NuSTAR J163433-4738.7: A Fast X- Ray Transient in the Galactic Plane The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Tomsick, John A., Eric V. Gotthelf, Farid Rahoui, Roberto J. Assef, Franz E. Bauer, Arash Bodaghee, Steven E. Boggs, et al. 2014. “NuSTAR J163433-4738.7: A Fast X-Ray Transient in the Galactic Plane.” The Astrophysical Journal 785 (1) (March 19): 4. doi:10.1088/0004-637x/785/1/4. Published Version doi:10.1088/0004-637X/785/1/4 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:30168192 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#OAP ACCEPTED BY APJ Preprint typeset using LATEX style emulateapj v. 5/2/11 NUSTAR J163433–4738.7: A FAST X-RAY TRANSIENT IN THE GALACTIC PLANE JOHN A. TOMSICK1,ERIC V. GOTTHELF2,FARID RAHOUI3,4 ,ROBERTO J. ASSEF5 ,FRANZ E. BAUER6,7,ARASH BODAGHEE1,STEVEN E. BOGGS1,FINN E. CHRISTENSEN8,WILLIAM W. CRAIG1,9 ,FRANCESCA M. FORNASINI1,10 , JONATHAN GRINDLAY11,CHARLES J. HAILEY2,FIONA A. HARRISON12,ROMAN KRIVONOS1,LORENZO NATALUCCI13,DANIEL STERN14,WILLIAM W. ZHANG15 Accepted by ApJ ABSTRACT During hard X-ray observations of the Norma spiral arm region by the Nuclear Spectroscopic Telescope Array (NuSTAR) in 2013 February, a new transient source, NuSTAR J163433–4738.7,was detected at a signif- icance level of 8-σ in the 3–10keV bandpass. The source is consistent with having a constant NuSTAR count rate over a period of 40ks and is also detected simultaneously by Swift at lower significance. The source is not significantly detected by NuSTAR, Swift, or Chandra in the days before or weeks after the discovery of the transient, indicating that the strong X-ray activity lasted for between ∼0.5 and 1.5 days. Near-IR imaging obser- vations were carried out before and after the X-ray activity, but we are not able to identify the counterpart. The Γ +1.5 combined NuSTAR and Swift energy spectrum is consistent with a power-law with a photon index of =4.1−1.0 +2.1 (90% confidence errors), a blackbodywith kT =1.2±0.3keV,or a bremsstrahlungmodel with kT =3.0−1.2 keV. The reduced-χ2 values for the three models are not significantly different, ranging from 1.23 to 1.44 for 8 de- +2.3 23 −2 +15 22 −2 grees of freedom. The spectrum is strongly absorbed with NH = (2.8−1.4) × 10 cm , (9−7 ) × 10 cm , and +1.7 23 −2 (1.7−0.9) × 10 cm , for the power-law, blackbody, and bremsstrahlung models, respectively. Although the high column density could be due to material local to the source, it is consistent with absorption from in- terstellar material along the line of sight at a distance of 11kpc, which would indicate an X-ray luminosity >1034 erg s−1. Although we do not reach a definitive determination of the nature of NuSTAR J163433–4738.7, we suggest that it may be an unusually bright active binary or a magnetar. Subject headings: stars: variables: general — X-rays: stars — surveys — Galaxy: stellar content — X-rays: individual (NuSTAR J163433–4738.7) 1. INTRODUCTION of the entire Galactic Plane and has discovered hundreds of Hard X-ray surveys of the Galaxy provide an opportunity new sources (Bird et al. 2010; Krivonos et al. 2012), includ- to discover populations of extreme sources. The promise ing new types of High Mass X-ray Binaries (HMXBs), pulsar of such surveys has been partially realized with the Inter- wind nebulae (PWNe), and magnetic Cataclysmic Variables national Gamma-Ray Astrophysics Laboratory (INTEGRAL, (CVs). The Nuclear Spectroscopic Telescope Array (NuSTAR, Winkler et al. 2003), which carried out a 20–100keV survey Harrison et al. 2013), which launched in 2012 June and cov- ers the 3–79keV bandpass, is the first focusing hard X-ray telescope in orbit. While it has a much smaller field of view 1 Space Sciences Laboratory, 7 Gauss Way, University of California, than INTEGRAL, it has much lower background and greatly Berkeley, CA 94720-7450, USA; [email protected] improved sensitivity. Thus, one of NuSTAR’s science goals is 2 Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA to extend our view deeper into the Galactic plane to look for 3 European Southern Observatory, Karl Schwarzschild-Strasse 2, 85748 hidden hard X-ray populations. Garching bei München, Germany During its first year of operation, NuSTAR has initiated 4 Department of Astronomy, Harvard University, 60 Garden Street, surveys of ∼1 deg2 areas both in the Galactic Center (e.g., Cambridge, MA 02138, USA 5 Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Moriet al. 2013; Nynkaet al. 2013) and