Discovery of two new Fast X-ray Transients with INTEGRAL: IGR J03346+4414 and IGR J20344+3913
aa r X i v : . [ a s t r o - ph . H E ] A ug Mon. Not. R. Astron. Soc. , 000–000 (0000) Printed 21 March 2018 (MN L A TEX style file v1.4)
Discovery of two new Fast X-ray Transients withINTEGRAL: IGR J03346+4414 and IGR J20344+3913
V. Sguera , L. Sidoli , A. Paizis , A. J. Bird INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Gobetti 101, I-40129 Bologna, Italy INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via E. Bassini 15, I-20133 Milano, Italy School of Physics and Astronomy, University of Southampton, University Road, Southampton, SO17 1BJ, UK
Accepted 2016 August 26. In original form 2016 June 1
ABSTRACT
We report on the discovery of two Fast X-ray Transients (FXTs) from analysis ofarchival INTEGRAL data. Both are characterized by a remarkable hard X-ray activityabove 20 keV, in term of duration ( ∼
15 and 30 minutes, respectively), peak-flux ( ∼ − erg cm − s − ) and dynamic range ( ∼ − erg cm − s − . The main spectral and temporal IBIS/ISGRI characteristics arepresented and discussed with the aim of infering possible hints on their nature. Key words:
X-rays: transient – X-rays: individual: IGR J03346+4414 – X-rays:individual: IGR J20344+3913
Since its launch in 2002, the IBIS/ISGRI detector (Lebrunet al. 2003, Ubertini et al. 2003) on board the INTEGRALobservatory (Winkler et al. 2003) has repeatedly proven itssuitableness for the investigation of the hard X-ray tran-sient sky at energies above 20 keV. Thanks to its powerfulcombination of large field of view (29 ◦ × ◦ , partially codedat zero response), fine angular resolution (12 ′ ) and goodinstantaneous sensitivity ( ∼ σ level with anexposure of ∼ ∼ − erg cm − s − ) outburstsas detected by INTEGRAL. Their main spectral and tem-poral characteristics are presented and discussed with theaim of infering possible hints on their nature. INTEGRAL
For our study, we used data collected with the ISGRI detec-tor which is the lower energy layer of the IBIS coded masktelescope. The reduction and analysis of the data have beenperformed by using the Offline Scientific Analysis (OSA)version 10.1. INTEGRAL observations in each telescope or- c (cid:13) Sguera et al.
Table 1.
Log of IBIS/ISGRI observationstelescope orbit range time ScWs exp(n ◦ ) (Ms) Anticenter(l ∼ ◦ ) ∼ ∼ ∼ ∼ ∼ (l ∼ ◦ ) ∼ ∼ ∼ Cygnus ∼ ∼ ∼ ∼ ∼ ∼ ∼ ∼ ∼ bit ( ′′ revolution ′′ ) are divided into short pointings (ScienceWindow, ScW) having a typical duration of ∼ ∼ ◦ and l ∼ ◦ respectively, for a totalexposure of ∼ ∼ σ in a single ScW.The search was initially performed in the energy band 22–60 keV; this choice takes into account the evolution of theIBIS/ISGRI energy threshold that occurred from revolutionnumber ∼
900 on. When an interesting excess was found,we also checked the detection at higher energies (i.e. 60–100keV) or in other different ranges (i.e. 22–30, 30–60 and 22–40keV). The sensitivity limit for a persistent source detectedat 5 σ level (22–60 keV) in only one ScW of about 2000 s du-ration is ∼
18 mCrab (Krivonos et al. 2010). We note thatthis approach is efficient in unveiling FXTs with durationsas short as a few tens of minutes, since the search occurson the same timescale as their outburst activity themselves.Integrating for longer periods just degrades the signal-to-noise ratio of the detection below detectability. The result-ing excesses were carefully inspected to identify them as realsources. They were visually examined to ensure an appropri-ate point spread function and to reject false detections suchas ghosts. In order to rule out an artificial nature due tobackground noise or structures/artifacts, we inspected theoverall ScW images rms and checked the residuals maps. The absence of any known or found systematic effect givesus confidence about the real nature of the two newly discov-ered FXTs.A detailed timing and spectral analysis was performedfor each newly discovered FXT. Due to possible cross-talkbetween sources in the same Field of View (FoV), we havealso investigated the variability pattern of all other brightersources in the FoV, besides the two sources of interest. Thelatter have shown a different time variability, enabling usto conclude that their light curves are uncontaminated andreliable.The X-ray monitor JEM–X makes observations simul-taneously with IBIS/ISGRI, although with a much smallerFoV, providing images in the softer energy band 3–35keV. Unfortunately both the here reported newly discoveredFXTs were outside the JEM–X FoV.Through the paper, the spectral analysis was performedusing XSPEC version 12.9.0 and, unless stated otherwise,errors are quoted at the 90 per cent confidence level for onesingle parameter of interest.
