Swift and SALT observations of the Multiple Outbursts of MAXI J1957+032

1.2 MB·

by  D. Mata Sánchez

MNRAS000,1–7(2017) Preprint22February2017 CompiledusingMNRASLATEXstylefilev3.0
Swift and SALT observations of the Multiple Outbursts of
MAXI J1957+032
D. Mata S´anchez1,2⋆, P.A. Charles1,3, M. Armas Padilla1,2, D.A.H. Buckley4,
G.L. Israel5, M. Linares1,2, T. Mun˜oz-Darias1,2
1Institutode Astrof´ısica de Canarias (IAC), E-38205 La Laguna, Tenerife,Spain
2Departamento de astrof´ısica, Univ.de La Laguna, E-38206 La Laguna, Tenerife,Spain
3Department of Physics & Astronomy, Universityof Southampton, Southampton SO17 1BJ, UK
4South African Astronomical Observatory, PO Box 9, 7935 Observatory,Cape Town, South Africa
5INAR-OsservatorioAstronomico di Roma, viaFrascati 33, I-00040 Monteporzio Catone, Italy
7
1 Accepted2017February21.Received2017February20;inoriginalform2017January12
0
2

b ABSTRACT
e The new recurrent X-ray transient MAXI J1957+032 has had four X-ray outbursts
F within 16 months, all very briefly detected (lasting <5 days). During the most re-
cent event (Sep/Oct 2016), we obtained with SALT the first optical spectrum of the
1
2 transient counterpart, showing the classic blue continuum of an X-ray irradiated disc
o
in an LMXB and no other features. At high Galactic latitude below the plane (-13 )

] reddeningislowbutthereisnoquiescentcounterpartvisibleonanyoftheexistingsky
E
surveys,nor any other known X-raysourcein the region.Swift monitoring of 3 of the
H 4eventsispresented,showingrapidlyfadingX-rayoutburststogetherwithsignificant
. UVOTdetectionsinthe UV(W1,M2,W2),UandBbands.Theopticalpropertiesare
h
most like those of the short-period LMXBs, which, combined with the softening wit-
p
nessedduringthedecaytoquiescencewouldplacethesystematd<13kpc.Theshort
-
o duration and short recurrence time of the outbursts are reminiscent of the AMXPs,
r which exhibit peak luminosities of ∼ 1%LEdd. Assuming this peak luminosity would
t
s place MAXI J1957+032at a distance of d∼5−6kpc.
a
[ Key words: accretion, accretion discs – X-rays: binaries

2
v
1
8 1 INTRODUCTION shared between the INTEGRAL, Swift and MAXI mis-
3
sions. Thanks to their wide field and all-sky monitor-
3 The highly variable X-ray sky is populated with X-ray
ing programs (Kuulkerset al. 2007, Krimm et al. 2013 and
0 transients (XRTs). These are usually classified as objects
Matsuokaet al.2009,respectively),thelastdecadehasseen
1. typically in quiescence with X-ray luminosities well be- the emergence of a number of new XRTs (Swift J1753.5-
0 low 1033ergs−1, which increase by several orders of mag- 0127,MAXIJ1659-152,SwiftJ1357.2-093313,MAXIJ1305-
7 nitude during outbursts. Within our Galaxy, luminous X-
704) at high Galactic latitude and subsequently found to
1 ray transient behaviouris demonstrated by both high-mass
haveshortperiods(≤5h).Combinedwiththealreadywell-
: and low-mass X-ray binaries (HMXBs and LMXBs respec-
v known XRTs GRO J0422+32 and XTE J1118+480, this
tively), where the former are predominantly BeX systems
i suggested the existence of a possible sub-group of LMXB
X involving neutron stars in long (tens of days or more) ec-
XRTs,whose high latitude(andconsequentlow extinction)
r centric orbits around a rapidly rotating Be star (e.g. Reig
made them important targets for multi-wavelength studies
a 2011). As we will show, the properties of J1957 are consis-
(seee.g.Shaw et al.2013).Furthermore,sincemanyofthese
tent with that of LMXB transients, which contain either a
XRTs are too faint in quiescence for current telescopes (es-
black hole (BH) or neutron star (NS) in a close (typically
pecially the short period systems which tend to have the
Porb ∼hours)circularorbitwith alow-mass, evolveddonor lowestmassdonors),itisveryimportanttoundertakerapid
(Remillard & McClintock 2006, Liu et al. 2007).
follow-up optical spectroscopy while the X-ray outburst is
Since the demise of the RXTE all-sky monitor
underway,asitcanthenbepossibletoexploittheX-rayir-
(ASM) in 2012, the X-ray sky monitoring task has been
radiation of thedonorto provideconstraints on themotion
of thedonor (see e.g. Cornelisse 2013).
