) using F bol = 1 . N BB ( kT BB ) × − erg cm − s − (Galloway et al. 2008). At peak, bolometric flux was ob- MNRAS , 1–9 (2020) stroSat observation of SAX J1748.9-2021 C oun t R a t e ( c t s − ) Time (sec) 0 20 400102030 3−6 keV 6−12 keV 12−18 keV 18−24 keV 24−30 keV 30−40 keVBurst 1 0 50 1000100020003000 C oun t R a t e ( c t s − ) Time (sec) 0 20 400204060 3−6 keV 6−12 keV 12−18 keV 18−24 keV 24−30 keV 30−40 keVBurst 2
Figure 4.
AstroSat -LAXPC (LAXPC10+LAXPC20) background-corrected light curve of the two X-ray bursts in different energy bands.The inset shows light curves in two energy bands 24–30 keV and 30–40 keV.
10 20 301020 B u r s t d eca y ti m e ( s ec ) Energy (keV)
Burst 1Burst 2
Figure 5.
The exponential decay time of two bursts as a functionof energy. served to be 2 . × − erg cm − s − and 2 . × − ergcm − s − for burst 1 and 2, respectively. We performed a broadband spectroscopy (1–50 keV) usingthe data during persistent emission obtained with SXT (1–7keV) and LAXPC (3–50 keV) aboard
AstroSat . Data be-low 1 keV was ignored due to low energy calibration issueof SXT. We used only LAXPC10 detector from LAXPCinstrument for broadband spectral fitting, LAXPC20 wasavoided due to instrument calibration issues at higher en-ergies. The LAXPC10 spectra was regrouped as 0–99 by 2channels, 100–199 by 4 channels and above 200 by 8 chan-nels. A systematic uncertainty of 2% was added to LAXPCspectra (Antia et al. 2017; Sreehari et al. 2019). The SXTspectra was grouped using grppha to have a minimum of 25counts per bin. While performing spectral fitting, we addeda multiplicative constant component to account for cross-calibration between two instruments. The parameter valueof the constant was fixed to 1 for LAXPC10 and for SXTallowed to vary. We also allowed the gain of the response fileof SXT to vary, with slope fixed to 1. We obtained a gain
Table 2.
The obtained best fit spectral parameters for SAXJ1748.9-2021. Reported errors and limits are at 90% for one pa-rameter.
Model Parameters SXT+LAXPC SXT+XMM+LAXPCTBabs N H (10 cm − ) 0 . ± .
10 0 . ± . kT BB (keV) 1 . ± .
08 0 . ± . . +1 . − . . ± . . ± .
04 1 . ± . kT seed (keV) 0 . ± .
06 = kT BB kT e (keV) > . > . +0 . − . . ± . ξ . ± . A Fe . +2 . − . Norm (10 − ) 1 . +1 . − . Constant C LAXPC C SXT . ± .
04 1 . ± . C XMM - 1 . ± . F . − keV . × − . × − (erg cm − s − ) χ /dof offset of ∼
37 eV. We have used tbabs to model interstellarneutral hydrogen absorption.The X-ray spectral continuum of SAX J1748.9-2021during its 2017 outburst was best fitted using a blackbodyand Comptonization model (Pintore et al. 2018). However,we noticed that during its 1998 outburst, the X-ray spec-tral continuum was best fitted with a single thermal Comp-tonized emission (in ’t Zand et al. 1999). Therefore, we beganto model the combined spectra from SXT and LAXPC us-ing a thermal Comptonized model nthcomp (Zdziarski et al.1996; ˙Zycki et al. 1999). We found that tbabs*nthcomp gavean unsatisfactory fit, χ /dof = 631/526. We also observedresiduals around 30 keV which is due to the Xenon calibra-tion edge (Antia et al. 2017) and modeled using a Gaussian.Addition of a second blackbody component (soft thermalcomponent) improved the fit and we obtained a value of χ /dof to be 599.8/524 (Figure 7). We would like to men-tion that we did not find any residuals around 6.4 keV (due MNRAS000
37 eV. We have used tbabs to model interstellarneutral hydrogen absorption.The X-ray spectral continuum of SAX J1748.9-2021during its 2017 outburst was best fitted using a blackbodyand Comptonization model (Pintore et al. 2018). However,we noticed that during its 1998 outburst, the X-ray spec-tral continuum was best fitted with a single thermal Comp-tonized emission (in ’t Zand et al. 1999). Therefore, we beganto model the combined spectra from SXT and LAXPC us-ing a thermal Comptonized model nthcomp (Zdziarski et al.1996; ˙Zycki et al. 1999). We found that tbabs*nthcomp gavean unsatisfactory fit, χ /dof = 631/526. We also observedresiduals around 30 keV which is due to the Xenon calibra-tion edge (Antia et al. 2017) and modeled using a Gaussian.Addition of a second blackbody component (soft thermalcomponent) improved the fit and we obtained a value of χ /dof to be 599.8/524 (Figure 7). We would like to men-tion that we did not find any residuals around 6.4 keV (due MNRAS000 , 1–9 (2020)
R. Sharma et al. C oun t R a t e Burst 122.5 k T N o r m R a d i u s −8 −8 F l ux
10 20 30 4011.5 χ r e d Time (sec) 100020003000 C oun t R a t e Burst 222.5 k T N o r m R a d i u s −8 −8 F l ux
10 20 30 4011.5 χ r e d Time (sec)
Figure 6.