We report the discovery of a new fast X-ray transientIGR J03346+4414 by analyzing IBIS/ISGRI data of theGalactic anticenter region in revolution number 1261. Thesource was detected at a significance of 7.2 σ (22–60 keV) ina single ScW (n. 10) starting at 9 Feb 2013 13:04 (UTC)and ending on the same day at 13:37 (UTC).As can be seen from the extracted 22–60 keV light curve(Fig. 1), the duration of the transient activity is only ∼ ∼
200 s) followedby a slower decay. At the peak, the source reached a flux of331 ±
34 mCrab ( ∼ × − erg − s − ). We point out thatthe duration of the transient activity ( ∼
15 min) is signifi-cantly shorter than that of the entire ScW number 10 whichcontain the detection of the source ( ∼
30 min). Bearing thisin mind, we performed an imaging analysis with the GTIby considering only the time interval of the transient activ-ity. By doing so, the source was detected at ∼ σ level (seeFig. 2) in the energy band 22–60 keV, while no detection wasobtained at higher energies 60–100 keV. We note that thesource was also significantly detected in both the softer (22–30 keV, 6.6 σ ) and harder (30–60 keV, 9.7 σ ) energy bands.The best position for the source is RA=53 ◦ .65(l=151 ◦ .25) and Dec=44 ◦ .24 (b=–9 ◦ .47) with a 90% con-fidence error circle radius equal to 2.1 arcminutes. No previ-ously known X-ray source is located inside this error cir-cle according to all the available X-ray catalogs in theHEASARC database. Following the source naming conven-tion, we named this newly discovered IBIS/ISGRI source asIGR J03346+4414.The IBIS/ISGRI spectrum extracted with the GTI(22–100 keV) is well fitted by a simple power law withΓ=1.99 ± .
35 ( χ ν =1.05, 9 d.o.f.) and 18–60 keV (20–40 keV)average flux of 1.9 × − erg cm − s − (1.1 × − erg cm − s − ). Fig. 3 shows the data-to-model fit with the correspond-ing residuals. Alternatively, we have also used a thermalmodel such as Bremsstrahlung which provides a good fit c (cid:13) , 000–000
500 1000 1500 2000 2500 3000 3500 4000 C oun t/ s e c Time (s)Start Time 16332 13:06:55:185 Stop Time 16332 14:13:35:185Bin time: 100.0 s
Figure 1.
IBIS/ISGRI 22–60 keV light curve (100 s bin time) ofthe newly discovered source IGR J03346+4414. For display pur-poses aimed at highlighting the transient nature of the source, thelight curve was extracted from the two consecutive ScW number10 and 11 (revolution 1261) although a source detection was ob-tained only from ScW number 10. : : . : . : . : . : : .
94 1 : : .
94 5 : : .
94 8 : : . Figure 2.
IBIS/ISGRI 22–60 keV significance image (ScW num-ber 10, revolution 1261) of the newly discovered source IGRJ03346+4414. It was detected with a significance of ∼ σ . no r m a li z ed c oun t s s − k e V − no r m a li z ed c oun t s s − k e V − Energy (keV)
Figure 3.
IBIS/ISGRI spectrum of IGR J03346+4414 (extractedwith the GTI from ScW number 10) fitted by a power law. Thelower panel shows the residuals from the fit.
Figure 4.
IBIS/ISGRI ScW image sequence (22–60 keV) fromnumber 25 to 27 (revolution 1614) of the newly discovered tran-sient source IGR J20344+3913 (encircled). It was detected in themiddle ScW with a significance of 6.6 σ . The source Cyg X–3 isalso detected in the field of view as a bright persistent source. C oun t/ s e c Time (s)Start Time 17351 1:48:27:186 Stop Time 17351 3:28:27:186Bin time: 300.0 s
Figure 5.
