We present a spectral and timing study of the NuSTAR and Swift observations
of the black hole candidate IGR J17091-3624 in the hard state during its
outburst in 2016. Disk reflection is detected in each of the NuSTAR spectra
taken in three epochs. Fitting with relativistic reflection models reveals that
the accretion disk is truncated during all epochs with $R_{\rm in}>10~r_{\rm
g}$, with the data favoring a low disk inclination of $\sim
30^{\circ}-40^{\circ}$. The steepening of the continuum spectra between epochs
is accompanied by a decrease in the high energy cut-off: the electron
temperature $kT_{\rm e}$ drops from $\sim 64$ keV to $\sim 26$ keV, changing
systematically with the source flux. We detect type-C QPOs in the power spectra
with frequency varying between 0.131 Hz and 0.327 Hz. In addition, a secondary
peak is found in the power spectra centered at about 2.3 times the QPO
frequency during all three epochs. The nature of this secondary frequency is
uncertain, however a non-harmonic origin is favored. We investigate the
evolution of the timing and spectral properties during the rising phase of the
outburst and discuss their physical implications.Comment: 11 pages, 9 figures, accepted by Ap

Bachetti, Matteo

Fuerst, Felix

Garcia, Javier A.

Grinberg, Victoria

Harrison, Fiona A.

King, Ashley L.

Madsen, Kristin K.

Miller, Jon M.

Tomsick, John A.

Walton, Dominic J.

Xu, Yanjun

The Astrophysical Journal

English

arXiv

We present a spectral and timing study of the NuSTAR and Swift observations of the black hole candidate IGR J17091–3624 in the hard state during its outburst in 2016. Disk reflection is detected in each of the NuSTAR spectra taken in three epochs. Fitting with relativistic reflection models reveals that the accretion disk is truncated during all epochs with R_(in) > 10 r_g, with the data favoring a low disk inclination of ~30°–40°. The steepening of the continuum spectra between epochs is accompanied by a decrease in the high energy cutoff: the electron temperature kT_e drops from ~64 to ~26 keV, changing systematically with the source flux. We detect type-C QPOs in the power spectra with frequency varying between 0.131 and 0.327 Hz. In addition, a secondary peak is found in the power spectra centered at about 2.3 times the QPO frequency during all three epochs. The nature of this secondary frequency is uncertain; however, a non-harmonic origin is favored. We investigate the evolution of the timing and spectral properties during the rising phase of the outburst and discuss their physical implications

García, Javier A.

Fürst, Felix

Caltech Authors - Main

Spectral and Timing Properties of IGR J17091–3624 in the Rising Hard State During Its 2016 Outburst

We present a spectral and timing study of the NuSTAR and Swift observations of the black hole candidate IGR J17091-3624 in the hard state during its outburst in 2016. Disk reflection is detected in each of the NuSTAR spectra taken in three epochs. Fitting with relativistic reflection models reveals that the accretion disk is truncated during all epochs with {R}{in}> 10 {r}{{g}}, with the data favoring a low disk inclination of ∼30°-40°. The steepening of the continuum spectra between epochs is accompanied by a decrease in the high energy cutoff: the electron temperature {{kT}}{{e}} drops from ∼64 to ∼26 keV, changing systematically with the source flux. We detect type-C QPOs in the power spectra with frequency varying between 0.131 and 0.327 Hz. In addition, a secondary peak is found in the power spectra centered at about 2.3 times the QPO frequency during all three epochs. The nature of this secondary frequency is uncertain; however, a non-harmonic origin is favored. We investigate the evolution of the timing and spectral properties during the rising phase of the outburst and discuss their physical implications