Time resolved spectroscopy of the two bursts observed with
AstroSat /LAXPC observation. to Fe K α ) as observed with XMM-Newton (see Pintore et al.2018). We estimated the upper limit on the equivalent widthof Fe K α emission line to be ∼
20 eV, where line energy andwidth was fixed to 6.5 keV and 0.2 keV, respectively. Theestimated upper limit on Fe emission line is well consistentwith the equivalent width ( ∼
15 eV) measured by Pintoreet al. (2018) with
XMM-Newton data.We report the average, unabsorbed 0.1–100 keV fluxduring
AstroSat observation was 9 . × − erg cm − s − ,corresponds to unabsorbed luminosity of L X ∼ . × erg s − for a distance of 8.5 kpc. XMM-Newton +SXT+LAXPC spectrum
We extracted the SXT and LAXPC spectra using data thatwas strictly simultaneous with the
XMM-Newton observa-tion. We removed the X-ray burst from the
XMM-Newton data to obtain the spectrum during the persistent emis-sion. These three spectra were simultaneously fitted withabsorbed blackbody and Comptonized blackbody model. Wefixed the value of calibration-constant to be 1 for LAXPCand we let it free to vary for SXT and EPIC-PN. Gaussianmodels at ∼ XMM-Newton (Papittoet al. 2009; Ferrigno et al. 2014) and Xenon calibration edgefeature of LAXPC (Antia et al. 2017), respectively, as seenin the residuals. We also found systematic residuals around6.5 keV (see Fig-8b), arising due the Fe K emission feature.This feature has also been reported by Pintore et al. (2018).Therefore, we added a Gaussian model component and ob-tained the emission line energy at 6 . +0 . − . keV having awidth of 0 . +0 . − . keV. The equivalent width of this ironline feature is about 63 eV ( χ /dof = 532 / . ± . χ /dof = 533 / xillvercp (Garc´ıa & Kallman 2010;Garc´ıa et al. 2013) to account for the emission line, assum- ing it to be originating due to accretion disc reflection. Thespectral shape of the xillvercp was assumed to be sameas nthcomp . The inclination angle of the accretion disc wasunconstrained during the fit, so we fixed it to 32 ◦ . xillvercp were logξ ,iron abundance ( A Fe ) and normalization. After adding thereflection component, fit improved to χ /dof = 527 . / ∼ × − . The disc was foundto be highly ionized with ionization parameter of ξ ∼ − . The blackbody temperature was found to be ∼ . >
14 keV on the electrontemperature of corona was obtained, consistent with Pintoreet al. (2018). Additionally, we found lower value of SXT gainoffset ∼
17 eV. The best fit parameter values are presentedin Table 2 and best fit spectrum is shown in Figure 8a withresiduals in 8c.We also report the average, unabsorbed flux in 0.1–100 keV energy range during
XMM-Newton and
AstroSat strictly simultaneous observation was 7 . × − erg cm − s − . As a note to mention, Pintore et al. (2018) reportedunder-estimated fluxes in 0.3–70 keV energy range by a fac-tor of ∼
2, which could be due to upper bound on the re-sponse matrix of
XMM-Newton at 16 keV. Using the sameparameter value reported in Pintore et al. (2018), we foundthe 0.3–70 keV unabsorbed flux of 7 . × − erg cm − s − for 9th October (XMM+ISGRI) observation, consistentwith XMM+SXT+LAXPC observation and 4 . × − ergcm − s − for 11th October (XRT+NUSTAR) observation. SAX J1748.9-2021 was observed in its sixth outburst in 2017.The
AstroSat observed it for ∼ . − from 63counts s − . The LAXPC light curves also showed the pres-ence of two type-I thermonuclear X-ray bursts.The 1–50 keV X-ray continuum of SAX J1748.9-2021can be well described with the blackbody and Comptonized MNRAS , 1–9 (2020) stroSat observation of SAX J1748.9-2021 −4 −3 P ho t on s c m − s − k e V − ( d a t a − m od e l ) / e rr o r Energy (keV)
Figure 7.