IBIS/ISGRI 22–60 keV light curve (300 s bin time)of the newly discovered source IGR J20344+3913. For displaypurposes aimed at highlighting the transient nature of the source,the light curve was extracted from the three consecutive ScWsnumber 25 to 27 (revolution 1614) although a source detectionwas obtained only in ScW number 26. no r m a li z ed c oun t s s − k e V − no r m a li z ed c oun t s s − k e V − Energy (keV)
Figure 6.
IBIS/ISGRI spectrum of IGR J20344+3913 extractedfrom Scw number 26 and fitted by a simple power law. The lowerpanel shows the residuals from the fit.c (cid:13) , 000–000
Sguera et al. : . : . : . : . : : . : . Figure 7. ( χ ν =0.9, 9 d.o.f.), however the temperature value is not wellconstrained (kT=70 +66 − keV).IGR J03346+4414 is not listed in the latest publishedIBIS/ISGRI catalog (Bird et al. 2016) despite extensive IN-TEGRAL coverage of its sky region ( ∼ ∼ σ upperlimit of ∼ × − erg cm − s − (20–40 keV)for persistent emission. When assuming the source peak fluxin the same energy band as measured by IBIS/ISGRI fromthe outburst reported here, we can infer a dynamic rangegreater than at least ∼ We report the discovery of a new fast X-ray transient foundby analyzing public IBIS/ISGRI data of the Cygnus regionin revolution number 1614. It was detected at about 6.6 σ level (22–60 keV) in a single ScW (n. 26) starting at 25Nov 2015 02:19 (UTC) and ending on the same day at02:52 (UTC). No detection is obtained at higher energies60–100 keV. We note that the source was barely detected inthe softer energy band 22–30 keV (4.3 σ ) while the signifi-cance was higher in the harder band 30–60 keV (5.5 σ ). Fig.4 shows the IBIS/ISGRI ScW significance image sequence(22–60 keV) from number 25 to 27.Fig. 5 displays the 22–60 keV light curve which illus-trates the fast flaring nature of the source. The durationof its total activity was about 0.5 hour, the source flared up reaching a peak flux of 138 ±
19 mCrab ( ∼ × − ergcm − s − ).The extracted IBIS/ISGRI spectrum (22–100 keV)is well fitted by a simple power law with Γ=2.85 +0 . − . ( χ ν =1.03, 9 d.o.f.) and 18–60 keV (20–40 keV) average fluxof 8.1 × − erg − s − (5.1 × − erg − s − ). Fig. 6 showsthe data-to-model fit with the corresponding residuals. Athermal Bremsstrahlung model provides a good fit as well,with χ ν =0.9 (9 d.o.f.) and kT=28.6 +24 . − . keV.The best source position is RA=308 ◦ .61 (l=78 ◦ .67) andDec=39 ◦ .22 (b=–0. ◦ .64) with a 90% confidence circle radiusequal to 3.8 arcminutes. No previously known X-ray source islocated inside this error circle according to all the availableX-ray catalogs in the HEASARC database. Following thesource naming convention, we named this newly discoveredIBIS/ISGRI source as IGR J20344+3913.In addition we note that, according to all the avail-able radio catalogs in the HEASARC database, the brightradio source GB6 J2034+3914 is located inside the IBISerror circle at a distance of 2 ′ .1. It is listed in the GreenBank radio catalog (Gregory et al. 1996) with a 6 cm flux of417 ±
39 mJy. Further away, two other bright radio sources(NVSS J203441+391441 and NVSS J203434+391617) listedin the NVSS radio catalog at 20 cm (Condon et al. 1998)are located inside the IBIS error circle at distance (flux) of3 ′ .3 (84 mJy) and 3 ′ .4 (70 mJy), respectively. Fig. 7 showsthe IBIS/ISGRI error circle superimposed on the NVSS20 cm radio map of the sky region. The x and the crosspoints mark the position of the NVSS and Green Bank ra-dio sources, respectively. We note that the two NVSS radiosources are close ( ∼ ′ from each other) and represent thebrightest peak of an extended radio emission as detected at20 cm. The position of the Green Bank radio source (crosspoint) is equally distant ( ∼ ′ .4) from the two NVSS ra-dio sources. We point out that the 6 cm Green Bank radiosurvey had an angular resolution of about 3 ′ .5 and it de-tected sources with a positional uncertainty as high as 1 ′ .Bearing this in mind, it cannot be excluded that during itssurvey the Green Bank telescope detected a radio source(GB6 J2034+3914) which is actually the same source as oneor both the NVSS radio objects (NVSS J203441+391441and NVSS J203434+391617). This possibility is further cor-roborated by the fact that GB6 J2034+3914 is listed in theGreen Bank catalog as flagged, to indicate that there is ei-ther significant extension to the source or that the source ispartially resolved blend of two or more sources.IGR J20344+3913 is not listed in the latest publishedIBIS/ISGRI catalog (Bird et al. 2016) despite extensive IN-TEGRAL coverage of its sky region ( ∼ ∼ σ upper limitof ∼ × − erg cm − s − (20–40 keV)for persistent emission. When assuming the source peak fluxin the same energy band as measured by IBIS/ISGRI fromthe outburst reported here, we can infer a dynamic rangegreater than at least ∼ c (cid:13) , 000–000 Table 2.