BACHETTI, Matteo

OA@INAF - Istituto Nazionale di Astrofisica

2017Publication Year2020-07-27T09:10:19ZAcceptance in OA@INAFSpectral and Timing Properties of IGR J17091-3624 in the Rising Hard State During Its 2016 OutburstTitleXu, Yanjun; García, Javier A.; Fürst, Felix; Harrison, Fiona A.; Walton, Dominic J.; et al.Authors10.3847/1538-4357/aa9ab4DOIhttp://hdl.handle.net/20.500.12386/26638HandleTHE ASTROPHYSICAL JOURNALJournal851NumberSpectral and Timing Properties of IGR J17091–3624 in the Rising Hard StateDuring Its 2016 OutburstYanjun Xu1 , Javier A. García1,2,3 , Felix Fürst4 , Fiona A. Harrison1 , Dominic J. Walton5 , John A. Tomsick6 ,Matteo Bachetti7 , Ashley L. King8, Kristin K. Madsen1 , Jon M. Miller9, and Victoria Grinberg101 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA2 Remeis Observatory and ECAP, Universität Erlangen-Nürnberg, Sternwartstr.7, D-96049 Bamberg, Germany3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA4 European Space Astronomy Centre (ESA/ESAC), Operations Department, Villanueva de la Cañada (Madrid), Spain5 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK6 Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA7 INAF/Osservatorio Astronomico di Cagliari, via della Scienza 5, I-09047 Selargius (CA), Italy8 KIPAC, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA9 Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA10 ESA/ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The NetherlandsReceived 2017 August 10; revised 2017 November 2; accepted 2017 November 12; published 2017 December 18AbstractWe present a spectral and timing study of the NuSTAR and Swift observations of the black hole candidate IGRJ17091–3624 in the hard state during its outburst in 2016. Disk reflection is detected in each of the NuSTAR spectrataken in three epochs. Fitting with relativistic reflection models reveals that the accretion disk is truncated duringall epochs with R r10in g> , with the data favoring a low disk inclination of ∼30°–40°. The steepening of thecontinuum spectra between epochs is accompanied by a decrease in the high energy cutoff: the electrontemperature kTe drops from ∼64 to ∼26keV, changing systematically with the source flux. We detect type-CQPOs in the power spectra with frequency varying between 0.131 and 0.327Hz. In addition, a secondary peak isfound in the power spectra centered at about 2.3 times the QPO frequency during all three epochs. The nature ofthis secondary frequency is uncertain; however, a non-harmonic origin is favored. We investigate the evolution ofthe timing and spectral properties during the rising phase of the outburst and discuss their physical implications.Key words: accretion, accretion disks – black hole physics – X-rays: binaries – X-rays: individual (IGRJ17091–3624)1. IntroductionMost Galactic black hole binaries detected to date aretransient X-ray sources that exhibit recurrent bright outbursts.During the onset of the month-to-year long outbursts, thesources transition from the low/hard state to the high/soft statethrough relatively short-lived intermediate states. In the hardstate, the sources typically have hard spectra with a power-lawindex 1.7G ~ and the thermal disk component is cool and faint(usually not detected above 2 keV). In the frequency domain,type-C quasi-periodic oscillations (QPOs) ranging in frequencyfrom ∼0.1 to 10 Hz have been frequently detected, and they areconsidered as potential probes of the strong-gravity dominatedflow dynamics (see Remillard & McClintock 2006; Fender &Belloni 2012, for relevant reviews).The standard paradigm of accretion disks in black holebinaries is that the inner disk radius extends to the innermoststable circular orbit (ISCO) in the soft state, whereas it istruncated at a larger distance during the hard state. There aretwo methods widely used to measure the inner disk radius fromX-ray spectral modeling: determining the disk emission areafrom its thermal disk blackbody component (e.g., Zhang et al.1997; McClintock et al. 2014), or estimating the relativisticdistortion