A strong emission line near 24.8 angstrom in the X-ray binary system MAXI J0556--332: gravitational redshift or unusual donor?
Dipankar Maitra, Jon M. Miller, John C. Raymond, Mark T. Reynolds
aa r X i v : . [ a s t r o - ph . H E ] O c t A strong emission line near 24.8 angstrom in the X-ray binarysystem MAXI J0556–332: gravitational redshift or unusual donor?
Dipankar Maitra , Jon M. Miller , John C. Raymond , and Mark T. Reynolds Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USAHarvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138,USA [email protected]
ABSTRACT
We report the discovery of a strong emission line near 24.8 ˚A (0.5 keV) inthe newly discovered X-ray binary system MAXI J0556-332 with the reflectiongrating spectrometer onboard the
XMM-Newton observatory. The X-ray lightcurve morphology during these observations is complex and shows occasionaldipping behavior. Here we present time- and rate-selected spectra from the RGSand show that this strong emission line is unambiguously present in all the XMMobservations. The measured line center is consistent with the Ly- α transition ofN vii in the rest frame. While the spectra contain imprints of absorption linesand edges, there appear to be no other significantly prominent narrow line due tothe source itself, thus making the identification of the 24.8 ˚A line uncertain. Wediscuss possible physical scenarios, including a gravitationally redshifted O viii Ly- α line originating at the surface of a neutron star or an unusual donor with anextremely high N/O abundance ( >
57) relative to solar, that may have producedthis comparatively strong emission line.
Subject headings: accretion, accretion disks — binaries: general — X-rays: bina-ries — X-rays: individual (MAXI J0556-332)
1. Introduction
Monitor of All-sky X-ray Image (MAXI) detected a new X-ray source on 2011 January11, subsequently named MAXI J0556-332 (Matsumura et al. 2011, hereafter we will refer tothe source as J0556). The discovery was immediately followed up by pointed observationsin other wavelengths. X-ray observations made with
Swift indicated that the interstellar 2 –extinction toward J0556 is small (N H ∼ cm − ; Kennea et al. 2011) compared to mostGalactic X-ray binaries (XRBs), which typically have N H ∼ cm − . The low N H , largelyowing to the high Galactic latitude of the source ( b =-25.183), implies that minimal line-of-sight absorption will facilitate efforts to study its low-energy X-ray spectrum. The X-rayspectral and timing properties of J0556 suggest that it is an XRB currently going through anepisode of active accretion. The optical counterpart for this source was found to be a stellarobject with USNO B1.0 magnitudes of m R =19.91 and m B =19.52, and an optical spectrumobtained by Halpern (2011) showed emission lines from H α , He I, and He II, indicating thepresence of an accretion disk near the compact accretor. Spaceborne X-ray and groundbasedI-band monitoring (using the CTIO/SMARTS 1.3m telescope) shows that the source is stillactive as of 2011 October 20.The nature of the compact accretor or the companion donor star in J0556 is currentlyunknown, although most evidence point towards a neutron star (NS) accretor. No “type 1”bursts have been seen from the source so far, but the X-ray timing properties and evolutionon color-color diagram are similar to those of NS XRBs (Homan et al. 2011). The optical-to-X-ray and radio-to-X-ray flux ratios are also similar to Galactic NS XRBs (Russell et al.2011; Coriat et al. 2011). The distance to the source is also currently unknown. Analyzingthe archival data of the source from the Catalina Real-Time Transient Survey revealed thatno optical outburst had been detected since August 2005 (Mahabal et al. 2011). Data from RXTE/All Sky Monitor show that this is its first X-ray outburst since January 1996.
