Multiwavelength Observations of Swift J1753. 5-0127

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Oct 31, 2013 - The continuum is well fit with disk models dominated by vis- .... tinue to drop to shorter wavelengths, but instead showed a recovery and an increasingly ... UV observations were simultaneous, while the optical/NIR data were ...
Multiwavelength Observations of Swift J1753.5-0127

arXiv:1311.0031v1 [astro-ph.HE] 31 Oct 2013

Cynthia S. Froning1,2 [email protected] Center for Astrophysics and Space Astronomy, University of Colorado, 593 UCB, Boulder, CO 80309-0593 Thomas J. Maccarone [email protected] Department of Physics, Texas Tech University, Box 41051, Lubbock, TX 79409 Kevin France [email protected] Center for Astrophysics and Space Astronomy, University of Colorado, 593 UCB, Boulder, CO 80309-0593 Lisa Winter [email protected] Space Weather and Effects Division, Atmospheric and Environmental Research, Superior, CO Edward L. Robinson [email protected] Department of Astronomy, University of Texas at Austin, Austin, TX 78712 Robert I. Hynes [email protected] Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803 and Fraser Lewis

–2– [email protected] Faulkes Telescope Project, University of South Wales; Astrophysics Research Institute, Liverpool John Moores University ABSTRACT We present contemporaneous X-ray, ultraviolet, optical and near-infrared observations of the black hole binary system, Swift J1753.5-0127, acquired in 2012 October. The UV observations, obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope, are the first UV spectra of this system. The dereddened UV spectrum is characterized by a smooth, blue continuum and broad emission lines of C IV and He II. The system was stable in the UV to 3σ) below 10% on 50 sec time scales. Above that limit, the COS data were steady over the course of the observations. There were no trends in the FUV light curves and all fluctuations were consistent with statistical noise. Finally, we examined the C IV line profile over time to see if we could identify radial

–8– velocity variations in the line over the orbital period. The orbital period of Swift J1753.50127 is unknown: a 3.24 hr photometric periodicity has been observed and was interpreted as a superhump modulation that occurs in low mass ratio systems when a 3:1 orbital resonance is established within the accretion disk, causing the disk to precess and exhibit a periodic emission modulation (Zurita et al. 2008; Haswell et al. 2001; Whitehurst 1988). Using an empirical relation between the orbital and superhump periods determined by Patterson et al. (2005), Zurita et al. found an orbital period range for Swift J1753.5-0127 between 3.18 < Porb < 3.24 hr. We examined spectra extracted in time subintervals from 100 to 800 sec. Unfortunately, the combination of low signal to noise in the individual spectra, the relatively low contrast between the line and continuum (the peak line flux is only 50% above the continuum flux) and the contamination of the line center by the interstellar absorption precluded obtaining reliable velocity shifts from C IV. The He II line was also too faint to yield significant results.

3.2.

Interstellar Absorption and Reddening

There have been various estimates for the interstellar absorption and reddening along the line of sight to Swift J1753.5-0127. Fits to X-ray spectra in which hydrogen column density, NH , was allowed to vary have resulted in values of NH = 1.7 – 2.3×1021 cm−2 (Hiemstra et al. 2009; Miller et al. 2006; Morris et al. 2005). Cadolle Bel et al. (2007) and Durant et al. (2008) determined reddening values from the equivalent widths of the optical Na I D lines using the prescription of Munari & Zwitter (1997) and converted these to NH after Bohlin et al. (1978). Using this method, Cadolle Bel et al. (2007) found E(B–V) = 0.34±0.04 and NH = 2.0 × 1021 cm−2 , while Durant et al. (2008) obtained E(B–V) = 0.42±0.02 and NH = 2.45 × 1021 cm−2 . Durant et al. speculated that the absorption internal to the system may vary over time, given the difference in their results from those of Cadolle Bel et al. Although there is some variation in determinations of NH , all of the derived values are comparable to the average total Galactic absorption along this line of sight (Morris et al. 2005; Durant et al. 2009). We obtained values for both E(B–V) and NH from our UV spectra by fitting 2175 ˚ A dust feature and the damping wings of the interstellar Lyα absorption line profile, respectively. To determine the reddening, we fit the 2175 ˚ A feature in the NUV. Figure 2 shows the UV spectrum of Swift J1753.5-0127 in the region of the 2175 ˚ A dust feature. The upper panel shows the observed spectrum normalized to a mean value of 1.0. Using the ccm unred