effects on the disk reflection features (e.g., Fabianet al. 1989; Reynolds 2014).While it is generally well supported by observations that thedisk radius extends to the ISCO, at high accretion rates in thesoft state (e.g., Steiner et al. 2010; Tomsick et al. 2014; Waltonet al. 2016), the disk truncation interpretation of the hard statespectrum is still highly debated (e.g., Kolehmainen et al. 2014;Fürst et al. 2015; García et al. 2015). Cool disks are common inthe brightest phases of the hard state; modeling thermalcontinuum emission in this state is difficult, but some continuapoint to small inner disk radii (e.g., Miller et al. 2006; Rykoffet al. 2007; Reynolds & Miller 2013). Evidence that disks mayremain at the ISCO in bright phases of the hard state alsocomes from reflection spectroscopy (e.g., Miller et al. 2015). Inaddition, it has been noticed that the measurements of truncateddisks could be biased by photon pile-up distortions in somecases (e.g., Miller et al. 2010), with the best evidence of disktruncation (a narrow Fe K line) only observed in GX339-4 bySuzaku at the luminosity approximately an order of magnitudebelow the brightest parts of the hard state (Tomsicket al. 2009). With the high sensitivity of NuSTAR for reflectionspectroscopy and its triggered read-out (Harrison et al. 2013),new observations in the hard state can help to better understandthe disk evolution of black hole binaries.IGRJ17091–3624 is a transient Galactic black holecandidate discovered with INTEGRAL in 2003 (Kuulkerset al. 2003). During its brightest outburst in 2011 (Krimmet al. 2011), IGRJ17091–3624 was regularly monitored withSwift and RXTE. The source revealed a rich variety ofvariability behavior, with both low- and high-frequency QPOsdetected (Altamirano et al. 2011b; Rodriguez et al. 2011;Altamirano & Belloni 2012). After following the canonicalevolution track for a black hole candidate at the beginning ofthe outburst, the source entered the soft state and was found todisplay a number of peculiar variability patterns. Most notably,The Astrophysical Journal, 851:103 (12pp), 2017 December 20 https://doi.org/10.3847/1538-4357/aa9ab4© 2017. The American Astronomical Society. All rights reserved.1the so-called “heartbeat” or ρ state was detected, which hadonly been seen previously in the bright Galactic binaryGRS1915+105 (Altamirano et al. 2011a). In addition, severalnew variability states were recently discovered inIGRJ17091–3624 that had never been reported in GRS1915+105 (Pahari et al. 2012; Court et al. 2017).Despite its similar timing properties to GRS1915+105,IGRJ17091–3624 is more than 10–50 times fainter. Assumingthat the exceptional variability patterns are a result of emittingclose to the Eddington limit, IGRJ17091–3624 either hosts anextremely small black hole 3< Me (for a distance of less than17 kpc), or is very distant (Altamirano et al. 2011a). However,both scenarios are in tension with other independent observa-tions: for example, based on the empirical relations between thephoton index and QPO frequency combined with constraintsfrom spectral modeling, the black hole mass ofIGRJ17091–3624 is estimated to be in the range of8.7Me–15.6Me (Iyer et al. 2015); meanwhile, the sourcedistance inferred from simultaneous multi-wavelength observa-tions is between ∼11 and ∼17kpc (Rodriguez et al. 2011).Assumptions about other properties of the system, such as alow black hole spin or a high disk inclination have beeninvoked to help reconcile this discrepancy (Capitanioet al. 2012; Rao & Vadawale 2012). So far, with no directmeasurement of the basic properties of IGRJ17091–3624 (thedistance, the inclination, and the black hole mass and spin allremain unknown), it is difficult to distinguish among thedifferent scenarios proposed and investigate the physical originof its peculiar behavior.In 2016 February, IGRJ17091–3624 started to showrenewed activity (Miller et al. 2016), and subsequent Swiftmonitoring confirmed that the source rose through the hardstate before transitioning to the soft state at a comparable