2. Data Reduction and Analysis
XMM-Newton observed J0556 on MJD 55608 and 55653 (hereafter referred to as Obs1and Obs2 respectively), each observation being of about 40 ksec duration. We analyzed thedata obtained by the reflection grating spectrometer (RGS; den Herder et al. 2001) using
XMM-Newton
Science Analysis Software (SAS; v.11.0.0) and following the standard extrac-tion procedures outlined in the
XMM-Newton
User Guide . The background light curveswere examined for the presence of proton flares. While there were no flares during Obs1,data from the initial 14 ksec of Obs2 had to be discarded due to high background. The SASextraction regions for source and background spectra were inspected using the spatial- andenergy-dispersion plots and left unchanged from the SAS default. The observed fluxes ineach RGS CCD were compared with their 2% pile-up flux limits ( § XMM-Newton
User’s Handbook), and we found that pile-up was significantly less than 2% even for http://xmm.esac.esa.int/external/xmm user support/documentation/sas usg/USG/ ∼
20 ksec during the middle of the observation as well as ex-tended epochs without any dips. This confirms previous shorter X-ray observations of J0556where the dips were seen occasionally (e.g. as reported by Strohmayer 2011; Belloni et al.2011). Given the marked changes in the source behavior, it is likely that the spectral prop-erties were changing during Obs1. Therefore we extracted spectra from 3 separate intervalsbased on the combined RGS count-rate ( cps ) and time ( t seconds since the start of the ob-servation): (1) steady-high – when 1,000 < t < cps>
45, which constitutes most ofthe time when the source flux was steady; (2) unsteady-high – when t > cps> unsteady-low – when t > cps>
35, selecting the photons collected during the minima of thedipping period. These selections are marked in Fig. 1 by differently colored regions. Oncethe background flare was excluded, the Obs2 light curve was steady and did not show anydips or any other signs of large variability. Therefore we created one single time-averagedspectrum of the entire Obs2 after excluding the initial flare.We analyzed data from the full RGS bandpass (7–36˚A) for both RGS1 and RGS2 units.Since the second order RGS spectra do not cover the wavelength region near the 24.8 ˚A line,we restrict our analysis to the first order spectra only. The spectra were grouped for all fitswith the criterion that there were at least 30 counts/bin. Fits to the X-ray spectra wereperformed in wavelength space using ISIS (v.1.6.1-36; Houck & Denicola 2000) and XSPEC(v12.7.0; Arnaud 1996). The solar abundance table given by Anders & Grevesse (1989) wereused for all fits.Apart from the 24.8 ˚A emission line which is readily visible in the RGS spectra (Fig. 2),there are also many low-significance “bumps and wiggles”. However there appear to be nostrong narrow spectral features associated to the source that are of comparable strengthto the 24.8 ˚A line. Signatures of the intervening interstellar medium (ISM) are seen viathe presence of neutral and ionized atomic oxygen edges and absorption lines in ∼ vi resonance line (28.78˚A in rest frame) and intercombination lines (29.082 and 29.084 ˚A). Note that the possibleN vi feature does not show the forbidden component at 29.535 ˚A. But that is not surprisingbecause the emission region in a compact XRB would be expected to be much denser than10 cm − . Also, if the excess is indeed due to N vi , this would support a nitrogen-rich donor 4 –scenario. While there are no other narrow lines of comparable strength, there is a broadfeature near 12.2 ˚A ( ∼ ∼ Letter , and will be presented in a later work (Maitra et al., in prep.). Howeverwe would briefly like to note that the phenomenological thermal+power law fits to the PNdata shows no strong evidence for hardening of the power law component during the dips.The power law photon index during dips (unsteady-low) is Γ=2.65 ± ± BB =0.70 ± BB =90 ± BB = 0.81 ± BB = 143 ± tbnew ; an updated version of Wilms et al.2000). The best-fit model parameters for the various observations/selections are given inTables 1 and 2. The quoted errors for the best-fit models correspond to a 90% confidencelimit for the continuum model parameters, and 68% confidence limit for line parameters.While fitting, the normalization of RGS2 was allowed to vary w.r.t. that of RGS1. Thefits show that the RGS2 normalization factor is in the range of 0.94–0.97. Fig. 3 shows the24.8 ˚A line flux and 26–28 ˚A continuum flux for the various observations/selections. Notethat the ratio of line-to-continuum flux during Obs2 (0.11 ± ±