–9– task from the IDL Astronomy User’s Library4 we dereddened the spectrum, stepping E(B–V) from 0.00 to 1.00 in steps of 0.01 to find the reddening value that minimized the χ2 deviation about a power law fit to the residual. RV was held fixed at 3.1. The best fit was E(B–V) = 0.45 with χ2ν = 1.55 (566 dof )for a 1400 – 3100 ˚ A fit range. The reddening is consistent with the value found by Durant et al. (2008). Following the lead of Fitzpatrick (1999), who noted the large scatter about the mean in the amplitude of the 2175 ˚ A feature in Galactic sight lines, we assume 20% uncertainty on our derived reddening and adopt E(B–V) = 0.45 ± 0.09. Figure 3 shows the Lyα absorption line and the best fit to the line profile. We fit the interstellar H I column density using a linear approximation of the observed (reddened) FUV continuum of Swift J1753.5-0127 and a Voigt profile for the Lyα resonance line. First, the geocoronal airglow emission (1213.5 – 1218.3 ˚ A) was removed from the core of the interstellar absorption trough. A linear fit was then made across the Lyα region (1174 – 1270 ˚ A), anchored on the blue end by the average continuum flux from 1175 – 1186 ˚ A and ˚ on the red end by the average continuum flux from 1254 – 1258 A. The Lyα absorber is characterized by a column density, NH (in units of cm−2 ), a Doppler-b parameter (in units of km s−1 ), and a velocity offset relative to the rest wavelength of the Lyα transition (in units of km s−1 relative to λrest = 1215.67 ˚ A). The velocity was fit by eye to produce a profile that evenly filled in the red and blue edges of the Lyα absorption trough (vLyα = −10 km s−1 ). The b-value was chosen to be typical of the local ISM (10 km s−1 ; Redfield & Linsky 2004), although the b-value does not have a large influence on the heavily damped line profile observed for Swift J1753.5-0127. The interstellar column density was then varied until a best-fit value was found, with errors bars defined as values of NH that produce fits consistent with the 1-σ photometric error bars in the line-core and wings. This procedure yielded NH = 2.0 ± 0.3 × 1021 cm−2 . If we use the reddening to infer NH from the relation of Bohlin et al. (1978), we obtain a value larger than the one directly found from the Lyα line profile fitting, although they agree within the uncertainties on NH and E(B–V). Relating gas and dust absorption is subject to a number of uncertainties that could cause the slight discrepancy: we do not know the value of RV for this sight line and there is considerable scatter in the relationships between reddening and the size of 2175 ˚ A feature (as noted above) and between gas and dust for individual sight lines (∼30% scatter about the Bohlin et al. relation). The gas along the line of sight to Swift J1753.5-0127 may also be more metal rich than the average Galactic value, resulting in a larger reddening and a higher NH found from the X-ray fitting compared to the fit to the Lyα line profile (whereas the Lyα interstellar line directly traces the neutral hydrogen 4

http://idlastro.gsfc.nasa.gov/homepage.html

– 10 – along the line of sight, the NH from the X-ray model fits includes absorption from helium and metals). Ultimately, the various methods for determining NH and E(B–V) for the sight line to Swift J1753.5-0127 have proven satisfactorily consistent within the uncertainties.

3.3.

Accretion Disk Model Fits to the UV Spectrum

If we assume that the UV emission is dominated by the accretion disk, we can use disk model fits to the dereddened spectrum to determine black hole masses and accretion rates as a function of the distance to Swift J1753.5-0127. Most parameters are unknown or poorly constrained for this system, however, so our fits will be more instructive in placing limits on interesting values than in providing precise determinations. We fit the dereddened UV continuum with a grid of steady-state accretion disk model spectra constructed from summed, area-weighted blackbody spectra. The blackbody spectrum for each disk annulus was chosen to correspond to the theoretical, steady-state thin accretion disk temperature for that annulus. The models did not include disk irradiation. Following Cheng et al. (1992), we can describe the observed flux density from a thin, steady-state, viscously-heated accretion disk as: cos i 1/3 fν = f0 2 (mm) ˙ 2/3 ν15 d