Swift/XRT (PC) observation log of the two ToOs targeted on the IBIS/ISGRI sky positions.Target Obs. ID Start Time End Time XRT/PC Exposure Time (s)IGR J03346+4414 00034387001 2016-03-06 00:36:28 2016-03-07 00:52:53 2932IGR J20344+3913 00034386001 2016-03-04 01:51:47 2016-03-04 03:43:54 2208
Table 3.
Swift/XRT results of the two ToOs centered on the two IBIS/ISGRI sources, the last row reports the properties of the X-ray source detected in the IGR J20344+3913 field: 3 σ upper limits to the Swift/XRT count rate (0.3–10 keV) of the two IBIS/ISGRIsources are listed in column 2, calculated for a non-detection within the 90% IBIS error circle. Assuming the average Galactic columndensity towards the targets (column 3, Dickey & Lockman 1990), we list in column 4 and 6 the 3 σ upper limits to the fluxes correctedfor the absorption, assuming a power law spectrum with a photon index Γ=1 (UF ) and Γ=2 (UF ), respectively. L and L are thecorrespondent luminosities, at 10 kpc (IGR J03346+4414) and 5 kpc (IGR J20344+3913 and the new XRT source).Src XRT Rate (0.3-10 keV) N H UF (0.3-10 keV) L UF (0.3-10 keV) L (counts s − ) (cm − ) (erg cm − s − ) (erg s − ) (erg cm − s − ) (erg s − )IGR J03346+4414 < × − × < × − < × < × − < × IGR J20344+3913 < × − × < × − < × < × − < × new XRT source (8.8 ± . × − × × − × × − × We requested ToO observations at the sky positions of thetwo new IBIS/ISGRI sources with the
Swift satellite, tolook for possible soft X-ray counterparts, and eventually re-fine their celestial coordinates. Indeed, to date no soft X-rayobservations (except the
ROSAT
All Sky Survey) have evercovered the IBIS/ISGRI sky positions. The two ToOs wereperformed between 2016 March 4 and 7 (see Table 2 forthe summary log) in photon counting (PC) mode. We used
HEASOFT version 6.18 and the most up to date calibrationfiles to perform the
Swift data reduction and analysis.
Swift results
We extracted Swift/XRT sky images in the energy range 0.3-10 keV. Using ximage we searched for soft excesses by meansof the tool detect . No excesses with a minimum signal-to-noise ratio ( snr ) of 3 were found inside the two IBIS/ISGRIerror circles. Therefore, we estimated 3 σ upper limits to thesoft flux, for a source centroid variable within their errorcircles, using the tool sosta (see Table 3 for these results),which makes use of a local estimate of the background. Us-ing WebPIMMS and adopting the average absorbing columndensity along the line of sight (Dickey & Lockman 1990) weestimated the 3 σ upper limits (UF) to the soft fluxes cor-rected for the absorption, adopting a power-law model withtwo values for the photon index (Γ=1 and Γ=2). The re-sults are reported in Table 3, where also the 3 σ upper limitsto the X-ray luminosity are listed. For IGR J20344+3913which is located in the Cygnus region, we assumed a dis-tance of 5 kpc in the hypothesis that it resides in the Cygnusspiral arm (Hachisuka et al. 2009, Kothes et al. 2014). ForIGR J03346+4414, we assumed a canonical value of 10 kpc for the distance since no assumption can be made on itslocation arm.No other excesses were found in the whole Swift/XRTfield-of-view of the two observations, except for the one tar-geted on IGR J20344+3913, where a faint source ( snr =3.6)was detected at the following sky position: R.A. (J2000) =20:33:57.3, Dec (J2000) = +39:15:11.6 (error radius of 7 ′′ ,estimated with the tool xrtcentroid ). The 0.3-10 keV netsource count rate is (8.8 ± . × − counts s − (correctedfor the PSF, sampling dead time and vignetting). In Table 3(last row) we report on the estimated fluxes and luminosi-ties for a power-law continuum. This faint X-ray source liesabout 6 ′ from IGR J20344+3913 (well outside its error cir-cle radius of 3.8 ′ ), so it cannot be associated with it. Asearch in the on-line catalogues at all wavelengths revealedthat this new XRT source is positionally coincident (off-set of 6 ′′ ) with the source USNO-B1.0 1292-0414079 (alsoconsistent with 2MASS 20335767+3915073), showing thefollowing magnitudes: J=10.925 ± . ± . ± .
