INTEGRAL and Swift/XRT observations of IGR~J16460+0849
aa r X i v : . [ a s t r o - ph . H E ] J u l Astronomy & Astrophysics manuscript no. 10866shu c (cid:13)
ESO 2018November 5, 2018
INTEGRAL and Swift/XRT observations of IGR J16460+0849(Research Note)
Shu Zhang , Yupeng Chen , Jianmin Wang , , Tipei Li , , Junqiang Ge Laboratory for Particle Astrophysics, Institute of High Energy Physics, Beijing 100049, China Theoretical Physics Center for Science Facilities (TPCSF), CAS Center for Astrophysics,Tsinghua University, Beijing 100084, ChinaReceived / Accepted
ABSTRACT
Aims.
IGR J16460+0849 is reported in the 3rd IBIS catalog with the shortest exposure of about 10 ks among all thedetected sources, which makes it the most interesting target to be investigated with a deeper exposure. The currentlyavailable data have increased to an exposure time of a few hundred ks. This stimulates us to investigate the sourceagain by using all the available data.
Methods.
We analyze all available observations carried out by the International Gamma-Ray Astrophysics Laboratory(INTEGRAL) on the unidentified source IGR J16460+0849. The data were processed by using the latest version OSA7.0. In addition we analyze also all the available Swift/XRT data on this source.
Results.
We find that IGR J16460+0849 has a detection significance of ∼ σ in the 20-100 keV band during theobservational period between March 2003 and September 2004, when it was exposed by ∼
19 ks. Thereafter the sourcewas not detected anymore, despite an additional exposure of ∼
271 ks. This suggests a flux variability on a timescale ofyears. The spectral analysis shows that the IBIS/ISGRI data are well presented by a power-law shape, with a photonindex obtained as 1.45 ± Key words.
X-rays: individual: IGR J16460+0849
1. INTRODUCTION
Totally 421 sources are listed in the 3rd IBIS/ISGRI cata-log (Bird et al. 2007). Among them are 171 Galactic accret-ing systems, 122 extragalactic objects, and 113 of unknownnature. The diagram of detection significance versus net ex-posure for the unidentified sources is shown in Fig. 1. Thenet exposure refers to the corrected on-source exposure, e.g.omitting observations taken during periods of solar activ-ity or near the spacecraft perigee passages when the back-ground modeling is difficult (Bird et al. 2007). Most sourceshave significances at the 5-7 σ level and are detected underrelatively large observational time of above hundreds ks. Inthese cases, an additional exposure will not lead to a largeincrement in source significance. However, the exceptionsare IGR J06292+4858 and IGR J16460+0849. They are de-tected with a net exposure of only 14 ks and 11 ks, respec-tively. While for IGR J06292+4858 almost no additionalexposure is available since Bird’s report in 2007, the situ-ation has improved a lot for IGR J16460+0849. ThereforeIGR J16460+0849 is the promising candidate for furtheranalyses in the hard and soft X-ray bands, by using allcurrently available INTEGRAL and Swift/XRT data.In this note we report the results from our INTEGRALand Swift/XRT analyses on IGR J16460+0849, carried out Send offprint requests to : Shu Zhang
Correspondence to : [email protected] by using the most updated analysis tools. The paper isstructured as follows: the observations and the data anal-yses are described in Sect.2, the results are presented inSect.3, and finally in Sect.4 we give the discussion and thesummary.
2. OBSERVATIONS AND DATA ANALYSIS
INTEGRAL (Winkler et al. 2003) is a 15 keV - 10 MeV γ -ray mission. The main instruments are the imager onboardINTEGRAL (IBIS, 15 keV - 10 MeV) (Ubertini et al. 2003)and spectrometer (SPI, 20 keV - 8 MeV) (Vedrenne et al.2003). They are supplemented by the Joint European X-ray Monitor (JEM-X, 3-35 keV) (Lund et al. 2003) andthe Optical Monitor Camera (OMC, V, 500-600 nm) (Mas-Hesse et al. 2003). At the lower energies (15 keV - 1 MeV),the CdTe array ISGRI (Lebrun et al. 2003) of IBIS hasa better continuum sensitivity than SPI. The satellite waslaunched in October 2002 into an elliptical orbit with aperiod of 3 days. Due to the coded-mask design of the de-tectors, the satellite normally operates in dithering mode,which suppresses the systematic effects on spatial and tem-poral backgrounds.The INTEGRAL observations are carried out in theso-called individual SCience Windows (SCWs), with atypical time duration of about 2000 seconds each. Only Shu Zhang et al.: INTEGRAL and Swift/XRT observations ... (RN)
IBIS/ISGRI public data have been taken into account,while JEMX data are not available because the source wasoutside its FOV. The data reduction has been performedby using the standard Online Science Analysis (OSA) soft-ware version 7.0, the latest released version. The resultsare obtained by running the pipeline from the flowchartto the image level. The flux and the detection significanceare measured from the mosaic map at the source positionreported previously by Bird et al. (2007).
