# Measurement of the absolute Crab flux with NuSTAR

## TL;DR

This study uses NuSTAR's direct detector illumination at a large off-axis angle to precisely measure the Crab's absolute flux and spectral parameters, confirming long-term stability and improving detector calibration.

## Contribution

First measurement of Crab's absolute flux with NuSTAR at large off-axis angles, demonstrating a method for accurate flux calibration and spectral analysis.

## Key findings

- Crab flux measured with better than 4% accuracy
- Spectral index consistent with historical measurements
- Detector absorption parameters improved

## Abstract

We present results from a NuSTAR observation of the Crab made at a large off-axis angle of 1.5\degree. At these angles X-rays do not pass through the optics, but rather illuminate the detectors directly due to incomplete baffling. Due to the simplicity of the instrument response in this configuration and the good absolute calibration of the detectors, we are able to measure the absolute intrinsic flux of the Crab to better than 4%. We find the spectral parameters of the powerlaw to be Gamma=2.106 +/- 0.006, N=9.71 +/-0.16, in agreement with the values measured 42 years ago by Toor & Seward (1974). This suggests that the observed variability of the Crab is not part of a long term trend, but instead results from fluctuations around a steady mean. The NuSTAR observation also enabled improved measurement of the detector absorption parameters without the added complications of the mirror response.