The high-latitude XRT MAXI J1957+032 (here-
⋆ E-mail:[email protected] after “J1957”) was discovered in 2015, independently
(cid:13)c 2017TheAuthors
2 Mata S´anchez et al.
by both MAXI (Negoro et al. 2015) and INTE- usingXselect. We used a circular region of ∼40 arcsec ra-
GRAL (Cherepashchuket al. 2015, who designated diuscentredatthesourceposition(theinner10arcsecwere
the source IGR J19566+0326), and optically identi- excludedfor00033770019 observation,57662.75,affectedby
fied (Rau,Yates& Greiner 2015) with a faint r′ ∼ 20 pile-up). For the PC observations, three circular regions of
star, there being no counterpart on sky-survey images similar size and shape, positioned on an empty sky region,
(Chakrabarty et al. 2016). However, that and the subse- was used for the background. On the other hand, an an-
quent two outbursts over the next year (Sugimoto et al. nulus centred on the source with ∼82 pixels for the inner
2015, Tanakaet al. 20161), decayed rapidly (within a radius and ∼118 pixels for the outer radius was used for
matter of days), making extensive follow-up study almost the background of the WT observations. We created expo-
impossible. Nevertheless,Marchesini et al. (2015) described suremapsandancillaryresponsefilesfollowingthestandard
a low-resolution (∼17˚A) optical spectrum of J1957 taken Swift analysis threads2, and we acquired the last version
duringtheOct 2015 outburst (whereGuveret al. 2015 had of the response matrix files from the High Energy Astro-
reported R=18.3 four days after the start of the outburst), physicsScienceArchiveResearch Center(HEASARC)cal-
whichcontainedonlyabsorptionfeaturesconsistentwithan ibration database (CALDB). Finally, we grouped the spec-
F/G-type star, and no emission lines. This was surprising, tra to have a minimum of 20 photons per bin to be able
as virtually all LMXB XRTs in outburst have optical to consistently use the χ2. However, due to the low num-
spectrathataredominatedentirely bytheX-rayirradiated berofcountscollectedduringvariousobservations,wefitted
accretiondisc,producingastrongbluecontinuumonwhich these data using both C-statistic and χ2 but grouped them
is usually superposed strong Balmer, HeII and Bowen withaminimumof5photonsperbin(seeTable1).There-
emission features (see e.g. Charles & Coe 2006). sults using both methods were consistent with each other
Furthermore,tohave3X-rayoutburstswithinayearis (e.g; Wijnands & Wang 2002; Armas Padilla et al. 2013).
unusualamongsttheclassofLMXBXRTs;so,whenanew, We fitted the spectra using xspec (v.12.9, Arnaud 1996).
brighterX-rayoutburstofMAXIJ1957+032wasreportedin Allobservations were well-fit with asimple power law (PL)
Sep2016(Negoro et al.2016),weimmediatelyinitiatedour plus absorbing column (PHABS in xspec) assuming the
SALT Transients ToO program (Buckleyet al. 2016) in or- cross-sectionsofVerneret al.(1996)andtheabundancesof
dertoobtainfurtheropticalspectraofthisintriguingobject. Wilms et al.(2000).Weassumedaconstanthydrogenequiv-
Those data, together with the associated Swift monitoring alentcolumndensityofNH =1.7×1021 cm−2,inferredfrom
of J1957, are thesubject of this paper. ourhighest signal-to-noisespectrum(Fig. 2)andconsistent
withthelowreddeningexpectedforthatdirection(seenext
section).Thephoton-indexofeachspectrum’sfitvariesfrom
1.90±0.02to2.34+0.27 (from highesttolowest luminosity).
2 OBSERVATIONS −0.25
Theunabsorbedfluxesare reported in Fig. 1. Wenotethat
Duetotheshort durationandfaintness ofJ1957 outbursts, observation00033770018 (57661.09) consistsoftwo∼350s-
both MAXI/GSC (2-20 keV) and Swift/BAT (15-50 keV) longgroupsofdetections∼13hapart(similartothegapbe-
light curves do not exhibit any remarkable feature (e.g. re- tweenthefirstgroupandthepreviousobservation,∼14h),
brightening)otherthanthereportedstartingtimesofthe4 so we decided to plot two independent points in the light
events.Thesecorrespondto57153.58(2015-05-11,epoch1), curve.
57301.52(2015-10-06,epoch2),57394.35(2016-01-07,epoch FortheobservationswhereJ1957isnotdetected,wede-
3)and57660.35 (2016-09-29, epoch4);3ofthemmarkedin terminedthe95percentconfidenceupperlimit countrates
Fig. 1. usingtheprescriptiongivenbyGehrels(1986).Weestimated
the corresponding unabsorbed flux upper limits using the
WebPIMMS HEASARC tool. We co-added 10 Swift/XRT
2.1 Swift XRT (PCmode)observations(11.7ksec),inordertoconstrainthe
quiescent luminosity of MAXI J1957+032. We can thereby
2.1.1 Spectral analysis placea95%confidenceupperlimitonthe0.5–10keVquies-
centunabsorbedfluxof1−3×10−13ergcm−2s−1 (assuming
Following the outburst alerts the X-ray Telescope (XRT;
Burrows et al. 2005) on board Swift (Gehrels et al. 2004) NH =1.7×1021cm−2 and a power law photon index in the
1–3range).Comparing withtheepoch 4detectionat aflux
covered 3 of the 4 reported events. A total of 28 obser-
of∼10−9ergcm−2s−1,thisrevealsthat theX-rayluminos-
vations (2 in window timing [WT] mode and the rest in
ity droppedby at least 4 orders of magnitudein just about
photon counting [PC] mode) were performed but, due to
a week.