3. Abundance estimates using XSTAR photoionization code
In order to obtain a self-consistent understanding of the elemental abundances in theobserved spectra we used the XSTAR photoionization code (Kallman & Bautista 2001). Agrid of 480 XSTAR models was created from the following parameters: (1) model columndensity of the emitting plasma was sampled between 10 − cm − , (2) the ionization pa-rameter (log( ξ ) where ξ = L/nR ) was sampled between 0–4, (3) N abundance was sampledbetween 1–100 times relative to solar, and (4) O abundance sampled between 0.01–1 timesrelative to solar. A hydrogen nucleus density of 10 cm − , and a covering fraction of 0.5 wereassumed. A high turbulent velocity of 1,000 km s − was used to simulate the line broaden-ing. Phenomenological blackbody+power law fit to the 0.6–10 keV EPIC-PN data obtainedduring the Obs1/steady-high state ( kT BB =0.86 keV, N BB =147, Γ PL =2.20, N PL =0.42) wereused as user-defined incident radiation field in XSTAR. For a source distance of 1 kpc (notethat Welsh et al. 2010 reported a cloud of Na I and Ca II at ∼
100 pc in the direction ofJ0556; moreover, given the high galactic longitude of J0556, a distance of ≫ × erg s − . The remaining XSTAR parameters were set to their default values.An XSPEC table model created from this grid was fit to Obs1/steady-high state. The bestfit suggests a model column density of 3.2 × cm − and log( ξ )=2.3. The 90% confidencelower limit on log( ξ ) is 2.1 (mainly based on an upper limit on the N vi /N vii ratio), butthe upper limit on log( ξ ) could not be constrained from the data. Since there is only onestatistically significant narrow line in the spectrum, it is not possible to constrain the abun-dance of any single element. Rather, the fits are only able to give limits on the relative N/Oratio. The fits suggest N/O overabundance >
57 ( >
27 at 90% confidence) with respect tosolar. This translates to an absolute N/O of > > viii Ly- α andsolar nitrogen abundance. The best-fit to this redshifted O viii model with χ / ν =4702/3608is statistically slightly less favored than the N-rich model with χ / ν =4673/3607. We notethat the best-fit log( ξ ) was pegged at the maximum value allowed by our redshifted oxygentable model, implying a rather high log( ξ ) >
4. Discussion and Conclusions
J0556 is one of the few exceptional XRBs whose low-energy X-ray spectrum can bestudied in detail due to low interstellar extinction. Fits to its RGS spectra imply a columndensity of (2.1–4.6) × cm − , which is at least an order of magnitude below most XRBs.Both XMM-Newton observations of J0556 show a strong emission line near 24.8 ˚A. If theline is from N vii , this would require the donor to have an extremely high N/O abundance( >
57) relative to solar, based on the weakness of oxygen lines of similar charge states.Typically, for an XRB with a solar-type donor, the oxygen abundance is ∼ × greater thanthat of nitrogen, and the X-ray spectra show lines from H- and He-like ions of oxygen.Even in the intermediate-mass X-ray binary Her X-1, wherein N/O is 4 × solar, O lines areobserved to be as strong or much stronger than N lines from similar charge states (see, e.g.,Jimenez-Garate et al. 2002). While N overabundance is not uncommon in low-mass XRBs(e.g., in 4 of 5 UV spectra analyzed by Raymond (1993) the N/C ratios were several timessolar, with a 9:1 ratio in Cyg X-2 being highest), the extremely high N/O in J0556 makes itan unique XRB.Given the lack of source distance and any signature of orbital periodicity so far, it isonly possible to speculate on the nature of