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PDF Hosted at the Radboud Repository of the Radboud University Nijmegen PDF hosted at the Radboud Repository of the Radboud University Nijmegen The following full text is a publisher's version. For additional information about this publication click this link. http://hdl.handle.net/2066/92441 Please be advised that this information was generated on 2021-09-26 and may be subject to change. The Astrophysical Journal Letters, 729:L21 (5pp), 2011 March 10 doi:10.1088/2041-8205/729/2/L21 C 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A. THE X-RAY QUIESCENCE OF SWIFT J195509.6+261406 (GRB 070610): AN OPTICAL BURSTING X-RAY BINARY? N. Rea1, P. G. Jonker2,3,4, G. Nelemans4,J.A.Pons5,M.M.Kasliwal6, S. R. Kulkarni6, and R. Wijnands7 1 Institut de Ciencies de l’Espai (CSIC–IEEC), Campus UAB, Facultat de Ciencies, Torre C5-parell, 2a planta, 08193, Bellaterra (Barcelona), Spain; [email protected] 2 SRON-Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands 3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 4 Department of Astrophysics, Radboud University Nijmegen, IMAPP, P.O. Box 9010, 6500 GL, Nijmegen, The Netherlands 5 Departament de Fisica Aplicada, Universitat d’Alacant, Ap. Correus 99, 03080 Alacant, Spain 6 Department of Astronomy, California Institute of Technology, Mail Stop 105-24, Pasadena, CA 91125, USA 7 University of Amsterdam, Astronomical Institute “Anton Pannekoek,” Postbus 94249, 1090 GE, Amsterdam, The Netherlands Received 2010 November 11; accepted 2011 January 18; published 2011 February 15 ABSTRACT We report on an ∼63 ks Chandra observation of the X-ray transient Swift J195509.6+261406 discovered as the afterglow of what was first believed to be a long-duration gamma-ray burst (GRB 070610). The outburst of this source was characterized by unique optical flares on timescales of second or less, morphologically similar to the short X-ray bursts usually observed from magnetars. Our Chandra observation was performed ∼2 years after the discovery of the optical and X-ray flaring activity of this source, catching it in its quiescent state. We derive stringent upper limits on the quiescent emission of Swift J195509.6+261406, which argues against the possibility of this object being a typical magnetar. Our limits show that the most viable interpretation on the nature of this peculiar bursting source is a binary system hosting a black hole or a neutron star with a low-mass companion star (<0.12 M) and with an orbital period smaller than a few hours. Key words: stars: magnetic field – X-rays: individual (SWIFT J195509.6+261406) – X-rays: stars Online-only material: color figures 1. INTRODUCTION The source distance was constrained by several different methods to be within 3.7–10 kpc (mainly red clump study, On 2007 June 10 the Swift Burst Alert Telescope (BAT) trig- and detailed measurements of the absorption column in the gered on GRB 070610, a typical long-duration GRB (gamma- millimeter waveband; Castro-Tirado et al. 2008). Furthermore, ray burst; see Gehrels et al. 2007 for a recent review), with the stringent optical and IR limits derived in the quiescent an ∼4.6 s high-energy prompt emission (Pagani et al. 2007; level (H > 23; R > 26.0 and i > 24.5; Kasliwal et al. 2008; Tueller et al. 2007). Follow-up soft X-ray observations with the Castro-Tirado et al. 2008) constrain the type of any companion Swift X-Ray Telescope (XRT) started soon after the event, dis- star to either a main-sequence star with spectral type later than covering only one variable X-ray source within the BAT error M5V (which means a mass <0.12 M) or to a semi-degenerate circle, namely, Swift J195509.6+261406 (hereafter Swift J1955; hydrogen poor star (Castro-Tirado et al. 2008). Kasliwal et al. 2008). This transient X-ray source was very dif- The large variations of its optical and infrared counterpart ferent from what expected for the X-ray afterglow of a long during the decay to quiescence, its distance and Galactic nature, GRB: it was decreasing in flux rather slowly, and it showed a set this transient apart from the typical optical afterglows of strong X-ray flaring activity. long-duration GRBs (see Liang et al. 2007 for a recent review). The source became undetectable by Swift-XRT on 2007 The resemblance of the optical bursts of Swift J1955 with June 29, ranging from a 0.5–10 keV flux of ∼10−9 to the short X-ray bursts from magnetars (see Mereghetti 2008 for <10−12 erg s−1 cm−2 in 19 days. While in outburst, Swift J1955 a recent review) led to the idea of a new kind of X-ray and had an X-ray spectrum that