the outburst rapid decline, J1957 is detected in only 10 of
these observations. We processed the data making use of
theHEASoftv.6.18software.Thedatareductionwascarried
2.1.2 Timing Analysis
out running the xrtpipeline task in which standard event
grades of 0-12 and 0-2 were selected for the PC and WT We searched for pulsations and quasi-periodic oscillations
mode observations, respectively. For each observation the (QPOs)inthethreeSwift/XRTobservationswithwindowed
0.5–10 keV spectrum, light curve and image were obtained timing (WT) mode data (1.78 ms time resolution). These
were takenon 57160 (2015-05-18, 1.3-ksec exposure),57660
(2016-09-29, 1 ksec) and 57661 (2016-09-30, 0.8 ksec). We
1 Note that the 2-20 keV fluxes quoted for Jan 7 and 8 are in-
correct. They should have been 0.066 and 0.071 phcm−2s−1 re-
spectively 2 http://www.swift.ac.uk/analysis/xrt/
MNRAS000,1–7(2017)
Multiple Outbursts of MAXI J1957+032 3
Table1.SwiftobservationslogandspectralresultsforJ1957usingaPHABS*POmodel.WeassumedaconstantNHof1.7×1021cm−2,
inferredfromthehighestsignal-to-noisespectrum(00033770017).
ID MJD Mode Exposure Γ unabs,Fx (0.5-10keV) χ2ν (dof)
[ks] [ergcm−2 s−1 ]
00033770001 57155.96 PC 2.9 2.13±0.07 (1.55± 0.08)×10−11 0.97(167)
00033776001 57156.42 PC 0.44 1.5±0.6 (8.5+6.2 )×10−12 0.36(5)
−3.7
00033777001 57156.43 PC 0.45 2.5±0.8 (5.2+2.6)×10−12 0.91(6)
−1.6
00033770009 57304.67 PC 0.96 2.0±0.1 (1.61± 0.16)×10−11 1.11(67)
00033770010 57305.61 PC 0.98 2.6±0.9 (1.5+0.7)×10−12 1.54(5)
−0.5
00033770017 57660.69 WT 0.97 1.90±0.02 (9.13± 0.13)×10−10 1.02(364)b
00033770018a 57661.09 WT 0.77 1.92±0.02 (7.09± 0.12)×10−10 1.04(304)b
00033770018-1a 57661.09 WT 0.45 1.92±0.03 (8.65± 0.18)×10−10 1.18(252)b
00033770018-2a 57661.62 WT 0.33 1.93±0.04 (5.53± 0.18)×10−10 0.89(153)b
00033770019 57662.75 PC 0.37 1.89±0.2 (1.58±0.22)×10−10 0.84(80)
00033770020 57663.08 PC 1.7 1.94±0.07 (2.80±0.17)×10−11 1.22(164)
00033770021 57664.14 PC 1.7 2.3±0.3 (3.03± 0.49)×10−12 0.97(27)
aObservation00033770018 wasdividedintwosub-events ofdetections ∼13hapart(seeSection2.1.1).
bSpectrawithaminimumof20photons perbin.Therestaregroupedwithaminimumof5photons perbin
(seeSection2.1.1).
Figure 2. Unfolded spectra of observation 00033770017
(57660.69, 2016-09-29). The solid line represents the best-fitting
Figure 1.Upperpanel:DetailedSwift/XRT(0.5–10keV)mon- using a PHABS*POWERLAW model. The sub-panel shows the
itoring of J1957 events 1, 2 and 4. The most detailed coverage data-to-modelratio.Datahavebeenre-binnedinXSPECforclar-
is of event 4, where we also plot exponential decays of e-folding ity.
timescales2.1±0.9and0.44±0.15daystotheslowandfastde-
clines respectively. Empty circles refer to WT mode and filled
circles to PC mode observations. The start times of the cov- FFT length and exact frequency range. We note that, con-
eredoutburstsaremarkedbyverticaldashedlines.Lowerpanel: sideringthepulsedfractionsandspinfrequenciesofAMXPs
Swift/UVOT(B,U,UW1,UM2andUW2filters)follow-uppoint- (e.g.Patruno& Watts2012,andreferencestherein),thisre-
ingsareshown. sult does not exclude the presence of similar pulsations in
J1957.
