Multiwavelength Observations of A0620-00

slugcomment: [current draft: October 22, 2016]


[still needed]

black hole physics, stars: individual: A0620-00, X-rays: binaries


[still needed]

This paper presents observations of the X-ray Binary A0620-00 taken simultaneously in X-rays with the Chandra Observatory, in radio with the Jansky VLA, and in optical and infrared with the SMARTS 1.3m telescope. Section \ref{sect:obs} describes our observations in detail, Section \ref{sect:sed} discusses the Spectral Energy Distribution for A0620-00, Section \ref{sect:correlation} discusses these results in the context of the radio/X-ray correlation for X-ray Binaries, and Section \ref{sect:concl} contains our conclusions.


X-ray Observations


We observed A0620-00 using the ACIS detector on board the Chandra X-ray Observatory [ADS/Sa.CXO#obs/14656 (catalog ObsId 14656)] on 2013 December 9, starting at 00:52 UT, for \(\sim\)30 ks. Standard level-2 data products were used, and were processed and calibrated with the most current version of the CIAO software (4.7, CALDB 4.6.5).

Since A0620-00 is known to vary over time, we wanted to determine whether this most recent X-ray observation differed significantly from previous ones. We therefore downloaded the two previous observations of this source from the Chandra Data Archive: ADS/Sa.CXO#obs/00095 (catalog ObsId 95) (2000 February 29), and ADS/Sa.CXO#obs/05479 (catalog ObsId 5479) (2005 August 20). These older observations were reprocessed using the same calibration information, software versions, tasks, and parameters as our most recent observation.

The high energy particle background is known to increase rapidly on the ACIS detectors below 0.3 keV and above 8 keV. We therefore restricted most analysis to the energy range \(0.3-8\) keV to minimize this background. Background flares were removed from our observations by examining all data within the energy range \(0.3-10\) keV, and removing time periods with count rates more than one sigma away from the mean. In this manner 0.25 ks were removed from ObsID 14656, leaving 29.4 ks for our analysis.

Total counts and count rates for A0620-00 were calculated using a circular aperture with radius 10 pixels. Background counts and rate were measured in an annulus centered on A0620-00 with inner radius 11 pix and outer radius 20 pix. The source had 434 total counts, 0.0148 ct s\({}^{-1}\), in the aperture during our observation. For comparison, these values are shown in Table \ref{table:xraycounts} along with the total source counts and count rates for the two previous A0620-00 observations (calculated using the same apertures given above).

\label{table:xraycounts}Chandra Observations of A0620-00
Observation Length Total Counts Count Rate (ct s\({}^{-1}\))
Date ObsID OriginalaaActual length of exposure. AnalyzedbbExposure length after removal of background flares. Src / Bkgnd Src / Bkgnd
2000 Feb 95 42.1 ks 32.4 ks 139 / 103 0.0043 / 0.0022
2005 Aug 5479 39.6 ks 39.6 ks 376 / 143 0.0095 / 0.0036
2013 Dec 14656 29.7 ks 29.4 ks 434 / 62 0.0148 / 0.0021

The spectra were extracted from all three Chandra observations, and they are shown (binned to 10 cts/bin) in Figure \ref{fig:xrayspectrumlin} (linear axes) and Figure \ref{fig:xrayspectrumlog} (log axes). We attempted fitting various models to the spectrum including a power-law model and thermal models. Models were fit with the column density as a free parameter and also with it fixed to the optical value, \(N_{\mathrm{H}}\) = \((1.95\pm 0.21)\times 10^{21}\) cm\({}^{-2}\) (converted from reddening value in Cantrell et al. (2010) using formula in Güver et al. (2009)). The power-law model provided the best fit to our spectrum. Fitted values for the column density were largely consistent with the optical value, although not always for the thermal models. Calculated model parameters, both free and fixed, are shown in Table \ref{table:xraymodels} for all three observations of A0620-00.

Figure 1: X-ray spectra.
(a) Linear axes; \(x\)- and \(y\)-axes are the same for all three panels. (b) Log axes; \(x\)- and \(y\)-axes are the same for all three panels.
\label{table:xraymodels}Modeling of X-ray data for A0620-00
ObsID \(N_{\mathrm{H}}\) Photon Index / Amplitude / \(\chi^{2}_{\mathrm{red}}\)
(\(10^{21}\mathrm{cm}^{-2}\)) Temperature (keV) Normalization
(\(\times 10^{-6}\))
Power Law

Note. – need to paste in updated numbers

After determining the best-fitting model for each spectrum, we calculated the \(1-10\) keV flux and luminosity, which are shown for all three observations in Table \ref{table:xrayflux}.