014 (2MASS catalogue, Cutri et al. 2003)and B1=13.39, R1=12.10, B2=13.90, R2=11.87, I=11.21(USNO-B1.0 catalogue, Monet et al. 2003). In the GuideStar Catalogue (GSC2.3, Lasker et al. 2008) it is classi-fied as a star with a magnitude V=12.47. Fig. 8 shows aclose-up view of both XRT and UVOT images, around thisfaint X-ray source. Adopting to its visual magnitude andan X-ray flux of 10 − erg cm − s − , we calculated a ratiolog(f X /f V )= − .
64, which is compatible with a stellar origin(Maccacaro et al. 1988), although the X-ray flux is likelyoverestimated because of the assumption of the power-lawcontinuum (Table 3). If the X-ray source is really associatedwith this star, it should be located within ∼
100 pc, to enablea low X–ray luminosity of the order of 1.3 × erg s − . c (cid:13) , 000–000 Sguera et al.
Figure 8.
Swift /XRT observation targeted on the source IGR J20344+3913.
Left panel:
Close-up view of the center of the XRTobservation: the large big red circle marks the 3.8 ′ error radius of IGR J20344+3913, while the small red circle (radius, R=7 ′′ ) indicatesthe only source detected during this XRT snapshot (the XRT image has been smoothed with a Gaussian filter only for graphical reasons,to better show the counts excess). Right panel : UVOT image, where the same XRT error regions have been displayed in red. The XRTsource is positionally coincident (within 6 ′′ ) with the star USNO-B1.0 1292-0414079 (2MASS 20335767+3915073, see text). Also thebright star TYC 3153-365-1 (VT mag=11.686 ± . From analysis of archival INTEGRAL data, we have pre-sented IBIS/ISGRI results on the two newly discoveredFXTs IGR J03346+4414 and IGR J20344+3913. They havebeen detected only once and never again on February 2013and November 2015 respectively, being characterized by flar-ing activity lasting in the range 15–30 minutes and hav-ing average flux of the order of ∼ − erg cm − s − (22–60 keV). Recent ToO observations performed with theSwift/XRT satellite failed to detect any quiescent or lowlevel soft X-ray emission from either of the two FXTs, pro-viding stringent 3 σ upper limits of the order of a few times10 − erg cm − s − (0.3–10 keV). Similarly, we obtainedstringent upper limits on their persistent emission above 20keV as well (of the order of ∼ − erg cm − s − ) allowingus to derive high dynamic range values of 1360 and 2400,respectively. After discovering the two FXTs analyzing ourIBIS/ISGRI dataset (revolution number in the range 1535–1629 for the Cygnus region and 918–1262 for the Galacticanticenter), we performed a search at the known source co-ordinate position for additional possible flares detected inthe entire IBIS/ISGRI archive (Paizis et al. 2013) cover-ing revolution number in the range 25–1519 ( ∼
12 years ofdata), i.e. prior to our current dataset. For both FXTs, nodetection was found at ScW level above a significance valueof 7 σ in several different energy bands (17–50 keV, 17–30keV, 30–50 keV). The exposure times obtained from suchentire archive revolution 25–1519 were of the order of ∼ ∼ At first glance, its location off the Galactic plane (b ∼ –9.5 ◦ )would naturally imply an extragalactic blazar nature, how- ever this scenario is strongly weakened by the lack of a strongradio source inside the error circle according to all the avail-able catalogs in the HEASARC and NED database.On the other hand, the fast X-ray transient behavior,the IBIS/ISGRI spectral shape, the high dynamic range, allare compatible with a Galactic SFXT scenario. The loca-tion of the source off the Galactic plane is apparently irrec-oncilable with a HMXB nature since such sources are typi-cally located on the Galactic plane in star forming regions.However we note that, although very unlikely, some HMXBscould eventually be located off the plane due to an effect ofperspective if they are particularly nearby ( i.e. distance < χ Per located atb ∼ –17 ◦ . Under this assumption, IGR J03346+4414 wouldbe characterized by a 22–60 keV average (peak) outburstluminosity of < × erg s − ( < × erg s − ) whilethe Swift/XRT flux upper limit would translate into an av-erage luminosity of < × erg s − . All such values arequite low for a SFXT scenario although they could eventu-ally be still compatible with the weakest hard X-ray flaresdetected from typical SFXTs and with the rare quiescentstate observed for SFXTs, respectively. According to theHEASARC database, 40 2MASS sources and 121 USNO-B1.0 optical stars are located within the 2. ′ ∼
