A NuSTAR Observation of the Black Hole Candidate 4U 1755-338

ATel #13665; P. Draghis, J. M. Miller, M. Balakrishnan, E. Kammoun, M. Reynolds, A. Zoghbi (University of Michigan)
on 24 Apr 2020; 17:01 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

Tweet

The black hole candidate 4U 1755-338 recently entered an outburst phase, after 24 years of quiescence (ATEL #13606).

We observed 4U 1755-338 with NuSTAR for a total of 32 ks, starting on 2020-04-09 07:36:09 UT. The source produced an average count rate of 24 c/s, when combining the two FPM NuSTAR sensors. The light curve shows no strong variability over the duration of the observation. We extracted the source spectrum using a 100 arc second (radius) region, and a separate background region of equivalent size. Prior to spectral fitting, the spectra were binned to require a minimum signal to noise of 10 in each bin.

The source is clearly detected out to 25 keV, so we fit the spectra over the 3-25 keV band. We fit the spectra from the two FPM sensors jointly, allowing a multiplicative constant to float between the two. In XSPEC parlance, the model used was constant*tbabs*(diskpbb+powerlaw). The "diskpbb" model is a version of the familiar "diskbb" model that measures the relationship between disk temperature and radius via T ~ r^-p, where p = 0.75 is the canonical value. All model parameters (except the multiplicative constant) were linked between the two spectra. The constant was fixed to 1 for FPMA and left free for FPMB, returning a value of 0.973 +/- 0.003.

This model yields a good description of the spectrum, returning chi2/dof = 495/397 (1.25). The fit prefers a low value for the ISM column density, hitting a lower bounding value of N_H = 5 E+21 cm^-2 suggested by Church & Balucinska-Church (1997). The model returns a temperature of the blackbody of T_in= 1.10 +/- 0.03 keV and an exponent of the radial dependence of the disk temperature of p = 0.57 +/- 0.08, suggesting that the disk is irradiated. The photon index of the power-law is Gamma=2.44 +/- 0.18. There is no clear indication for disk reflection, and no clear indication of a disk wind in the NuSTAR spectra.

The lack of strong variability, the disk temperature and power-law index, and the lack of strong reflection are all consistent with our observation having occurred in the "high/soft" (aka "thermal dominant") state. Based on this model, an absorbed flux of F = 1.09 E-09 ergs/cm^2/s is inferred, with an unabsorbed flux of F = 1.61 E-09 ergs/cm^2/s in the 0.5-100 keV range. The high/soft state is typically associated with Eddington fractions above ~0.1. For an assumed black hole mass of 3 Msun, 4U 1755-338 would have to be at a distance of 15 kpc for this to hold. For an assumed mass of 10 Msun, a distance of 26 kpc is implied. These values greatly exceed a prior estimate of 6.5 kpc and would expand the size of the fossil jets inferred in XMM-Newton images (Angelini & White 2003).

We thank Fiona Harrison and the NuSTAR planning team for executing this observation.

References:
Angelini, L., & White, N., 2003, ApJ, 586, L71
Church, M. J. & Balucinska-Church, M., 1997, A&A, 317, L47