fluxlevel with its outburst in 2011. In this paper, we performspectral and timing analyses of three NuSTAR and simultaneousSwift observations taken during the rising phase of the hardstate. In Section 2, we describe the observations and the datareduction. Section 3 provides the details of our spectral fittingand timing analyses. We present a discussion in Section 4 andsummarize our results in Section 5.2. Observations and Data ReductionIGRJ17091–3624 was observed by NuSTAR (Harrisonet al. 2013) three times in the hard state during the risingphase of its 2016 outburst, on March 7, 12, and 14, about oneweek before the source started to enter the soft state (seeTable 1 for the list of observations considered in this work).Swift/XRT monitored the entire outburst of IGRJ17091–3624with frequent 1–2ks snapshots from February 26 to October 4.Figure 1 shows the evolution of the soft and hard X-ray bandflux during the outburst measured by the Swift X-ray Telescope(XRT; Burrows et al. 2005) and Burst Alert Telescope (BAT;Krimm et al. 2013). The Swift/XRT data were taken in theWindowed Timing mode to avoid photon pile-up. There aresimultaneous Swift/XRT data for all three NuSTAR observa-tions, providing coverage of the soft X-ray band (see Table 1).2.1. NuSTARWe processed the NuSTAR data using v.1.6.0 of theNuSTARDAS pipeline with NuSTAR CALDB v20170120.After the standard data cleaning and filtering, the total exposuretime is 43.3, 20.2, and 20.7ks for the three observations. Foreach observation, the source spectra and light curves wereextracted at the position of IGRJ17091–3624 within the radiusof 100″ from the two NuSTAR focal plane modules (FPMA andFPMB). Corresponding background spectra were extracted fromsource-free areas on the same chip using polygonal regions. InTable 1NuSTAR and Simultaneous Swift ObservationsEpoch Instrument ObsID Start Time E. T. C. R.(UTC) (ks) (s−1)1 NuSTAR 80001041002 2016 Mar 07 02:01 43.3 16.5Swift/XRT 00031921099 2016 Mar 07 01:49 1.46 7.32 NuSTAR 80202014002 2016 Mar 12 14:11 20.2 21.4Swift/XRT 00031921104 2016 Mar 12 13:53 1.95 8.93 NuSTAR 80202014004 2016 Mar 14 19:26 20.7 23.6Swift/XRT 00031921106 2016 Mar 14 21:39 1.03 10.4Note. The exposure time and count rates of NuSTAR quoted here are the values from FPMA.Figure 1. (a) Swift monitoring of the 2016 outburst of IGRJ17091–3624. The15–50keV BAT light curve is from the Swift/BAT hard X-ray monitor, whichis rescaled to the unit mCrab based on the Crab rates in the instrument band(Krimm et al. 2013). The XRT light curve is converted to mCrab from theenergy flux in 2–8 keV. (b) Photon index derived by fitting the Swift/XRT datawith an absorbed power-law model. The gray shaded areas mark the threeNuSTAR observations, which caught the source in the hard state before itstarted to enter the soft state around MJD 57470.2The Astrophysical Journal, 851:103 (12pp), 2017 December 20 Xu et al.the first NuSTAR observation (ObsID 80001041002), the sourceregion was contaminated by the stray light from a nearby brightsource GX 349+2. In this case, we also placed the backgroundregion in the part of the field illuminated by the stray light. Wenote that the influence from the stray light is minimal, as thebackground only contributes ∼2% to the total count rate.The NuSTAR spectra were grouped to have a signal-to-noiseratio (S/N) of at least 20 per bin after background subtraction.For the NuSTAR timing analysis, we first applied barycentercorrection to the event files to transfer the photon arrival timesto the barycenter of the solar system using JPL PlanetaryEphemeris DE-200. We then generated cross-power densityspectra (CPDS) using MaLTPyNT (Bachetti 2015) from thecleaned event files. MaLTPyNT, developed for NuSTAR timinganalysis, properly treats orbital gaps and uses the CPDS as aproxy for the power density spectrum. The CPDS uses thesignals from two independent focal plane modules, FPMA andFPMB, and the real part of the CPDS provides a measure of thesignal that is in phase between the two modules. Thus, thismethod helps to remove the