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Spectrogram of a single tone complex signal has two dark lines?

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Spectrogram of a complex signal has two dark lines

I am trying to plot the spectrogram of a complex signal I generated. I have written code to generate this signal and plot the spectrogram. It works. However I see that there are two dark lines present within the spectrogram. I'm curious to know the reason for the existence of these. I have also noticed that by varying the NFFT, the dark lines tend wobble around.

enter image description here

I paste my code below:

import matplotlib.pyplot as plt
import numpy as np
import scipy.signal as signal



def plot_spectrogram(data,NFFT,Fs,ex ):
    plt.specgram(data, NFFT=NFFT, Fs=Fs)
    plt.title("Spectrogram of data")
    plt.ylim(-Fs/2, Fs/2)
    plt.show()
    plt.close()

    if ex:
        exit()


###  Parameters
F_offset = 250000           
Fs = 1140000       

###  Generate a digital complex exponential with phase -F_offset/Fs
fc1 = np.exp(-1.0j*2.0*np.pi* F_offset/Fs*np.arange(len(x1)) )
plot_spectrogram(fc1, 512, Fs,ex=True)