SWIFT is a γ -ray burst explorer launched 2004 November20. It carries three co-aligned detectors (Gehrels et al.2004), the Burst Alert Telescope (BAT, Barthelmy et al.2005), the X-Ray Telescope (XRT, Burrows et al. 2005),and the Ultraviolet/Optical Telescope (UVOT, Roming etal. 2005). We took only Swift/XRT data into account, ba-cause BAT data were not available. The XRT uses a grazingincidence Wolter I telescope to focus X-rays onto a state-of-the-art CCD. XRT has an effective area of 110 cm , aFOV of 23.6 arcminutes, an angular resolution (half-powerdiameter) of 15 arcseconds, and it operates in the 0.2-10keV energy range, providing the possibility of extendingthe investigation on the unidentified source to soft X-rayswith the available SWIFT observations.There is one SWIFT snapshot available for IGRJ16460+0849 with an exposure of 4949 seconds. The ob-servation was carried out on 2007 October 23 in the pho-ton counting mode and has ID 00037052001. We analyzedthe Swift/XRT 0.2-10 keV data by using the latest releasedanalysis software, provided in HEAsoft version 6.4.
3. RESULTS
The available INTEGRAL observations, when IGRJ16460+0849 falls into the partially coded field of view ofISGRI (offset angle less than 19 degrees), comprise about290 SCWs, adding up to a total exposure time of 511ks (until 2006 August 25). The details of the analyzedINTEGRAL observations on IGR J16460+0849, includingthe exposure and the time period, are summarized in Table1. Most of these observations were carried out in the 5x5dithering mode. We subdivided the data into 5 groups ac-cording to the observational sequence. For most of the timethe source was at the edge of the field of view, at an offsetangle of more than 14 degrees. In what follows the resultsare reported from the observations during which the sourcehas the offset angle of less than 14 degrees.The imaging analyses show that the source is only de-tected by summing the first three observational groups(MJD 52725-53259). It is not detected afterwards, althoughthe exposure is increased by a factor of 14. The best sourcedetection, ∼ σ at 20-100 keV (Fig. 2), is derived in themosaic map of the first 3 observational groups. The fluxin units of mcrab and the detection significance derivedfrom each observational group in the 20-100 keV band aresummarized in Table 1. We also generate the images inthe 20-40 keV and 40-100 keV bands, with the combinedobservations of the revolutions 0056-0235 (MJD 52725.0-53259.0). We derive the fluxes of 7.4 mcrab and 12.4 mcrabfor the two energy bands, respectively. We produce in Fig. 3 the SCW lightcurve for the obser-vational groups containing the revolutions 0056-0235 (MJD52725.0-53259.0), during which the source is observed tohave the highest detection significance. There are only a fewSCWs with source offset angle of less than 14 degrees exist-ing in each of the three observational groups. This preventsus from making a rough estimate of the duration of thesource activity on a short time scale. To improve the statis-tics, we add the SCWs in each observational group togetherand produce the group lightcurve (Fig. 3). This lightcurveshows that the source stands out persistently during thefirst three observational groups 0056-0235 (MJD 52725.0-53259.0), and does not become detectable thereafter. Theaverage flux drops from roughly 9 mcrab of the first threegroups to an undetectable level of the later, clearly suggest-ing a flux variability over a time scale of years.The source spectrum is extracted from the observa-tional sequence of the revolutions 0056-0235 (MJD 52725.0-53259.0) where the highest detection significance is pre-sented. The detection significances of the source in the en-ergy bands 20-40 keV, 40-60 keV, and 60-100 keV are mea-sured as 2.9 σ , 2.3 σ , and 2.0 σ , respectively. These data arewell-fitted with a simple power-law model (see Fig. 4), withthe photon index derived as 1.45 ± χ of ∼ The Swift/XRT imaging analysis (5 ks exposure time) doesnot show any detected source inside the IBIS error circle (5arcminutes radius). We conclude that IGR J16460+0849was not detected by Swift/XRT below 10 keV during thissnapshot of 5 ks. Knowing the sensitivity of the XRT, wecan put an upper limit on the source flux. With an XRTsensitivity of about 2 × − erg cm − s − in an exposuretime of 10 ks (see Capalbi et al. 2005), we estimate an upperflux limit of ∼ × − erg cm − s − from an observationof 5 ks. By extrapolating the ISGRI spectrum to energiesbelow 10 keV, we have an integral flux of ∼ × − erg cm − s − for the band 0.2-10 keV. We would thereforeexpect a bright X-ray source from Swift/XRT exposure of5 ks if it was active at the time. The non-detection againshows that the source was variable and its flux dropped toa level below Swift/XRT threshold.