spurious distortion to the power-density spectrum introduced by the instrumental dead time (fordetails, see Bachetti et al. 2015). We generated the CPDS withthe binning time of 2−8s and in 512s long intervals. For all thepower spectra, we used the root mean square (rms) normal-ization, and geometrically rebinned the power spectra with afactor of 1.03 to increase the S/N of the frequency bins andproduce nearly equally spaced bins in the logarithmic scale.2.2. SwiftThe Swift/XRT data were processed using xrtpipelinev.0.13.2 with XRT CALDB v20160609 to produce clean eventfiles. We extracted the source spectra with xselect from acircular region with a radius of 20 pixels centered on theposition of IGRJ17091–3624. The background was extractedfrom an annulus area with the inner and outer radii of 80 pixelsand 120 pixels, respectively. The data were filtered for grade 0events only and we used swxwt0s6_20131212v015.rmffor the response matrix. Ancillary response files were generatedwith xrtmkarf, incorporating exposure maps to correct forthe bad columns. Finally, we rebinned the data to have a S/Ngreater than 5 for each energy bin.3. Analysis3.1. Spectral ModelingWe model the simultaneous NuSTAR and Swift/XRT spectrain XSPEC v12.9.0n (Arnaud 1996) using 2c statistics. Forspectral fitting, we adopt the cross-sections from Verner et al.(1996) and abundances from Wilms et al. (2000). All parameteruncertainties are reported at the 90% confidence level for oneparameter of interest. Cross-normalization constants areallowed to vary freely for NuSTAR FPMB and Swift/XRTand assumed to be unity for FPMA. The values are within 5%of unity for Epoch 1 and Epoch 2, as expected from Madsenet al. (2015). In Epoch 3, the Swift normalization is ∼13%higher than NuSTAR. We note that it is probably caused by theexposure correction uncertainty when the source lies close tothe CCD bad columns, which is sensitive to the exact sourceposition and can be affected by the spacecraft pointingstability.11All three NuSTAR observations detect IGRJ17091–3624across the entire instrument bandpass. To highlight thereflection features, we fit the NuSTAR spectra with a simpleabsorbed power-law model, TBabs*powerlaw in XSPEC,using only data in the 3–5keV and 8–12keV energy range. Asshown in Figure 2, the residuals display asymmetric broad ironlines around 4–7keV and show evidence for changes in thecutoff energy as the source luminosity increases. From thissimple model, we also detect a slight steepening in the power-law slope: Γ increases from ∼1.55 to ∼1.60 from Epoch 1 toEpoch 3. No parameter uncertainties are calculated here, as thefits are too poor to determine meaningful parameteruncertainties.3.1.1. Joint Fits with Disk Reflection ModelsGiven the disk reflection features observed (see Figure 2),we model the NuSTAR and Swift spectra simultaneously withthe self-consistent reflection model relxill v0.5b (Dauseret al. 2014; García et al. 2014). We fit the NuSTAR and Swiftspectra in the bands 3–79keV and 1–10keV, respectively. Weignore the Swift data below 1keV to avoid possible low-energyresiduals known to be present in the XRT Windowed Timingmode data of some absorbed sources.12 We note that due to theFigure 2. (a) Unfolded IGRJ17091–3624 spectra from the three NuSTARobservations. (b)–(d) Residuals in χ from the simple absorbed power-lawmodel. FPMA spectra taken in Epoch 1, 2, and 3 are marked in red, blue, andgreen, respectively. For comparison, FPMB spectra are all in gray. The data arerebinned for display clarity.11 http://www.swift.ac.uk/analysis/xrt/digest_sci.php 12 http://www.swift.ac.uk/analysis/xrt/digest_cal.php3The Astrophysical Journal, 851:103 (12pp), 2017 December 20 Xu et al.

Spectral and Timing Properties of IGR J17091-3624 in the Rising Hard State During Its 2016 Outburst

Spectral and Timing Properties of IGR J17091-3624 in the Rising Hard
  State During its 2016 Outburst

Spectral and Timing Properties of IGR J17091-3624 in the Rising Hard State During its 2016 Outburst

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