the donor star in this system based on its strongN/O overabundance and color (see below), and exotic stars such as hot, core-helium burningsubdwarfs (sdB, sdO), or degenerate white dwarfs (WD) appear to be strong candidates. Hotsubdwarfs were first discovered at high Galactic latitudes by Humason & Zwicky (1947), andsubsequent studies have shown that about half of them are in binary systems with WD orlow-mass main sequence stars. It is generally thought that the hydrogen-rich outer envelopeof the progenitor hot subdwarf in a binary system is lost via either Roche-lobe overflow orcommon-envelope ejection mechanism, thus exposing CNO products dredged up from thecore (see, e.g., Han et al. 2002). Naslim et al. (2010) have recently determined abundancesfor 6 sdB stars. The absolute N/O ratios were 10, 13 and 20 for three of them (SB 21,LB 1766, and BPS CS 22940–0009), and oxygen was not detected in the other three. Thesestars were Ne-rich as well, so future observations of the Ne spectrum of J0556 would beespecially interesting. One particular sdB star, PG 1219+534, is observed to have N/O & vii line. Based on the diagnosticsgiven by Nelemans et al. (2010), a high N/C would also point towards a helium WD.Assuming that the magnitudes reported in the USNO-B1.0 catalog for the optical coun-terpart were made during quiescence, the temperature inferred from the de-reddened B-R 7 –color is ∼ ∼ few hours. Observations of suchUCBs are sparse, and their evolution is only recently being explored thoroughly (see, e.g.,Tauris & van den Heuvel 2006; Nelemans et al. 2010). From a sample of 50 sdB binariesGeier et al. (2008) found that possibly four could have a neutron star or black hole donor.However, despite a sensitive search, no radio pulsations have yet been detected from thesefour systems (Coenen et al. 2011). If the donor in J0556 is a hot subdwarf, this would bethe first direct evidence of a binary system with a hot subdwarf donor and a compact (blackhole or neutron star) accretor.Another intriguing possibility, assuming solar-abundance plasma, is that the observedline is a gravitationally redshifted O viii Ly- α line (rest-frame λ =18.967 ˚A) originating fromthe surface of the NS. A photon emitted with a rest-frame energy of E from the surface ofa slowly rotating, spherically symmetric NS of mass M NS and radius R NS is observed by anobserver at infinity to have an energy E where E/E = p − (2 G/c )( M NS /R NS ). Based onthe most precise mass estimates from pulsar timing experiments, observed NS masses rangebetween 1.25–2M ⊙ (see, e.g., Kramer & Wex 2009; Demorest et al. 2010). Assuming theobserved 24.8 ˚A line in J0556 is gravitationally redshifted O viii Ly- α , the above mass rangewould imply a radius of 8.9–14.2 km. In this redshifted oxygen line scenario, the width of theline could be used to put an upper limit on the relative size of the emission region as follows:assume that the width of the observed line profile is mainly due to superposition of linesoriginating at various heights above the NS. Since a photon emitted from the surface (i.e.from a radius of R NS ) is more redshifted than a photon emitted from R NS +∆ R , the observedwidth of the narrow line may be related to ∆ R as ∆ R/R NS ∼ σ/ ( E − E ) ∼ . R , since it ignores any bulk motion. However, a redshiftedO viii line scenario would also require other redshifted lines of oxygen to be present in thespectra, which are not seen. Also, the hint of N vi resonance and intercombination linesfavors the N/O overabundance scenario. Planned high-resolution optical/UV spectroscopy 8 –will be key in understanding the nature of this unique source.We thank the anonymous referee for helpful comments. It is a pleasure to thank NorbertSchartel and the XMM-Newton planning team for carrying out the ToO observations. Wewould also like to acknowledge the use of the daily monitoring data obtained by the RXTEwhich has provided excellent all-sky coverage of X-ray sources over the past 15 years.
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This preprint was prepared with the AAS L A TEX macros v5.2.