Best fitted time-averaged broadband spectra of SXTand LAXPC modeled with bbodyrad+nthcomp . −4 −3 P ho t on s c m − s − k e V − (a) −2024 χ (b) χ Energy (keV) (c)SXTXMMLAXPC
Figure 8. (a) Broadband spectra of SAX J1748.9-2021from
XMM-Newton , SXT and LAXPC fitted simultaneously.(b) Residuals ( χ =(data-model)/error) with bbodyrad+nthcomp model. (c) Residuals with bbodyrad+nthcomp+xillvercp model.The figure has been rebinned for the plotting purpose. blackbody. The best-fit spectral parameters showed thatthe source was in the hard spectral state during the As-troSat observations (Table 2). AMXPs show the hard spec-trum with electron temperature of 30–50 keV (Falanga et al.2005; Gierli´nski & Poutanen 2005; Papitto et al. 2010, 2013a;Wilkinson et al. 2011; Sanna et al. 2018a,b; Di Salvo et al.2019). Also, it was in hard state during the
NuSTAR ob-servation taken after 1 day of
AstroSat observation (on2017-10-11), where the source flux reduced by 40% (Pintoreet al. 2018). Thus, it seems that the 2017 outburst of SAXJ1748.9-2021 did not show any spectral state change as ob-served during its previous outbursts in 2001, 2005, 2010 and2015 (Patruno et al. 2009; Li et al. 2018; Wu et al. 2018).Previously, SAX J1748.9-2021 was found to be only in thehard state during 1998 outburst (in ’t Zand et al. 1999).However, we see that the 1998 outburst of SAX J1748.9- 2021 also showed a similar behaviour, where peak luminos-ity reached ∼ erg s − and the outburst lasted for ∼ ξ ∼ − (e.g., Papitto et al. 2010; Di Salvo et al. 2019). The ironabundance, A Fe = 1 . +2 . − . , obtained from the fit is consis-tent with Solar values, although the uncertainty is large.The Comptonized emission associated to a hot corona orthe accretion column is characterized by a photon index of1 . ± . . ± .
04 keV and emission ra-dius of ∼ ∼
442 Hz are significantlydetected in the
AstroSat dataset. Timing analysis of thecollected events allowed us to obtain an updated orbital so-lution of the source, compatible within the errors with thesolution obtained for the 2015 outburst of SAX J1748.9-2021(Sanna et al. 2016). No X-ray pulsation has been detected ontimescales shorter than the whole observation. The strengthof the X-ray pulsation did not allow a detailed study of thesignal as a function of energy. However, the pulse profilesobtained in the energy bands 3–7 keV and 7–20 keV sug-gest a constant fractional amplitude around 0 .