Z

xout (ν)

xin (ν)

x5/3 dx ex − 1

(1)

where f0 ≃ 2.9 × 10−26 ergs cm2 s−1 Hz−1 , d is the distance in kpc, i is the binary inclination, m (= MBH / M⊙ ) is the mass of the black hole, m ˙ is the mass accretion rate −9 −1 15 in units of 10 M⊙ yr , ν15 = ν/10 Hz, and x = hν/kT (r) where T (r) denotes the blackbody temperature (K) and radius (in units of 1011 cm) of each annulus. The disk temperature at each annulus is a function of MBH , m, ˙ and the radius. The overall temperature distribution in the disk depends on the inner and outer disk radii adopted. For the inner disk radius, rin , we examined models between two extreme values: rin = 1.23rg and 500 rg , where rg is the gravitational radius for a given black hole mass. The values represent differing predictions of the inner radius of the thin disk as extending to the Innermost Stable Circular Orbit (ISCO) for a maximally-rotating Kerr black hole or truncated at larger radii under the ADAF model paradigm (Reynolds et al. 2010; Zhang et al. 2010). We set the outer disk radius, rout , to 60% of the Roche lobe radius. Following Zurita et al., we fixed Porb = 3.23 hr and the donor star mass to M2 = 0.3 M⊙ , which we used in conjunction with

– 11 – the black hole mass to set the binary geometry and rout .5 Because the UV flux is dominated by emission from the inner disk annuli, the exact value of the rout does not affect our fit results. For these model parameters, the accretion disk model spectra for Swift J1753.5-0127 follow a ν 1/3 power law profile in the FUV spectral region. We can therefore use the normalization of the disk models to the observed FUV flux to constrain the black hole mass as a function of accretion rate, distance, and inclination. For the continuum region centered on 1470 ˚ A (1425–1520 ˚ A; ν = 2.04 × 1015 ), we measure an average flux density of fλ = 6.48 × 10−14 ergs cm2 s−1 ˚ A−1 . Converting to the frequency domain and substituting into equation 6 of Cheng et al., we can express the mass of the black hole in Swift J1753.5-0127 as: 3   −1 MBH m ˙ d −3/2 = 0.546[cos i] M⊙ 1kpc 10−9 M⊙ yr −1

(2)

The binary inclination, distance, and mass accretion rate for Swift J1753.5-0127 are not known. The system does not show eclipses, which restricts inclinations to i . 80◦ . Reis et al. (2009) found i = 55◦+2 −7 based on model fits to X-ray reflection features. We adopt this as our default inclination but also examine how varying the inclination affects the other model parameters. (Note that the reflection fits assume that the disk extended to the ISCO at the time of the observations, which may not have been the case; see § 4.) Swift J1753.5-0127 has remained in the low/hard state throughout its outburst. Maccarone (2003) found that XRBs transition from the soft to the hard state between 1–4% of the Eddington luminosity. The transition from hard state to soft can occur over a somewhat larger range of luminosities due to hysteresis effects (Maccarone & Coppi 2003). Here, we adopt an upper limit on the mass accretion rate of m ˙ ≤ 0.05m ˙ Edd , where m ˙ Edd = LEdd /(0.1c2 ), assuming a radiative efficiency of 10%. For the units in equation (2), this corresponds to m ˙ ≤ 1.097 × m. Since our data were taken at a X-ray luminosity several times smaller than the peak value (when the system remained in the low state), this upper limit is conservative. Based on these parameter assumptions, we have generated upper limits on the distance to Swift J1753.5-0127 for given black hole masses based on fits to the dereddened UV continuum. These are given in Table 3. For the upper limits, we also assumed that the disk 5

Shaw et al. (2013) have claimed a 402 d modulation in the X-ray light curves of Swift J1753.5-0127. If this is attributed to a disk precession period, the inferred mass ratio for the system may be very low, q ∼ 0.002, and M2 may be much smaller than 0.3 M⊙ . This has a negligible effect on our models here, however, as the UV emission is only weakly dependent on the size of the outer accretion disk.

– 12 – radius extends to the ISCO to maximize the potential disk emission. The distance could be higher for a lower inclination disk so we also present the upper limit on the distance for the extreme case of a face-on disk with i = 0◦ . Figure 4 shows the 12 M⊙ disk model compared to the data. The accretion disk models with viscous heating and no irradiation provide qualitatively good fits to the shape of the FUV continuum. The observed spectrum may be somewhat more blue than the model at the shortest FUV wavelengths, but we don’t make a quantitative conclusion on this point, because a small error in the adopted dereddening and/or the presence of unidentified emission features (such as C III 1175 ˚ A and N V 1238, 1242 ˚ A) could bias the continuum shape in this region. We are unable to distinguish between an accretion disk extending to the ISCO or one truncated at larger radii from the UV data alone (although see § 4 for comparisons with the broadband SED). In either scenario, the UV is on the ν 1/3 power law portion of the disk SED, with peak emission occurring in the EUV for the truncated disk or in soft X-rays for the ISCO disk. The two radii give different results when fit to the data, however, since the ISCO model generates more UV flux: e.g., for a 9 M⊙ black hole at a 6 kpc distance, the inferred mass accretion rate must be 25% higher for the truncated disk compared to the ISCO radius to match the observed UV flux.