11 –Table 1. Best fit continuum parameters for the RGS data H × Γ E b Γ N (cm − ) (keV)Obs1/steady-high 3 . +0 . − . . +0 . − . . +0 . − . . +0 . − . . +0 . − . Obs1/unsteady-high 2 . +0 . − . . +0 . − . . +0 . − . . +0 . − . . +0 . − . Obs1/unsteady-low 3 . +0 . − . . +0 . − . . +0 . − . . +0 . − . . +0 . − . Obs2 3 . +0 . − . . +0 . − . . +0 . − . . +0 . − . . +0 . − . Errors are 90% confidence limits. Col. (1) gives the observation and time/rate selectionas discussed in §
2; col. (2) is the best-fit N H in units of 10 atoms cm ; cols. (3,5) arepower law photon indices for E < E b and E > E b respectively; col. (4) is the break pointfor the energy; col. (6) is the power law normalization at 1 keV, i.e., photons keV − cm − s − at 1 keV.; Table 2. Best fit narrow line parameters for the RGS data σ × Flux Equiv. width χ /ν (˚A) (˚A) (photons cm − s − ) (m˚A)Obs1/steady-high 24 . +0 . − . . +0 . − . . +0 . − . +15 − . / . +0 . − . . +0 . − . . +0 . − . +16 − . / . +0 . − . . +0 . − . . +0 . − . +26 − . / . +0 . − . . +0 . − . . +0 . − . +18 − . / Errors are 68% confidence limits. Col. (1) same as in Table 1; cols. (2–5) best fit Gaussian line center,width ( σ ), line flux, and equivalent width for the narrow line; col. (6) χ /d.o.f for the best fit to the 7–36 ˚ARGS data.
12 –
20 25 30 35 40 45 50 55 0 5000 10000 15000 20000 25000 30000 35000 40000 C o m b i ned R G S c oun t/ s Seconds since MJD 55608.182051Steady-high Unsteady-lowUnsteady-high
Fig. 1.— RGS light curve of the first
XMM-Newton observation (Obs1) showing its complexmorphology, especially epochs of dipping and non-dipping behavior. The variously coloredregions represent the intervals from which spectra were analysed. 13 – × − − λ F λ ( e r g s c m − s − ) C V I K − edge C V K − edge C V I L y − α N V II K − edge N V I K − edge N V II L y − α N V I r , i ,f O V III K − edge O V II K − edge O V III L y − α O V II r , i ,f Ne X K−edge N e I X K − edge N e X L y − α N e I X r , i ,f N a X I K − edge Na X K−edge N a X I L y − α N a X r , i ,f N a X I L − edge M g X II L y − α M g X I r , i ,f Mg XII L−edge − χ Without N VII line
10 15 20 25 30 35 − χ Wavelength (Å)
With N VII line
Fig. 2.—
XMM-Newton /RGS spectrum of J0556, in 7–36 ˚A range, obtained on MJD 55608during the steady-high period (i.e. the cyan region in Fig. 1).
Top panel : RGS1 data inred and the RGS2 data in blue (binned for visual clarity only). Data from both RGS unitswere fitted jointly as described in text. The joint best-fit models for RGS1 and RGS2are shown by red and blue histograms respectively. Rest-frame energies of few differentlines/edges typically prominent in this range are labeled. The energies of the resonance (r),intercombination (i), and forbidden (f) lines are also shown. The broad feature in the 10–14˚A range was modeled with a Gaussian centered at 12.2 ˚A and width ( σ ) of 1.4 ˚A. Middlepanel : Residuals in units of standard deviation, when the normalization of the narrow lineat 24.8 ˚A (0.5 keV) was set to zero.
Bottom panel : Residuals in units of standard deviation,to the best-fit continuum + line model. 14 – . Å e m i ss i on li ne f l u x ( P ho t on s / s / c m ) Photons/s/cm )Obs1/steady−highObs1/unsteady−highObs1/unsteady−lowObs2 Fig. 3.— Photon flux from the 24.8 ˚A emission line vs. 26–28 ˚A continuum flux during thevarious observations/selections. 15 – -4 -3 -2 S w i ft/ BA T ( - k e V c p s / c m ) MJD10 -2 -1 M AX I ( - k e V c p s / c m ) -1 X T E / AS M ( . - k e V c p s ) I - band ( m ag ) H - band ( m ag ))