could be described by a rather optical transient event in a Galactic magnetar (Castro-Tirado hard power law corrected for the photoelectric absorption et al. 2008; Stefanescu et al. 2008). On the other hand, its X-ray 21 −2 (NH = 7×10 cm and = 1.7). Due to spatial and temporal flaring activity was also proposed to resemble the emission of coincidence (it was the only transient source in the BAT error the fast X-ray nova V4641 Sgr (Markwardt et al. 2008; Kasliwal circle), GRB 070610 and the X-ray transient Swift J1955 have et al. 2008), an unusual 9 M black hole in orbit with a 5–8 M been associated with high probability (Kasliwal et al. 2008). B9 III companion star (in’t Zand et al. 2000; Orosz et al. 2001) The most interesting and peculiar features of this transient In this Letter, we present the results of an ∼63 ks Chandra source came from optical and infrared observations. Many observation of Swift J1955 (see Section 2) aimed at unveiling its telescopes, triggered by the GRB-like event, promptly observed X-ray properties during quiescence (see Section 3) and compare the position of Swift J1955 during the outburst. A highly variable them with the current quiescent levels of the magnetar and X-ray optical and infrared counterpart was observed, showing large binary populations (see Section 4). flares for about 11 days after the GRB-like event, when it went back to quiescence. These large flares were characterized 2. OBSERVATION AND DATA ANALYSIS by a very short timescale: during the largest flare, the source increased its optical flux by more than a factor of 200 in less The Chandra X-Ray Observatory observed Swift J1955 for than 4 s. Furthermore, a broad quasi-periodic oscillation was ∼63 ks with the Advanced CCD Imaging Spectrometer (ACIS) observed in the optical band at ∼0.16 Hz (Stefanescu et al. instrument (ObsID 10042) from 2009 August 3 16:09:57 to 2008). August 4 09:55:59 (Terrestrial Time) in VERY FAINT (VF) 1 The Astrophysical Journal Letters, 729:L21 (5pp), 2011 March 10 Rea et al. timed exposure imaging mode. The source was positioned on the place the source ∼4–5 kpc. The main problem to assess a back-illuminated ACIS-S3 CCD at the nominal target position distance error bar is that the Galactic plane in the direction (R.A.: 19 55 09.653, decl.: +26 14 05.84 ±0.27; J2000), and of Swift J1955 extends only until ∼5 kpc, behind which there we used a sub-array of 1/8 leading to a time resolution of is the bulge, with its intrinsically lower column density. This 1.14 s. Standard processing of the data was performed by the makes it extremely hard to define an upper limit to the distance, Chandra X-Ray Center to Level 1 and Level 2 (processing since in the bulge a column density versus distance relation is software DS 8.0). The data were reprocessed using the CIAO not well defined. Hereafter, we will discuss our results assuming software (version 4.1.2). We used the latest ACIS gain map, and a distance range of 3.7–10 kpc, considering the farthest limit in applied the time-dependent gain and charge transfer inefficiency the Milky Way as a distance upper limit. corrections. The data were then filtered for bad event grades and The 95% upper limit on the quiescent X-ray luminosity of × 30 2 −1 × 30 2 −1 only good time intervals were used. No high background events Swift J1955 is 2.8 10 d5 kpc erg s or 1.9 10 d5 kpc erg s were detected, resulting in a final on-time exposure of 62.732 ks. for the power-law or blackbody models, respectively (see also Section 3). Considering the whole 3.7–10 kpc range (see above), 3. RESULTS the derived quiescent X-ray luminosity range is between 1.5 and 11.4 × 1030 erg s−1 or 1.0 and 7.9 × 1030 erg s−1, again assuming We did not detect any X-ray source at the position of a power law or a blackbody, respectively. the optical counterpart to Swift J1955 (Kasliwal et al. 2008; Castro-Tirado et al. 2008). In particular, we detected no 4.1. On the Magnetar Interpretation 0.3–10 keV photons within a 1 circle centered on the optical position. We took a 95% upper limit of three photons (Gehrels The discovery of fast optical bursts in Swift J1955 led 1986) and inferred a 4.78 × 10−5 counts s−1 upper limit on the a few authors to claim the magnetar nature of this source X-ray quiescent count rate of Swift J1955. (Castro-Tirado et al. 2008; Stefanescu et al. 2008), based on Using PIMMS8 we estimated the 95% upper limit on the the similarity of the optical light curve with the typical short source flux assuming: (1) an absorbed power-law spectrum X-ray bursts from magnetars.
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