used 30-s and 120-s-long Fast Fourier Transforms (FFTs),
including energies between 1 and 10 keV, keeping the orig-
2.2 Swift/UVOT
inal time resolution and inspecting individual FFTs as well
as observation averages. We detect neither coherent pulsa- Therewerealsooptical/UVobservationsofJ1957associated
tions nor aperiodic variability in the 0.1–281 Hz frequency with many of the XRT pointings, and these were accessed
range. The most constraining upper limits come from the from theSwift online archive. We used theuvotsource soft-
57660 observation, when the average 1–10 keV count rate ware(releasedaspartoftheHEASOFTpackage)toextract
was∼16.5c/s.FollowingVaughanet al.(1994),weestimate fluxesforJ1957usinga5arcsecaperturecenteredatthepo-
upperlimitsonthepulsedfractionofasinusoidalsignal.We sitionreportedinRau et al.(2015).J1957 wasnotdetected
findupperlimitsonthe0.1–281 Hzpulsedfraction between in the V band, our best constraint being V>17.85 (MJD
2.2% and 5.4% (95% confidence level), depending on the 57665.5). At different times it was seen in all other bands,
MNRAS000,1–7(2017)
4 Mata S´anchez et al.
Figure 3.Leftpanel:PartoftheSALTICAMr’bandimage(left)oftheregionofJ1957(markedwith“X”andablackcircle)showing
theorientation oftheRSSlongslit(redbox) andthefieldstar“F”that wasincluded. Theblackboxisblownup(right)soastoshow
the locationof J1957 andits nearby starsmarked“1”and“2”,circledinred.Rightpanel: RSSlow-resolutionspectraof J1957 (upper)
andthesimultaneouslyobservedbrighterfieldstar(“F”)whosefluxhasbeenreducedbyx7tofacilitatecomparison.“F”isalate-Gstar
intheUSNOcatalogue (seetext)andthekeyspectralfeaturesaremarkedasgreen-dashedlines.Telluricfeatures aremarkedwiththe
symbol ⊕ and sky subtraction residuals as red-shadowed bands. There are no cosmicfeatures in J1957, but it is noticeably bluer than
“F”.Colorfiguresareavailableintheonlineversion.
with the brightest detections being B=18.5 ± 0.3 (MJD (2.0 arcsec to the NE of J1957). These data, together with
57664.1), U=18.7±0.3 (MJD 57665.5), UVW1=18.5±0.3, associated arcsandbiasframeswereprocessed andreduced
UVM2=18.3±0.2 and UVW2=18.1±0.2 (MJD 57663.1). withthestandardSALTpipeline(Crawford et al.2010),fol-
All values are Vega magnitude and were obtained during lowed by cosmic-ray removal using the IRAF3 task“L. A.
epoch 4. Usingtherelation NH =2.0±0.5×1021 cm−2AV Cosmic”(vanDokkum2001)on the2D image. Wethenex-
(Watson 2011) and the Galactic hydrogen column den- tractedthespectrumofJ1957andotherstarswithintheslit
sity NH = 0.95×1021 cm−2 (which yields EB−V = 0.14, using standard IRAF routines. Wavelength calibration was
Willingale et al. 2013); we obtain AV ∼ 0.5 and AB ∼ 0.6. performed using arc spectra with the MOLLY routines arc
This implies the brightest B-band detection corresponds to and acal, and theresults are in Fig. 3.
an intrinsic value of B = 17.9±0.3. We note that using a The spectrum of J1957 is essentially featureless (apart
5 arcsec aperture includes star“1”(see Fig. 3), classified as from telluriclines),notablysoincomparison withfieldstar
anA-Fstar(R=18.3).Itmakesasmallcontributioninthe “F”,whichwassimultaneouslyintheslit.ThisstarisUSNO-
Bband(B>19),hasnoeffectintheUV-bands,butaffects B1.0 0935-0543890, of observed spectral type G5-K0 and
V-bandobservationswhich will bedisregarded. with B=18.3, B-R=1.2, which we have used with our spec-
tra to estimate B-V=0.3 for J1957 (significantly bluer than
“F”in our spectrum for which we obtain B-V=0.7). X-ray
2.3 SALT Imaging and Spectroscopy activeLMXBshaveopticalspectradominatedbytheX-ray
irradiateddisc,andoftenhavestrongBalmerandHeIIlines
We obtained optical imaging of J1957 with the Southern in their spectra. However, the more compact systems, es-
African Large Telescope (SALT, Buckleyet al. 2006) on pecially those seen at high inclination, have much weaker
57662.76 (2016-10-01, epoch4),2.4 dafter theinitial X-ray emission lines, often almost completely undetectable (e.g.
outburst report (Negoro et al. 2016). We used SALTICAM Baglio et al. 2016). Indeed,ultracompact systems accretion
(O’Donoghueet al. 2006) to obtain a 60 s r’ band image discs should be depleted of hydrogen, as the companion
(Fig.3),whichshowedthepreviouslyidentifiedopticalcoun- star is itself degenerate (see Nelemans & Jonker 2010). We
terpart (Guveret al. 2015; Marchesini et al. 2015) to be at constrain the equivalent width of the Hα line to be lower
r’ = 18.5±0.15, similar to the R=18.27 reported 4.2d after than 3˚A at 3σ confidence level (assuming a conservative
theOctober2015outburst(Guveret al.2015),andbrighter FWHM<4400˚AdeterminedbythewidestHαprofilemea-
than the r’ = 20.03 measured 3.7 after the 2015 May out- sured so far in a LMXB, see Mata S´anchezet al. 2015).
burst (Rau et al. 2015). Asalreadyreported(Buckley et al.2016),ourspectrum
Withthisclearindicationofrenewedopticalactivity,we contrastswiththeA-Fstarabsorption featuresreportedby
thenimmediately followed this with a 1000s exposurespec-
trum using the SALT Robert Stobie Spectrograph (RSS).