20 kpc (which is very unlikely). Amuch nearer main sequence B star (about 5 kpc) affectedby a similar extinction could account for both the IR colorsand the faint IR magnitudes, as well. This latter possibil-ity would indicate a Be HMXB nature. Unfortunately thehigh number of NIR/optical sources within the large ISGRIerror circle prevents us to pinpoint the correct counterpartand we cannot constrain the source nature. To this aim, itis mandatory to reduce the value of the error circle radiusto arcsecond size. c (cid:13) , 000–000 An alternative and interesting Galactic scenario is thatinvolving nearby flare stars. In this case flaring activity orig-inates in main sequence or pre-main sequence K-M stars,from plasma magnetically confined in compact structures inthe stellar outer atmosphere and heated at very hot tem-peratures of the order of 10 –10 K (see Maggio 2008 fora review). Stellar flares in the soft X-ray band 0.2–10 keVhave been extensively studied by essentially every major X-ray mission. Measured soft X-ray fluxes can be as high as ∼ − erg cm − s − with corresponding X-ray luminosi-ties as high as ∼ erg s − for very nearby stars (i.e.distances typically in a range from a few parsec to a fewhundreds of parsec). On the contrary, in the hard X-ray do-main (E >
20 keV) only a few firm events have been detectedby the PDS onboard BeppoSAX (Schmitt & Favata 1999,Pallavicini et al. 2000), Swift/BAT (Osten et al. 2007, 2010,2016, Copete et al. 2008) and INTEGRAL/IBIS (Bird etal. 2016). This is because the hard X-ray emission is muchless intense than soft X-ray (e.g. by a factor of ∼ ) andthe available hard X-ray instruments have too limited sen-sitivity, such that only the largest flares (which are intrin-sically rare) can be unambiguously detected. Hard X-rayemission from stellar flares is mainly thermal in origin, be-ing best interpreted by single or multi temperature modelsof an optically thin plasma (e.g. Bremss, APEC, MEKAL inXSPEC terminology). However non thermal emission is alsoexpected in the form of an additional power law tail compo-nent which is weak and very difficult to disentangle from thethermal component, in fact to date non thermal hard X-rayemission from stellar flares has escaped firm detections. If weconsider the Swift mission, to date it has detected ∼ > × − erg cm − s − in the 15–50 keV band). In most casesonly high temperature thermal emission was detected withvalues as high as ∼ × K, however in at least one casea possible excess over the thermal emission component wasobserved and interpreted as most likely non thermal emis-sion (Osten et al. 2007). The typical durations of the eventsdetected by BAT is from several minutes to several hours,conversely in the soft X-ray band stellar flares can last evenmuch longer (from few hours to few days). The duration ofthe flare detected by INTEGRAL from IGR J03346+4414( ∼
15 minutes) is compatible with that of stellar flares astypically observed in the hard X-ray band, moreover its lo-cation off the Galactic plane could be eventually explainedby its nearby location. If we assume a reasonable distanceof 100 pc, the corresponding 22–60 keV peak (average) X-ray luminosity would be ∼ × erg s − ( ∼ × erg s − )which is consistent with expectations for typical stellar flaresdetected in the hard X-ray band. As comparison, the onlystellar flare detected by INTEGRAL/IBIS to date is thatfrom the flare star GT Mus (172 parsec distance) whichreached a 20–40 keV average luminosity of ∼ × ergs − (Bird et al. 2016, 2010). As for the spectral shape, thehard X-ray spectrum of IGR J03346+4414 is well fitted bya power law model with a soft photon index of ∼
2, consis-tent with expectations of typical stellar flare detected in thehard X-ray band. Alternatively, a single temperature model(Bremsstrahlung) also provided a good fit with a particu-larly high temperature of kT ∼
70 keV or T ∼ × K, al-though the nominal value is poorly constrained in the range ∼ × K. In addition here we note that an equally good fit ( χ ν =0.9, 9 d.o.f.) was also achievedwith a APEC thermal model, the temperature was not con-strained but its nominal value ( ∼