3.4.

The Spectral Energy Distribution

In Figure 5 we plot the broadband spectral energy distribution (SED) for Swift J1753.50127 at the time of our observations. Table 4 gives the data we used for the SED for future reference. The NIR/optical/UV data were dereddened assuming E(B–V)=0.45. For the Xray data, we plot the power law component of the model fit to the data and its uncertainties. In the figure, we also plot previous SEDs acquired in 2005 (three months after outburst start) and in 2007, with the optical/NIR points dereddened using our E(B–V) value for a consistent comparison (Cadolle Bel et al. 2007; Durant et al. 2009). A comparison of the X-ray SED from the three epochs shows that the X-ray fluxes have declined. The system has also faded in the optical/NIR. The NIR fluxes we observed are ≃90% and the optical fluxes are ≃65% of those observed in 2005. The 2012 data are also fainter than the 2007 observations, with fluxes that are ≃75% of the 2007 data in the optical. The shape of the optical/NIR SED has not changed significantly as it has faded, however: all three epochs show approximately power law shapes, although a single power law cannot fit all of the optical/NIR data in any epoch. The new FUV observations show that the power law shape extends to ≃1150 ˚ A at least: the spectral break between the X-rays and lower energy data occurs at higher energies than the FUV.

– 13 – 4. 4.1.

Discussion

The UV Spectrum

In this manuscript, we have presented the first UV spectrum of Swift J1753.5-0127, accompanied by contemporaneous X-ray, optical, and NIR observations. The dereddened UV spectrum is characterized by a continuum that smoothly increases to the blue and broad line emission from C IV, He II, and possibly N V. There are now high quality FUV observations of three black hole XRBs in outburst: Swift J1753.5-0127, XTE J1118+480, and XTE J1859+226 (Haswell et al. 2002). The emission line spectra are different in each case. In XTE J1859+226, the UV spectrum has strong lines of C III, C IV, N V, O V, He II, and Si IV. That spectrum is richer in emission lines than Swift J1753.5-0127, and the N V line flux is nearly equal to that of C IV, whereas we do not confidently detect N V here. As noted by Haswell et al. the N V line can be suppressed relative to C IV by either photoionization by a harder X-ray ionizing spectrum or by relatively metal-poor abundances. On the other hand, XTE J1118+480 shows the opposite effect, with much stronger N V emission and no C IV, an effect that has also been observed in A0620-00 (Froning et al. 2011) and is attributed to CNO processing in the accreted material. If Swift J1753.5-0127 had followed the evolutionary path proposed by Haswell et al. for XTE J1118+480 and A0620-00, wherein mass transfer was initiated after the donor star had already begun to move off the main sequence, their model would predict a current C/N abundance ratio for Swift J1753.5-0127 of log(C/N) ≤ −2.3, lower than in XTE J1118+480, where C IV is undetectable (Haswell et al. Fig. 3; see also their discussion of why the low C V to N V ratio is unlikely to be caused by photoionization effects). Instead, the UV line spectrum of Swift J1753.5-0127 suggests little CNO processing, which is consistent with a system that initiated mass transfer at a shorter orbital period when the lower-mass donor star was still on the main sequence. More generally, the comparison of the three black hole XRB spectra in the UV shows that the complicated interplay of evolutionary history, metallicity, and photoionization leads to diverse spectroscopic signatures; observations of more systems will be valuable to constrain the physical processes underlying the line emission. Swift J1753.5-0127 was stable during our observations to 21 and a distance d > 1 kpc for a M2 main sequence donor star. Their comparison of the outburst luminosities to various empirical relations for XRBs gave a distance range of ≃2.5–8 kpc. The interstellar hydrogen absorption column

– 16 – density in the spectrum, NH , of Swift J1753.5-0127 is comparable to the total Galactic column density in this direction, also suggesting a distance d ∼6–7 kpc (Morris et al. 2005; Cadolle Bel et al. 2007; Durant et al. 2009). A 6 kpc distance would correspond to a black hole mass ≥10.4 M⊙ . We compared our disk model results to the system parameters adopted by Zurita et al. (2008): MBH = 12.0 M⊙ and d = 5.5kpc. If we adopt those values, we must set m ˙ ≃ −8 ◦ 2 × 10 M⊙ / yr, or 0.08m ˙ Edd , to match the UV flux (for an i = 55 disk). Some of the ADAF models of XRBs have predicted that the accretion rate could be as high as m ˙ = 0.08 − 0.1m ˙ Edd in the hard state (Esin et al. 1997; Zhang et al. 2010). If the accretion rate is