This was taken with the 300 l/mm grating (mean resolving 3 IRAF is distributed by National Optical Astronomy Observa-
power of 320) covering 360-770nm and through a 1.5 arc- tories,operatedbytheAssociationofUniversitiesforResearchin
secslit(thePSFwas1.3arcsecFWHM),orientedasshown Astronomy,Inc.,undercontractwiththeNationalScienceFoun-
in Fig. 3 so as to exclude light from the nearby star “1” dation.
MNRAS000,1–7(2017)
Multiple Outbursts of MAXI J1957+032 5
Marchesini et al.(2015)usingtheNordicOpticalTelescope as well as brighter LMXBs. However, none of these have a
(andALFOSC)atasimilarresolution 8.2daftertheepoch recurrencetime as short as J1957.
2 outburst (2015 October). We re-analysed the ALFOSC Perhaps the most comprehensive monitoring campaign
data of this epoch, and from a 250 s image taken immedi- on Galactic X-ray transients took place in the period 1996-
ately after the spectrum, obtained R=21.4±0.2 for J1957. 2000 when theBeppoSAXWideField Cameras (which had
Thisledtotherealisation thatthereportedNOTspectrum a40o fieldofview)observedtheGalacticCentreregionnine
was not of J1957, but rather of star“1”, 2.0 arcsec NE of times for a total exposure of ∼ 4Ms. As summarised by
J1957. We measured star“1”to have r’ = 18.0±0.15 and in’t Zand (2001), the combination of the BeppoSAX WFC
R=18.3±0.1, respectively,from theSALTICAMandAL- observations and the RXTE/ASM database led to the dis-
FOSCimagestakenayearapart,andisconsistentwiththe coveryof31LMXBXRTsthatwereactiveduringthistime
fluxesmeasured in theNOTspectrum.All of thephotome- interval. From this study (see their Table 2) there are 4
try reported hereused thesame reference star, USNOB1.0 XRTs whose duration and peak X-ray flux are compara-
0934-0560330. ble to J1957. These are KS 1741-293, SAX J1748.9-2021,
SAXJ1750.8-2900 andSAXJ1810.8-2609, allofwhoseout-
burstslastedlessthan10days.However,subsequentINTE-
GRAL observations of the former (DeCesare et al. 2007)
3 DISCUSSION
showed more extended periods of activity, and the latter
Havingresolvedthequestionofthespectroscopicidentifica- two (Natalucci et al. 1999, 2000) both displayed Type I X-
tion of J1957, and demonstrated that its optical spectrum ray bursts with close to Eddington-limited properties. De-
is consistent with other LMXB XRTs in their active phase, spite J1957 not exhibiting any of those features, we note
the Swift/XRT light curve highlighted the extremely rapid thateitherwecouldhavemissedtheignitionofahypothet-
decay of the outburst (and which Fig. 1 shows was likely ical burst or maybe its outbursts are too short to trigger
the case for all the outbursts). The essential properties of it. The remaining system SAX J1748.9-2021 is an AMXP
this object are straightforward to summarise: (i) outburst (accreting millisecond X-ray pulsar) spinning at 442Hz in
light curves that increase rapidly (within a MAXI orbit we an 8.8hr binary, and while it has recurrence times of order
change from non-detection to the peak luminosity) and de- a year, the duration (of 8 days) was so similar in timescale
cay steeply, lasting < 5d in total; (ii) no detected rapid andlight-curvetoJ1957thatwedecidedtoconductacloser
variability or typeI X-rayburstsduringtheirbright phase; comparison with such objects.
