Multiwavelength Observations of A0620-00

slugcomment: [current draft: July 5, 2018]


[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 (citation not found: 1998A&Abessell) (optical),