\label{table:xrayflux}X-ray Fluxes and Luminosities for A0620-00
Date FluxaaUsing \(N_{\mathrm{H}}\) = \((1.95\pm 0.21)\times 10^{21}\) cm\({}^{-2}\); see §\ref{sect:obs-xray} for details. LuminositybbUsing distance \(d=1.06\pm 0.12\) kpc and black hole mass \(M_{\mathrm{BH}}=6.6\pm 0.25\) M\({}_{\sun}\)(Cantrell et al., 2010)—.
(erg s\({}^{-1}\) cm\({}^{-2}\)) (erg s\({}^{-1}\)) (\(L_{\mathrm{X}}/L_{\mathrm{Edd}}\))
2000 Feb
2005 Aug
2013 Dec

Optical and Near-Infrared Observations


Optical and near-infrared observations were taken on the SMARTS 1.3m telescope (Buxton et al., 2012) at Cerro-Tololo InterAmerican Observatory. For this project A0620-00 was observed in \(B\), \(V\), \(I\), \(J\), \(H\), and \(K\) using the dual-channel imager ANDICAM. A0620-00 was observed continuously for \(\sim\)6 hours on the night of 2013 Dec 8/9, scheduled to coincide with the Chandra X-ray observation. We also observed A0620-00 in those same filters a few times a night for approximately a week before and a week after the Chandra X-ray observation, to be sure we would be able to identify whether the target was optically “active” or “passive” (Cantrell et al., 2008) on the night of our multiwavelength campaign. Optical and infrared (OIR) data were reduced and photometry was done using standard tasks in IRAF, following the procedures in Cantrell et al. (2008); Cantrell et al. (2010).

OIR magnitudes were dereddened using \(E(B-V)=0.29\pm 0.03\) (Cantrell et al., 2010) and \(R=3.1\). Extinction values \(A_{\lambda}\) were calculated using \(A_{\lambda}/A_{V}=a(x)+b(x)/R_{V}\), with relations for \(a(x)\) and \(b(x)\) from O’Donnell (1994) for the optical and Cardelli et al. (1989) for the infrared. Dereddened magnitudes were converted to flux density in Jy using the zero points given in Bessell et al. (1998) (optical), Frogel et al. (1978), and Elias et al. (1982) (infrared). Magnitudes and flux densities, as-observed and dereddened, are shown in Table \ref{table:oirdata}.

\label{table:oirdata}Optical/near-infrared measurements of A0620-00
As Observed Dereddened
Filter Magnitude Flux Density Magnitude Flux Density
(mJy) (mJy)
\(B\) 18.81 0.121 17.64 0.357
\(V\) 17.81 0.274 16.91 0.624
\(I\) 16.18 0.815 15.64 1.34
\(J\) 15.21 1.38 14.96 1.74
\(H\) 14.64 1.37 14.48 1.59
\(K\) 14.25 1.23 14.15 1.35
Nonstellar Only
\(V\) 0.129 0.293
\(I\) 0.348 0.571
\(H\) 0.378 0.438

Note. – rearrange: group by Mag or Flux, then give as-obs. and dered.
also: is it worth converting nonstellar flux back to mags to put in this table?

Subtracting the Star


We wanted to examine the spectral energy distribution (SED) for A0620-00 for nonstellar emission only, without contamination from the secondary star. Therefore we used the Ellipsoidal Light Curve code (Orosz et al., 2000) and the stellar parameters given in Cantrell et al. (2010) to generate star-only ellipsoidal light curves for each of the six filters in which we observed A0620-00. These star-only light curves were normalized to the zero-disk stellar magnitudes given in Cantrell et al. (2010), then dereddened and converted to flux densities in the same manner as described above. Star-only flux densities were subtracted from total observed flux densities to give the nonstellar flux density for each observation. These nonstellar values are shown in Table \ref{table:oirdata}, along with both the observed and the dereddened total OIR emission.

Radio Observations


We observed A0620-00 with the Karl G. Jansky Very Large Array (VLA) on 2013 December 9, from approximately 03:28 to 09:27 UT, with the array in ’B’ configuration. We observed in the C-band and K-band, splitting our C-band observations into two 1-GHz basebands centered at 5.25 GHz and 7.45 GHz. Our observations totaled just over an hour on-source in each of the C- and K-bands, with the remaining time used to observe several calibrator sources: 3C147, J0641-0320, and J0656-0323.

All processing and analysis were done using the Common Astronomy Software Applications (CASA, v4.2.1) package (McMullin et al., 2007). Each band (5.25, 7.45, 22 GHz) was processed and imaged separately. For each, RFI was removed and then data were calibrated and imaged11Imaging was done using the clean task in multi-frequency synthesis mode, with two Taylor terms and Briggs weighting (robustness parameter = 1.0). [[include image or contour plot]]

The rms of the noise was calculated in a large source-free box (\(\sim\)XX arcsec on a side) to the south-east of A0620-00, and the flux density of A0620-00 was calculated using an ellipse-fitting routine. Two additional sources were detected to the north and north-east of A0620-00, and the flux densities of each of these was also calculated in the same way. Flux densities, and their associated uncertainties, for all three sources in all three frequency bands are shown in Table \ref{table:radioflux}.

\label{table:radioflux}Radio Flux Densities for A0620-00 and Neighboring Sources
Source 5.25 GHz 7.45 GHz 22 GHz
(\(\mu\)Jy) (\(\mu\)Jy) (\(\mu\)Jy)
A0620-00 \(21.6\pm 4.4\) \(9.1\pm 4.5\)
N Source \(75.4\pm 4.4\) \(46.5\pm 4.5\)
NE Source