(iii) have an associated optical brightening from quiescence
of ≥3 mags; (iv) hard X-ray spectrum (not that of a disc
3.2 Is J1957 an AMXP?
black-body).Withthesepropertiesathand,wecanruleout
thefollowing classifications for J1957: The most extensive review of AMXPs is that of
Patruno& Watts (2012), in which we found N6440 X-2 to
• HMXB/BeX system: the outbursts of J1957 have al-
beverysimilartoJ1957bothinrecurrencetime(∼1month,
ready been noted to be non-periodic (Tanakaet al. 2016),
theshortest recurrence time of any known XRT) and dura-
andthelargerangeofopticalvariability(combinedwithour
tion (3–5 d) of its outbursts (Heinkeet al. 2010). The peak
SALTspectrum)fromquiescencerulesoutthepossibilityof
luminosities of the AMXPs are typically around 1%L
an•easrulpy-etrybpuersdteorn:ort.;he longest “superburst” is ∼ 0.3d (∼1036ergs−1 fora1.4M⊙ NS),whichwouldplaceJ1957Eaddt
d∼6kpc.Indeed,comparingdirectlywithN6440X-2places
(in ’t Zand 2011), much less than the 5 days of each J1957
J1957 atd∼5kpc.Anotherkeysimilarity hereisthelight-
outburst, and there would be enormous theoretical difficul-
curve shape of at least two AMXP (SAX J1808.4-3658 and
tiesinproducingsufficientnuclearfuelforsuchasuperburst;
IGR J00291+5934) during their (short, 1–2 weeks) periods
of activity (see Fig. 1). They are characterised by an initial
slowdecayfromthepeakluminositythatendswithasharp
3.1 LMXB/XRT: comparison with Galactic
drop (see Hartman et al. 2008, Hartman et al. 2011a). The
Centre Region X-ray Transients
kneeofthelightcurveatacriticalfluxhasbeeninterpreted
In spite of the extensive RXTE/ASM database of XRT either as centrifugal inhibition of accretion or as the prop-
light-curves (see e.g. Yan & Yu 2015) there are no BH agation of a cooling wave in the disc (Gilfanov et al. 1998).
LMXB/XRT which display properties comparable to those Assumingthat these phenomenatakeplace at thesame lu-
of J1957. Within that same database, only the NS system minositylevel,we inferadistancetoJ1957 ofd∼5−6kpc
MXB 1730-335 (better known as the“Rapid Burster”) has (using d =3.5kpc for SAX J1808.4-3658 and d =4kpc for
a long-term light-curve similar to that of J1957. Indeed, in IGR J00291+5934).
thelatter phases of theRXTE mission, MXB 1730-335 un- There are three ultracompact AMXPs (Porb < 1.3h)
derwent 6 outbursts in barely 2 years, each of which had known to have exhibited short (below 2 weeks) out-
a bright phase of usually <10 days. However, in its bright bursts: N6440 X-2 (Altamirano et al. 2010), XTE J1751-
phase, this object emits most of its flux in theform of very 305 (Grebenev et al. 2005, Patruno& Watts 2012) and
rapid,typeIIX-raybursts(e.g. Mahasena et al.2003) that Swift J1756.9-2508 (Patrunoet al. 2010); which favours
last only for 10s or so. J1957 exhibitsno suchvariations. the association of short outbursts with short orbital pe-
Morerecently,Degenaar et al.(2015)reporteda9years riod LMXBs. Nevertheless, we note that at least 3 other
Swift X-ray monitoring of the Galactic centre. During this AMXPs with longer periods that have exhibited similarly
campaign, various short outbursts (∼ 15 days long) were short outbursts:: IGR J00291+5934 (Hartmanet al. 2011b;
detectedpeakingatLX ∼1035ergs−1,bothfrom veryfaint Porb = 2.5h reported by Markwardt et al. 2004), Swift
MNRAS000,1–7(2017)
6 Mata S´anchez et al.
J1749.4-2807 (Porb = 8.82h, see Wijnands et al. 2009 and
Altamirano et al. 2011) and SAX J1748.9-2921 (Porb =
8.77h, Patrunoet al. 2009). On the other hand, there are
short period AMXPs like XTE J0929-314 (Remillard et al.
2002) and XTE J1807-294 (Markwardt et al. 2003) which
exhibitedoutburstslastingseveralmonths.Despitethefact
thattheseexampleshighlighttheinexactnessofthecorrela-
tion between short outbursts and short orbital periods, we
proposeJ1957asashortperiodLMXBduetothecombina-
tion of short duration outbursts, high recurrence time and
featureless optical spectrum.
Finally, assuming that the optical emission during ac-
tive periods is dominated by an X-ray irradiated accretion
disc, then we can use the X-ray/optical correlation equa-
tion of van Paradijs & McClintock (1994): M = 1.57 −
V
2.27 logΣ, where Σ = (LX/LEdd)1/2(Porb[h])2/3 to derive
an M for J1957 of 3.4, assuming a 2h period. With our
V
brightest B magnitude (see Sec. 2), this would imply a dis- Figure4.HardnessratiofortheXRTdetectionsinthelastout-
tance of about ∼ 7−9kpc, somewhat closer if the period burst, defined as the ratio between (2-10 keV) and (0.5-2 keV)
were shorter. fluxes.Thelargesterrorpointcorrespondstotheobservationaf-
fectedbypile-up(seeSec.2).