65 keV) was consistentwith the Bremsstrahlung measurement. Both temperaturevalues would clearly be representative of an ”superhot” ther-mal plasma (Hudson & Nitta 1996). This high plasma tem-perature value could eventually be deemed unlikely and re-jected as unphysically high, but we note that temperaturesas high as ∼ K have been measured from many previouslarge stellar flares. For example, a temperature of ∼ × Khas been measured with Swift/BAT from the source II PEGduring a huge flare (Osten et al. 2007) as well as from thesource DG CVn (Osten et al. 2016). In addition, the Chan-dra Orion ultra deep project (Getman et al. 2008) allowedthe detection of several stellar flares with peak plasma tem-peratures in excess of ∼ × K, with the most extremecase even reaching a value of ∼ × K. In our specificcase, the high measured temperature of ∼ × K could beexplained either by the hard tail of a ”superhot” thermalplasma (among the hottest thermal emission from possiblya flaring star, to our knowledge), or by a non-thermal com-ponent in addition to a thermal plasma emission which wecannot constrain, given the high energy spectrum available(E >
20 keV).
The fast X-ray transient behavior, IBIS/ISGRI spectralshape (e.g. Sguera et al. 2008), location on the Galac-tic plane and high dynamic range strongly suggest thatIGR J20344+3913 is a Galactic SFXT. The 0.3–10 keV lu-minosity upper limit ( ∼ × erg s − at 5 kpc distance)is compatible with the source being a SFXT in the classicalintermediate X-ray state or during the rarer quiescence. Theinferred duty cycle is remarkably low ( ∼ ∼
16 Ms to date (i.e. obtained by sum-ming 12 Ms from entire archive revolution 25–1519 and 4Ms from our current dataset revolution 1535–1629). Consid-ering that classical SFXTs are characterized by a duty cyclevalue typically in the range (0.1–5)% when observed above20 keV by INTEGRAL (Paizis & Sidoli 2014), the value ofIGR J20344+3913 is significantly lower than this range. Onone side this could cast some doubts on its SFXT nature,on the other side it could be due to a marked scarcity ofdetected outbursts which could have been missed by INTE-GRAL eventually because of an observational effect, i.e. thesource’s particularly large distance would allow the detec-tion of only the brightest (and rarest) outbursts , much likethe case of the candidate SFXT IGR J18462 − × erg s − whichis effectively representative of the brightest (and rarest) out-bursts typically detected from SFXTs. This is particularlytrue for the highest luminosity value of the order of ∼ erg s − , which tends to favour a large distance of the or-der of 10 kpc. A huge number of NIR/optical sources fall c (cid:13) , 000–000 Sguera et al. within the 3.8 ′ error circle radius of IGR J20344+3913: 3632MASS sources and 130 USNO-B1.0 stars. Among them,several could be the donor star in a HMXB. For instance,2MASS 20342790+3913165 is one of the brightest NIRsources (J=10.583 mag, H=9.595, K=9.262 mag) and it islocated at 0.3 ′ from the IGR J20344+3913 best position.Its properties would be consistent with a B-type supergiantstar located at a distance of 7.6 kpc suffering an extinc-tion of Av=8.6 mag. Conversely, if we assume that it is amain sequence B star, then it should be located at about1.9 kpc (with Av=6.5 mag). In conclusion, a SFXT is a vi-able scenario but unfortunately the large ISGRI error circleprevents us to pinpoint the correct NIR/optical counterpartand so we are unable to constrain the source nature. To thisaim it is mandatory to reduce the value of the error circleradius to arcsecond size. Additionally, we note the intrigu-ing presence of a strong radio source (listed in both NVSSand Green Bank catalogs) inside the IBIS/ISGRI error cir-cle of this candidate SFXT. However, the possibility thatsuch association could be simply spurious must be takeninto account, and to this aim we calculated the probabilityof finding a NVSS radio source inside the IBIS/ISGRI errorcircle by chance. Given the spatial density of NVSS radiosources (as taken from Condon et al. 1998), we estimated aprobability of ∼ ∼ ∼