4. Discussion and summary
Bird et al. (2007) reported IGR J16460+0849 as anINTEGRAL hard X-ray source at a detection significanceof 4.7 σ in the 20-100 keV from a net exposure of ∼
11 ks.Our analyses agree with Bird’s result, using the latest soft-ware release. We find a source at a significance level of ∼ σ in the sum of the observations between March 2003and September 2004. The source was not detected later,and the exposure increased to ∼
271 ks. This suggests thesource is most likely variable on the timescale of years ifthe detected signal is real.As listed in the 3rd IBIS catalog (Brid et al. 2007),IGR J16460+0849 belongs to a group of sources with lowsignificances < σ . About 10-20 percent of them mightresult from false detections (Bird et al. 2007). We noticethat the map (Fig. 2), where the source has the highestsignificance in detection, is rather noisy. To check whether hu Zhang et al.: INTEGRAL and Swift/XRT observations ... (RN)
10 100 1000 10000net exposure (ks)01020304050 s i gn i f i c an c e ( s i g m a ) IGR J06292+4858IGR J16460+0849
Fig. 1.
Source significance ( σ ) vs corrected onsource expo-sure (ks) for sources listed in the 3rd IBIS catalog (Bird etal. 2007).this is statistical, we generated the histogram of the signif-icance distribution. We find that the significance value forIGR J16460+0849 is consistent with the wing of a Gaussianshape, which fits this distribution well (see Fig. 5). The 1 σ variance of this distribution is about 1.27 and, accord-ingly, the probability of having a detection of 4.4 σ de-creases to 4.4/1.27 ∼ α, δ ) =16h45m57s; +08d49m05s (l/b = 26.297 ◦ /31.853 ◦ ) for IGRJ16460+0849 with an error circle at the 90 percent confi-dence level of about 5 arcminutes. A SIMBAD search withinthis error location yields no known source. The source loca-tion at middle Galactic latitude most probably indicates anextragalactic origin, so far for all AGNs reported in 3rd IBIScatalogue, additional exposure leads to large increment intheir significance, which clearly agrees with their AGN na-ture. However, on the contrary, for IGR J16460+0849, ad-ditional IBIS exposure does not lead to an increment ofits significance but instead leads to a significant decrease.This agrees with the evidence that IGR J16460+ 0849 isvery likely a variable source spending a considerable frac-tion of the time in a “non active state”, during which it isbelow the Swift/XRT and IBIS threshold.In summary, the most recent analyses carried out withthe latest software releases again show a hint of a sourcesignal from IGR J16460+0849. This is consistent with theprevious report from Bird et al. (2007). The high Galacticlatitude of IGR J16460+0849 suggests an extragalactic ori-gin, but more data and further studies are needed to shedlight on its nature. Acknowledgements.
We thank the anonymous referee and Dr. WernerCollmar for the constructive comments that helped in improving ourpaper. This work was subsidized by the National Natural Science
Table 1.
IBIS/ISGRI observations log of IGRJ16460+0849.
Rev. Date Expo.( a ) Flux Sig.MJD ks mcrab σ ± ± ± b ) 53592.7-53972.3 0(133) – –0056-0235 52725.0-53259.0 20(106) 9.0 ± ± b the source is never at an offset angle less than 14 degrees.Note: The flux and the significance are presented in the energyband 20-100 keV for the data where the source was withinan offset angle of 14 degrees. RA (deg)
DEC (deg)
IGR J16460+0849 RA (deg)
DEC (deg)
IGR J16460+0849
Fig. 2.
The 20-100 keV significance maps of IGRJ16460+0849 as obtained from the combined revolutions0056 - 0235 (MJD 52725.0-53259.0). The maps on the leftand on the right are derived from data with a source offsetangle less than 14 degrees and less than 19 degrees, respec-tively. The contours start at a significance level of 2 σ withsteps of 0.5 σ . MJD (days)52700 52800 52900 53000 53100 53200 53300 F l u x ( m c r a b ) Group
MJD (days)52800 53000 53200 53400 F l u x ( m c r a b ) Group
Fig. 3.
ISGRI longterm lightcurves of IGR J16460+0849in the 20-100 keV band, on the basis of SCW (top) andobservational group (bottom), for the time period between2003 and 2006. The revolution numbers are shown at thetop of the bottom panel.
Shu Zhang et al.: INTEGRAL and Swift/XRT observations ... (RN)
Energy (keV) ) - ke V - s - F l u x ( ph c m -5 -4 Fig. 4.
The energy spectrum of IGR J16460+0849 obtainedfrom the observations of the revolutions 0056 - 0235 (MJD52725.0-53259.0). The line shows the best fit with a power-law shape. significance_distribution
Entries 2401Mean 0.06437RMS 1.288
Significance-6 -4 -2 0 2 4 6 N u m b e r significance_distribution Entries 2401Mean 0.06437RMS 1.288
Fig. 5.
Gaussian fit to the significance distribution of the20-100 keV mosaic map for the time period of revolutions0056 - 0235 (MJD 52725.0-53259.0).
Foundation of China, the CAS key Project KJCX2-YW-T03, and the973 Program 2009CB824800. J.-M. W. thanks the Natural ScienceFoundation of China for support via NSFC-10325313, 10521001, and10733010.