3.3 The X-ray spectrum ACKNOWLEDGEMENTS
Hysteresis patterns between Compton-dominated and DMS acknowledges Fundacio´n La Caixa for the financial
thermal-dominated states have been observed in support received in the form of a PhD contract. PAC is
both BH and NS LMXBs (e.g. Belloni et al. 2011; grateful for the support of a Severo Ochoa Visiting Re-
Mun˜oz-Darias et al. 2014). These hysteresis phases, which search Fellowship and the hospitality of the Instituto de
are not witnessed in J1957, are observed at luminosities Astrof´ısica de Canarias, where the work reported here was
larger than ∼ 2%Ledd (see also Maccarone 2003). On the undertaken. We also acknowledge support by the Spanish
other hand, a softening phase during the decay towards Ministerio de Econom´ıa y competitividad (MINECO) un-
quiescence is detected at lower luminosities (Plotkin et al. der grant AYA2013-42627. TMD acknowledges support via
2013, Wijnands et al. 2015). In J1957 we do observe a a Ram´on y Cajal Fellowship (RYC-2015-18148). DAHBac-
softening in the hardness ratio as the luminosity decreases knowledges support by the National Research Foundation
by,at least, a factor of ∼50 (see fig. 4). andalsothanksM.KotzeforsupportingtheSALTobserva-
Indeed, if the softening of the source begins at 1.6× tions. MOLLY software developed by T. R. Marsh is grate-
10−10ergcm−2s−1, given that Wijnandset al. (2015) found fully acknowledged. This research has made use of MAXI
this event always below 3×1036ergs−1, which would place data provided by RIKEN,JAXA and the MAXI team. We
J1957atd<13kpc.Ontheotherhand,thenon-detectionin alsoacknowledgetheuseofpublicdatafromtheSwift data
quiescenceofJ1957 intheUSNO-Bcatalogue(Monet et al. archive. Some of the observations reported in this paper
2003),whoselimitingmagnitudeinVis∼21,impliesadrop were obtained with the Southern African Large Telescope
in luminosity ofmore thanthreemagnitudesin theoptical. (SALT).
If the donor is a dwarf star at a distance d < 13kpc, its
absolute magnitude is constrained to MV >5.4. Using Cox
(2000) tabulated values, thisresults in aspectral typelater
REFERENCES
than G7 (K6if d∼5kpc).
AltamiranoD.,etal.,2010,ApJ,712,L58
AltamiranoD.,etal.,2011,ApJ,727,L18
Armas Padilla M., Degenaar N., Russell D. M., Wijnands R.,
4 CONCLUSIONS 2013,MNRAS,428,3083
ArnaudK.,1996,inJacobyG.,BarnesJ.,eds,AstronomicalSo-
Our X-ray and optical analysis suggests that MAXI ciety of the Pacific Conference Series Vol. 101, Astronomical
J1957+032 is a short period LMXB, given the short du- DataAnalysisSoftwareandSystemsV.p.17
ration and frequent recurrence of its outbursts, as well as BaglioM.C.,D’AvanzoP.,CampanaS.,GoldoniP.,MasettiN.,
thefeaturelessopticalspectrum.Thespectralanalysisleads Mun˜oz-Darias T., Patin˜o-A´lvarez V., Chavushyan V., 2016,
us to place the system at d<13kpc, which combined with A&A,587,A102
Belloni T. M., Motta S. E., Mun˜oz-Darias T., 2011, Bulletin of
the non-detection of the quiescence counterpart results in
theAstronomicalSocietyofIndia,39,409
a donor star spectral type later than G8. After comparing
BuckleyD.A.H.,SwartG.P.,MeiringJ.G.,2006,inSocietyof
with the properties of other known XRTs, we find similari-
Photo-OpticalInstrumentationEngineers(SPIE)Conference
tieswiththoseoftheAMXPclass,whichwouldplaceJ1957
Series.p.62670Z, doi:10.1117/12.673750
at d∼5kpc.Furthermulti-wavelengthobservationsareen- Buckley D. A. H., Kotze M. M., Charles P. A., Sanchez D. M.,
couragedespeciallytakingintoaccounttheshortrecurrence Munoz-Darias T., Israel G., Masetti E. J. M. N., Jonker P.,
times thissystem has exhibitedin thepast ∼1.5 years. 2016,TheAstronomer’sTelegram,9649
MNRAS000,1–7(2017)
Multiple Outbursts of MAXI J1957+032 7
BurrowsD.,etal.,2005,SpaceSci.Rev.,120,165 ShawA.W.,etal.,2013,MNRAS,433,740