in a region which arXiv:1402.2292v2 [astro-ph.HE] 25 Mar 2014 Portales, Av. Ejército 441, Santiago, Chile samples the spiral arm population in order to probe poten- 6 Instituto de Astrofísica, Facultad de Física, Pontifica Universidad tially different environments where X-ray binaries are found. Católica de Chile, 306, Santiago 22, Chile A spiral arm region centered on Galactic coordinates of 7 Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO l = 337.5◦ and b = 0◦ was chosen for having the highest 80301, USA 8 DTU Space, National Space Institute, Technical University of Den- known density of OB star associations (Russeil 2003) and mark, Elektrovej 327, DK-2800 Lyngby, Denmark HMXBs (Bodagheeet al. 2007, 2012). This is part of the 9 Lawrence Livermore National Laboratory, Livermore, CA 94550, “Norma” spiral arm region, which was identified early in the USA INTEGRAL mission as having an unusually high density of 10 Astronomy Department, University of California, 601 Campbell Hall, hard X-ray sources (Tomsick et al. 2004; Dean et al. 2005; Berkeley, CA 94720, USA 11 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA Lutovinov et al. 2005). The combination of active star forma- 02138, USA tion and evidence that compact objects have already formed 12 Cahill Center for Astronomy and Astrophysics, California Institute of suggests that a survey by NuSTAR may uncover compact ob- Technology, Pasadena, CA 91125, USA 13 jects associated with populations of massive stars such as Istituto Nazionale di Astrofisica, INAF-IAPS, via del Fosso del Cav- magnetars or faint HMXBs that are early in their evolutionary aliere, 00133 Roma, Italy 14 Jet Propulsion Laboratory, California Institute of Technology, process and may have neutron star or black hole accretors. Pasadena, CA 91109, USA The full surveys of the Galactic Center and the Norma re- 15 NASA Goddard Space Flight Center, Greenbelt, MD 20771 gion will be carried out over a period of ∼2 years; here we 2 Tomsick et al. report on the discovery of a transient source made during the N first part of the survey. In the following, §2 describes obser- vations made with NuSTAR, Swift, and Chandra, as well as the procedures we used to reduce the data. The results are E presented in §3, and we discuss possibilities for the nature of the transient in §4. 2. OBSERVATIONS AND DATA REDUCTION As the first part of the NuSTAR survey of the Norma region, nine ∼20ks NuSTAR observations were performed between 1 arcmin UT 2013 February 20 and February 24. Each 13′-by-13′ field FPMA FPMB of view was partially overlapping with adjacent pointings, and 0 0.4 0.8 1.2 1.6 the entire region covered was ∼0.2 deg2. The results from all nine pointings will be reported in Bodaghee et al. (submit- FIG. 1.— NuSTAR discovery images in the 3–20 keV energy band ted). Here we focus on the observations that covered a new for NuSTAR J163433–4738.7. The source was detected in both of transient, NuSTAR J163433–4738.7. These observations are the NuSTAR Focal Plane Modules (FPMA and FPMB) in a 22.6 ks exposure taken on 2013 Feb. 23-24. The images have been rebinned listed in Table 1, including two that were obtained during the so that the pixel size is 2.′′5 and smoothed with a 6-pixel Gaussian. survey and a follow-up NuSTAR observation on 2013 March The scale on the bottom of the figure is in counts per pixel, and a 23 that was coordinated with Chandra. logarithmic scaling is used. The apparent elongation of the source is In addition, ∼2 ks Swift X-ray Telescope (XRT) observa- due to the distorted PSF shape at large off-axis angles. tions of the region were carried out during 2013 February 21- 24, and four of these observationscoveredNuSTAR J163433– Catalog sources in the field. 4738.7. Table 1 lists these along with three other Swift obser- 3. RESULTS vations that were acquired after our survey as part of another observing program. We also analyzed archival data cover- The new transient was discovered from an inspection of the ing the source, including two Chandra observations (ObsIDs image from the 22.6ks NuSTAR observation that took place 12529 and 12532 with exposure times of 19.0ks and 19.5ks, starting on 2013 February 23, 14.52 h. As shown in Fig- respectively) that were acquired in 2011 as part of a survey of ure 1, the source was detected in FPMA and FPMB. To deter- mine the significance of the detection, we extracted 3–10keV the same region being covered by NuSTAR (Fornasini et al., ′′ submitted).