5% for thefundamental component. This result is in contrast with thepulse profile energy dependence reported for the previousoutbursts (see e.g. Patruno et al. 2009; Sanna et al. 2016)where the fractional amplitude has been observed to increasefrom 0.1% at 0.5 keV to 4% at 20 keV.In AMXPs, accretion taking place on the NS is guidedby the magnetic field of the NS. This magnetically chan-neled accretion means that the accretion disc radius is out-side the NS surface and smaller than the co-rotation radius(Pringle & Rees 1972; Illarionov & Sunyaev 1975). Mukher-jee et al. (2015) estimated the upper limits on magnetic fieldstrength of 14 AMXPs, by assuming that the inner edge ofthe disc can not be outside the co-rotation radius. As X-ray pulsations has been observed during
AstroSat observa-tion, imply on going magnetically channelled accretion onto the NS. At the lowest X-ray luminosity (accretion rate),the accretion disc cannot be outside the co-rotation radius.Using the above assumptions, we estimated the upper limiton the magnetic field strength of NS from the flux obtainedwith XMM+SXT+LAXPC data ( F min = 7 . × − ergcm − s − ). Using equation (10) of Mukherjee et al. (2015),the upper limit on magnetic dipole moment estimated to be1 . × G cm , consistent with the estimates of Sharmaet al. (2019). This will give B < . × G for SAX J1748.9-2021 which is nearly a factor of 2 lower than the previousestimate of Mukherjee et al. (2015).
We performed the time-resolved burst spectroscopy andenergy-resolved burst analysis on the two X-ray bursts ob-served with
AstroSat /LAXPC. From the time-resolved spec-troscopy performed on the 1 sec bin of burst, a complex vari-
MNRAS000
MNRAS000 , 1–9 (2020)
R. Sharma et al. ation in emission radii of blackbody was found in burst 2,suggestive of mild PRE phase. The second burst was brighterthan the first one. The burst 1 was detected upto 30 keVonly, but burst 2 showed emission in 30–40 keV energy rangealso. The burst observed with
Beppo-SAX during 1998 alsoshowed the emission >
30 keV, where the burst emission isCompton up-scattered by the hot corona (in ’t Zand et al.1999). The bursts were also found to influence the hard tosoft state transition time. The soft photons from the burstscool the corona faster to push the state transition to theshorter time scale (Li et al. 2018). The burst decay timestrongly depends on the energy and the decay time of burst2 was lower than the burst 1. The peak temperature andbolometric flux of 2 .
88 keV and 2 . × − erg cm − s − ,respectively were obtained for burst 2. Previously, observedPRE bursts of SAX J1748.9-2021 showed the peak flux of ∼ . − × − erg cm − s − (Galloway et al. 2008).The local accretion rate per unit area onto the com-pact object can be estimated using ˙ m = L pers (1 + z )((4 πR )( GM/R )) − . Using the gravitational redshift of1 + z = 1 .
31 for a canonical NS with a mass M = 1 . M (cid:12) and a radius of R = 10 km, we found ˙ m (cid:39) . × gcm − s − . The observed recurrence time depends on ˙ m as∆ t rec ∝ < ˙ m > − . measured from the burst observed dur-ing 2015 outburst (Li et al. 2018). From the estimated av-erage accretion rate of 2017 outburst, the burst recurrencetime estimated was ∼ ∼ BeppoSAX in 1998 outburst showed the recurrence timeof (cid:39) . ACKNOWLEDGEMENTS
RS acknowledges the financial support from the UniversityGrants Commission (UGC), India, under the Senior Re-search Fellow (SRF) scheme. AB is grateful to both theRoyal Society, U.K and to SERB (Science and EngineeringResearch Board), India. AB is supported by an INSPIREFaculty grant (DST/INSPIRE/04/2018/001265) by the De-partment of Science and Technology, Govt. of India and alsoacknowledges the financial support of Indian Space ResearchOrganisation (ISRO) under
AstroSat archival Data utiliza-tion program. AB would also like to thank Prof. K. P. Singhfor several discussions regarding SXT data analysis. Thispublication uses data from the
AstroSat mission of ISRO,archived at the Indian Space Science Data Centre (ISSDC)and
XMM-Newton for which the data was obtained fromHigh Energy Astrophysics Science Archive Research Center(HEASARC), provided by NASA’s Goddard Space FlightCenter. We thank the LAXPC Payload Operation Center(POC) and the SXT POC at TIFR, Mumbai for providingnecessary software tools. We would like to thank IISER, Mo-hali for extending its library facilities. The authors thankthe anonymous referee for the valuable comments on themanuscript.
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