Chakrabarty D., Jonker P. G., Markwardt C. B., 2016, The As- SugimotoJ.,etal.,2015,TheAstronomer’sTelegram,8143
tronomer’sTelegram,9591 TanakaK.,etal.,2016,TheAstronomer’sTelegram,8529
CharlesP. A.,CoeM.J.,2006, Optical, ultravioletandinfrared VaughanB.A.,etal.,1994,ApJ,435,362
observations ofX-raybinaries.pp215–265 VernerD.,FerlandG.,KoristaK.,YakovlevD.,1996, ApJ,465,
Cherepashchuk A. M., Molkov S. V., Lutovinov A. A., Postnov 487
K.A.,2015, TheAstronomer’sTelegram,7506 WatsonD.,2011,A&A,533,A16
CornelisseR.,2013,inPuglieseG.,deKoterA.,WijburgM.,eds, Wijnands R., Wang Q. D., 2002,
AstronomicalSocietyofthePacificConferenceSeriesVol.470, TheAstrophysicalJournal,Volume568,Issue2,pp.L93-L96.,
370YearsofAstronomyinUtrecht.p.263 568,L93
CoxA.N.,2000,Allen’sastrophysicalquantities Wijnands R., Rol E., Cackett E., Starling R. L. C., Remillard
Crawford S. M., et al., 2010, in Observatory Opera- R.A.,2009,MNRAS,393,126
tions: Strategies, Processes, and Systems III. p. 773725, Wijnands R., Degenaar N., Armas Padilla M., Altamirano D.,
doi:10.1117/12.857000 Cavecchi Y.,LinaresM.,BahramianA.,HeinkeC.O.,2015,
DeCesareG.,BazzanoA.,Mart´ınezNu´n˜ezS.,StrattaG.,Tarana MNRAS,454,1371
A.,DelSantoM.,UbertiniP.,2007,MNRAS,380,615 WillingaleR.,StarlingR.L.C.,BeardmoreA.P.,Tanvir N.R.,
DegenaarN.,WijnandsR.,MillerJ.M.,ReynoldsM.T.,Kennea O’BrienP.T.,2013,MNRAS,431,394
J., Gehrels N., 2015, JournalofHighEnergyAstrophysics, WilmsJ.,AllenA.,McCrayR.,2000,ApJ,542,914
7,137 YanZ.,YuW.,2015,ApJ,805,87
GehrelsN.,etal.,2004,ApJ,611,1005 in’tZandJ.,2011,preprint, (arXiv:1102.3345)
Gilfanov M., Revnivtsev M., Sunyaev R., Churazov E., 1998, in’t Zand J., 2001, in Gimenez A., Reglero V., Winkler C., eds,
A&A,338,L83 ESASpecialPublicationVol.459,ExploringtheGamma-Ray
Grebenev S. A., Molkov S. V., Sunyaev R. A., 2005, The As- Universe.pp463–470(arXiv:astro-ph/0104299)
tronomer’sTelegram,446 vanDokkumP.G.,2001,PASP,113,1420
GuverT.,etal.,2015,TheAstronomer’sTelegram,8149 vanParadijsJ.,McClintockJ.E.,1994,A&A,290,133
HartmanJ.M.,etal.,2008,ApJ,675,1468
Hartman J. M., Galloway D. K., Chakrabarty D., 2011a, ApJ, Thispaper has been typeset fromaTEX/LATEX fileprepared by
726,26
theauthor.
Hartman J. M., Galloway D. K., Chakrabarty D., 2011b, ApJ,
726,26
HeinkeC.O.,etal.,2010, ApJ,714,894
KrimmH.A.,etal.,2013,ApJS,209,14
KuulkersE.,etal.,2007,A&A,466,595
LiuQ.Z., vanParadijs J.,van denHeuvel E. P.J., 2007, A&A,
469,807
MaccaroneT.J.,2003, A&A,409,697
MahasenaP.,InoueH.,AsaiK.,DotaniT.,2003,PASJ,55,827
Marchesini E. J., Israel G. L., Masetti N., Rodriguez G., 2015,
TheAstronomer’sTelegram,8197
MarkwardtC.B.,JudaM.,SwankJ.H.,2003,IAUCirc.,8095
MarkwardtC.B.,GallowayD.K.,ChakrabartyD.,MorganE.H.,
StrohmayerT.E.,2004,TheAstronomer’sTelegram,360
Mata S´anchez D., Mun˜oz-Darias T., Casares J., Corral-Santana
J.M.,ShahbazT.,2015, MNRAS,454,2199
MatsuokaM.,etal.,2009,PASJ,61,999
MonetD.G.,etal.,2003,AJ,125,984
Mun˜oz-DariasT.,FenderR.P.,MottaS.E.,BelloniT.M.,2014,
MNRAS,443,3270
Natalucci L.,CornelisseR.,Bazzano A.,Cocchi M.,UbertiniP.,
HeiseJ.,in’tZandJ.J.M.,KuulkersE.,1999,ApJ,523,L45
Natalucci L., Bazzano A., Cocchi M., Ubertini P., Heise J., Ku-
ulkers E., in ’t Zand J. J. M., Smith M. J. S., 2000, ApJ,
536,891
NegoroH.,etal.,2015,TheAstronomer’sTelegram,7504
NegoroH.,etal.,2016,TheAstronomer’sTelegram,9565
NelemansG.,JonkerP.G.,2010,NewAstron.Rev.,54,87
O’DonoghueD.,etal.,2006,MNRAS,372,151
PatrunoA.,Watts A.L.,2012, preprint, (arXiv:1206.2727)
Patruno A., Altamirano D., Hessels J. W. T., Casella P., Wij-
nandsR.,vanderKlisM.,2009,ApJ,690,1856
Patruno A., Altamirano D., Messenger C., 2010, MNRAS,
403,1426
PlotkinR.M.,GalloE.,JonkerP.G.,2013,ApJ,773,59
RauA.,YatesR.,GreinerJ.,2015,TheAstronomer’sTelegram,
7524
ReigP.,2011, Ap&SS,332,1
RemillardR.A.,McClintockJ.E.,2006,ARA&A,44,49
RemillardR.A.,SwankJ.,StrohmayerT.,2002,IAUCirc.,7893
MNRAS000,1–7(2017)