28 keV), plasma temperature (T ∼ × K), flux/luminosity value, duration of the flaring ac-tivity, are all compatible with expectations from this inter-pretation.An alternative Galactic scenario is that involving Sym-biotic X-ray binaries (SyXBs). They are a subclass of lowmass X-ray binaries in which material is accreted onto aneutron star compact object from the wind of a late type Mgiant companion. Only a few SyXBs are known to date (seelist in Masetti et al. 2007, Kuranov & Postnov 2015), theyare mainly characterized by relatively low X-ray luminos-ity both at soft and hard X-rays (typically L x ∼ –10 erg s − ) and show long as well as short term X-ray vari-ability. Occasionally, they can exhibit flaring activity typ-ical of neutron stars accreting matter from a stellar wind.The majority of known SyXBs have been detected by IN-TEGRAL as persistent hard X-ray sources, as such theyare listed in several IBIS catalogs published to date (e.g.Bird et al. 2007, 2010, 2016) with typical fluxes of the or-der of ∼ − –10 − erg cm − s − (20–40 keV). This is atodds with the INTEGRAL/IBIS non detection of persistentemission from IGR J20344+3913 (20–40 keV upper limit of1.1 × − erg cm − s − ) despite its sky region has been ex-tensively exposed (i.e. the source is not listed in the latestpublished INTEGRAL/IBIS catalog Bird et al. 2016). Inaddition, the extremely short and energetic flare observedfrom IGR J20344+3913 is not consistent with expectationsfor typical duration observed for SyXBs in the hard X-rayband (i.e. from several days on).Alternatively IGR J20344+3913 could be a blazar be- hind the Galactic plane, hypothesis supported by the pres-ence of a bright radio source inside its error circle (420 mJyat 6 cm) as listed in the Green Bank radio catalog. TheSwift/XRT flux upper limit (2.2 × − erg cm − s − ) iscompatible with this blazar interpretation since it is no-tably in the range of soft X-ray flux values (0.6–8) × − erg cm − s − typically measured from blazars detected byINTEGRAL (Malizia et al. 2016), and therefore it is likelythat even small decrements of the X-ray flux (which is likelyto happen given the variable nature of blazar) could havebeen sufficient to hamper the detection by Swift/XRT in avery short exposure. However there are several shortcom-ings for such blazar scenario: i) as reported above, it islikely that the association between IGR J20344+3913 andthe radio source is spurious; ii) the rather short duration ofthe IBIS/ISGRI flare ( ∼
30 minutes) would be among theshortest flaring episodes ever detected at hard X-rays froma blazar. In the literature, only one similar case is reported,i.e. a ∼
30 minutes flare from a firm blazar likely detected byINTEGRAL (Foschini et al 2006); iii) our reported dynamicrange of IGR J20344+3913 at hard X-rays (about three or-ders of magnitude) is extreme if compared to blazars, in factas from observations of blazar X-ray variability their typicaldynamic range is fairly small (from a few to a very few tensin the X-ray band). Higher blazar dynamic range values (upto about two orders of magnitude) have been achieved atgamma-ray energies only in very exceptional cases, like theremarkable blazar 3C 454.3 which is to date the most vari-able and bright gamma-ray blazar detected by both Fermiand AGILE (Vercellone 2012).
We reported on the discovery of two new Fast X-ray Tran-sients (FXTs) from analysis of archival INTEGRAL data.The main spectral and temporal characteristics of bothIGR J03346+4414 and IGR J20344+3913 are best compat-ible with a Galactic origin such as a SFXT or a nearby flarestar. Conversely, the extragalactic blazar scenario presentssome shortcomings. Regardless of their nature, their pe-culiar characteristics (e.g. high plasma temperature forIGR J03346+4414 ) as well as their unusually short andbright outbursts make them particularly interesting. Thiskind of source is very difficult to discover and characterizedue to the very transitory nature and especially the very lowduty cycle. The instrumental characteristics of IBIS/ISGRIonboard INTEGRAL are particularly suited in serendipi-tously detecting and discovering such short duration randomevents. It seems plausible that other such sources wait to bediscovered, further exploitations of the entire INTEGRALdata archive may yield additional discoveries of this kind ofinteresting X-ray transients.
ACKNOWLEDGMENTS
We thank the anonymous referee for useful comments whichhelped us to improve the quality of this paper. We thankthe
Swift team, the PI, the duty scientists and science plan-ners for making the two ToO observations reported here c (cid:13) , 000–000 REFERENCES
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